| Printers | ||
| MindMachine Associates Ltd | Document Ref: print02 |
Printers are an essential part of most computer systems. People generally prefer to read things on paper rather than on screens, and it is still easier to move information on paper than electronically. For many organisations printed material may be one of their main points of contact with customers.
Printers have a boring public image, particularly in a computer industry gripped by Internet fever. A computer printer is essentially ruled by mechanical and chemical technology – not generally thought of as hot-beds of innovation by comparison with semiconductor research.
Printer design has changed radically during the last 20 years. The typewriter dominated office life in the 1970s. Computer systems either used modified typewriters or gigantic band-printers for output. Laser and inkjet printers have taken over so completely that a 1970s’ typewritten script is now seen as retro kitsch – perhaps someone will develop a font to reproduce it. As for hand-writing, university faculties and schools often ban non-printed work.
Print technologies change rapidly. Daisy-wheel machines are museum pieces. Dot-matrix and band-printers have niches printing business forms, but are rarely first choice for documents. Another decade of battle is likely between laser and inkjet technology to determine which will dominate the market. Most printing is produced by laser photo-static machines at present.
One view of Information Technology might suggest that printers should disappear – their role taken by Internet browsers and Electronic Data Interchange (EDI). Most administrators would probably welcome a reduction in the burden of paper needing to be filed, and cutting down on paper might help preserve forest timber for more sensible uses. However there are lots of difficulties in the way of EDI so printers are likely to remain necessary for a while.
Printed paper will be a popular communications medium into the foreseeable future. The only real challenge to print on paper would have to be a cheap, light, foldable computer screen. Light Emitting Polymers do promise this, so computer-screen wallpaper is conceivable technology for the long term, but it is not likely in the near future.
Existing printing techniques are very flexible. Nearly every surface in an indoor environment is covered with either paint or some sort of printed pattern. Photographs, X-rays, decorations on crockery, labels on CDs and the labelling on boxes are all printing or related technology. Computer printing on cloth is moving from just applying logos to T-shirts to making pre-production samples, and will clearly scale up to full production. How long will it be before DIY shops offer custom-made wallpaper? Integrated circuits and electronic circuit boards are manufactured by techniques owing most to old-fashioned plate printing. Cheap, low volume production of multi-layer circuit boards could make interesting differences to the electronics industry. For instance, the emergence of new printing devices could move a significant fraction of manufacturing out of global-scale plants and back into local economies.
Printer technology is a cross-disciplinary art, using intelligent electronics to co-ordinate the work of complex sets of sensors and actuators. A significant driving force in the emergent field of nano-technology has been construction of print-heads. Research into the possibilities of ink and toner materials is a significant challenge to chemistry.
This report is mainly concerned to giving an overview of today’s technologies for printing on standard paper – impact, inkjet and laser. A typical aim for printer users is to produce something that looks as good as the pages of a magazine. Today’s printer technologies need to advance some way to meet this demand.
Since printing technology is not a settled issue it is probably wrong to expect a long life from any of the current designs. In the early 1980’s the Tally MT440 printer was designed for a 15 year life – a few are still in use. No modern printer is likely to last so long.
Choice of Printer:
Private individuals normally have the money and space for only one printer. For them, this report can be summarised in one phrase - buy an inkjet. We advise looking at the several varieties of Epson Stylus- the mono printer is cheap, the colour version reproduces pictures very well. Hewlett Packard also have a good track-record – we use an HP870C. Some people talk of "laser printer quality". Inkjet printers now tend to produce rather better looking material than lasers, often with a rather better defined black. Laser printers are generally a bit cheaper to run, but they are more expensive to buy so for low-volume use the inkjet will almost always be the better option.
Multi-function Printer / Scanner/ Fax machines are also be worth considering, especially for home-based business. Any computer with a modem can receive faxes, and can fax documents from the word-processor. A combined printer-scanner can have the flexibility of a desktop fax machine and double up as a copier.
Beware of bargain-basement offers from little-known manufacturers – consumables often cost a lot and may be hard to get in a year or so.
Organisations usually need lots of printers fulfilling several different roles and inkjets are not likely to be a first choice for most printing needs. The table gives a summary of what printers suit what tasks in condensed form:
| Function | Technology | Typical Models |
| Low cost, low quality printing, for picking lists, delivery notes, low volume invoices etc | Dot Matrix | Epson LQ2170, Panasonic, Citizen, Star LC10 etc |
| Home correspondence, College Assignments, some colour graphics. | Colour Inkjet | Canon BJ200 series, Epson Stylus, HP870 |
| Small business correspondence, some A4 graphics | Inkjet or Small Laser printer | Epson Stylus, HP870. |
| Small business correspondence and accounts with multi-part paper (Sage, Pegasus, SIMS) | Dot Matrix | Epson LQ2170, Brother 4318. Panasonic KXP36?? |
| Office correspondence etc for a mid-sized network. | 10-20 page per minute laser printer | HP4MV, Kyocera FS1650, Dataproducts 1560 |
| Despatch notes and invoicing for a regional depot. Payroll for large business. | High speed dot matrix | Brother M4318, MT490 ND680, PPI 405, IBM4232 |
| Box labels, consignment notes etc for major warehouse | Shuttle Printer | Tally 660 or Printronix. IBM4234 |
| Public Utility Billing, Mass mailing. | Bandprinter | Tally 660, Printronix, Fujitsu M304X series |
| Colour printing | Inkjet, Thermal Transfer or Laser | Epson Colour Stylus, Fargo Dye Sublimation. Minolta Colour Page-Pro |
There is more on where to buy printers and what price to pay at the end of this report.
Running Costs //?? This has no implementable information //
One of the main ongoing costs of running a computer is buying printer consumables. The cost of running an ordinary printer can be anywhere between 0.1p and 7p per page. It is not uncommon for a workgroup laser printer to produce 5,000 pages of output a month, so consumables alone could be costing over £1,000 per year. With a 70-fold difference in operating costs for common printers it is possible for large organisations to save millions of pounds by using the right printer for each task.
Smaller organisations may not have the space or budget for several printers, but a badly chosen printer can impose unnecessary running costs. Buying an extra printer – having both a colour inkjet and a mono laser – may save money within a year or so.
Printer reliability is an important issue. If a workgroup printer fails then PCs nearby can be virtually useless. A printer’s lifetime costs need to be considered, not just it’s purchase price.
The total costs of printing can be difficult to assess, but they should include:
- Cost of the printer itself - including paper trays and interfaces.
- Costs and life of "consumables" - ink cartridges, ribbons, printheads, toners, developers and drums. This is where printer manufacturers make their money.
- Costs of paper - including whether it needs to be to a particular specification and the overprinting costs of material like letterhead paper.
- Print volumes – ranging from under 100 pages a month for home users to 10,000 a month on office workgroup machines and 50,000 per month on machines used for catalogues.
- Requirement for colour – all printer technologies can produce some sort of colour printing – at least in principle. Inkjet printers have a massive advantage because the extra circuitry costs very little and all the extra works are in the cartridge. The cost of colour cartridges is quite low – but they usually have a short life.
- Requirement for multiple copies – only impact printers can produce 2 to 6 copies with one pass of the printhead.
- Requirement for special paper sizes- A3 printers cost a bit extra, A0 printers are in a completely different price class to A4.
- Paper cover – standard text covers only 5-7% of the paper with ink. Printing white on black covers 95% and the ink will therefore cost nearly 20 times as much. Printing solid colour will have similar costs.
- Electricity consumption – large laser printers typically consume half a kilowatt intermittently and about 100 Watts on average (about 10 –20 p per day). Printers with power saving standby modes can drop power consumption to under a Watt
- Costs of maintenance – including the nature of warranty and potential losses due to printer down-time.
- The expected life of the printer – most will be technically obsolete in 5 years, some will die sooner due to
- The potential benefits of document appearance. Some documents are high profile – sales material and quotations need to create a good impression. Other material is just part of the business process- delivery notes, invoices, purchase orders – most businesses would judge it to be poor taste to send these on letterhead – but they may be spending as much sending pre-printed multi-part forms. Some business try to stick to white-paper forms
Information on paper - the basics
There has been talk of the paperless office ever since the first commercial computers were produced. The reality for most organisations, however, is a mixture of computer and paper systems. Keyboards and printers are the main bridgeheads between the two media. Scanners, OCR software, E-mail the Web and EDI are coming into use – but a frequent reaction on finding an interesting web-page is to print it out.
In the absence of adequate Electronic Data Interchange systems, printers are also the link between an organisation and the outside world. A well printed document should help build a good image for an organisation. A well-presented web-site is still second-fiddle to paper.
Printed material has been around for a long time, so there are a lot of cultural expectations as to what it should look like. For instance, people would not accept a newspaper printed entirely in capitals on green lined computer paper. Such a newspaper might be perfectly readable and contain all the text of the "Sun" or the "Times", but it just wouldn’t feel right. It would be possible to produce a will or the deeds to a house on a cash-register tally roll - perfectly printed and perfectly legal, but not acceptable. Equally, people don’t expect a supermarket receipt to be printed on A4- even though it could be much more informative.
For some time the computer printer was quite successful in producing a paper style and cultural image of its own - the green lined paper mentioned above. People tolerated enormous sheaves of "print-out" on fan-fold paper even though they were difficult to handle and store. A whole genre of special furniture was created to store print-out. However the need for special stationery is fading, many printers now deal with standard paper sizes.
Although there have been a great many paper sizes and styles the trend is to handle just two: A3 for diagrams, A4 for written material. Standardising on paper sizes simplifies purchasing and storage.
A normal human eye can cope with any size of writing in between the smallest footnote and tabloid headlines. In correspondence and reports text is usually arranged in lines about 1/6th of an inch high, with approximately ten characters to the inch. The size of A4 paper therefore dictates that a page is about 80 characters wide and at most 66 lines long - a page of solid print can therefore contain 5280 characters.
The human eye reading a page does not generally pick up details less than 1/300th of an inch across, so printed material will look reasonably good if it achieves or exceeds this resolution. The smallest pens used by professional draughtsmen are 0.1mm (1/250th inch). Almost all computer printers work by placing dots on the paper. Old dot matrix printers achieve about 100 dots per inch and the print looks crude. Recent printers are achieve 200 dots per inch or better. Most dot matrix printers could, in principle place 600 dots per inch on the page – but this would not make much difference to the look of the print because the dots remain physically quite large.
A piece of A4 printed at 300 dots per inch contains about 8 million dot positions – equivalent to a megabyte of memory. Ten years ago this much memory was expensive, so many older printers have far less.
People quite often want bigger pages and there are uses for finer lines. A3 paper at 1200 dpi requires 253,440,000 dots – about 32 megabytes. Introducing CYMK colour could quadruple that number to nearly a billion dots. Clearly if these dots are produced individually the act of printing is not a trivial mechanical or data-processing task. Actions set in motion by pressing "print" in Windows are deceptively simple.
Human requirements in terms of perception and tradition set what the market requires of computer printers. Printer manufacturers move to meet demand. Making printers involves a substantial amount of electronics and mechanics, and a large investment to make the special consumables. Printer manufacturers are then able to compete on price using the scale economies they achieve in manufacturing and distribution. But the manufacturers cannot simply set about dreaming up the ultimate printer, then build this to a price. All printer designs are limited by what is technically possible at a given price. This is a continually moving target as better semiconductors and micro-engineered mechanical components become available.
Business printers intended for desktop use have generally fallen in price about 3-fold during the last decade, with capabilities that would have cost over £1000 in 1988 costing about £300 in 1998. Laser printer prices fell more rapidly - the capabilities that would have cost £5000 in 1988 cost about £300 in 1997.
Consumer market printers can be exceptionally cheap. The Star LC10 colour dot matrix machine retailed for around £100. Cheaper machines will probably be produced- the manufacturing complexity of an inkjet printer is similar to that of a portable cassette recorder, so given similar volumes a consistent retail price well under £50 should be attainable.
Printer manufacturers are beginning to run into a downward price spiral where each undercuts the other until a £50 price is achieved. At the moment they are avoiding this by putting more features into their machines –such as photo-realistic colour and the ability to use metallic inks.
Special- purpose printer prices do not necessarily fall. There are fewer economies of scale and not so much competition. Tally roll printers for shops, wide A0 printers for draughtsmen, credit card and CD- label printers carry high prices that might not decline.
Printing has always been part of the computing business. Some of the earliest designs for automatic printers date back to Charles Babbage’s designs in the 1830s. Early electronic computers in the 1950’s and 60s made use of modified typewriters and teletypes - the workhorses of the computer industry for many years were the teletype KSR33 and the IBM Selectric Golfball Typewriter. Laser printers dominate the market today – and may continue to do so where low page prices are a requirement. Inkjet technology can be more flexible. A colour inkjet printer can produce pages nearly good enough for a commercial magazine.
This briefing document gives an outline of the main computer print technologies:
- High speed, high volume basic printing for business forms is provided by bandprinters.
- Dot matrix printing offers medium speed, letter quality low cost copy on multipart stationery. Dot matrix machines are good for business forms and correspondence.
- Thermal printers provide a high level of flexibility and a very cheap mechanism. They are used in faxes, label printers and instrumentation.
- Inkjet printers can provide fairly high speed at fairly high copy cost, but they give laser quality and convincing performance in colour. Inkjet is a young technology that may take most of the market in the long term.
- Laser printers can provide high speed and high precision. However the laser printer’s basic electrostatic mechanism tends to imply less long-term design flexibility than can be achieved with inkjets.
- Printers generally need support from a computer’s operating system. DOS has no control over printers, the work is all done by individual programs. Microsoft Windows 3.1 has a rather inflexible print manager–Windows 95 is better. Windows NT 4.0 printing works well.
- The environmental impact of computer printing can be substantial - generally printers with lower running costs probably have a lower environmental impact.
- Substituting Electronic Data Interchange, bulletin boards, Internet or publishing on CD-ROM.
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Band-printers
Impact Printers
Band-printers are part of a family of printing devices called impact printers – printers that rely on transferring ink using a hammer. The design first appears with the typewriter and ticker-tape in the later 18th century. In the 1970’s impact printers expanded to a major industry, but they have been overtaken by other designs and are now a minor part of the market.
Impact printers mark the paper using a shaped print-character hitting a ribbon. The pressure transfers ink from ribbon to paper. The technology dates back to the early typewriters in the 1880s. Typewriter technology was used by the computer industry – some early computer printers were quite clearly modified typewriters. The Teletype KSR33 and IBM Selectric golf-ball were work-horses for many years. The basic problem was that this technology was slow, under 20 characters per second – at best a half-page per minute.
Daisywheel, golf-ball, drum and band printers all have different mechanisms, but share the general principle that the shape of the character is formed in one piece. This produces good-looking printed characters with only rudimentary electronics. The cost is usually more complex mechanics. One aspect of the complexity is the need to produce a perfectly formed typeface. In general, electronic systems fall in price but mechanical systems do not. The high price of mechanical assemblies has mostly killed the lower speed devices. Band-printers are the survivors.
Band-Printers
Band-printers are used in large organisations because they can achieve high-speed output at low cost. Band-printers themselves are among the most expensive printers on the market, costing in excess of £10,000 for a new machine, but the combination of high speed and low cost output justifies the high purchase price if a lot of printing is to be done.
Band-printer speeds are typically 1200 lines or 20 pages per minute. This equals all but the fastest laser printers. In practice a band-printer handling simple forms is usually very much quicker than a laser-printer because it can accelerate non-print areas of the form through the paper path in a way that a laser-printer usually cannot. The bandprinter has another phenomenal advantage over laser machines, a very low cost per page. A bandprinter can produce a page of print for about 1/10th of a penny, a laser printer page would typically cost 10 to 20 times as much. Furthermore the bandprinter can easily print on four-part paper, making the cost of each copy lower still.
Unfortunately bandprinter output is often ugly to look at. A typical machine can only manage a limited repertoire of 64 or 96 characters, and they are limited to output at 10 characters per inch. Crude graphics are achieved by subterfuges, like using arrays of asterisks, or overprinting one character with another to give shading effects. Users don’t want high quality so they economise on ribbon changes and maintenance.
Much bandprinter output is badly faded because the ribbon is exhausted, and the hammer-banks are often ill-tuned so that some characters are barely legible.
During the last decade the band-printer has lost market share to high speed dot-matrix devices, to the shuttle printer and even to special high speed laser printers. There are change in work patterns, which move operations away from IT centres and their band-printers towards local offices with laser and inkjet machines. However, band-printers are built to an old and trusted design and they can have a very long life.
The general principle of a bandprinter is that paper is carried through a print station on forms tractors. The print station consists of a platen, hammer bank, print band, the ribbon and ribbon shield.
The platen is nothing more than a straight, solid bar of metal with a polished face, the paper travels across the platen which is mounted so that it will represent one line of print across the page. The platen’s job is to cleanly absorb the impact of the hammers.
In principle there is a hammer for every possible print position on the page. This means that to achieve 10 characters per inch there must be 10 hammers per inch across the paper, and since standard computer paper is 14" wide there will usually be 132 or 136 hammers. To achieve a print-speed of 1200 lines per minute each hammer may need to move twenty times per second, and achieve a reasonable impact on band, ribbon and paper. This requires a momentary drive power of 200-400 watts or more. The hammer coils need to be quite substantial to deliver this sort of power, and this is difficult if the entire assembly is to be 1/10th inch thick. The spacing can be relaxed to a more comfortable 1/5th inch by arranging hammer mechanisms alternately up and down. The Fujitsu M304* family has eight hammer banks, four suspended down and four held upright, designed so that the hammers are aligned in a straight row across the page. Perhaps contrary to expectations, the hammers do not need to move much, they travel forward only 1/10th inch or so, what matters is the sharpness of their impact. Hammer power levels are high, but the duty cycle is short.
The print band gives the bandprinter its name. It is a flexible steel band about 40" long with the character-set embossed on its surface. Since the character set is usually only 96 characters long the whole lot might be repeated four times along the band. The band will also generally have a timing- track of marks for each character position embossed on it, together with an index mark to show the control logic exactly where the beginning of the band is. The band rides on two pulleys, one of which will be driven by a motor. The speed of the printer is determined by how fast the hammers can fire and retract, and by how fast the band travels.
One way of speeding up the print process is to use "statistically organised" bands. The character sequence is organised using statistics on character frequency in English, so there is more probability of a required character being under a hammer. This optimises throughput.
The ribbon is simply caught between print-band and paper. The ribbon shield prevents the ribbon from rubbing against the paper at other times, causing random streaking.
For each print position, mechanical operations are as follows. The machines logic determines which character is wanted, and what character is currently going past on the band. When the two match, the hammer is fired; this impacts paper, ribbon and platen together. The hammer must immediately retract or the continued travel of the band will smudge the outline of the character. This requirement means that the forward and backward travel of the hammer must be very rapid - rather more so than implied by the rate of 20 characters per second for each print position.
Electronic operations are fairly simple. The printer assembles a line of print it has received from the computer. It then matches each character of the line with the characters travelling past on the band. When the characters match, the hammer fires. When a whole line has been printed the machine can move on to the next line.
Recent Bandprinter designs use one or two microprocessors for control, but older designs achieved the same speeds using a small amount of old fashioned computer logic. Although it is fast, the bandprinters electronic inner workings are relatively simple.
Mechanically band-printers are quite complex. Hammer banks with 132 mechanisms are obviously quite elaborate devices. The basic paper transport mechanism is like that of any other printer – just bigger and more powerful for faster paper movement. Larger printers often have motorised forms stackers. A 1200 line per minute printer handling invoice stationery can output 3 or 4 pages per second so if the paper isn’t neatly stacked it quickly gets out of control..
Inflexible working is the main problem – graphical capabilities and fonts are limited to what is on the band. There is a design modification that can overcome this:
The band itself can contain nothing but dots. The characters are made up using a dot-matrix pattern, rather than by being pressed from a solid form. One obvious limitation is that each dot-position needs a hammer movement. If the characters are formed on a 9 x 7 matrix then 63 hammer-firings are needed to form each letter –and this would make the printer much slower than a machine using solid forms. This may not be as much of a limitation as it sounds because the mechanical actions needed to fire a dot-hammer can be speeded up considerably if it only comprises a dot. IBM 4234 printers use this kind of mechanism and achieve an interesting balance between speed and graphical capability.
Environmentally band-printers are quite benign – considering their ability to consume whole forests worth of paper. If a task demands a lot of printing then that paper was going to be used anyway. The ribbons are often quite large and expensive, but they use fairly conventional printing ink embedded in fabric and are not generally thought to be hazardous.
Almost uniquely amongst modern computer equipment band-printers sometimes contain some metal- so whilst it may not be worth much as scrap the parts have potential for re-cycling. The complexity problem applies – many of the components are intricate and dismantling them for efficient metal recovery would take several hours manual work.
Dot Matrix Printers:
Any shape can be made out of a complex pattern of points or "pixels" - picture elements. The technique is very commonly used; it is the basis of television and is used to display computer information on screens. It is possible to represent the upper-case alphabet on a matrix of 5*7. On a matrix 7*9 it is possible to form upper and lower case characters quite readably. Pixel graphics is the normal way to print characters – it is sometimes called "raster graphics".
Dot-matrix printers form characters by making a pattern of dots on the paper. The dots are formed by pins, usually carried in a print-head. In principle the print-head could carry a single pin which would scan the paper and form the image. In practice it is normal to incorporate enough pins in the print-head to form a whole line of alphanumeric print at one pass. Dot matrix print-heads commonly have 7, 9, 18, 24 or 48 pins.
Dot-matrix printers vary greatly in price and capability. Costs vary between £100 and £5000, and print-speeds vary between 80 and 800 or more characters per second (1 page to 10 pages per minute). Cheap machines are suitable for use as personal printers, the print-head and other mechanisms have a limited life. Higher speed devices are suited for use in corporate payroll and invoicing - competing directly against band-printers.
In a dot matrix printer the paper moves vertically across a platen, as it does in a bandprinter. At the point where the print-head contacts paper the pins are usually arranged in a vertical column. The print-head scans horizontally across the page on the carriage, forming the line of characters as it goes. The more pins there are within the print-head the better the print quality.
The pins in a dot-matrix printer are quite light, and can therefore move fast, at speeds between 1,000 and 2,000 Hz. This sort of speed is necessary, because characters formed on a dot-matrix machine will need at least 10 times as many hammer actions as those on a daisywheel or bandprinter. Printer designers have difficulty getting the speed of individual pins beyond 2khz so achieving high print speeds means providing more pins- there is more on this below.
The print process is as follows. The paper is positioned at a blank line, and the input logic assembles the line to be printed in memory. The print-head sets off from the left margin, and as it travels across the paper its position is tracked. For ten characters per inch the tracking might be in 1/100ths of an inch, but for closer character spacing it will be more accurate. Each print-head position corresponds to a column of dot-positions within the character to be printed. Columns of pixels are made by firing the pins in each position.
The shapes of the characters are usually stored at manufacture in a ROM in the printer. (In some recent printers the shape of the characters can be downloaded into RAM). As the print-head is moved to successive locations the character and dot position required are looked up and drive circuits push the pins accordingly. A basic 9 pin print head might fire the top pin 9 times to produce the top bar on a "T", but scarcely ever fire the two bottom pins that produce the "tail" on letters like "q" or "g".
The high speed action required from the pins on a dot matrix printer demands quite a lot of electrical power - pins may draw peak powers exceeding 100 watts although the duty cycle on each coil will probably be under 5%. Looking up the dot pattern required for every character and dot position also requires a moderate amount of processor power - a dot matrix printer will commonly have one processor for general control and another handling the print process.
The dot matrix printer has become so commonplace that its way of operating has defined the market. The dot matrix machine can manage various character sets selected by escape sequences and control codes. The development of the dot matrix machine has produced an expectation that printers will function by receiving trains of characters with embedded control codes.
Dot Matrix machines can produce graphics. Instead of a pattern being looked up for every print position, the print-head is told exactly when to print dots in a manner corresponding with the picture. This is not quite as simple as it sounds, because the printer memory needs to store all the dot positions required by 9 or more horizontal lines before the head sets off from the margin. Some old printers have small memories, so they shuffle the head across the page an eighth of a line at a time. Another problem is that the pins are arranged to deal with text at 6 lines per inch, with gaps between the lines. The platen may have to move and the print-head go back to fill in gaps. Some dot matrix printers perform abysmally when dealing with Windows output; inefficiency in the control process is the reason. The Epson MX100 (an old machine) is very slow printing graphics. The Epson LQ1170 gives a really impressive Windows printing performance at inkjet –like speeds.
The maximum speed of a dot-matrix printer is set by:
- the speed that the pins can move – not much faster than 2-3 khz
- the number of pins in a head - 24 or 48 at a maximum
- the quality of print required - how many dots are required to form an image
A double-column head has an additional advantage because it can be made to function as a single head producing good quality, or to have the effect of two heads giving high speed but lower quality. Each column of pins fires at 2kHz, but the pair behaves as one column running at 4kHz.
Mechanical problems make it difficult to put more than two columns of pins into one print-head, but this needn’t limit printer speed. It is quite possible to put two print-heads into a machine, one dealing with the left hand of the print, the other with the right. Newbury Data 680 printers have two heads each with two columns. Printers with four heads have been produced and were quite fast. Large numbers of heads make the carriage heavy, however, so the horizontal motor needs to be more powerful to handle the weight.
It is also possible to arrange the pins across the page to simultaneously print a row of dots. Since each character will be ten or more dots across, a 132 character page will be 1320 dots across- this would need rather too many pins to be provided on a practical printer, although it would be phenomenally fast.
One solution is to arrange a page-width of pins on a moveable cradle. The number of pins is cut tenfold to a manageable 132 or less, and the cradle is driven rapidly backwards and forwards. The result is a device called a shuttle printer.
Shuttle printers can produce ordinary characters and graphics with equal ease. The technology is well developed, so the machines can deliver a low price per page. Unfortunately shuttle printers are mechanically quite complex, so they are expensive.
In principle it might be possible for the pins and ribbon of a dot-matrix printer to be miniaturised add-infinitum to produce any dot-pitch. High quality machines tend to have 24 pins arranged in a column 1/6th of an inch high, giving a vertical resolution of just 144 dots per inch. By making the pins rather smaller and offsetting them some dot matrix machines claim a resolution of 360 dots per inch. Using a carbon-coated film instead of fabric ribbon improves the print further. 48 pin printers have been tried, but the miniaturisation of the coils, pins and jewels in the print-head seems to have been too extreme and some designs have been unreliable. It is also difficult to repeatedly position the print-head to within 1/300th of an inch. In practice, impact dot-matrix machines produce rather poorly defined line art pictures. A pin can only be fired or not - so it is not possible to produce shades of grey and a dot-matrix machine can only make a crude attempt at printing photographs.
Colour is relatively easy to achieve with a dot-matrix design. The usual approach is a ribbon-lift mechanism. The ribbon has stripes in several different colours – cyan, magenta, yellow and black for instance. If the printer needs to mix bands to achieve the colour the head makes successive passes with the ribbon lift positioned.
Dot-matrix and shuttle printers transfer ink onto paper by impacting on a ribbon. Impact on a ribbon was the mechanism used by the original typewriters, it is widely understood and cheap, but it does not seem capable of producing images that look good to the human eye.
Impact printer designs have reached a point where few real improvements seem possible without changes in the underlying technology.
The problem with impact printers using pins, bands or daisy-wheels is that they rely on non-repeating oscillatory motion at a scale of about 1-2 millimetres. The power delivered at one end of the oscillation must be sufficient to drive ink off a ribbon into the paper, and it must be possible to start and stop motion within a cycle. It does not seem possible to get the frequency of oscillation above 3,000 per second. It also seems impossible to reduce the tip of the print element below 1/10th millimetre – around 250 dots per inch. A smaller print element tip would wear rapidly in contact with the paper and be very prone to accidental damage. Unless these limits are overcome impact printers cannot give fine-grained images.
New materials could give new life to the old designs – what seems to be needed is a pinpoint-thin object capable of moving much faster than 3,000 times per second with a "throw" rather more than a millimetre. Piezoelectric ceramics and electro-mechanical polymers might help transcend today’s limits.
In the absence of such a design manufacturers have turned to other print techniques
Photochemical Printers:
A cheap, fast, reliable method of producing fine grained images is required. One obvious method is to use a cathode ray screen of the kind used for monitors and TVs, and try to capture this photographically.
Specialist devices of this kind used to be quite popular in scientific labs and the graphical trades. For instance many older oscilloscopes and other instrumentation screens can have a special Polaroid camera mounted on them.
Output direct to microfilm was popular before computer terminals were commonplace – some large organisations tried to move the greater part of their paperwork onto microfilm in a move reminiscent of today’s interest in workflow systems. Microfilm is still sometimes used to provide an audit trail, and it is likely that film media will remain readable in the indefinite future when today’s tapes and CDs are obsolete.
Print houses generally have specialist machines to produce lithographic plates.
Honeywell made a printer that could output directly from cathode ray tube onto a role of photosensitive paper. This weird printer was popular in training departments where it could be used to directly capture what a student had done from screen to page; unfortunately the price of over £2,500 and the average repair price of over £500 was not so popular. Photo-chemical technology has never caught on as a basis for computer output devices because the price and inconvenience have rarely seemed attractive.
Thermal Printers:
Thermal printing has similarities to photography but the paper is thermo rather than photo-sensitive. Photosensitive materials are difficult to handle because we live bathed in light - we can’t see without it. Thermal devices operate in the infra-red spectrum. Thermo-sensitive papers do not turn black immediately on exposure – although they do go brown with age.
A thermal print-head contains small infra-red emitters which produce a dot-pattern on the paper. Thermal print-head elements have no moving parts and are less costly to manufacture than mechanical devices. The print-head is basically a piece of ceramic with embedded heaters. Mechanically this object may be no more impressive than a pencil-thin ceramic bar with metal brackets and some relatively simple control and drive electronics.
One great advantage of thermal printing is that it is possible to make a thermal print-head to match the required paper width - every dot position across the page has a corresponding element in the head. All that needs to be added to make the printer is a motor to drive the paper across the print-head. Thermal printing has been widely used, particularly in fax machines. Weighing and labelling machines also often have a thermal printer. Using several wide heads it is possible to make a fairly fast printer capable of enormous page-widths, Roland have made some of their large plotters this way.
Thermal printing is silent – the only noise is that of the feed motor and the creaking of the rubber traction wheels on the paper.
Because they have few moving parts thermal printers can be very reliable and cheap to make. However the part which inevitably goes wrong is the print-head, and when it fails it can cost nearly as much to replace it as to buy a new printer.
There are several problems with thermal printing. The paper has to be coated with a material that is very sensitive to heat at a certain wavelength. The coating material adds to the cost of the paper. The print process is fairly fast, but even with an element in the head to correspond with every dot across the paper it merely competes with the fastest dot-matrix or with laser printers. Thermo-sensitive paper is not completely stable in a standard office environment. Over time, the images tend to fade and the paper yellows badly in sunlight, in the worst case putting a cup of hot coffee down on thermal paper will print black rings on its surface and blot out the writing.
The print-head is in direct contact with the paper. Over time the paper polishes and erodes the ceramic, so parts of the head do not make good contact with the page. Print-heads eventually wear away and the only possibility of repair is replacement. Thermal head manufacture is a complex process so if the part is obsolete repair is not possible.
The paper used in thermal printing clearly does involve some exotic compounds, although there seems to be some secrecy about what the ingredients are. There do not seem to be any allegations that fax paper is a health or environmental hazard.
The standard complaint about thermal printing is the cost of the paper. "Plain paper" faxes using inkjet or laser output gained a foothold in the fax market because of the cost of the paper. In the last couple of years the price of thermal paper has fallen – and is often quite comparable with laser printing and cheaper than using an inkjet. Beware of low-cost plain-paper faxes that use inkjet technology and cost more to run than a thermal-paper model would have done. Thermal printing will probably continue to be used for applications where simple reliable printing is the only demand - retail product weighing and labelling, portable printing and instrumentation logs. The fax market may disappear because of the growing use of computers for document-imaging and E-mail as a way to move information.
Thermal wax transfer uses a print-head similar to that in the thermal printer, but the process involves melting a substance in a dot-matrix pattern onto the paper. The wax is carried on a plastic backing ribbon. The process is normally used only where brightly coloured images are desirable. One layer each of cyan, magenta and yellow are moved between the head and the page, with the relevant areas being transferred by heat. To print a colour image on a piece of A4 paper the printer will use 33 inches of backing material. The consumables for this kind of printer are clearly costly. However thermal wax printers can produce some interesting effects- one is that an image can be placed onto a backing sheet and then transferred by hot ironing onto a tee-shirt (inkjets can do something similar with special paper)
Two technologies are now dominating the printer market, inkjet and laser printing.
Laser Printers:
Laser printers currently lead the computer printer market. There are probably more laser printers in use than any other print technology. Inkjet printers are cheaper to buy, but they are slower and significantly more expensive to run. Inkjet printers are cheap to make so they are likely to outnumber laser machines, but they have always been expensive to run so volume printing is likely to come from laser printers.
Laser printers are based on the same process as the photocopier. A few early laser printers were built from photocopier innards. Kodak sold a machine that incorporated both photocopier mirror scanner and laser scanner in one chassis.
Fifteen years ago the market demand was for photocopiers, with a significant developing market for computer printers.
In today’s market the main demand is for printers, but there is still strong demand for copiers.
The technology used in copiers and printers is much the same, but older copiers tend to be a bit more complicated. Photocopiers need an optical / mechanical scanner to reduce the page image to a thin stripe that can be copied onto the drum.
The combined copier –fax-printer devices on the market today are generally computer CCD scanners sitting on top of a printer and combined in one neat, office-friendly unit. The innards of such a machine are purely a printer – so the laser printer has taken over the photocopier’s technology and it’s role.
Overview of the Laser Print Process:
The "laser" plays a small part in the overall printing process, which is based on photo-conductivity and static electricity. Photo-static imaging process is a more accurate description – but lacks brevity and marketing punch. The basic principles used in the laser printer actually date back to the 1940’s and might properly be known as Xerography.
At the heart of the photocopier and laser process are two physical principles
- Photo-conductivity. Some semiconductors and plastics are photo-conductive, they act as insulators in the dark, but become conductive when exposed to light. A photo-conductive material can be charged to a high voltage in the dark. A picture can then be painted in light on the surface, and this will discharge static electricity wherever the light hits. The image is now stored in static on the photo-conductor surface.
- Electro-static attraction – materials can carry a static electric charge and be attracted or repelled from one another depending on it’s polarity and strength. A fine dust called "toner" is given a charge so that it is attracted to the areas of the photo-conductor that carry the image and repelled from others.
When the drum is in darkness it’s surface acts as an insulator. The drum surface can therefore be made to pick up a strong charge of static electricity as it passes near a wire charged to several hundred volts.
The image to be printed is painted on the drum by scanning it with laser light. The laser scans from side to side forming a pattern called a raster. Other sources of light can be used – some printers use an array of tiny LEDs. Photocopiers often illuminate the document brightly and use a lense to focus a thin strip of document onto the drum.
Exposing the surface of the drum to light from the laser discharges it’s surface charge into the metal beneath. When it has been raster scanned the drum carries an invisible image in static electricity.
The image in static electricity needs to be developed – made visible. The developer allows small particles of plastic dust to migrate onto the static-charged surface of the drum.
The developer is usually a hopper filled with a fine plastic dust called "toner" – named because it is the colour-bearing material. The developer rollers bring an even coating of fine plastic dust close to the drum. The plastic dust grips the charged areas – but not by those with no charge.
The image is now sharply defined in toner powder, and the drum rotates further until it is over the paper. At this point, drum and paper are also over a charge-wire or roller sometimes called the "transfer station". The transfer station’s charge attracts the toner off the OPC and onto the paper. Because the drum rotates at the same speed as the paper is moving the image is transferred between them with precision. The paper now carries the image outlined in plastic dust. The paper moves to the fuser.
The drum rotates on past eraser lights and a cleaning blade which removes any surplus toner from the OPC. The drum is then ready to go through the print cycle again.
The plastic dust on the paper will smear if it is touched. This image is fixed to the paper by a device called the fuser. The fuser has two rollers, one of which is heated to a point where the plastic dust will melt. The plastic melts onto the paper and so is "fused" to it. The printed paper is now ready for use.
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Obviously this description of the photo-static imaging process is very broad. There are a great variety of photo-conductive materials, developer mechanisms, transfer voltages and fusers in use.
More than 20 laser-printer manufacturers are competing to produce machines that print faster, give higher resolution images, better grey-scale handling for photographs, lower copy prices, and easier handling for users. Laser-print technology has advanced rapidly in the last fifteen years– one result is that printer prices have fallen significantly.
A point worth noting with laser printers is that the manufacturer label on the front and the maker of the actual print-mechanism are often very different.
Some companies have a historic strength producing photo-copiers but initially missed the potential of the laser printer market. Canon and Minolta both seem to be examples.
A large part of laser-printer performance is determined by software. Two printers with the same engine can behave very differently. Hewlett – Packard have used Canon Laser print engines to achieve a strong market position. In principle the Canon machines work just like their HP equivalents – but the software has often been a bit less easy to work with.
This distinction between what the print-engine can do and what users expect of printers is widening as the software found on PCs becomes more complicated.
Laser Process Details
| Check //////// | |
| Raster Image Processor | |
| Communication Hardware | |
| Communication Software | |
| Laser and Optics | |
| OPC drums and belts | |
| Developer | |
| Transfer | |
| Fuser | |
| Paper feed mechanisms | |
Raster Image Processing
The image in the computer has to be downloaded into the printer and converted to the patterns expected on a printed page. This is not quite as simple as it sounds. The transformation may be complex and there can be a lot of information.
Computers can hold material in several different ways – for instance:
- Word-processors are likely to hold information as strings of ASCII or Unicode.
- Vector-graphic programs like Autocad hold information mainly as numbers.
- A scanned image will probably be held as a bitmap.
Somewhere between the computer and the raster scanning mechanism in the print-engine the codes used inside the computer have to be transformed into the pixels to be placed on paper. This is the task of the raster image processor.
Some sort of raster image processor (RIP) function is carried out in many printers including those using dot-matrix and inkjet technology. Laser printers are different because the conversion has to be done quickly. Laser printers often have three circuit boards –
- communications interface dealing with Input output
- RIP taking printer control codes and turning them into a printable image
- engine controller sequencing the operation of motors and clutches
The RIP needs this power because a page can contain a lot of pixels. An A4 page measures about 8" x 11", and at 300 dots per inch it will therefore contain 7,920,000 pixels – nearly a megabyte of data. An A3 (16 x 11) page at 2,400dpi contains 128 megabytes of data.
Until quite recently many PCs did not have a spare megabyte of memory in which to hold a page image – many still do not have 128 megabytes for large and detailed images. It would also take a long time to send such an image from PC to printer over some kinds of communications interface.
Dealing with the different ways that computers might hold information, and a legacy of different communication methods means the print process is usually much more complicated than just sending a stream of pixels to the printer.
Material to be printed may come to the printer in several different ways:
- bit-mapped graphics: the computer sends a stream of pixels to the printer, usually packed 8 to a byte. If the entire page image is sent this way the computer acts as it’s own RIP and no other is needed. Because there are so many bits on a page (typically 8 million for low-resolution A4) moving them may take some time.
- ASCII characters to be mapped one by one from a ROM in much the same way as would happen with a dot-matrix
- downloaded font: similar to mapping from ROM. The computer sends the bit-patterns to be used for each character at the beginning of a task, and the printer stores and handles them as though it were using a ROM.
- an Adobe Postscript or other page-description language. Page layout is broken down into a succession of phrases describing the image on the page in the pigeon-English of a programming language. A page description language can be very efficient and precise for basic graphics, but may still need to use bit-maps to handle photographic material.
Bit-Maps Versus Print Languages
A low resolution A4 page of output from a laser printer is made up of about 8 million pixels that can each be set dark or light. This information can be held in computer so that a pixel on the page corresponds to a memory-bit in an array. The computer holding the image can be inside the printer – or it can be the machine on the users desk.
It would make sense for the user’s computer to generate the page image and send this to the printer. This is done:
- by low-cost "GDI" printers that rely on the users computer for their processing power
- by some printers intended for professional design that have an external RIP
Laser printers came into use before graphical operating systems were common. Competition was from dot-matrix and daisy-wheel machines, and the primary workload for these was textual – for the most part they imitated a teletype. The typical desktop computer ran Microsoft DOS and had a memory of 640 kilobytes or less. DOS machines could not hold a bit-map graphic for an entire page. PCs were only exceptionally used for graphics with programs like AutoCad. The launch of the Apple Mac computer in 1984 created the first serious interest in graphical computing, and it was another decade before the take-up of Microsoft Windows made graphical computing universal.
Printer design is still adapting to a world where documents are prepared on powerful PCs with a graphical interface. For instance
- A typical laser-printer is shared between several users, so it makes some sense to place the large raster image memory in the printer and share it.
- The Microsoft Windows operating system is widely used but is not universal – there is some competition from Apple and Linux.
- Printers tend to wear out more quickly than other components of a computer system, and quite a lot of computer systems last a long time.
Bit-Maps
Bit map graphics are widely used:
- A lot of graphical material is handled as bit-maps. Information from a scanner or a camera naturally originates and makes its way through the computer in this form – although it may be compressed as a JPEG or GIF for storage.
- Some printers are entirely fed by bit-map images. Until recently printers that used pure bit-maps were rather unusual – the machines that print out microfilm images were an example. In the last five years it has become more common for low-cost printers to be fed nothing but the graphic image to print – these are known as GDI printers.
- Bit maps need significant memory space – a 1024 x 768 colour photo take more than 2 megabytes. It is only in the last decade or so that such memory has been commonplace and low-cost
- Although the computer might well hold some material in bit-map form this may not be printable. The bit-maps held in the computer often contain grey-scales and colour. The laser printing process cannot easily use grey-scales, so to produce a photographic appearance the computer bit-map has to be transformed to a dither-pattern in which 1 pixel from the original bit-map is made into several.
- Communication from computer to printer can take a significant amount of time. Faster interfaces such as Ethernet, EPP and USB make communicating bit-maps more practical.
- The range of meaningful computer operations that can be performed on bit-maps is limited. A computer can easily search for the word "tiger" in ASCII code, but it is much more difficult to search for a bit-map picture of a tiger.
Allocating 2 megabytes of memory to hold an image is trivial in comparison with the 64Mb memory of typical PCs.
PCs need powerful processors to handle Windows-style display screens. Since the display screen may be expected to handle moving images and a printer cannot do so the power of the PC processor needs to be significantly greater than what is needed for printing.
Programs to make intelligent use of bitmaps are becoming available. Optical Character Recognition (OCR) software has been around for many years. Visioneer’s scanning software uses ZyIndex to unobtrusively index material in the background. Programs that can index photographic material are beginning to appear.
GDI Printers
A new generation of printers using the PC processor to do the work of the raster image processor appeared as Microsoft Windows machines became common
These machines are often called "GDI" printers because they rely on the Microsoft Windows Graphics Device Interface for their print capabilities. Microsoft has since re-defined their graphics interface - but the name still sticks.
GDI construction reduces the amount of electronics needed in the printer because the user’s computer acts as a Raster Image Processor (RIP).
In principle the printer could be an engine with scarcely any electronic components – control for the motors could come from the PC.
The GDI approach to printer construction seems to make a lot of sense
- PCs based on powerful processors, with ample memory running Microsoft Windows graphical operating system are in widespread use.
- There is no need for the expense of a powerful processor and large memory for the RIP.
- Driver programs in the computer have close control over the printer – so if there are any unsatisfactory aspects to printer operation (as often happens) upgrading is just a matter of downloading updated drivers from the Internet.
- Users want more complicated tasks from printers:– forms layouts, colour on overhead transparencies, full colour pictures, use of specialist inks. New needs can be met by a complicated printer control panel – but it is better to build the control into a local PC.
- Printers are often difficult to control. A graphical representation of the printer and its works on the screen is likely to be more help than LCD panels and flashing lights– putting the manual into on-line help files might also be useful.
- The Internet makes it possible for manufacturers to support upgraded printer firmware and drivers without the need for human technical support. Some large printers might benefit from remote diagnostics.
- Reliance on the computer operating system can be very heavy. The first generation of GDI printers were designed for MS-Windows 3.1 and may not work under Windows 95 and ’98. The printer manufacturer might provide updated drivers for printers two years old – they are not likely to do so when five years has passed and the printer is "obsolete".
- There may be no support at all for Apple Mac and Linux.
- There may be no support for Microsoft NT. To get the graphics drivers to operate quickly Microsoft allowed them to run in "Kernel" mode on the processor – a potential breach of NT’s integrity. If print-drivers do the same the integrity of NT begins to spring holes.
- The extra load of the print-process on the computer may or may not be perceptible. A user with a GDI printer may set a print task away and then wait to do anything else. If the printer is available on a network then the computer user will notice a slow-down when it is in use.
- If the computer crashes then the printer crashes too – not desirable in a network printer. Microsoft Windows generally seems more crash-prone the more tasks it is asked to do, so GDI printers are likely to cause crashes.
ASCII Codes and Emulations
Most computer printers still include a rather basic method of communication that dates back to the early years of the 20th Century and the invention of the teletype.
Early computer printers were often based on teletype mechanisms and could only print a fixed set of alpha-numeric characters. The computer sent a code representing the character to be printed and the printer carried out the action. The 26 upper and lower case characters, decimal numbers and common punctuation marks can be represented by about 100 codes.
The 7-bit character code still widely used by computers is known as ASCII (American Standard Code for Information Interchange). A printer using ASCII code prints character positions left to right across the page, from top to bottom of the page. There are special non-print characters for new-line, new page and to control the communication process itself.
A standard A4 page of printing has 66 lines 80 characters long – so at most it contains 5,280 characters. Holding and transferring this volume of data is undemanding.
Teletypes, daisy-wheel printers and band printers hold the shape of the character as a mirror-image raised pattern on their print mechanisms. Dot matrix, inkjet and laser printers hold the dot pattern for the characters to be printed in pre-programmed memory. To form the characters the printer software maintains counters that step through character positions, columns and rows. VDU’s and graphics adapters work in much the same way.
Laser printers are not quite like a dot matrix printer or a display because the dot patterns that have been looked up are moved to the raster-image memory before use. The temporary memory or "raster image" is necessary because once the print-engine is set in motion the laser must be fed with pixel information continually. The print engine cannot stop whilst the processor looks-up information in the character table. The raster-image of the page in memory is simply relayed to the laser to be painted on the drum.
ASCII character transmission was just about adequate for typewriter based mechanisms where the only font available was built in to the machine. It was less satisfactory as graphics came into widespread use.
Printer designers first extended the range of characters available. Since computers generally hold data in 8-bit bytes the 7 bit ASCII character set could be extended. Some graphical symbols and characters common in European languages were added. The most common character set is that used in the monochrome display adapter (MDA) on the first IBM PCs.
The "block graphics" characters can produce lines and boxes that can be used to outline boxes on forms, but they are not very satisfactory for anything else.
A range of fonts can be handled if they are programmed into the printer’s ROM. The computer sends a selection-code telling the printer which font to use. The code usually starts with a special ASCII character "Escape" which tells software looking at the data stream that something different is to be done with the character sequence that follows. Printer instructions are sometimes called "escape sequences"
Rival printer manufacturers produced several different kinds of selection-code. The IBM ProPrinter and Epson FX printers were popular. Although these were not laser printers it made some sense for printers to be able to use the codes.
To sell printers in another manufacturers market the first requirement was to imitate their code, so printer manufacturers commonly "emulate" one-another’s codes. The computer industry therefore talks of "emulations" – meaning escape sequences and control codes.
Most emulations include a code that allows the computer to download bit-map graphics. They may also include codes for vector graphics and other drawing elements. In principle it is possible to produce any sort of graphics using ASCII codes and bit-mapped graphics. However slight differences in the way emulations have been programmed sometimes gives mistakes in the output.
If a printer is sent codes for an emulation it does not support then it will generally print junk.
Downloaded Fonts
Printer fonts can be selected from a pre-programmed memory using escape codes. Most printers can also set aside an area of memory and the computer can download fonts into it. The font will need to be downloaded every time the printer is powered up, and if the computer and printer have a slow communications link then the download time may be noticeable.
Downloading fonts gives some increase in flexibility – for instance the new fonts don’t have to be alpha-numeric – they can be made up of graphical symbols.
A further increase in flexibility if the printer RIP can be told what scale to reproduce the fonts.
One of the benefits of laser printer technology, however, is that graphics need be no more troublesome to print than any other shape. This benefit cannot be properly used if the communications between computer and printer are all based around character-set fonts.
Page Description Languages
Kyocera was one of the first companies to produce low-cost high performance laser printers. They also created a language "Prescribe" that could place graphics and fonts on the same page. Prescribe used English phrases and mnemonics that resembled the Basic programming language and was reasonably easy to use.
Hewlett Packard’s LaserJet series used Page Control Language (PCL).
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A dot matrix printer will generally receive enough data to print one line before the carriage sets off from the left margin. There have been designs that can print a character at a time if required, but these are rare. A laser printer cannot easily stop once it has started, the succession of rollers that form the pickup roller, drum, developer and fuser need to be synchronised with one another and to move at a constant speed, which in turn is synchronised with the polygon mirror. The printer’s control processor must be sure that all the data for the page makeup will be ready when it starts to print. The need for synchronisation as a page moves produces a speed problem. In a dot matrix printer the limiting factor on how fast the printer operates is normally the speed of the pins, the processor can be weak, because it can easily keep up with what the pins are doing. In a laser printer once the page has been downloaded the user wants print immediately, but the raster image processor has to work out the layout. Laser printer processors are usually fairly powerful to allow this. Slow old machines like the HPII use 68000 16 bit processors. The Ricoh / DEC LN03 uses an 80186, the rather more recent Ricoh LP1200 uses a 68030, and the Compaq Pagemarq / Xerox 4025 series use the AMD29000 series RISC processors.
The processors used in laser printers are quite commonly equal to those that run a users PC, and there may be a substantial amount of memory as well – typically 8 megabytes and possibly more for machines intended to handle graphics. This duplication is clearly wasteful and expensive. Until quite recently a desktop PC with 8Mb of memory would cost £1000 +. Printers with powerful processors cost more.
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With the layout approaching completion the printer can set the paper in motion and write the first line of dots onto the drum.
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Communication
Parallel Interface
The de-facto standard for printer communication is the parallel interface – sometimes called a "Centronics" interface after the company that originated it.
The parallel interface uses a lead that usually has two different plugs:
- The printer plug is 36 way – (typically Amphenol 5730360). Nearly half the connections in this plug are used for grounds.
- The PC plug is a 25 way male D plug – a few of the grounds are sacrificed to allow this smaller plug to be used.
The parallel interface has several limitations.
- Plugs and cable are big and use crude screw locks. Most printers are not small, but clumsy plugs don’t meet modern expectations.
- Cable length is limited to 5 metres or less. Many sources quote 2 or 3 metres but the more relaxed standard generally works. This is too short in many cases.
- The interface can function at up to 2 megabytes per second – but can be much slower with older designs of PC and printer.
- Bi-directional transmission is a relatively recent capability – older PCs and printers often do not support it. Of course most data is going from PC to printer – but return information can show what paper is currently loaded and the progress of the job.
RS232
RS232 was designed for modems but has been a popular option for connecting printers
- Fairly long lines are possible. A line length of 15 metres is within spec, but cables up to 50 metres can work.
- Cables can be simple – if the printer supports X-on / X-off then just 3 wires are needed
- maximum speed is 10 kilobytes per second. At this speed a page of densely printed text takes just half a second or so to move, but a page of graphics may take two minutes or more.
- RS232 often allows a trade-off between line length and speed. Using a slow RS232 serial data-cable can add a very heavy overhead to the time it takes to print a page of graphics
In most cases there is no need for RS232. If long lines are required parallel-port extenders can operate up to a kilometre. If close integration with the rest of the network is required then Ethernet can operate lines 100 metres long and print-servers can cost under £100.
Ethernet
Ethernet is the most common networking standard – more than 80% of new network installations are Ethernet. Fast laser printers generally have an "option slot" in which an Ethernet adapter can be fitted – manufacturers sometimes include the interface as standard.
Ethernet was intended as a general-purpose standard, but it presents a couple of significant difficulties for printer support –
- Ethernet needs a fairly powerful microprocessor to support it. Furthermore the processor handling Ethernet must be able to handle network events whenever they happen. Printers often have a powerful processor - the Raster Image Processor (RIP). However this is usually dedicated to building the page-image in memory and cannot be interupted by the network. Printers with an Ethernet interface have a dedicated processor to support it.
- Several control parameters have to be set up. Printers with an Ethernet interface do not usually accept and print anything sent to the correct Media Access Control address (MAC). They generally expect data to obey a protocol –
- IPX – a semi-proprietary Novell protocol now sliding into obsolescence
- TCP/IP – an open standard but one that requires several parameters to be set
Ethernet is rather unusual amongst connection standards because it is "scalable". If 10mbps Ethernet is too slow plug in a switch and run part of the network at 100mbps. If 100mbps is too slow use Ethernet trunking or gigabit Ethernet.
Since Ethernet is so flexible and can scale up to meet users demands it is surprising it is not included in more printer designs. The key problem seems to be the cost of the control processor. Printer manufacturers quite commonly try to charge as much as £300 extra for an Ethernet adapter.
If Ethernet connection seems like a good idea but the printer either doesn’t have the option or it is overpriced a good alternative is to use an external print-server. External print-servers do exactly the same job as an internal interface but use a standard parallel cable in order to connect to the printer. Some Ethernet to parallel interfaces are built into a large parallel plug.
USB
Universal Serial Bus (USB) could well start to replace the parallel interface that has been universal until now. USB offers
- similar speed (up to 12 million bits per second)
- longer but less obtrusive cables and connectors
Appletalk / LocalTalk
Apple’s LocalTalk is RS422 using a mini-DIN plug and working at 230.4 Kbps. LocalTalk was a considerable advance when it was introduced - it might be described as a cross between RS232 and USB. Some inkjet and laser printers offer a LocalTalk interface – particularly those aimed at the graphics market where Apple machines have been enduringly popular. Apple have now adopted USB so LocalTalk is part of the "legacy" market.
Lasers and light-sources:
The laser’s function is to paint the image onto the drum in more or less the same way that the beam in a TV tube paints the screen. The speed and accuracy of the whole machine is set by how well the laser does its job.
Lasers are used because they give a fairly powerful beam of focused light, and because the beam can be very rapidly modulated - turned on and off. Other possible light sources are not designed to be modulated at the required rate of several megaherz. A 20 page per minute A3 machine scanning at 600 dots per inch has under 3 seconds to paint nearly 64 million dots onto the OPC and so requires a laser modulated at over 20 million bits per second. Fibre optic communications lasers are designed to operate at 100 Megahertz or more (up to 4 GigaHert) nd the need for speed contributes markedly to their cost. Printer lasers do not need to be quite so fast, but they have to deliver a sizeable quanta of light to the OPC material so they need to rather more powerful. The demand for fast laser printers is one of the driving factors behind low-cost semiconductor laser technology.
The laser beam is a single point of light, and this is made to scan across the drum by mirrors.
It would be possible to sweep the laser across the drum by physically aiming it, but although the laser is quite a small device most ways of moving it would be too slow. Consider that a slow laser printer produces 4 pages per minute. Even if those pages follow one another immediately head to tail there will be 44" of paper moving through the printer. At 300 dots per inch that means 13,200 lines must be scanned in a minute. In fact there are substantial time delays between the passing of each sheet, so the laser has to scan twice as fast when paper is in position- say 25,000 lines per minute. There are not many rotating components that can move at 25,000 rpm – the fastest disk drives only achieve 10,000 rpm. A 20 page per minute printer would need more like 125,000 rpm – not attainable with simple components.
A major component of most laser printers is a device called the polygon mirror. Many people have seen the balls of mirrors that hang in discos and dance-halls. When these rotate in the beam of a spotlight , they sweep a series of small patches of light around the room. A watch-glass will also act as a mirror, slight changes in the angle of the wearers wrist can be made to sweep a point of sunlight around a room.
The polygon mirror in a laser printer scans the beam across the page. By having five or more mirrors it reduces the speed of the drive motor to a practical 5,000 RPM - fast for a motor, but not impossibly so. If the mirror has too many sides very small optical inaccuracies would distort the output, if it has too few it has to spin impracticably fast. Obviously a 20 page per minute printer has more facets on the mirror, a faster motor, or both.
The speed of the polygon motor is a limiting factor on laser design. In many machines a high pitched whine starts just before the machine prints, this is the motor spinning up. The motor’s speed has to be very accurately controlled or the laser image will jitter across the page, so there are commonly four windings and two sets of sense information from Hall effect devices feeding back to the motor control board. Polygon motors spin so fast that they have to have a limited duty cycle, otherwise their windings overheat and bearings might fail. Polygon motor failure is quite common in laser printers and generally very expensive to repair.
Running the printer for prolonged periods beyond its duty cycle will cause the polygon motor to overheat, the lubricants in the bearings will degrade and lead to failure.
The polygon mirror’s speed is quite closely controlled but any inaccuracy will cause jittering in the horizontal positioning of pixels on the page. Vertical lines will appear jagged or un-focussed.
Laser printers generally sense the start-position of each horizontal sweep of the polygon mirror with an optical detector. The printer RIP starts reading a line from the raster-image memory when a pulse is received from the start of scan detector.
LED Printers
Laser and polygon mirror both run at or near the limits for current manufacturing technology, and so they are expensive. Because of this there are several other approaches to providing an image. The most common is the LED printer used in some OKI, Texas and Kyocera machines.
In the LED printer a bar of LEDs is suspended over the drum. For a page 8" across there will need to be 2,400 LEDs to achieve 300 dpi. All that is visible is a row of about 100 lenses less than 1/10th inch across, with perhaps 24 LEDs behind each.
The LED head is a small object but nevertheless it’s manufacture is plainly quite complex. It is probably cheaper to make than a laser scanner assembly because the main construction techniques are similar to integrated-circuit manufacture and can be highly automated.
The electronics in an LED head are more complex than those in a laser printer - a set of drive and addressing circuitry is needed to activate each LED. The sheer number of LED’s might create reliability problems, but each LED can work much more slowly than a laser-diode would need to so no individual component needs to be stressed.
LED printers should generally be more reliable than laser equivalents because the rather stressed mechanics of the polygon mirror and it’s bearing are eliminated.
Manufacturers seem to face some difficulties in scaling the technique up to deal with 1,200 dpi – 9,600 LEDs in an 8" (200mm) row. The LED printer’s niche tends to be low-quality machines.
The Qume crystalprint uses an even more exotic technique. It contains a tungsten-halide projector lamp facing into a mirrored tube stretching across the drum. One wall of the mirrored tube is made up of liquid crystal devices which can be turned on and off to paint the image onto the drum.
More exotic techniques are possible.
Porous silicon technology can apparently allow laser-diodes to be made as cheaply as LEDs and to run much more efficiently than the current generation of laser devices.
If porous silicon fulfils it’s promise then a static 8" LED –Laser head could output far more light, allowing the printer drum to move much faster.
If there is a limit to how many LEDs can be incorporated in an 8" head is might still be possible to achieve high dot-per-inch densities by imitating the shuttle printer on a small scale. The whole LED assembly could move slightly through 4 positions a fraction of a millimetre apart to produce higher dot densities.
It would also be possible to aim several lasers through the same polygon-scanning unit so that several rows of the raster are produced at once – rather in the way that dot-matrix printers scan out a whole print-line at a time.
Photo-Conductor
There are lots of different photo-conductors, just as there have been a variety of photographic mediums (ranging from egg-albumin to Kodacolour). The original material used in photocopiers was selenium, but this provided a crude image and there were health questions about it’s use. Nobody seems to have had the nerve to try Cadmium, which could do the job very well but is very poisonous (Cadmium cells are used in simple light-detecting circuits). The majority of recent drums use a material called OPC - Organic photo-conductor, a plastic. OPC materials may well be cadmium bearing. There are several varieties of OPC, usually green or brown in colour. All OPC materials share several problems:
- They are quite soft and can be easily scratched or chemically damaged by grease from finger-prints. Marks and grease reduce effectiveness even if they do no more damage.
- OPC surfaces become progressively less sensitive over time, and so the copies begin to fade. This can be compensated for by continually polishing surface material away.
The photo-conductor needs some sort of conductive support and must be precisely positioned with respect to the optics, corona wires and the developer. There are two common ways to provide this:
- An aluminium or steel drum gives a very stable precision surface. The problem with a drum is that it gives a small surface area in a given space.
- A belt made of metal or metallised plastic can give a large surface area, but needs a fairly elaborate set of rollers, tensioners and supports for accurate positioning.
Most practical drums are part of a plastic carrier even if they are not bound up in a cartridge. The carrier often holds auxiliary components.
There is usually a brush made of carbon fibre or conductive foam to carry the electric charges released by the laser away from the drum or belt surface. Charges cannot reliably be carried through metal bearings because the lubricant in the bearing is an insulating layer. Contact brushes often look unimpressive, but if they are contaminated with dirt they create faults that are very difficult to diagnose. Perfectly good consumables are sometimes thrown away before they need be because the brushes need cleaning.
Some carriers include the "corotron" wires that place charges on the drum. The corona wires themselves are hair-thin, usually held in place by grooves in the plastic support, and kept under tension by a spring. The wires don’t wear out but they do lose performance when they get dirty, and they are so low-cost that they can be treated as disposable.
The limitations on sensitivity mentioned above mean that in most laser printers the drum has a life of 5-10,000 copies after which it will be disposed of - or possibly recycled.
One way to extend the life of the OPC material is to make it rather thicker than necessary and then gradually scrape it away. Many printers take this approach to some extent – the cleaning blade that primarily removes surplus toner abrades the OPC surface and the material is disposed in the waste toner. Some photocopier designs have arranged scraper blades to quite deliberately cut away at the OPC drum and give it a longer life.
The Kyocera FS1500 uses an amorphous silicon drum (aSi). This material has the advantage that it is quite hard wearing -similar to glass - and does not degrade over time due to exposure to oxygen or light. On the other hand aSi is a brittle substance and manufacturing a perfect drum is quite difficult. The other slight problem is that to work effectively the drum has to be heated to about 70 centigrade – this is done by heater windings inside.
Obviously the choice of drum material has an impact on the polarity and voltage of the charge that needs to be applied to make up a satisfactory image. Insensitive photo-conductors will need higher voltages and a greater quanta of light. Leaky materials will lose some of the image definition – so might thick layers of material. Thin layers of material abraded by scraper blades and ceramic in the toner will have a short life.
One problem is that high voltages in the corona wires strip oxygen atoms out of the atmosphere making ozone. The smell of ozone often wafts from photocopiers and laser printers when they are busy. Contrary to popular opinion, ozone is poisonous and carcinogenic. To reduce the risk of ozone pollution manufacturers generally fit activated carbon filters to machines. Unfortunately the carbon filters are often only changeable by engineers, who are pressed for time and may not bother. Filters also get filthy with atmospheric dust and toner attracted by the high voltages used by the corona wires and this can reduce their effectiveness prematurely. Users can be exposed to an unnecessary hazard if the ozone filter in a printer does not work. A smelly laser printer or photocopier is a health risk, and the machines should only be used in a well ventilated room!
Reducing the voltage below a certain point greatly reduces ozone production - newer designs try to achieve this.
A typical laser printer will make three High Tension voltages (HT). One to charge the drum, another to charge the developer, and a third for the transfer of the toner to paper. Voltages are generally made by several separate inverter circuits, then distributed around the machine by wires, strips of metal and springs. The contacts between different parts of the laser printer body and its consumables often look thoroughly unimpressive, they are nothing but tin strips. These strips get covered in toner or tarnish; they then make poor contact and voltages leak away. Periodic cleaning of the inside of the printer can improve print quality by preserving the voltages.
The high voltages are usually taken to corona wires. High voltage fields move best from sharp points, and a thin tungsten wire provides a knife-edge for voltage transfer. Since they carry a high charge the corona assemblies attract dust, and this impairs their operation because it causes the charge to leak. Vertical streaks on laser print are usually caused by nodules of dirt or trapped hairs on corona wires.
Many recent printers use charged conductive rubber rollers for the transfer voltage. This technique was used by Kyocera on the FS1500 both for transfer and waste toner. Rollers make close contact so they allow the voltage used to be reduced significantly. Reducing the voltage avoids ozone production and reduces the design problems of distributing high voltages around the machine. The only real problem with conductive rollers is that their effectiveness can be impaired if they are dirty, scored, or greasy from fingerprints.
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Developer and Toner
The developer in a laser printer turns the electro-static charge on the drum into an image using toner-powder. Toner is presented very close to the charged drum and where there is a charge difference some toner is transferred from developer to drum.
Toner is a fine powder with the consistency of corn-flour. Many different materials could be used in toner, with different melting points and grain sizes. Seemingly identical print engines used by different manufacturers can be designed for quite different types of toner, and use of the wrong grade can require a very expensive strip-down and cleaning of a machine. The developer and fuser could be wrecked.
Until recently manufacturers consistently based developer designs on a technique that might be described as iron-filing velvet. The toner is mixed with iron filings. The filings are kept coated by stirring in the developer hopper. The iron filings in the developer are picked up by a magnetised roller at the front of the developer that rotates at a speed to match the drum. The amount of material carried by the developer is controlled by a gate - usually a brass strip adjusted over the developer roller. The developer roller rotates to carry fresh material. In some designs of developer the magnets inside the roller are static so that the iron filings are always at the same angle when they contact the photo-conductive drum. In other designs the magnets within the roller rotate more quickly so that waves of toner-bearing material pass across its surface.
Developer designs originally arranged for the developer to brush very lightly against the drum. Some later designs have impacted the developer quite hard onto the drum – it is a surprise that the image doesn’t smear.
Some recent developers use a resin coat instead of the iron-filing velvet.
The dimensions and electrostatic properties of toner particles will obviously be important in determining image quality. Fine-grained toner (and developer) can give a finer-grained image. Low melting-point toners should allow the paper to move faster through a cooler fuser unit.
Toner is a precision material, and the grade has to be right for the machine. All machines produce waste toner and some drop it into a separate bottle.
There are several approaches to building the toner/developer drum system. All of these components are consumable, in the sense that they will be used up or wear out.
In many machines each component is separate. Typically the software in a printer will keep track of when a new consumable has been installed and will then count through its lifetime, either warning when it is exhausted or else stopping the machine.
Another approach to keeping the right mixture of toner powder and iron-filings in the developer is to measure its quality. Electrical resistance, magnetic and sound properties will all change depending on the iron-filing toner mix. Most printers seem to use a magneto-resistive sensor. A motor or clutch will give the developer a dose of toner when the toner mix is too weak.
Another approach is to integrate all the consumables into one cartridge- this is the approach taken in the Canon engines starting with the LBP8 / HPII and in the Fuji/Xerox machines. One big advantage of cartridges is that no consumable ever need fall below its optimum performance, and it simplifies fault finding because there is only one module to change. The disadvantage is that parts of the cartridge will be perfectly useable when it is scrapped - so it seems ecologically unsound. Printer manufacturers commonly run recycling schemes and users can get their cartridges refurbished by a host of little local firms.
Photo-conductor Cleaner
Laser printer drums usually turn several times to produce one A4 page. As the drum passes the transfer station the toner image is pulled onto the paper – or at lest most of it is. In practice the transfer is rarely complete.
The cleaning mechanism removes any residual toner from the surface of the photo-conductor.
The cleaning mechanism usually has three main parts
- Eraser lamps – a row of LEDs that bathe a horizontal strip of the drum in light to remove any static charge
- Scraper blade that forces any toner off the drum
- Waste transport mechanism that removes the waste to a storage place
The scraper blade usually seems to be made of a fairly stiff transparent silicon-rubber. A stiff mylar might seem more appropriate to the scraper function – perhaps it is too abrasive or the relatively soft material makes better contact.
The waste transport mechanism almost invariably seems to be a plastic screw-feed.
The total amount of waste produced seems to vary with printer designs, but to be 10 – 20% of the toner input. In single cartridge printer designs the waste is just packed at one end and is thrown away with the cartridge. In some printers the waste storage bottle is separate.
If the waste storage bottle is separate there is often some device to sense when it is full. The Kyocera FS1500 series count how many copies have been done since the last bottle was changed – they sense this using a silver label which reflects light onto an opto-detector when a new bottle is put in. Photocopiers often shine an optical sensor across the top of the bottle and when it is blocked they report that the bottle is full.
Photo-copier users sometimes tip the waste toner back into the toner supply hopper. This practice is more difficult with laser printers because there are usually interlocks on the cartridges that make the toner difficult to get at. Even if it is possible to get to the toner never pour waste toner back into a copier or printer.
- The composition of waste toner will have been degraded.
- The waste scraper blade and screw compress the material and produces lumps.
- Waste toner has been exposed to oxidation which may change its chemical composition,
- There will be dust from the OPC polishing process, kaolin from the paper and broken down filings from the developer present in the waste
Refurbished Laser Printer Cartridges
Printer manufacturers are often dismissive about the quality of refurbished cartridges – but they would be, they make a great part of their profits from sales of new consumables.
Unfortunately engineering experience makes us sceptical about the quality of refurbishment.
Refurbishing a laser-printer cartridge ought to be much more complicated than just refilling the toner hopper.
Some manufacturers (such as Canon) run their own refurbishment schemes. No doubt they can ensure that they have all the right parts available to do a good job.
Refurbishment companies may be able to get hold of an appropriate grade of toner, so this allows them to do a basic job. From their point of view there can be good money involved – some of the bigger cartridges retail for about £100 but contain £5 worth of toner.
Unfortunately they may find it less easy to get new OPC drums and scraper blades. From the evidence we have seen scraper blades are a common source of trouble, with grainy-looking print and intermittent avalanches of waste-toner down into the transfer station a regular problem.
Users are often quite unaware that a badly done refurbishment is costing them more money by causing an undue number of maintenance call-outs.
Toner & Health
Laser printers and photocopiers are common; so most people now have some exposure to toner. The toners used in older machines were certainly hazardous to health, but manufacturers became aware of the problem and either minimise exposure using cartridges, or claim that their own brand of toner is safe.
In fact most toner dust is probably a mild health hazard, it is sufficiently fine to wash into pores on the skin, and because any powder has a huge surface area it will easily gives off any volatile component in its makeup. Some recent toner materials intended for machines with a 1200 dpi resolution are probably smaller than the "PM10" particles from diesel engines, considered to be a hazard to sufferers of asthma and emphysema. Massive exposure could even be a cause of mesothelioma.
Accidental experience suggests that it is quite possible to be made mildly ill by massive exposure to toner-dust, but such exposure only occurs when a large quantity is spilled and blown about.
Never use a standard vacuum cleaner for toner – it will penetrate the pores on the bag and be blown out in the exhaust. Scoop up large quantities using a paper bag or envelope that can be sealed before disposing. Toner will generally not mix easily with water - quite a high concentration of soap is needed to mop it up, nevertheless a damp soapy cloth does seem to pick it up quite well. Do not use hot water – the toner powder will melt and adhere to clothing and skin. Avoid solvents, most toner powders break down in isopropyl alcohol making a blue stain.
Paper Feed Mechanism
There are an infinite variety of paper sizes and styles, but the average laser printer is designed to handle only a few. Most allow for A4 in Europe and the US standards of "legal" and "letter". To meet these standards the paper path has to be just over 8" wide. By designing the paper path to be 11" wide the manufacturers can allow users to put A3 paper through the machine - and substantially increase its utility. Engineering diagrams commonly need a minimum of A3. Achieving A2 print would need a 16" drum and only exceptional designs allow this.
The average laser printer is designed to hold A4 paper with its 8" side presented to the paper path. The printer can be set to print this as "portrait" or "landscape" because the laser mechanism is quite indifferent as to what pattern it creates in the raster image memory- the printer just needs to be given the correct instructions. If the printer has an A3 capability the 11" side will be turned to the paper path, and in this case A4 will pass through the same way round so as to make better use of the OPC surface. Printers with an 11" path are inherently faster for A4 because they only need to roll 8" of paper through the paper path.
Laser printers almost invariably use cut-sheet paper, which is much more popular with users than fan-fold tractor-feed stationery.
Cut-sheet presents some handling difficulties, because it is difficult to control the flow of a sheet through the print mechanism.
There are lots of different paper-tray designs, but there is a common underlying theme. A spring loaded plate forces the paper up against "D" shaped pickup rollers. Sheets may stick together for several reasons - they may be damp, the cutting process may force edges together, or friction between sheets may cause several to move at once. The tray mechanism therefore invariably includes a "separator" - a device that causes sheets to ruck as they leave the tray and breaks them away from their successors.
A ream of paper will generally have an arrow on the pack to show which way up it should be placed. The arrow points to the print surface, which is sometimes more polished than the other side. The print-surface will generally be the underside of the paper in the input tray because the rollers in the feed path often turn the paper through a bend before it reaches the print-station.
Five years ago paper quality often gave serious reliability problems. Some paper manufacturers sold "laser printer paper" as a product quite distinct from photo-copier paper – even though the two processes are much the same. Most people now just use photocopier paper. So much bland and uninteresting copier paper is in circulation that people tend to forget that different paper qualities can look and feel quite distinctive – but fancy paper can give trouble in laser printers.
Large organisations sometimes have epidemics of print-jams when a new shipment of paper arrives. A large organisation buys paper by the lorry-load, and the main decision making criterion is often price per ream. Over a few weeks the new paper penetrates to every department, and the photo-copiers and laser printers start giving trouble. This problem is less common than it used to be. Paper manufacturers and distributors seem to have improved the quality of ordinary photo-copier paper and it now rarely gives trouble.
There are national preferences in paper quality. UK paper users like a highly polished very smooth paper. This finish is achieved by coating the paper with kaolin (china clay) which is absorbed into the paper structure. The kaolin can cause problems – for instance there can be an excessive dust build-up in the drum cleaner.
Pickup rollers on some printers can give trouble, and this too may be connected with the UK fondness for kaolin smoothed paper. The rubber on the rollers contains a softener agent, and it is important that the rollers are the right composition or they don’t make enough frictional contact with the page. Some rubbers age – they can be seen to have a cracked surface. If kaolin powder builds up on the rollers they may build up a coating of kaolin mixed with the softener agent, and this is then polished as the slippage in the rollers gets progressively worse. As the number of misfeeds increases and the rubber gets polished the printer reaches a point when it will scarcely feed at all.
Once in the feed path the paper sheets are driven by rubber and metal rollers.
There may be several sets of rollers depending on the shape of the paper path. The most significant is the paper registration station.
The registration station is usually located just before the transfer station. The purpose is to set the paper off through the transfer station held as straight as possible and as tightly as possible. If the paper is not straight or feeds unevenly then there will be distortion in the print.
The registration station very often uses a page width metal and rubber roller pair. The kicker-roller in the feed tray would feed the paper slightly too far for these rollers, but when it hits them they are stopped so the paper recoils a bit and settles with its edge aligned straight in the rollers. The presence of the paper pushes a sensor–dog down and this notofies the print engine that it can start the polygon mirror scanner motor and the print engine.
Many recent laser printers try to economise on desk-space and this is done by having an "S" shaped paper path. The paper is taken from a tray on the underside of the printer, doubles back on itself, then doubles back again to feed out of the top.
A common design feature is to have a front-loading tray that can handle rather heavier paper because the feed-path is flatter.
The rollers are generally connected together by a cog chain driven by a main motor that also drives the fuser, OPC and developer. The relative movement of all these parts is determined by the ratio of the cogs. Individual pickup rollers are usually connected to this chain by clutches. If the cog-chain feeds paper unevenly there will be horizontal smudging of the print, usually particularly discernible at the top and bottom of pages where the rollers grip the paper rather weakly.
Maximum paper thickness in a laser printer is usually 150 gram per square meter (gsm) – a thin card. The maximum thickness is usually set by the feed-rollers ability to bend the paper through the feed path – there may also be an issue as to how large the transfer station gap is.
Minimum paper thickness is usually about 70gsm - just a bit thicker than "bank paper". The minimum paper thickness is partly set by what the feed rollers would be likely to tear, and partly by the likelihood of paper being deflected from it’s proper path by electrostatic fields inside the printer.
Some organisations have office practices that require copies to be made on coloured paper. The typing-pool practice of using 60gsm bank paper developed in the days of carbon-paper copies is carried through. Bank paper often seems to work OK in a laser printer for several weeks, but it can create disasters. On the odd occasion bank paper will disappear up into the drum assembly of a printer, jamming it severely.
Multiple Trays:
Basic printers provide one or two paper input trays. A lower tray is usually capable of holding 100 – 200 sheets. A front-feed tray can hold perhaps a dozen sheets. The printer will either take paper from the front-feed tray whenever there is some in, or it can be specifically told to do so from the MS-Windows printer control panel.
Network-printers often need to hold more paper – nobody is specifically looking after them. It is also safe to assume that nobody will be near-by to load letterhead into the front tray when it is needed, so a network printer needs two large trays.
Some printers such as the HP4Si come with dual trays built in. Others such as the Kyocera FS series can have a stack of trays built alongside or underneath.
Fuser
Fusers are generally made from two rollers; one is often made of a fairly soft silicon rubber, the other of Teflon coated aluminium or steel. The Teflon coated roller is heated to about 150C - the exact temperature depends on the grade of toner used and on the rate at which the paper is expected to pass through the fuser.
The fuser is normally heated by a tungsten halide lamp. When the printer is first turned on this lamp will stay on for some time providing initial heat to the fuser, after this initial warm-up the lamp will light every thirty seconds or so depending on the amount of paper passing through. The fuser temperature is controlled by a thermistor circuit which feeds back into the engine controller board.
The fuser uses a high proportion of the power required by the printer, and the time to bring it up to temperature can be an annoyance to the user. To make the fuser come to temperature rapidly all that is needed is a more powerful lamp with thinner metal components. If the fuser can reach temperature very quickly it does not need to stay hot all the time - the processor can allow it to cool down between prints and this will save energy.
In case the temperature control mechanism fails a thermal fuse is usually provided to prevent the fuser mechanism melting and the printer catching fire. Both the thermistor and the thermal cut-out are pressed up against the fuser body, and they tend to wear through the Teflon coat. By far the commonest causes of fuser failure are marks in the Teflon caused by this wear.
There is naturally some tendency for the heated toner passing through the fuser to adhere to the rollers. This is reduced by coating the rollers in a very thin layer of silica oil. The coating comes from a cleaning pad usually located in the top of the fuser. A very small amount of oil will serve for several thousand pages of work, but if the pad is not replaced spent toner will begin to build up on the rollers. High-capacity printers and photocopiers often have a bottle of fuser-oil which needs to be periodically changed.
There is some evidence that failing to replace the cleaning pad also causes the thermistor and cut-out to wear into the Teflon rather faster than they otherwise would.
Some recent printers do not have a Teflon coated roller, instead they have a lose Teflon sheet which rotates around a halogen or ceramic heater- some of the smaller HP machines have this kind of fuser. Since the Teflon sheet is not rigid it seems possible to dispense with the fuser-oil, and the fuser may be able to turn on and off more rapidly – helping save energy.
Fusers have a long life but they do wear out – they are sometimes described as an engineer-changeable consumable. Fuser failure can cause a lot of aggravation because they are an expensive part – the retail price on fusers is often around £200.
A typical manufacturers rating for a fuser built with a halogen-heated teflon-coated roller is 150,000 pages. Practical engineering experience suggests that earlier failure is quite frequent. There are probably several causes:
- One might be the UK use of high kaolin paper. This builds up on the thermal sensors and begins eroding the teflon.
- Another might be failure to replace the silica-oil cleaning pad in the top of the fuser.
- A likely cause of sudden failure is that a foreign object such as a staple has passed through the machine. Engineers quite regularly find small metal objects inside printers and it would not be surprising if the odd one passed right through the fuser and back out.
- A surprising frequent cause of damage is users taking a paper-knife to the mechanism to try and free up a paper-jam. Seeing what looks like a metal roller they push hard – and score the teflon coat.
The halogen lamp in a fuser will ultimately fail, although many last beyond the design-life of the printer. The lamp element will go brittle with age so the point of failure is often after a printer has been moved – often because of engineering work for some other problem.
Fuser Repair
Laser Printer fuser units users can sometimes be repaired – although the economics of this depends on how much a replacement unit costs and how long it takes a technician to strip and rebuild a fuser. The parts most likely to fail are rollers and lamps and these can often be bought and replaced.
When replacing a roller look at the surface of the sensors and ensure they are clean – if they are not then they will start gouging the surface of the new roller straight away.
In a few cases the bearings of a fuser become damaged. The bearings are often brittle plastic and are quite likely to snap – if this happens the fuser is a write off.
The bearings also sometimes melt – if this has happened beware of fitting a new fuser because the real fault may be that:
- the traic that controls the halogen lamp has failed open-circuit.
- the sensor that controls the fuser temperature is no making proper contact with the roller
- the sensor has drifted well out of specification.
Colour Laser Printers
The toner used in photo-static printing can be any colour – it is perfectly possible to have a printer working entirely in blue.
Full colour printing means using three colours, usually Cyan, Magenta and Yellow. These three colours are blended in proportion to make any other. The blending is not perfect- particularly for producing grey and black. Users tend to produce a lot of black and white material even on a colour printer, and this would use a lot of each colour. To improve the economics and colour rendering printers normally have a black cartridge as well so the colour process is often known as CMYK (Cyan, Magenta, Yellow and blacK).
It would conceivably be possible to line up four monochrome laser engines to produce colour print but the resulting machine would be a monster and prone to paper-jams.
The technique normally adopted in colour laser printing is an intermediate image store.
There is normally a single main OPC belt or drum and this is charged up by a corona wire and scanned by a laser in the same way as any other printer.
The developer presented to the OPC is selected either by
- a rotating carousel, the Minolta Page-Pro does this
- setting voltage and motor action on others so that they have no effect –as with the QMS CX
In principle the image from each pass could be transferred straight onto paper, but this would mean shuffling the page back and forwards several times through the transfer station. The paper would not "grip" the image very well – a static charge on paper tends to dissipate across the surface.
The intermediate store takes the image from each colour pass and holds it in place. Minolta use a Belt made of OPC material for this, QMS use a drum coated with rubber. Once all four images have been built on the intermediate store the paper is moved through the transfer station and the intermediate store drops the image onto it.
The page with the toner image then travels on into the fuser, which melts the CMYK layers and they blend to provide the colour required. At this point the only difference between mono and colour printers is that the fuser must not carry any surplus from the relatively thick layer of toner around and deposit it in the wrong place. Colour laser-printer fusers tend to be rather more elaborate than their mono counterparts, using larger rollers, a metalised heat transfer band and/or more silica oil for the cleaning process.
Colour laser printers are big because they need to accommodate four developer mechanisms and an intermediate belt. They also need extra electronics – a faster RIP and more memory to deal with the colour-separated image. Colour laser printers will get cheaper, but not to the low level mono machines have achieved.
Page-per-minute and price-per-page ratings for colour laser printers can be quite misleading.
Colour page images are more complicated than mono pages not just because of the four-colour process but because users are very much more likely to include a lot of graphics. The printer is likely to need a great deal of memory. One problem can be that the print drivers in a PC can need a lot of RAM to work with. Another is that some colour printers are shipped with as little as 4 megabytes of RAM and the RIP attempts to cram it’s work into this using compression – this can slow printing down. If four copies of a 5 page document are needed it may be much quicker to send each page five times than to send five copies of the document.
Operating costs also depend on circumstances. The exact colouring of the toner is important and fading due to oxidation can be a problem. Printers typically discard the top layer from the developer when they have been idle for some time and use fresh material. If the printer is used for a single page now and then this wasting of material can double operating costs.
Further Light on Lasers
The photostatic printing process known as "laser printing" is now very well established as the standard computer printer process. Laser printer technology has matured over fifteen years and become surprisingly reliable. However laser printing is limited by several factors.
The properties of the photo-conductor material are very significant. If the photo-conductor could take up a clear, sharp image from a very limited light source then optical / electrostatic printers could function quickly regardless of the light source used.
Since the photo-conductors don’t perform ideally the power and modulation frequency of the light is important. The current generation of laser diodes emit light from an aperture in their side and are rather inefficient. Communications and data recording devices need faster, more powerful lasers. Techniques to produce these are emerging (porous silicon etc) and it seems likely that the brighter, more efficient, cheaper lasers will be applied to printers. A five to ten-fold increase in laser printer speed might be practical if the brightness of the laser light can be increased. Precautions against laser-light contacting users or engineers eyesight will have to be reinforced.
The laser and polygon mirror serve much the same purpose as the print-head in a dot-matrix machine, so greater speeds could also be achieved with multiple lasers.
The obvious way to produce multiple lasers is by making a page-width bar of lasers like that used in LED machines. If it were possible to produce modulation and light-quanta levels similar to those from lasers then a ten-fold increase in speed could easily be achieved. Polygon mirrors would then be unnecessary.
The polygon mirror itself can be improved. With current drive and bearing technology, and current optical engineering techniques a fifteen to twenty sided mirror rotating at about 15,000 rpm is a practical limit. New bearing technologies in development could quadruple the mirror speed. With nano-engineering techniques currently emerging from the laboratory 150,000 rpm and many more sides might be achieved.
Given faster lasers and finer optics it is possible to improve the resolution of laser printers might be improved beyond the 2,400 dpi expected of photo-typesetters.
The laser and electrostatic process is not inherently digital - it could be made to produce a photographic grey-scale.
If grey-scale material at 2,400 dpi can be achieved the human eye would accept laser printed material as being as good as any existing printed matter.
The remaining problem would then be the toner/developer system - delivering a finer toner to allow finer print. There doesn’t seem to be a particular problem with speeding up the developer or reducing the size of the toner powder.
It isn’t clear whether there is an easy way to reduce the cost of toner.
There probably is a downward limit on page cost somewhere near the 0.7 p per page currently attained by the cheapest printers such as the Kyocera FS1650. The limit is set by the cost of the toner powder, the plastic containers needed to move it, and by wear and tear on developer, drum and fuser.
The process used by most laser printers is virtually identical to photocopying. The laser’s only role is to act as a fairly powerful and controllable light source, and as mentioned it is replaced by a projector lamp in the Qume Crystalprint. The laser in the average printer has an output of 5mW - not enough to do much harm if pointed at skin. However the laser light is generally infra-red, so it is invisible. There is a potential danger because even at this low power lasers can damage the retina of the human eye - so it is important never to over-ride the safety switches in a futile attempt to actually see the beam.
It is possible to build a laser printer that simply burns the image onto the paper by modulating and steering the beam with mirrors. In a machine of this kind the laser obviously has to be up-rated -perhaps a couple of kilowatts might be sufficient. Laser printers of this kind were tried by Siemens, who had three installed at DSS Longbenton in the mid ‘80s. Obviously it would be possible to build a machine capable of hundreds of pages per minute if the mirror steering can act quickly enough. Such a device might replace conventional newsprint presses.
Copiers and Combinations
Photocopier and laser printer share the same technology so it might make a lot of sense to include both functions in one unit.
Their existence owes most to the awfulness of today’s Windows based scanning and printing software - separate scanners and printers generally aren’t very good at producing a 1-page copy. Combined scanner printers are good at this – but their capacity for computer control of an image is more limited than would be true of a separate scanner. Top-end high speed colour photocopiers are often "network ready" as printers.
Combining copier, printer and fax into one multifunction box is probably a temporary marketing position
Inkjet printers:
Inkjet printers have been the subject of ten to twenty years research by most of the major printer manufacturers. The inkjet process has a simplicity and intellectual elegance sufficient to suggest that it may take over the whole future of printing. The legend is that an engineer discovered the inkjet principle when a soldering iron accidentally touched the needle of a syringe full of ink, causing it to jet out over the desk.
The basis of inkjet printing is very similar to that used by dot-matrix machines - a print-head is swept back and forth across the page making a pattern of dots. Everything about an inkjet machine can be very similar to a dot matrix printer except the head. The Epson SQ series serve to emphasise this – they are inkjets, but look very similar to the dot-matrix LQ Series.
Ink
The relatively direct use of ink is an advantage. Ink production is a well-researched practical art. The basis of printing since the invention of the process by the Chinese and its importation into Europe by Guttenberg and Caxton has been the application of ink to paper.
Ink is usually a suspension of particles in a liquid. Many possible formulations for ink exist, but a common characteristic is that there is a "vector" that evaporates and a dye that remains behind.
In a printing press a "plate" applies the ink. The ink adheres to the plate and is then transferred onto the paper – children imitate the process with potato cut-outs. In older "impact" computer printers the ink is carried by a ribbon which is pressed against the paper by a matrix head, golfball or print-band. The print-head is a surrogate for printing plates, it presses the image onto the paper. The fabric ribbon raises the cost of the printing process a bit. There are two main costs to impact printing – new ribbons and new print-heads. Laser printers require complex consumable kits, so they are unlikely to undercut ink-based processes in cost per page. If inkjet printers could produce reliable high quality output at an ink-printer cost then they would dominate the market. It seems plausible that inkjet printers could replace photocopiers, and even match the operating speed of offset lithographic machines whilst giving page-by-page flexibility. For the present, however, the operating costs of inkjet printers are much higher than those for laser printers and the laser engine makers seem confident of retaining their lead.
The inkjet printer applies ink to paper in a very controlled manner, without the intervention of any sort of mechanical plate or needle. The process is nearly silent, and ought to be very reliable because it involves no great mechanical action. The problem is to make print-heads reliable and ink cheap without involving users in messy procedures.
What inkjet printers currently achieve is a machine that is inexpensive to buy, but rather costly to run. A mono inkjet typically costs under £200 compared with £300 - £1500 for a laser machine. The inkjet price per black and white page is 3-6p against 1-3p for laser printers. A colour inkjet typically costs between £250 and £500 - but an equivalent laser would cost £5000; price per colour page is between 6p and 60p. The figures given are drawn from engineering experience with the machines. All the manufacturers are loath to admit it, but the current generation of inkjet machines are rather expensive to maintain and run. There are two interconnected reasons – print-heads are unreliable, and manufacturers are profiteering on the price of ink.
Inkjet Print-heads
In an inkjet head there need be no moving parts except liquid ink, which is cheap to make and not too difficult to handle and store.
Several principles are used in inkjet designs:
- Tubes hold liquid ink and a heat-element that makes steam-bubbles to eject ink.
- Epson uses a piezo-electric element to eject ink from the elements.
- Phase-change devices turn a solid ink to liquid or vapour at the moment of use.
Most inkjet heads use a thermal mechanism. The head is usually built on a piece of aluminium that acts as mechanical support and heatsink. The head itself will be silicon or ceramic, with heaters embedded in the walls of 40 to 60 capillary tubes. Electrical connections to the drive logic control each heater. The head also has to carry plumbing connecting each tube to the ink reservoir.
A practical inkjet head is a compromise between print quality, speed, reliability, ease of manufacture and ease of use.
If the printer is to be fast then the more individual print-elements there are the better. This is precisely the same problem as for dot matrix and thermal machines. Ideally what is wanted is a head as wide as the paper, with as many elements as there are to be dots across the page.
Thermal printers can use wide print-heads because the device is simple - it is possible to make a head with 2,400 elements or more (300 dots per inch, 8 inch print width)
Inkjet heads are thermal devices, but with the addition of the plumbing for the ink, making them far more complicated to manufacture. Manufacturers are though to be investigating the possibility of heads with thousands of elements, but none have come to market as yet. If page-width inkjet heads can be produced then the printers will be able to match or exceed the performance of the fastest laser printers, and it is likely that the cost per page could be lower.
A head with only one element on a carriage would be impracticably slow - the dot-firing rate for inkjets seems to be rather similar to that for dot-matrix needles at around 2-4kHz. Actual print-heads are a compromise between the ideal of a 2,400 dot page-width head and the fact that the probability of failure increases with each element added.
The number of elements in a head differs between printers but is generally between 48 and 120 elements per head. Given this number of elements and a resolution of 300dpi older inkjet printers achieve speeds of 2-4 pages per minute. Manufacturers have recently increased the number of elements in their head designs. Epson’s latest machines are capable of 1,440 dpi.
One problem is print-head life; this may be threatened by wear, tube blockages, airlocks and electrical failure.
- An inkjet head has to contain some electronics for the heaters or piezo-electric elements. The print-head itself is usually based on a silicon chip, so building some circuitry onto it is natural. Even if a head only contains 50 elements this implies 50 power-carrying connections to the control circuitry – together with grounds and sensors. Rather than try to make quite so many connections reliably manufacturers have often opted to put a shift-register, latches and power transistors onto the print-head chip and load the data in serially. This head-mounted decoding and driver circuit controls the individual elements. Nevertheless there are often a couple of dozen connections to the head – often made by small gold-plated springy plastic pads. The head itself will be bonded to a small circuit board carrying printed wires from these pads to the head, examination shows that manufacturers often use tiny bond-wires to link the printed wiring to the semiconductor chip. Altogether there are a lot of interconnections that can easily fail.
- Airlocks might not only block the flow of ink temporarily but allow ink to dry locally deep inside the head so that it is difficult to flush out. One possibility is that an airlock next to a heater element would allow localised overheating, causing an element in the head to fail.
- A more serious problem is likely to be blockages. Any contaminant in the ink, such as sediment or foreign matter introduced by fitting a cartridge onto an old head could block one of the capillary tubes. In an attempt to avoid this, the manufacturers use ink filtered to sub-micron particle sizes, and print-heads integrated with a sealed ink tank are widely used
- When the printer is out of use the ends of the capillaries will dry out. Most printers make some provision to shield the head from the atmosphere when it is not in use. Recent printers usually park the head over a silicon rubber cup - if it is to do this the printer must be allowed to follow a power-down sequence and not just turned off at the wall. Older printers often have a stainless steel shutter that is spring loaded to close into position. The exact formulation of the ink - for instance the balance between water and isopropyl alcohol, will have an effect on the drying time – the ideal ink would dry immediately on contact with paper, but not at all in air.
- Even a hard ceramic material will gradually wear away as it passes across paper, and if the dimensions of the inkjet tubes change then the accuracy of the ink delivery will decline.
Canon has tried several approaches. The BJ200 has an integrated head and ink cartridge- although if this is prized apart it is actually made of a head and a reservoir. The BJC600 uses separate cartridges closely coupled with the head on the carriage. The BJC800 has large cartridges in the base connecting to the head by pipes.
Epson have produced a printhead that uses a piezo-electric effect to fire ink at the page. Piezo-electric transducers can have a very long life and can operate very fast – the crystal timebases used in computer circuits vibrate on a small scale millions of times per second
No one approach seems to entirely overcome reliability problems. From time to time cartridges have to be wasted with the ink only part used or perfectly good heads are thrown away because the cartridge can’t be re-inked.
Anyone refilling a cartridge needs to bear the problems of wear, blockages, formulation and drying time in mind.
Ink
Almost any coloured liquid can be used as ink, but manufacturing something to perform the task perfectly seems difficult.
There are two premier properties required in inkjet formulations: mobility in the head and immobility on the paper. Whilst the ink is in the printhead it should be very mobile and not adhere to or react with any of the working parts. In a thermal printhead the ink must also contain a component that will evaporate suddenly when the element heater is tuned on so the ink ejects. When the ink hits the paper it should instantly turn to stone, any flow within the paper beyond that needed to make it adhere is undesirable.
Ink generally has two components: a vector and a dye.
The vector is the liquid that the dye is dissolved or suspended in, usually either water or alcohol. Water will dissolve the widest range of materials, is odourless and evaporates quite slowly; but it reacts with metallic components in the mechanism and can break up the fibrous mat of the paper if a lot of ink is used. There are a variety of alcohols, they will dissolve waxes and other organic materials and they dry quite quickly. Water and alcohol in solution have some of the properties of each in proportion to the mixture. Water and alcohol also have different surface tensions so mixture can vary the energy input needed to eject ink. Most inkjet inks are mainly water based with some isopropyl alcohol, ethylene glycol and other organic solvents.
Printer manufacturers generally rely on absorption into the paper and evaporation to dry the ink. If the paper is absorbent, the vector evaporates slowly or the dye is in solution then the ink will smudge in the paper. Material in suspension will settle out more quickly and not disperse through the paper. If ink diffusing into paper is a real problem then heating the paper will help – the HP1200C uses a halogen lamp to drive wetness out of the paper just after printing.
Dye properties include:
- Precisely reproducible colour and colour density
- Mixable with other colours
- chemical inertness especially to other inks and to common paper ingredients
- insolubility once used,
- colour stability in ultraviolet,
Dyes in solution will not settle out, unfortunately it is difficult to achieve a truly intense colour with a water-based solution. Suspensions deliver much more solid colour. Unfortunately suspensions may tend to settle or floculate and will then either lose colour or clog the print-head.
The simplest dye is a solution or suspension of very small particles in liquid that will settle into the fibrous kaolin filled mat provided by paper. No chemical reaction is expected between the dye and the paper. The vector used for this sort of process tends to be mainly water. Most actual inks are much more complicated – separating ink components out using blotting-paper chromatography can be quite amusing.
Complex processes can be devised. The printing can be done onto a sheet which accepts ink, which is then placed face down on cloth. Ironing the sheet causes the ink loaded polymer on the backing material to melt, transferring the pattern onto the fabric.
Specialist inkjet cartridges intended to print directly on plastics and fabric are now becoming available – in principle an inkjet cartridge can be made to print directly onto anything which can go through the printer mechanism. Specialist inks are likely to become a significant market.
The basic message of this section is that paper and ink are not simple materials. Fast machine printing requires detailed attention to chemistry and engineering.
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User expectations are always a problem. For instance, should ink perform well when paper is damp?. The alcohol-based inks used on offset litho machines often perform very well in soaking conditions as posters and placards demonstrate. Biro and dot-matrix inks are also waxy in their consistency and do work surprisingly well on damp paper. Laser printer output is vinyl powder hearted into the paper and a laser or photocopy sheet will work excellently as a poster. People have become used to this performance. The appearance of an inkjet-printed envelope can be spoiled slightly if the postman delivers it in the rain. Because inkjets can produce solid printed posters people can be surprised when inkjet printing runs and smudges in the rain.
Inkjet design and layout
Delivering liquid ink to the head can be done in two ways:
- Incorporate the ink cartridge into the carriage – possibly as part of the head.
- Take flexible rubber tubes to the carriage from a static cartridge.
The option of putting the ink-cartridge on the carriage has been widely chosen. The problem with this is that the cartridge cannot contain much ink or it will be too heavy. Typically the cartridge allows enough for 100-200 pages of output.
There are two advantages in keeping the ink in the printer base, it takes weight off the carriage and allows a large, high capacity, long life cartridge capable of 500 or more pages to be used. Using a large cartridge should reduce the price per page. Unfortunately these advantages may be counterbalanced by the problems of keeping a flexible pipe attached to a fast-moving head under just the right pressure. The connecting tubes have a tendency to develop air-locks, block, dry out or even split.
Inevitably the ink in the heads will either lose its capillary connection or dry out from time to time. Mechanisms to shield the head against drying out have been mentioned.
Shielding the head from the atmosphere will not guarantee ink-flow. The printer also needs one or more mechanisms to flush dry ink and airlocks out of the pipes. The simplest mechanism is a rubber wiper blade which is passed over the head several times. Some printers have an additional pump mechanism. The Canon BJ200 has an intriguing pump mechanism under the rubber cup that the head parks on, it is activated at power up or under user control. There may be several cleaning cycles. At power-up the printer will pull some ink through the head to ensure there are no air-locks. At power-down some printers pull further ink through to empty the capillaries. There are usually one or two levels of user-initiated cleaning:
- a quick cycle might merely wipe the head with the rubber blade to clean of sticky ink and re-establish flow.
- A "new cartridge" cycle will employ the pump and use rather more ink but has more chance of dislodging a major blockage.
Phase Change Inkjets:
Inks which are liquid at room temperature rely on the evaporation of a solvent to leave material fixed to the paper. One disadvantage is that the rate of evapoation relies on temperature and humidity, so some printing may be smudged.
Phase change inkjets use solid ink sticks on which the tip is melted just before use. A phase-change machine will generally melt the ink at a temperature of 70-80 centigrade just before use. The mechanism is then similar to that of a liquid inkjet, but the rapid drying avoids some of the problems of liquid inks. In principle it is possible for a phase-change inkjet to emulate a phase change thermal printer making a machine capable of very high quality colour printing. Phase-change inks can also contain metallic flakes which can give attractive and unusual effects on paperwork.
More Forceful Inkjets:
The ink-jet technology described so far is used in standard desktop devices. It is also possible to build inkjet-like devices for use in applications where the surface to be painted is not paper – bottles, cables and anything which might need date/time, batch number or other information printing. This kind of mechanism is used in factories to stamp batch and date numbers onto goods.
There is also a refinement of inkjet technology intended for very high speed operation. A single nozzle emits a fine stream of ink droplets at some speed and carrying a high electric charge. Electrostatic plates steer the ink droplets to the correct place on the paper. This process bears some resemblance to the way cathode ray tubes work, it could be quite fast - but it could also be incredibly messy.
All sorts of refinements on the inkjet principle can be devised. On might be a machine using pastry or icing sugar as ink to date-stamp pies – or even to individualise cakes. The print elements could be large, using solenoid actuators. It might be possible to put edible ink into standard Hewlett –Packard style cartridges.
The HP cartridge could also be mounted in a little cart, on which one of the wheels carries a position encoder – When pushed across any surface the cartridge would print the message.
Inkjet mechanisms construct an image by squirting small blobs of material onto a surface. The same principles could also be used to make three-dimensional objects. Successively printing the same pattern in the same position will build up an object layer by layer. Common inkjet cartridges produce images with resolutions less than 1000/th of an inch, so complex and detailed objects could be made. Complex integrated circuits might be made using organic semiconductors. At a much larger scale the same sort of ideas could be applied to resins, molten steel or concrete.
Paper styles and use:
Although paper is often just a vehicle for information people have very strong views on what it should looks like. A newspaper on green-lined computer paper, or a will on cash register tally-roll might be functional but they are unlikely to be acceptable.
For most tasks most people are adopting a standard compromise – A4 80 gram white "photocopier paper". Even with such a boring material paper can cause an amazing amount of trouble to printer users an support engineers. More information on how paper works might sometimes be helpful.
Paper is made largely from wood fibre. Parts of the wood break down in the pulping process to give a cellulose adhesive that holds the fibres together. Very high quality papers might also contain a woven material holding the sheet together –traditionally linen.
Most paper contains a certain amount of kaolin (white clay) within the fibre mat. British paper types tend to contain rather more kaolin than US papers because there is a traditional demand for higher levels of polish. The kaolin will absorb liquid before it can disperse through the fibre, but a polished surface may slow absorption so that ink smears.
The wood –fibre in paper is often chemically bleached, perhaps using chlorine or sulphur dioxide. Chemical brighteners might also be added – a brightener tends to flouresce slightly towards the blue end of the spectrum – the same principle used by some washing powders. Coloured papers will also incorporate a dye. Many papers are vulnerable to breakdown by environmental factors- newsprint will yellow if exposed to light and microbes will often invade, particularly if the material gets damp. Paper cannot easily be kept dry because the fibres are often hygroscopic. When paper is damp the kaolin on its surface will tend to bind sheets together, and fibres may break lose from the sheet interfering with ink-flow.
Paper is generally made in large rolls and cut to the required shape. The latter stages of the cutting process generally involve a stack of many sheets of paper being cut simultaneously. The cutting can be done by a guillotine, a saw, or by a water-jet.
Guillotines will obviously bend the edges of the paper in such a way that they will curve slightly and may be difficult to separate. Sawing will both bend edges and make dust, some of which will penetrate into the paper stack and re-appear during use. Water-jets are the most expensive cutting devices but make no dust. Dust in the paper and bending of the edges can have a marked effect on printing processes. One of the commonest reasons for engineers to be called out to printers and copiers is that users have changed the grade of paper used.
Printer users have quite a lot of incentive to try out different papers. For instance in the market for A4 copier paper there is commonly a three-fold price difference between papers, with bulk unbranded supplies of available for under £1.50 per ream and a single ream of quality paper costing £4. Since the best bargains are bulk-buys organisations can find themselves burdened with large quantities of paper that will not feed through machines reliably. Who carries the burden of this problem can depend on the exact terms of copier and printer maintenance contracts.
There are about five main printer technologies - bandprinter, dot matrix, thermal, laser and inkjet. In principle, any printer can take any kind of paper - for instance it would be possible to design a bandprinter with a cut sheet feeder, and there are laser printers that cope with continuous forms. However there is a tendency for older bandprinters and dot-matrix machines to use continuous forms whilst lasers and inkjets use A4 cut sheets.
It is possible for a printer to work with no real knowledge of the shape of paper- but only if the layout requirements are very basic. Most users want the text to begin at the top left of a page and just about fill the space, then to skip to the next page giving a neat layout. Users do not want print to go beyond the margin of the page - which would mean they lose information . A lot of this process can be controlled entirely by the computer, but the printer needs to know at least where the top of the first page is, and how long each succeeding pages is. The printer must be given some basic information about the shape of the page.
Twenty years ago it was common for printers to have a "formatter tape". Users would have sales invoice, purchase order, stock control and other pre-printed stationery and would load the correct tape into the printer at the same time as the stationery. The printer controlled layout using information on the tape. The tapes often broke, they could be difficult to program, and they made printer operation more difficult than necessary. These days printers are always controlled by paper size switches, parameters stored from the control panel or increasingly often by using a Windows program that ships with the print-driver program. Some knowledge of the kinds of paper used is necessary to set the printer correctly.
A4 paper will be familiar to most people - this report is normally printed on it. The standard paper sizes start with A0- this is vast and only used for charts and plans. Take a piece of A0 and fold it in two halfway down the long side and that makes A1. Do the same again and that makes A2. Repeating the action makes A3, A4 and A5, these are the sizes seen in common use. In Europe A4 has gradually replaced older paper sizes such as manuscript and foolscap. It is worth noting that in the US paper sizes such as "legal" and "letter" persist.
Some professions have great affection for odd old paper sizes – the legal and accountancy professions are especially keen on tradition. There will certainly be a printer that can cope, but it may not be a mass market device, so it might be costly
Tractor Feed:
Tractor feed paper has a row of sprocket holes down each margin spaced at 1/2" intervals - no other size has been used in recent times. Sheets are not detached from one-another but folded at perforations, usually there are 2000 sheets in a box.
The industry standard size for tractor feed listing paper is 14", wide enough for 136 columns at 10 columns per inch, and for 272 characters at 20 cpi – useful for spreadsheets.
80 column paper usually has a fine line of perforations down the margins to allow users to separate the tractor feed holes, producing paper approximating to A4 and improving the look of reports
The printer grips the paper by sprocket-pins. Some low cost printers had sprocket-pins at either side of the platen, which meant that only one size of paper could be used. More usually the sprocket-pins are carried on two adjustable tractor blocks that can be positioned to handle any width of paper.
The tractors can be positioned after the platen, or before it.
Tractors are traditionally placed after the platen - where they are called "pull tractors". This means that the paper is under tension as it feeds through the print position making it less liable to wrinkle. If the paper wrinkles as the printhead passes over it the characters will be mal-formed, but much worse is that the head will generally tear the paper, crumple and tear the ribbon and possibly bend and jam printhead needles. Avoiding paper-wrecks is therefore quite important.
But there is a problem, with a pull tractor at least one sheet of every printout has to be wasted. This does not matter too much if the average printout is 10 or 20 pages long, but if the printer produces one page warehouse picking lists every few minutes wasting half of the paper becomes irritating.
"Push tractors" are placed before the print-position, either below or behind the platen. It can be more difficult to load the paper into a push tractor, but once it is there the paper can be torn off at the perforations the moment it has left the print station. Push tractors allow continuous feed paper to output a single sheet at a time, so that dot matrix machines can be used in work such as producing despatch notes, point-of-sale invoices or pharmaceutical labels.
Listing paper has never been very satisfactory to users, its purpose is to make printer design simpler, but the paper is clumsy and the tractor-holes look ragged. It would be possible to apply any of the print technologies including laser to continuous sheet paper, but the market now favours cut sheet paper.
Pre-printed forms:
Tractor–feed remains supreme in one market, multi-part business forms. There is no way to print on multi-part paper except using an impact printer. The nature of business processing tends to produce batches of print so loading and unloading tractor feed is not unduly inconvenient.
It is actually quite difficult for a normal print-works to make a multi-part pre-printed business form. Printing companies naturally expect to handle single part good quality cut sheets, and they need some different equipment to handle light-weight tractor feed. Normally each part except the last will be made of carbon-less copy paper, which has microscopic ink capsules embedded in its weave. Each part will be overprinted with the outline of the form and they must then be bound together, the tractor margins are usually used to do this, either with a very light adhesive or by perforating the paper so that it holds together.
The price of pre-printed stationery can be surprisingly high, typically £200 or more for 1000 forms - 20p per sheet. These costs are often much higher if artwork and plates have to be made – rising so that little change will be seen from £500. Long print runs bring the price down to 3p to 10p per form.
Because it can cost a lot to set up pre-printed stationery accounting software houses often do this for their customers. In fact this can often be where a substantial part of the profit for a software business lies.
Software houses own the copyright on their pre-printed forms. Customers cannot simply take the design and get it re-printed, even if they felt the substantial setup prices were justified. If the design of software makes a pre-printed form necessary then software houses can charge a substantial premium for it. Perhaps it is no surprise that some software simply will not operate effectively without pre-printed stationery.
Cut Sheet Feeding:
Cut sheet handling is rather more difficult than tractor feed. The first computer printers to provide cut-sheet handling as standard were laser machines, which work in a similar manner to photocopiers and largely copied their mechanical construction. The primary selling-point for early laser printers was quality of output, so their ability to use good quality paper was important. Cut sheet feed was available for daisy-wheel and dot-matrix printers as an add-on option at a premium price.
Cut sheet feeders hold a neat stack of paper in a tray. The tray will have adjustable spacers to ensure that the paper stays in the correct positions. Under the paper will be a lift plate which keeps the paper pushing upwards. At the "exit" end of the tray the paper is pressed up against one or two hinged catches that obstruct its escape as long as nothing pushes it. These are the paper separators.
When a sheet is to be fed it is held up against two or more rubber rollers. These pickup rollers are often shaped like a "D" and the flat side faces the paper until the moment a sheet is to be fed. As the rollers rotate they engage the paper, pushing against it. The paper is held back by the separator and will distort slightly, breaking away from the stack and moving further into the machine where it is engaged by other rollers that carry it through the remainder of the process.
Controlling the action of the pickup and other rollers requires a sequence of actions. The paper is kicked out, then the pickup rollers stop. The feed-rollers now take over - the paper must be held under enough tension that it does not slip, but not so much that it will be distorted as it travels through the machine. The tension must be very even or the paper will skew to one side or another. Quite often there will be a series of guides and paper gates all of which must be correctly adjusted. This is what makes cut sheet handling complex and prone to failure.
Some early cut-sheet feeders by firms such as Rutishauser used dedicated microprocessors and motors to achieve the action, making their feeders quite expensive (£400 plus). More recently, printer manufacturers tend to have incorporated a cut sheet feeder into the design, or to make the printer processor and line-feed motor do much of the work. Epson’s LQ series feeders are fairly cheap because they are entirely mechanical, with the timing actions being produced by rotating the platen forwards and backwards to engage different mechanisms through planetary wheels. The only problem is thatnon-powered cut sheet feeders tend to be slow because of the platen movement needed to engage a variety of sun and planet gears.
Laser printers with high work-rates can usually be a component in a stack of cut-sheet paper trays which can also incorporate duplex units (double sided printing), output collators and even bookbinding.
The design of cut sheet feed mechanisms has now been refined to the point where they are quite reliable and cheap to produce. Because people tend to prefer their work on neat, crisp standard A4 paper, most laser and inkjet printers make no provision for any other sort of output.
Duplex:
One obvious defect with almost all of the current generation of printers is that they print on only one side of the paper. One sided printing is normal for correspondence where wasting paper has always been seen as a matter of courtesy, but it makes other documents look unprofessional – and of course it nearly doubles paper consumption. Two-sided printing can be achieved by simply putting each page through the printer manually the right way up – which can be confusing because some printers feed the paper upside down.
Recent MS-Windows drivers try to help by having a select-box for all, odd or even pages so that a whole report can be fed back through the machine. Unfortunately this can still be confusing and the risks of a paper-jam rise disproportionately because the paper tends to curl after it has passed through the fuser.
The ideal way to print double-sided is with a duplex unit which takes the output and directs it through the printer a second time. The duplex unit is much better than messing about trying to ensure each page goes through in the correct order and the right way up, although it still tends to be a bit prone to jamming due to paper curl.
Duplex units are surprisingly rare for "Catch-22" reasons: few users buy duplex units because they are expensive, duplex units are expensive because so few people buy them.
Multiple Trays:
Letterhead paper is very widely used to create company image. Almost every organisation has some sort of coloured logo that is to be incorporated in every document in the front page. Letterhead paper is usually pre-printed in colour, it may be on heavier than normal paper and it may be embossed. The page orientation problems of manual duplex printing are also a concern for letterhead, but letterhead is also needed a great deal – often every second page, especially on network printers.
The simplest arrangement is for the printer to have a tray set aside for letterhead and to mark which pages of a document are to use that tray. MS-Windows printer drivers for multi-tray printers offer these choices.
Some organisations have several different letterheads and require a file copy on different coloured paper. Paper-feeder stacks provide for this sort of use, but it is worth noting that the cost of the paper feeder trays might well exceed the price of the printer itself – and that the probability of failure rises with the number of trays. A key problem with multiple tray systems is that users get over-ambitious. Trays are generally rated to handle paper grades between 80 an 100 grams – but users persist in trying to put 60 gram copy paper and 110 gram letterhead through feeders. The fact that CSFs will often handle paper grades outside their rating does not mean they should be expected to- often as the machine ages slight wear on components causes it to malfunction and it may not be possible to restore the original beyond-specification workability.
White Paper Offices:
All kinds of pre-printing tend to be quite costly – small firms can find that letterhead costs more than 20p per sheet. Overprinted tractor feed can have similar costs. Calculating the price of computer printing should probably take these exterrnal costs into account.
Some organisations have decided to stop using pre-printed forms altogether – any paper output by the business will have to come from computer printers or photocopiers. The idea is sometimes called a white paper office because fresh white paper is the only kind of stationery used
A good colour inkjet printer can produce reasonably convincing letterhead – although the quality might not match that provided by a professional printer using special inks. The price per page for inkjet letterhead can be around 3-4p, but this might very significantly undercut the price of professional printing.
One very important factor in white-paper procedures is that it is easy to correct mistakes. Too often people only find the mistakes in form design when they are already using them and have a 20,000 copy print-run to work through. Long print-runs can also be a nuisance because basic information changes so often – businesses move premises, change phone numbers, change their product lines and ways of operating – each can mean a re-design of the business forms.
Of course using nothing but computer and photocopier output can mean that print quality suffers. Computer output offers a very significant quality advantage that may more than compensate:- each form can be customised to the job in hand and have some boxes pre-printed with information already known to the system.
Printing forms with the data already held can save time re-writing the obvious. Just as importantly the process of transferring information into a computer is usually error-prone, and re-printing the information gives an opportunity for it to be corrected.
Although the idea of a white-paper office has some merit, professional printers are still likely to be needed:
In the last two decades most printers have equipped themselves with computer gear that is more powerful than that on the average desktop. Printers and graphic designers work closely together and tend to come up with a better looking product than an amateur with an inkjet. The better quality inks and stiffer papers used in professional processes are a small part of the overall package.
Most people are swamped with unwelcome paper. The single major cause of complaints to the Data Protection Registrar is unsolicited mail shots. A few weeks of catalogues, trade-press and mailshots can easily mount up into a congealed mass of dead material. In an information-overloaded environment the only way for information to succeed is to look unusual. Professional printers are usually best at making sure a message succeeds.
Printer Control Codes:
In principle, all that a computer user wishes to see on paper is exactly what they see on the screen. Until very recently this has been difficult to achieve. Computer screens were a poor reflection of anything that could be produced on paper. Paper printing includes fonts and typefaces, bold, underlining, superscript and subscript, symbols, graphics and borders. Old display screens could not achieve this.
The Apple Mac and Windows systems with VGA displays have changed this by introducing WYSIWYG - What You See Is What You Get. Screen and printed output have the same appearance. The problem is that the printer control languages that used to work well for material from crude old displays are poor at handling complex graphics on screen or paper.
The basis of printer control has generally been ASCII codes. Each character is represented by a seven or eight bit binary pattern, and most of the characters transferred are printed. Screen and printer each hold their own pattern for reproducing characters, so it is possible for the pattern seen on the screen and that output on paper to be rather different. Some characters give special instructions to the printer on character spacing, bold printing, double height, underlining or colour selection, or to knock it into a graphics printing mode. There have been innumerable different standards for printer control. OKI printers obeyed one set, IBM another and Mannesman yet another still. Each new generation of printers has introduced new capabilities and new codes.
Epson Esc-P2:
The most general system of control codes for dot matrix printers small is the Epson command set, the most recent version being ESC-P2. (There are a series of Epson Commands - MX100, FX1050, LQ850, LQ2050, LQ1170). Epson owns the copyright to the ESC-P2 command set - so in theory only their printers can produce them. However Epson used to copy IBM printer designs, and they now find other manufacturers copying them.
While the computer is transferring character-oriented material from a word-processor or a database the interface between computer and printer is quite efficient. An A4 page can contain just over 5,000 characters, so the interface will at most need to carry 5,000 bytes of information. In principle it is possible for an EPP interface to handle this volume of data in 2/1000th of a second, more usually it may take one second.
However with the growth of Windows applications the material to be printed often contains graphics. Most command-sets allow for this, an escape code allows the computer to download a binary pattern which is applied directly to the printhead. However it is worth noting that an A4 page will contain nearly 8 million dot positions, so nearly a million bytes of information will need to be transferred over the interface, this process may be rather slow.
Furthermore the printer is often rather poor at handling graphics. To print a line of normal characters the printer needs to acquire 132 characters before the head sets off across the page. To print graphics at 300dpi it will need to store about 4000 bytes in its buffer. Quite a few printers cannot do this, they have to print part of a line at a time. This wouldn’t be so bad, except that the printers carriage drive circuit may not keep very accurate track of spacing, so for each chunk that is printed the head has to be reset to the left margin again. Printing graphics can therefore take a long time and impose a lot of wear on a printer that is not well adapted to it. Most older dot matrix printers can be used for graphics, but they are dreadfully slow and often go wrong.
This situation is made worse because Windows programs use so many fonts. Suppose that the printer’s internal character set is Courier, and the users screen shows Arial. In the worst case the printer must simply substitute one font for another - and the printed page looks nothing like its screen counterpart. An alternative is to build the required font out of bit-mapped graphics to create the Arial characters. The computer therefore sends the whole text of the page as graphics. At least 10 times as much data will need to be moved, and the printhead will not be used in the most efficient way, so this will slow the print process down.
Some printers offer a rather more efficient way to produce fonts, the required font can be downloaded into the printer where it will be used to produce non-native fonts much more efficiently. This process can work providing the computers printer-drivers know the facility exists and the memory space taken by the font is not too large. Downloadable fonts aren’t much help with graphics, but they can regain efficiency for DOS.
One way to ensure that screen and printer graphics are identical is to use a common language. The most widely respected is Adobe Postscript.
Postscript:
Postscript is a full-scale computer language, every character, line and dot on the page is described in a sort of pidgin-English that sometimes breaks into streams of binary. Apple have been great promoters of Adobe Postscript, partly because they owned a share of the company but mostly because of its reliability in producing page layout. Other manufacturers have paid a licence to Adobe to use their language, but the price of the licence is often a significant fraction of the cost of the printer. Some manufacturers have tried to produce machines that will obey Adobe Postscript without legally infringing their copyright.
Postscript is complex, and its interpretation in the printer requires a powerful processor. A user with a postscript machine may have a better processor and more RAM in their printer than they do in their computer, in effect they have paid twice, but can only use half the investment at a time.
Windows largely created the graphics problem - before it became widespread graphics was a "ghetto" function carried on by people who had bought Apple Mac machines. Windows also provides a solution by providing a common Graphical Device Interface.
The Graphical Device Interface is a Windows software mechanism which treats screen, printer and any other output device that might be attached in the same way. A printer attached directly to the GDI can be entirely controlled by the main PC, and needs little in the way of logic itself and no control language - all of its controls are directly carried out from Windows. The disadvantage is that the user must be running Windows, the advantage is that the user can spend money on a more powerful PC with the savings from not having to buy an expensive printer logic.
Clearly the GDI printer is no use to someone running an Apple Mac (they have had their own equivalent overpriced machines for years). A GDI printer is also of little use to a DOS user - although some printers do come with a driver that will provide some DOS services. Microsoft have said that the GDI in Windows ‘95 will not be identical to that in Windows 3.1, so users will need to have a secure line to get the correct printer driver when they want to upgrade their version of Windows.
Standing somewhere between Epson codes and Postscript in effectiveness and widespread use is the Hewlett Packard code-set PCML..
Consumables and Recycling:
Printer buyers will spend hours selecting the best-priced printer and completely forget the ownership costs – cartridges and consumables, paper, staff and engineer time. Printer manufacturers and salesmen take advantage of this vulnerability and often sell the machines for just a little profit, but make up the difference selling consumables and maintenance.
If the manufacturers were aiming to keep consumables cheap for the end-user they would re-use the same designs. In fact just about every printer launched onto the market brings yet another design of consumable. The changing design of inkjet cartridges is forgivable, the technology is evolving and each new design may be a real advance over the old. The endless stream of dot-matrix ribbons is less understandable; each ribbon is in a different plastic cartridge, but delivers a nearly identical ink on a rather similar fabric medium. Ribbon design changes show manufacturers being irresponsible: ensuring the customer has to go on buying an expensive item that wastes plastic and fabric for little or no technical reason.
Bandprinter ribbons ought to be relatively easy to re-ink. Users do not seem to bother because although the ribbons are quite expensive (£30-£70), they are often immense, and last for several months. If a recycled bandprinter ribbon were to misfeed and damage the print-band the cost could exceed £500, so users are nervous. On the other hand the ink requirements of a bandprinter are simple, re-inking should be cheap, and is quite unlikely to do any damage.
Dot-matrix ribbons are more critical. Different specifications of weave are used for 9 and 24 needle printers. If the ribbon has any sort of disruption in its surface it could trap a needle and wreck the printhead. The ink also provides a lubricant layer between the printhead jewel and the needles. If the lubricant fails printhead wear will be excessive, or the needles could fall out of the printhead. Nevertheless ribbon re-inking is quite widely used.
Inkjet cartridge refilling is also widely practised. Some users take a DIY approach using Quink or Rotring ink and a hypodermic needle. Kits with specially made ink are available at about half the cost of new cartridges. Inkjet printer manufacturers claim that the formulation of the ink and the life of the printhead are very finely balanced, but some users find they can extend the life of a Hewlett Packard printhead several times by re-inking using Quink.
Laser printers include half a dozen consumables: toner, developer, drum, scraper blade, waste bottle and fuser. Each element has a different life potential.
- Fusers are always separate elements of the system with a design life of roughly 150,000 pages. The fuser often has a "cleaning pad" which lubricates the rollers to prevent toner adhering to them in it’s molten state. The cleaning pad is impregnated with silica oil and needs changing every 5,000 pages or so. The pad can usually be replaced quite easily. Although fusers are often expensive to replace when the parts are worn they canbe rebuilt. The halogen bulb, teflon coated roller and rubber roller can all be replaced if the manufacturer makes parts available.
- Drums typically have a life between 5,000 and 50,000 pages depending on the nature and thickness of the OPC and any arrangements made to clean it’s surface. Some Kyocers and Canon printers use an amorphous silicon drum than can last substantially longer than normal. Kyocera claim a drum life life of 300,000 pages.
- Developers churn iron filings, which must rust and degenerate over time. The working surface of the developer is usually an aluminium roller, which will wear out after a few hundred thousand pages.
- The waste toner mechanism is normally a silica-rubber scraper blade to remove material from the drum, together with a transport mechanism and a waste bottle. The blades are often part of the drum assembly, they do seem to wear out and de-nature with the material turning rather brown. Sometimes waste bottles can be simply emptied- but because toner is certainly messy and might be hazardous manufacturers normally arrange for the bottle to be sealed with a cap and disposed of in ordinary garbage.
Cartridge designers obviously aim for some sort of balance between the components, but many parts inevitably have life left when the toner runs out. Apart from any other consideration, one of the main manufacturing and environmental costs in a cartridge is simply making all the plastic and metal mouldings which are not suffering any wear.
Cartridge manufacturers are aware that the waste implied by throwing away cartridges seems offensive to environmental concerns. Manufacturers often do offer a recycling service, but often this is just a matter of free return of the cartridge to a central depot – there is no economic incentive. From a cartridge manufacturers viewpoint it almost certainly costs more to dismantle old cartridges, clean and inspect the parts and re-use those which pass quality thresholds than to make new devices. Manufacturers recycling may well be to separate the plastics and use them as feedstock, rather than to re-use the old parts.
Many recycling operations try to make much more use of components. There is great variability in the quality of recycled cartridges – some are thoroughly cleaned and have all the active components replaced and should be as good as new. The easiest recycling option is just to re-fill the toner compartment and empty the waste compartment, and this seems to be all some recyclers do. The problem is that whilst print quality may be acceptable at first it often degenerates too quickly. Waste toner scrapers and compartment seals are often damaged and poorly recycled cartridges can drop toner into the works of the printer causing dirty copy, mis-feeds and requiring an engineering call-out.
Manufacturers will often reject maintenance calls for printers that are using consumables not made by them – the call usually becomes chargeable when the engineer finds the alien device. This can be used to trap the unwary. A Dataproducts cartridge may be much cheaper than an Apple cartridge – but if it breaches warranty terms then the cost of the field service call could outweigh the saving in cartridges.
There are several companies who specialise in recycled and re-manufactured printer consumables.
Alladink
Printer Maintenance:
A computer might conceivably live out it’s four or five year service life without going wrong. Computers generally have only two moving parts – the fan and the disk, so if they were not so full of questionable connectors and flakey software they could be very reliable.
Printers are often expected to last rather longer and are inevitably prone to much more mechanical trouble. Even the simplest thermal printers contain a motor driven platen roller. Multi-tray laser printers contain dozens of moving parts powered by several motors. Mechanical failures are innevitable. Add into the formula the effects of badly written printer drivers and printers can be a nuisance.
Low-cost printers can almost be treated as disposable
The average printer buyer has an idea that a long warranty shows a manufacturers confidence in the product. There is also an idea that the warranty must be good value for money because it is free.
It would certainly be difficult to give a long warranty without some confidence in the mechanics. However printer warranties should be viewed more as interesting marketing tools than as a manufacturers guarantee of a working machine.
Printer manufacturers are keen to emphasise the quality of their equipment, so they may offer long warranties. However a major manufacturer cannot be seen to drop behind the pack in terms of warranty – if Hewlett Packard offer a three year onsite warranty then so will other manufacturers. The very competitive nature of the printer market means that warranty cannot be a free-gift from the manufacturer.
Buyers tend to settle on a printer design because the specification seems good. They will perceive a warranty as a guarantee of quality, and an "extended warranty" seems like a courtesy from the manufacturer rather than a rip-off.
In the small print manufacturers invariably state that only manufacturers original cartridges or authorised recycled cartridges can be used. A warranty call where the wrong brand of cartridge is found in a machine might result in "sorry- you must replace the cartridge" . This is quite likely to be followed up with an invoice for the manufacturers standard field-service call out – generally not less than £100 and possibly much more.
Spares:
It can be difficult to repair proprietary PC designs and monitors without spares, printers can be virtually irreparable.
A perfect example is the Compaq Pagemarq 20, a big, sophisticated 20 page per minute network laser printer. This is a good machine with a below-average collection of design faults, amongst them that the fuser rollers are driven by a white nylon cog which de-natures in the hot greasy environment it is required to work in. The cog eventually loses some of it’s teeth and the printer jams continually. Without a spare there can be no repair and a big expensive printer is finished.
Compaq are useless a source of spares – they only made the product for a year or so in 1993-4. I once phoned Compaq to ask about the printer and the dialogue went like this:
-I’d like to speak to someone about a problem with a Compaq printer
-We don’t make printers
-You made this one last year, it has a Compaq badge on the front
-No, we never have made printers, someone must have stuck the badge on the front.
The Pagemarq is actually a Fuji-Xerox engine and is also sold with Xerox, Apple and DataProducts badges. Xerox will not sell parts ("Why should we help people undercut our own service business") and Apple are notoriously unhelpful to anyone who isn’t an Apple authorised dealer. DataProducts have a UK based repairer who also sells spares – but for some time they claimed to have no stock of these cogs. In fact closer questioning showed they could "refurbish the motor drive chain" – which included replacing this cog if necessary (in fact failure of this cog is the only known fault in the drive chain). A "refurbishment" was well over £100, but after a little persuading they decided they could send one through the post. Luckily they now have a supply of cogs at more reasonable prices.
It would have been possible to get another cog milled in brass for about £100. This didn’t seem a good alternative because it might have conducted heat back to the next stage in the chain, could have damaged the fuser cog and might have proved noisy. In fact the new Fuji-Xerox cogs are made of a grey plastic which seems much more robust.
The Pagemarq has other problems.
Machines sometimes give the message "System Check" and simply will not work. Nothing seems to be wrong with the engine (there are actually a whole library of tests accessible through the front panel). The major electronics on the system board all seem perfectly workable. The fault is almost certainly either the checksum in the user setups stored in an EEPROM or in the control software stored in ROMS. The best solution at present seems to be an exchange system board from the US, which is quite expensive. What is actually being provided seems to be an upgraded set of ROMs.
Another problem is a tendency for the Ethernet port to sulk. The printer engine and system board work and pass self test. The Ethernet port can be seen responding on a network sniffer – but nothing is printed. The Network Interface board can be bought from the US as a spare but costs more than £350, the easy answer is to use the parallel port together with a print-server. Printing speeds do not seem to suffer very much and print-servers are half the price of the interface board.
The Compaq Pagemarq strongly illustrates the common problems and solutions in printer maintenance. The real manufacturing cost of the cog is unknown, but is unlikely to be more than £1 – yet without it the printer is dead. The only real limit on the price of the cog is the point where it seems preferable to scrap the printer - and a full functional replacement is likely to cost over £2,000 and could take several hour’s work replacing workstation printer-drivers. By these standards a £100 cog is a bargain.
Manufacturers follow a range of policies on the price and availability of spares.
A decade ago Mannesman Tally used to supply spares at a price which meant that the purchase price for all the parts needed to build a printer was three times the price of the printer itself. Superficially this seems rather expensive – but in fact it was probably the most user friendly policy any company ever adopted. A couple of years later this policy was quietly dropped and MT parts became amongst the more expensive. It is difficult to nominate a manufacturer with a reasonably fair parts policy currently in place – they all seem to be raising a major revenue stream through spares sales.
Tektronix made pen-plotters in the mid 80’s, they used electrostatic mats to hold the paper in place. The original price of the multi-pen plotter was over £3,000. Pen plotters are now obsolete, replaced by inkjets, but in the early 90s quite a few of these machines needed repair. Tektronix’ price for the electrostatic mat alone was £2,000. A copy of the manual was $130 US, or £160. "Surely that’s the wrong way round" I once said to someone in Tektronix spares – "it should cost less in pounds". "Not the way we do it" was the reply.
Hideously expensive spares might arguably be rather better than non at all. Xerox will not supply spare parts.
Hewlett Packard’s attitude to spares seems erratic.
Most manufacturers run a repair operation themselves. The main revenue stream probably comes from selling extended warranty and maintenance contracts.
A secondary revenue stream will come from workshop repair. One off repair usually involves a non- refundable evaluation fee for the quotation, followed by other charges for parts and labour. The evaluation fee is often set around £75, so that an extra £125 for a repair doesn’t seem such a bad option. Return un-repaired might also attract a penalty- such as a £15 packing and courier fee.
Manufacturers are generally hostile to the idea of other businesses repairing their equipment.
Set a high price on maintenance manuals -
Manuals
Warranty:
Extended Warranty:
Buying a printer:
The worldwide printer market is large - most computers have some sort of printer attached. Considering the substantial manufacturing and distribution economies of scale present in the industry there are a surprising number of printer manufacturers. In many industries there is one dominant force, half a dozen much smaller competitors and a scatter of specialists. The printer industry is dominated by Hewlett Packard with its Laserjet and Deskjet series but there are many others including:
Brother, Canon, Citizen, C.Itoh, Digital, Epson, Facit, Fargo, Fujitsu, Hewlett Packard, IBM, Kodak-Diconix, Kyocera, Lexmark, Lannier, Memorex-Telex, NEC, Newbury, Nokia, OCE, OKI, Olivetti, Panasonic, Printronix, PPI, Qume, Ricoh, Sharp, Star, Tally, Tektronix, Xerox.
To give some guidance
Fujitsu, Tally, Genicom and Printronix lead in high speed
Epson leads the pack in small dot matrix printers, although Brother, OKI, and Panasonic all have credible offerings.
Brother, Tally and Newbury have good, solid tested reliable designs for large dot matrix machines. Printronics and Tally take the lead in producing fast shuttle printers.
Hewlett Packard and Canon seem to lead in laser-printers. Kyocera’s designs using amorphous silicon may be attractive if price per page is a primary concern..
Hewlett Packard, Canon and Epson lead in production of inkjet machines, but nearly every other manufacturer offers some sort of inkjet model.
Tektronix are leaders in producing unusual "top end" machines for functions such as thermal process colour imaging. Their machines tend to be innovative but costly to buy. Spares are often nightmare prices.