Printers & Print Technology
Print Technology
This is a general look at how printing is done, particularly by computer -based or "digital" printers. Computer printers have been rather different from the kind of machine found in a commercial print works - there was no relationship in the way they were used or the way they worked. Most of the distinction is dissapearing:
Producing good-looking documents may still be an art but it no longer needs special machinery. Computer software can produce good looking documents that would merit professional "offtset" printing. Even rather ordinary software like Microsoft "Word" and PowerPoint can position pictures and page furniture. Graphic artists are more likely to use Photoshop and Quark Xpress to get the look they want rather than something a wizard provides. Using templates and style sheets corporate products can all be given a house style - even when the author knows nothing about layout. There may be more demand for the graphical arts than ever before but people increasingly want them to supply icons, GIFs and CSS or XLS.
Print technology is changing as well. Desktop printers can produce pages that nearly match what a commercial press might produce.
Commercial print works are increasingly turning to digital print systems. Digital printer systems are industrial versions of the laser and inkjet printer to produce short runs. They have the cost advantage that they don't rely on little cartridges. Professional printers have the quality advantage that they know all about their industry - from how an intended audience might like a document to look through to how to operate the machinery for the best effect
One thing the professional printer can do that the desktop publisher can't is to finish the document. Fifty pages bound with a spiral or a plastic strip is OK for a sales presentation or a manual but it isn't a book. The professional printer can make a product.
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There are lots of technologies that change the look of things - painting, plating and veneering for instance. Printing processes normally produce a complex pattern, such as text and graphics, usually on a paper surface. Cloth, plastic and metal can be printed as well. Printing CDs is a widespread need. Iron-on T-shirt transfers and vinyl signs for vehicles are examples of small industries based on computer printing.
Printing presses push a carved and inked surface against a sheet of paper to transfer an image where the two touch. The simplest mechanism uses a woodcut, one unchangeable block that produces one unvarying image. Woodcuts might be used to produce a book, but the effort of all that carving to make row after row of repeated alphabetic characters is offputing.
Moveable type makes printing much more practical. The alphabetic characters are made up one at a time on a letter form block. Letter-forms are locked in a frame to make up a plate, but they can be released and re-used on new jobs. The idea of printing is not just to transfer a pattern, but to do it using a flexible technology.
Digital printing turns everything to pixels - picture elements which are single dots of light or colour. Pixels are usually small - on a typical computer screen there are about 75 to 100 dots per inch and most printers produce more than 300 dots per inch. The human eye sees the overall pattern made by the pixels and rarely notices the dots themselves. A computer can set any pixel to any brightness or colour making it the ultimate flexible technology.
Why now
Why not keep it in the computer
At the moment we might distinguish two or three fairly distinct branches of print technology: commercial printing, computer printing and industrial printing.
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Commercial printing is largely based on traditional presses at present. Printing is a large market - the world total is believed to be about $400bn. The aim is usually to produce many identical copies of one document on paper or card at the highest speed and the lowest unit cost - books, magazines, catalogues. Like many industries commercial print is undergoing a bit of change due to computer methods. At present 60% or so of all printing is based on traditional methods, but within 5 years digital systems are expected to be the bulk of production.
The definitive tools are the plate-maker and the offset litho printer.
Plate-maker
Diazo plates are coated with an organic material and developed using a solvent. Diazo plates have a production life of around 150,000 impressions and a shelf life of around a year
Photopolymer plates are similar but more innert and abrasion resistant. Photopolymer has a production life of around 250,000 impressions and a long shelf life.
Ablation plates
Bimetal plates, Waterless plates and Silver halide plates
Press
Printers also use form cutters, creasing machines, guillotines and binders to give documents in their conventional shapes..
There is some overlap between commercial mass-production presses and computer printing, both in technology and in tasks.
Print processes have a long tradition: design, layout and typesetting were all things the printer did. For a simple low-cost print job today's print works wants camera ready copy as a PDF with uncompressed TIFF images. The pages will be "proofed" on an inkjet plotter with a specially calibrated RIP then sent to a platemaking machine. A large part of the process is computerised and standardised. Printing on a large scale is almost always done by a specialist operation that can afford the equipment. The customer has the option to do the design and layout themselves, or contract it out.
In fact, in the last couple of years the customer has had an option to do the printing as well. Photocopiers and colour copiers have been used for short print-run jobs for many years. Recent colour laser-printers are just about up to the job of producing 100 copies of a 50 page catalogue - although binding might be an issue.
Obviously the commercial printer doesn't want to lose business, so they have equipped themselves with big, fast colour laser printers that can collate and bind documents as well. E-mail the PDF and you can collect the document in a couple of hours.
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Computer printing generally aims to produce one (perhaps several) copies of a document on a standardised type of paper - letter in the USA and A4 in Europe. Speed, appearance and colour might all be important; so is cost to buy and cost to run.
Paper seems to be essential to computer work but annoying. A few years ago a lot of people were aiming to have paperless offices but so far it hasn't really worked out. People can't just interact with a computer wherever they are - keyboards and screens are big, clumsy things. Screens don't convey enough information, and people aren't as good at absorbing it as they are with paper. So most people use computers to generate documents which they then print and file - duplicating part of the effort and consuming large ammounts of paper.
Ready availability of wireless mobile computing, e-mail and the web might reduce the need for paper but there have been few signs of it doing so yet. Very little of the computer technology is friendly in the sense of being easily fixed when things go wrong. A lot of the technology is prone to misuse - the flood of spam e-mail is just one example.
Two print technologies meeting the main demand for basic computer printing: laser and inkjets.
Laser printers usually have the lowest ownsership costs - they are faster, more reliable and the material is cheaper. The laser printer is a close relative of the photocopier and the basic job is to put a fine plastic powder called "toner" onto the paper and then adhere it using heat and pressure. Laser printer mechanisms are quite complex and expensive but can produce print runs quickly. They are mainly used for business purposes
Inkjet printers are usually cheaper to buy - the main mechanism is quite simple. The inkjet uses liquid ink droplets forced through nozzles and soaked into the paper. Inkjet printheads scan across the paper rather slowly but the nozzles can be very small so a colour printer with the right grade of paper can produce material matching photographic quality.
The concentration of solid colour material in toner probably makes it an inherently cheaper material although liquid ink is an older thechnology and the one used in commercial printing. The issues that prevent large scale low cost inkjet operation are the need for a complex print-head with very fine channels and for very pure ink that will not block the channels Neither laser nor inkjet can match commercial printing technology for cost.
There are other computer print technologies. Band printers were the industry standard for fast low cost printing of things like invoices for many years. Dot-matrix printers are still common in things like point-of-sale terminals where multipart stationery is used. Thermal printers can be very simple to operate for fax machines, till-rolls and labelling. Thermal printers using transfer technology can give very bold, sharp barcodes. Dye sublimation printing can be used to give photo-realistic images on ordinary paper.
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Industrial printing includes a great variety of labelling machines and ticket printers. This can overlap with commercial print jobs making bottle and pack labels - for instance a reel of labels might be printed with a serial number in the print works rather than on the production line. Industrial print tasks also overlap with "consumer" technology - CD labellers, badge makers and so forth.
A common industrial requirement is to print the date and time of manufacture onto products. Printers that do this trend to be special inket devices mounted on a production line and powered by compressed air.
A whole new realm of micromanufacturing machines is beginning to merge.
Micromanufacturing machines are often relatives of printer technology, but with a special role. Laser-engravers, vinyl cutters, cloth sample printers, rembossing and embroidering machines. There is also an emerging market of 3-D printers.
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At the moment there isn't much overlap between commercial, computer and industrial print markets.
Offset litho reproduces large quantities of identical colour pages at as little as a penny each and below. The technology can do this with nearly complete page cover on good quality paper, bound and guillotined in book form. However because there are all sorts of setup costs - plate making and then mounting the plates in the machines - a minimum run is several thousand pages and a typical budget is £5,000 right through to hundreds of thousands. Costs fall as the print run grows.
Industrial print mechanisms tend to a realm of their own - of interest to factory production engineers. If anything beyond product numbering and datestamping is required the most reliable technology tends to be thermal label printers.
Computer printers include pen plotters, dot-matrix, inkjets, lasers and thermal printers. Non of these technologies currently comes close to competing with the offset litho in terms of getting large quantities of bright, neat, colourful material to people in a form they are used to.
Laser printers can produce something that looks very like a commercially printed brochure - at a much higher price per page. A single page in full colour can esily cost 10p. However the laser printer does have two advantages over offset litho
Page quantities can be one -off. So a catalogue can be produced tailored to a specific customer with their own pricing and terms.
Small organisations can own and run the printer in house. In a frenetic business environment the ability to produce new documents overnight can be vital.
Inkjet printers are cheap to buy but tend to be relatively slow and expensive to run. If the output is one page of photographic quality material then a suitable inkjet with special paper is right for the job. If printing is a rare requirement - ten pages a week for school essays - then the low cost to buy an inkjet makes it right for the job.
Thermal printers are mechanically the simplest. The most familiar thermal printer is probably the older fax machine (which tends to be cheaper to run and more reliable than inkjet models). Thermal printing is also common in industrial labelling. Most people think of thermal mechanisms as cheap, robust but not accurate. Thermal printing using dye sublimation also works well for photography.
Print bureau services help fill the gap between the mass-production of the offset litho and the individual tailoring of laser printers. They also offer the graphic design and layout skills that many businesses don't have in house.
Pen Plotters
Pen plotters are one of the earliest and most obvious ways for a computer to put ink on paper. A human arm is replaced by a pair of motors which can move a pen over the page to write and draw.
Flexibility is a key virtue - the computer gets all the abilities a human would get using a pen. It can write, draw, shade and stipple to produce text, diagrams and maps.
Colour is possible - there can be several arms with several pens, or a holder can select between several pens in a docking station.
Paper size is limited by the length of the plotter arms, but making long arms is just a matter of metalwork so machines can be built to handle very large pages.
However there are several disadvantages
Writing is not simple - the plotter control circuits will need 10 or 20 instructions to draw one alphabetic character.
The process is slow. Making the shapes of characters takes time - using powerful motors and a light, reliable pen a plotter might manage 3 or 4 characters per second. A page might take several minutes. A big pen plotter might take several hours to complete a complex A0 diagram.
The computer can't easily tell when the pen runs out or blotches - reliable pens that don't need constant attention are quite a recent invention.
Grey-scale isn't really possible - the pen either makes a dot or it doesn't, so photographs can't be reproduced.
Computer controlled pen plotters have largely dissapeared. They were widely used in architectural and engineering offices, but they have been replaced by the faster and more flexible inkjet plotter. One thing in their favour hasn't been mentioned - they were one of the few pieces of computer machinery that was genuinely interesting to watch in action.
The pen plotter concept does survive in today's vinyl cutters, CNC routers and engraving machines. Engraving machines are often "printers" - they plug into a computers print interface and reproduce drawings from Corel Draw - and they can take a document from Word. A laser engraver can produce a very nice looking woodcut, which might be used as a printing block.
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There are four common printer mechanisms: impact, inkjet, laser and thermal. These four very different principles are all in widespread use.
The impact printer was the most common - particularly in it's dot-matrix form. Dot matrix printers are still used for "robust" commercial activities like printing delivery notes.
Inkjet printers have been the fastest growing product - they are an almost universal choice for the "Small Office / Home Office" or "SOHO" sector. One big attraction of inkjets is that they make colour printing available at low cost. The main problem with inkjet printers has been the high price of consumeables, but competition might change that.
Laser printers have been the choice for general office work - fairly fast, and not too expensive to run. Historically they were usually monochrome because colour mechanisms are expensive and slower. Fast laser printers intended for bulk use usually still are monochrome.
Thermal printers are normally forgotten, but they are one of the commonest mechanisms in industry because they are quite fast and very reliable.
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Impact printers crush an inked ribbon into the paper so that some of the ink transfers. Ribbons are fabric or plastic and inks are typically a pigment in a greasy carrier. Impact printing uses straight-forward mechanical processes that derive from printing presses and typewriters. The mechnical process means printing is noisy. The main "consumable" part is just inked ribbon so the process can be low cost, just fractions of a penny per page. The print mechanism (a head or band) also wears out, but more slowly. The cost of printheads is often forgotten in calculating page costs but it can be significant.
Typewriter mechanisms
Impact printer technology has several subdivisions. Some early computer printers used 'flying key' typewriter mechanisms, but these were far too slow. Type-cylinders and "Selectric" golf balls were faster - up to 16 characters per second. In the 1970's microprocessor controlled "daisywheel" printers and typewriters became very popular, used in a printer the mechanism can print 30-40 characters per second.
Typewriter mechanisms have the form of the letters cast in plastic or metal. The letters are selected by sending binary codes. It would be expensive and impractical to have a powerful solenoid under every key so the mechanisms used in teleprinters were widely adopted. Several different mechanism were used for teleprinters. The simplest mechanism to understand has a matrix of characters. The matrix printhead is on a carriage which scans across the page and halts at each print position. When the head is positioned it moves along and up to put the character in position, then the character is struck by a hammer. A refinement of this mechanism wraps the characters around a solidly cast cylinder or a ball and uses that to strike the page. Teleprinter type cylinders and IBM "Selectric" golf-ball print mechanisms suit automation quite well, the codes select the row and column by rotating and lifting the head, then the whole head moves forward and strikes the head. The codes used in Teletypes became the ASCII code still used today.
Typewriter speeds are limited because the key has to fly a long way forward to strike the paper, then return. Selectric golf-balls and teletype mechanisms are heavier but only need to move a couple of millimetres with the torque of a motor behind them to give adequate striking power to squeeze ink out of the ribbon and onto the paper. Both the Selectric and the teletype peaked at speeds of about 16 characters per second.
Daiswheel printers arranged the characters as something valguely resembling petals around a wheel.The machine spins the wheel to position and a hammer strikes it. Daisywheel mechanisms themselves are quite simple but need some fairly complex control electronics so they didn't really catch on until the 1970s when microprocessor control became possible. The daisywhell was light so printers could achieve speeds up to 40 characters per second.
The castings for the letters can be made very clean, polished and precise so a well-adjusted typewriter mechanism can give very good-looking print, although this was often spoiled in practice by coarsness and unevenness in the ribbon. In the 1970s carbon-film ribbons were adopted because these gave a very clean image on the page.
Typewriter typefaces are normally fixed. One of the benefits of the Selectric and daisywheel designs was that the typeface could be changed. A different font could be used by putting in a new type element.
Ordinary type mechanisms can't produce much in the way of graphics - although some entertaining maps, graphs and pictures were printed using "ASCII art". ASCII art is still sometimes used in e-mail. Daisywheel printers can manage to produce maps and charts by picking out the pattern required in dots using the full stop.
Typists liked the controlability of typewriters so much that some offices still have one or two for odd jobs - special labels, or typing a letter they want to look "hand crafted".
Many people didn't like typewriters; it really was a skill getting a page of clean copy so most people stuck to longhand writing. It is certainly arguable that the frustration of writing longhand or error prone typing actually produced better material - or to put it another way, people don't seem to be celebrating a rennaisance in literature sincew word-processors took over.
However there is no longer much role for typewriter mechanisms in computer printing, they are too slow.
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Band Printers and drum printers merit a brief mention here. Both were designed for "industrial scale" continual print runs at rates above 20 pages per minute, usually on multipart green-lined computer paper.
Drum printers were giant relatives of the print-cylinder idea. The heart of the machine was a 14 inch page-wide drum about 10 inches (250mm) in circumference with the alphanumeric character set formed round it 136 times. Next to this sat a hammer bank, one hammer for each ring of characters and print position. The paper and ribbon sat between the hammers and the paper. The drum span rapidly and hammers struck the page as the correct letter passed underneath. In principle the mechanism was simple and reliable. Drum printers could be fast - if the drum span at 3,000 rpm then it could do 3,000 lines or 60 pages per minute. On the other hand the drums were expensive - a huge precision casting. A typical drum printer couldn't print in lower-case because accomodating lower-case characters as well would have meant and even bigger drum. The drum printer idea seems to be extinct.
Band printers have one thin metal band of type-characters - usually repeated three or four times in succession around a steel band about 3 feet in circuference. The steel band is mounted on pulley-wheels and runs horizontally under the page. On the other side of the page and ribbon is a hammer bank with 136 hammers. As the band spins a succession of characters goes underneath each print position and the hammers strike. Bands are more emphemeral than drums so they can't usually spin as quickly, but 1,000 lines or 20 pages per minute was typical for a densely printed page. Lightly printed pages could move more quickly still.
Band printers have been in use until recently in banks and government offices where long runs of monthly statements have to be produced. They are being replaced by high-speed laser printers.
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Serial dot matrix printers form the image on the paper using a technique resembling that used for TV pictures; they use a raster scan on paper.
The pattern is picked out by small pins that move backwards and forwards hitting the ribbon against the page. Printheads could have a single pin, but scanning this back and forth would make printing slow (it would be like printing everything using the full-stop on a daisywheel). Basic dot matrix printers have 9 pin printheads and the nine dots are used to form crude but readable text as the head scans across the page. Recent printers often have 24 pin printheads because this gives better looking print. 48 pin pritheads were tried, but reliability became a problem - the pins were made of fine wire and even toughened steel was too easily damaged. A 24 pin dot matrix printer with powerful motors can print at up to 1,000 characters per second on 5 part paper. Bulk print speeds are usually about 10 seconds per page - or 6 pages per minute - much faster if print density is quite light.
The dot-matrix printer is always actually producing a graphical image - but it is designed with the expectation that it will mainly be used for alphanumeric information. A typical printer is fed ASCII teletype code and the printer's control microprocessor looks up the pin-patterns to use in a memory table. A dot matrix printer can produce different fonts, bold letters, underscored and all sorts of variants by using different internal tables.
Graphics can be produced by feeding data direct to the pins. However there was no industry standard way to do this, or to select fonts, so manufacturers like Epson, Facit and Tally made up their own codes to do these things. The variety of codes and "emulations" caused all sorts of problems. Graphical output from a serial dot-matrix mechanism tends to be slow and rather dissapointing. Printhead pins are quite big so output is typically 100-200 dots per inch - each dot is quite visible.
Dot-matrix printers were an industry standard in the 1980's but were displaced from offices and home computing a few years ago. They are too slow and users find them difficult to look after - they get paper-jams, the ribbon comes off or gets worn.
Serial dot-matrix printers are still quite widely used for producing delivery notes and invoices. Special short-carriage mechanisms are used in tally-roll and ticket printers.
Shuttle printers (or parallel dot matrix) arrange a much larger number of pins horizontally across the paper. The large number of pins simultaneously working on the pattern allows a shuttle printer to work quickly. To match the output of a 9 pin dot matrix printer such a machine would need 1360 pins across a 14" page- and this would be difficult to arrange and expensive. A more practical arrangement places 136 pins on a "shuttle" that vibrates through about 1/10th of an inch, so each pin prints ten or more dot positions. Shuttle printers are faster than serial dot matrix machines, but they are complex and expensive.
Graphical output from a shuttle printer has some of the same limitations in terms of code selection and big noticeable dots. However with a whole bank of pins working at once a graphic can be produced quite quickly - so these printers can produce outline pictures and barcodes.
Shuttle printers have typically been used in distribution warehouses - both to produce labels and to create reports. The mechanism is inherently rather complicated, but it is quite robust and most faults can be cleared by cleaning.
Impact Print Costs
The only direct consumeable in impact printing is the ribbon, which is just a fabric carrier impregnated with a thick printing ink. The printer isn't really suited to producing lots of ink-consuming images or white-on-black print. A big ribbon on a spool with no plastic cartridge can be a very low-cost item; running costs under 0.1p per page of print are possible In practice users don't like handling spools of ribbon - the ink gets on their fingers and clothes, so manufacturers wrapped the ribbon up in fancy proprietary cartridges and prices tend to be quite a lot higher.
Another issue is the print-head. Print-heads tend to last a long time, but not forever, and a replacement part may well cost £200 or more. Fitting a printhead doesn't strictly needs a technician, but most do require screwdriver skills. The cost of printheads comes as a shock to users. Even with this disadvantage the cost per page can still be under 0.3 p per page
Impact Print Limits
The heart of a dot-matrix printer is the set of pins and the limits on print speed and quality come from how big they need to be and how fast they can move. Big pins tend to be robust, but produce crude -looking images and can't move quickly. Little pins make a nicer looking image, but wear quickly and break easily.
The pins in a typical printer need to be about 0.2 millimeter or more across or they are too fragile. Since the pins cannot be very small the image they produce will be crude - the best resolution is typically 250 dots per inch. A further limit on print quality is the weave of the ribbon. A coarse-grained ribbon tends to hold a lot of ink but cannot produce a good image. A fine ribbon is needed where fine pins are in use, otherwise the pins get trapped in the fabric and the printhead is wrecked.
The other limiting factor is speed. Pins cannot easily be made to operate more than 2-3 thousands times per second, so this speed factor and the number of pins in use limits the speed of the printer. The pins are normally driven by magnetic solenoids, so they could be made to move faster but only at the cost of greatly increasing the current used - which would mean the printhead getting hot very quickly. If the needles get too hot they will expand and jam, then the next movement will wreck them. Many printheads have heat-sinks to try to limit the temperature rise, some even have built-in fans. A printhead with a heat-sink and fan satrts to get heavy so either the carriage motors have to be made bigger to cope or carriage weight becomes a limit on speed.
Impact technology involves a lot of mechanical action so it is noisy, a needle vibrating 3,000 times per second against a piece of paper produces an unpleasant shriek - so a hundred or more in a shuttle printer produce a really unpleasant noise. Shuttle printer speeds can be 5,000 characters per second - a solidly printed page in 2 to 3 seconds and a lightly printed form in under a second.
Ink jet and laser printers have replaced impact printers in many circumstances; however impact printing is still the best method to produce large quantities of printing at low cost.
One limitation is that most impact printers are designed for continuous feed stationery - normally fanfold tractor feed paper. Impact printers cannot produce really good looking pages, so designers opt for the simplest paper feed mechanisms. Cut sheet feeders are often available for dot-matrix machines, but they are fairly costly optional extras.
Prices of impact printers are rising. Dot matrix printers were once mass market items with huge scale economies of production. Prices fell so low that a basic model could be bought for £50. Fast, robust dot matrix business printers were £2,000; shuttle and band printer prices ranged above £5,000 to £10,000.
Production volumes are not sufficient to maintain low prices. Some manufacturers have dot matrix printers in their catalogues, those still produced are aimed at specific business uses in warehousing and distribution, or for data logging.
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Ink jet printers shoot a bubble of ink out of hair thin capillary tubes onto the page - normally using heat or vibration as an actuator. An inkjet print head is made up of capillary nozzles containing a simple resistor or piezo-electric crystal to fire the drops. A print head usually has a large number of nozzles - the technology has improved to the point where there can be a hundred or more in a single thumbnail sized head. The only consumable needed in principle could be liquid ink, and if this were all inkjets could be quite cheap to run. In practice the print heads tend to have short lives - the capillary tubes block. Many ink jet printers use cartridges with print head and ink built into a single throw away unit. The need for little cartridges often with a built in head makes ink jet printing the most expensive technology, a basic page of printing may cost 3-4p, but a full colour solidly printed page can cost 50p or more.
A typical inkjet printer body is a lightweight version of those used for dot-matrix printers. One motor pushes the page through vertically, and another scans the head across the page horizontally.
The limit on printer speed is created by how many nozzles there are, how rapidly they can fire, and how often the head can pass over the page.
An inkjet printer generally scans the page in the same way as a dot matrix printer. The rate of dot production is similar as well, 2-3 thousand per nozzle per second. Inkjet mechanisms are almost silent because the only rapid movement is that of the ink - tiny droplets of liquid. The motors just sweep the head backwards and forwards across the page.
Inkjet printhead manufacture is closely related to semiconductors so unit costs depend on numbers made. A mass-market printhead will be very cheap to produce - perhaps a couple of dollars per unit. Unusual cartridges cost much more because the cost of design and production machinery has to be paid for.
Colour
One of the big advantages for inkjet printers is that making colour models is relatively simple.
Inkjet cartridges are consumeables, so printers can easily be fitted with several cartridges containing different coloured ink. A basic colour printer has three heads printing in Cyan, Magenta and Yellow ink. Mixing the three primaries as on-off dots only produces eight colours, so either the dot size has to be varied or a dither pattern produced at a scale smaller than the eye can see, so colour injets are often rated to produce thousands of dots per inch. Another issue with the "CMY" process is that it is an expensive and imperfect way to produce black - all three inks are used and the result actually tends to look brown. Black print on white paper is a very common requirement, so most printers have a fourth black ink head and the process is called CMYK. Some recent printers have half-tone inks to give really accurate colour rendering using a 7 colour process.
Designing an inkjet printer to hold extra cartridges is not a massive change. Essentially all that is different is that the carriage has more positions, and the processor has to generate signals to control the extra cartridges if they are present. It is up to the user to decide how much colour they use. Since each cartridge is a consumeable it doesn't have much impact on the price of a new printer - only on its running cost.
Users find dealing with four or more cartridges fiddly, so inkjet printer manufacturers may incorporate the CMY inks into a single printhead and the "k" into another. This kind of printer is a bit more costly to operate because some ink reservoirs will only be part used when another is exhausted and the head is thrown away.
Inkjet nozzles can be very small and dense packed in the head. Older machines give a resolution of 300 dots per inch - good quality text but poor graphics. Print speeds from older machines are as low as 100 characters per second -2 pages per minute.
Recent inkjets achieve between 1,500 and 4,500 dots per inch. The number of nozzles in the head allows the printer to produce quite a broad band of print for each scan of the head so print speeds up to 10 pages per second can be achieved. Manufacturers quoted speeds for inkjets can be very misleading. Most inkjet printers rely heavily on the host computer for the processing to create the page, and this can slow printing.
High resolution nozzles allow an inkjet printer to produce high quality text, colour graphics and photographs. Printers are usually designed to use cut-sheet stationery because this matches the expectations for high quality colour print. Using papers with a polished surface can give output that looks very like photographic printing.
The inkjet concept has a lot of potential.
To make a faster printer, build a bigger head - specifically make a page-width head. Using cut-sheet paper one just 8 inches (200 mm) wide would be adequate. Page-width inkjet mechanisms have not been common. The probability of a nozzle failing rises with the numbers involved and this means that very large printheads could be unreliable.
Inkjet prices
Prices for inkjet printers are the lowest available - the cheapest printers with colour capability cost under £70. At it's simplest an inkjet printer is just one motor driving the paper, another scanning the printhead back and forwards across the page. The user's computer does the main work determining the pixel-pattern on the page. A significant part of the works are in the disposable cartridge.
Consumeables are a different matter. Once the user has the printer, they have to buy cartridges. It is commonly thought that manufacturers don't make much profit on inkjet printer bodies, but do make a good profit on the cartridges. Obviously there is a limit to how well this can work - if printers are cheap and cartridges expensive a manufacturer could push a customer to buy a competitors printer.
Another limit is set by clone cartridges and refilling.
Refilling cartridges has been enthusiastically adopted by some users. The cartridge is a plastic tank filled with a well filtered ink, usually with a print-head built into the bottom. The printthead will often work reliably through several refills of the tank with cheap ink. Obviously manufacturers have learned to make refilling difficult - engineering the plastic bodies so there is no readily available hole. Some have gone so far as to put memories on the printhead so that the head cuts out after it's sell-by date. Refilling cartridges does usually mean a willingness to suffer bad print and ink spills - and the colours aren't likely to be quite right.
Clone cartridges are another limit on printer manufacturer profits. With a bit of analytic chemistry a clone cartridge maker can get the the ink formulation right - something no amateur refilling is likely to achieve. Plastic mouldings are easily immitated. Printhead circuitry obviously needs a bit of work, but there are several fabrication facilities that could make them.
There is now a huge market for ink cartridges. Original manufacturers rely on users:
not caring about cartridge costs - fairly normal in a large office
feeling nervous that clone cartridges will prove unreliable. Normal in a small office
By being a specialist producer of cartridges across a broad range of printers there is no reason why a clone manufacturer can't produce cartridges that are better than printer manufacturers originals.
Recently the printer manufacturers have tried to erradicate clones using the courts. One US court case has ruled that clone cartridged infringe Epson's copyright on cartridge design and the copiers must pay a fee to Epson. The effect has been that a couple of clone cartridge makers have withdrawn from the market so low cost options may be closing down.
Inkjet printer mechanisms contain a cetain amount of truly original intellectual property - for instance Epson's piezoelectric heads are quite different to HP and Caonon's thermal heads. On the other hand a lot of the principle are very well known and the patents are beginning to expire, so perhaps there will be a burst of new creativity in a few years time.
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Laser printers rely on the same photo-electrostatic principles used for many years in photocopiers. The basic mechanism of the laser printer is rather more complicated to outline than dot-matrix or inkjet printers, but the technology has been refined over many years and is reliable. The laser printing process needs several consumables and these all contribute to the cost of operation:
- toner is the equivalent to ink, so it has to be used up in operation.
- developer transfers the toner to the photoconductor drum, toner ultimately degrades
- photo-conductor material transfers toner to the paper - it degrades with time as well
- the machine makes some waste-toner, and a bottle to collect this has to be provided.
- the fuser rollers ultimately wear out and have to be replaced.
Laser printer toner is a fine dust of plastic powder. Toner dust can be given a static charge and it will be attracted to opposite charged surfaces, and repelled from like charges.
Photo-conductor materials can be given a static charge in the dark, when they are non-conducting. If a photo-conductor is exposed to light it will release the charge into an underlying metal layer.
Laser printers and photocopiers combine these two principles. A metal drum or belt carries a layer of photo-conductor. The photoconductor is charged in the dark, then a pattern is painted on it using:
- light reflected from a source document - in a photocopier
- or light scanned by a laser - genuine laser printers
- light from a row of tiny LEDs - some "laser printers" use this technique.
Next the photoconductor moves over the paper to be printed, and a transfer charge pulls the toner onto the paper, still held in the pattern of the scanned image.
The paper now holds the image as dry toner-powder on its surface. This is normally fixed to the page by running the paper through the hot rollers in the fuser.
A laser printers list of actions can be arranged as one near-continuous process, all of them involving rotary motions. A machine running like this can be very fast - there are laser printers capable of more than 100 pages per minute. The limits on laser printer performance are:
- toner, developer and photoconductor have to be very fine grained to give a high resolution image.
- if a single laser is the light source then image scanning requires rapid on-off modulation of the beam. It so happens that rapid modulation of laser light sources is also needed for communications cables, and video disk recorders - so it is a rapidly developing technology.
- the laser scanner contains a motor which must spin rapidly to give high resolution pages at high speed. As it happens laser printers share the need for high speed low-power motors with disk drives.
- a powerful processor and a lot of memory are needed to turn computer information into a page layout. The whole thrust of semiconductor manufacturing and computing is aiming in this direction so laser printer design wins here as well.
A laser printer mechanism generally has a bit more "substance" than an inkjet. The electronics can "rasterise" the image without relying on the host PC to do it. Laser printers are built for a commercial environment, so they tend to be more robust and rather more expensive to buy.
Colour lasers
The "CMYK" process can be used in laser printers, but there is no design where including colour does not mean a massive change in chassis design. Inkjet designers find colour printing a relatively easy option to include. Laser printer designers have to make a special design to deal with colour.
Colour printing with a laser printer involves having four toner / developer mechanisms, each laying down an image before the page goes to the fuser. This is more difficult than it sounds.
Obviously there could be four successive mechanisms - but each is quite expensive to make and the resulting printer will be rather large. Some printers do work this way.
Most lower-cost colour lasers arrange four colour toner-developer units on a carousel over a drum which will take an image from each in succession. The drum transfers the powder image onto a belt for intermediate storage. Finally the belt transfers the image onto the page. One of the main difficulties is to avoid the successive images blurring in static fields left over from the last image.
Colour laser printers operating at 600 or 1200 dpi produce very good-looking pages, but the carousel mechanism is innevitably quite expensive to build and can be quite slow to operate. However a colour laser printer will generally be much quicker than an inkjet at producing a page of equivalent complexity.
Laser Consumeables
Every brand and almost every model of laser printer has it's own set of cartridges in special shapes, using slightly different processes and grades of toner.
Even low-end laser printers are a bit more complex than inkjets. Fast laser printers have complex gear-chains to drive the drum, paper-path and fuser and a fairly powerful computer as a raster image processor - so they aren't all that cheap.
Manufacturers have the same pressure to sell printers at a low cost then make money on the cartridges. Laser printer cartridges can oftern produce a lot of pages per cartridge - 5,000 to 15,000 pages depending on design. The price of cartridges can be quite startling - £30 to £150 for manufacturers originals - depending on size and make obviously.
Laser printer toner cartridges are often available in several kinds
-manufacturers original - carrying the printer makers brandname
-refurbished - original cartridges where the plastic assembly has been stripped down and refilled. Because laser cartridges are so expensive it can be economically worthwhile recycling them. Some refurbished cartridges can carry the manufacturers name and logo.
-clone - cartridges designed to function like the manufacturers original.
Laser printer users have the same concerns as inkjet users about using anything other than manufacturers original cartridges. Originals are expensive, but clones might not work very well. Actually its a bit more problematic because:
- Clones and refurbs are significantly cheaper, but still a significant expense.
- Some cartridge refurbs are badly done - they leak toner and cause engineer call-outs. This problem tended to be particularly prevalent in the early days of refurbishment.
- Manufacturers might refuse warranty service if an engineer finds anything but a manufacturer badged cartridge in a printer.
The toner cartridges are just one part. Older mono printers often have a seperate developer unit but this becomes complicated with colour printers. Manufacturers discovered the idea of resin developers which are mechanically simpler. Most colour cartridges are toner and resin developer in one unit. Other consumeables include the imaging drum and on colour printers an imaging belt as well. Finally the image is bonded to the page in the fuser. Most fusers are basically a hot pressure roller and constant contact with the paper wears them out.
Strip all the consumeables out of a colour laser printer and there isn't much left - just the imaging electronics, a motor gear chain and the laser scanner unit. Basically the printer is a spine which the cartridges hang from.
Laser costs
Laser printers aren't likely to compete directly with inkjets in the home computing market.
The mechanism is simply more complicated so even if the manufacturer can make money on the cartridges they might not be prepared to risk giving the printers away virtually for free - as some do with inkjets (ie the printer costs less than a pair of cartridges)
The laser mechanism tends to be inherently faster than an inkjet. Rather than relying on the user's computer to make up the raster image, laser printers tend to have their own dedicated processor for the job. A four colour CMYK image on A4 at 1200 dpi can easily require a couple of hundred megabytes of memory - still a significant amount.
The different lives of toner, drum, belt and fuser might be too confusing for home users. Furthermore to get low running costs implies big cartridges costing more than £50 each, which would seem hideously expensive to a home user.
Electrostatic imaging mechanism can't provide such small dots so inkjets are likely to be better at things like photo-reproduction on special paper. Lasers are better at photo-production without needing special paper.
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Thermal image printing uses paper with a coating of chemicals that change colour on heating. The materials used are sensitive to infra-red. The printhead is a row of resistor or semiconductor heating elements embedded in a piece of ceramic.
Thermal printer mechanisms can be very simple. The printer holds a roll of paper. A tongue from the roll passes through the gap between a printhead and a rubber roller which keeps the paper pressed against the head and drives it through the machine. The only complexity is in the printhead.
Thermal printheads are generally a ceramic bar mounted on an aluminium heatsink with film resistors along one edge and a few controller chips. The controller turns resistors on and they heat up rapidly, off and they cool down rapidly. Where the resistors turn on there is a dark pixel on the page.
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The most familiar thermal image printer is a fax machine (which is a printer, scanner, modem and computer all bound up in a neat mass-market box). The fax machine illustrates several important features of thermal image printing:
- print mechanisms are small - just a motor to feed the paper and the ceramic bar of the printhead itself.
- printing is nearly silent - the only sound is the paper feed mechanism.
- speed can be up to 4 inches of paper per second - not fast but acceptable for a lot of tasks
- simplicity - there are no consumables to change except the paper, no printhead gap to set, no cleaning procedures to worry about.
The main problems with thermal image printing are:
coated paper feels odd - it could be printed on 80gm paper and cut into A4, but most people still would not like the odd shiny look of the pages.
images fade away if they are exposed to light and the paper turns brown.
Most people claim that the cost of printing is a problem, but thermal printers can now be amongst the cheapest to run. Paper at 1p per sheet may seem expensive - but a laser printer costs 1p or more to print a sheet, and an inkjet costs significantly more than that.
Although most people are familiar with the thermal printer's role as part of a fax machine they are very widely used in industry. Label printers, chart recorders, ticket printers, and point of sale receipts all commonly use thermal mechanisms. The vinyl cutters that make signs often have thermal printer mechanisms to produce colourful images. Thermal printing costs more than other processes, but the costs of operation are often considered to be just part of a production or sales process. The big benefit is flexibility and reliability - try producing varying length runs of inkjet or laser printed material in a dusty factory environment - the printer can't cope. A thermal label printer just get on with the job.
There are two different types of thermal printing:
Direct - the image is made directly on heat sensitive paper. The paper will fade to brown over a year or so and the contrast will become poor. Direct printing is the simplest so it is least troublesome. One problem with direct printing is that barcodes become less reliable if things are stored for a year or so.
Indirect - the image is made using a heat sensitive wax or resin on a thin film carrier and then transfered to plain paper - or to a vinyl material. The paper and the image don't fade, and there are scuff-resistant papers as well. The problem is that there is both a paper and a ribbon to consider.
Colour
Direct transfer more or less has to be monochrome.
Indirect printing can use coloured waxes and resins and produce rather nice vivid shiny colour images. Some of the first "consumer" colour printers on the market were produced by Fargo, and could use a backing film with four successive colours. Obviously the cost per image was rather high - although since the page could be 100% covered if that was needed it may actually have been cheaper than an inkjet in some circumstances. Using special paper and a domestic iron thermal images can be transfered to material like T-shirts.
Consumeables
Direct thermal printing has one very nice feature - a single non proprietary consumeable. Fax and printer rolls can be various widths and diameters, in principle there could be different chemical properties to the paper and the coating, in practice any suitable size will probably work well enough.
More or less the same holds for rolls of labels but there are additional things to watch out for
labels can be on the topside or underside of the backing papers - and some printers do care.
printers have to sense the end of labels. Some do this by sensing the reflection from a white label, some by sensing a change in the light passing through the paper at the start of a label and some by an opto-sensor shining through a slot in the backing - or being blocked by a black mark on the edge of the backing. On some printers you can choose amongst several possibilities in a menu.
Low cost label printers from Dymo and others work on the same principle as low cost inkjets: you pay very little for the printer - but a great deal for labels. These printers are carefully engineered so that only the manufacturers own labels fit, and the design is copyright. If all that is wanted is a quick way to produce a few neat address labels these printers are a fine solution to the job.
Larger thermal printers usually take label stationery in standard sizes. Actually there aren't many "standards" - what generally happens is that people order labels and ribbons from the printer maker. However it is possible to save money by ordering labels elsewhere - at the cost of more trouble when things don't work quite as expected. Although the materials used in label printers look simple print speeds, head temperature and contact pressures need to be adjusted correctly to get flawless print/
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Colour Support -
Colour support for inkjets: - Mono Inkjet. Swappable CMY Cartridge. Separate CMY&K Cartridges.
Colour support for lasers: - Mono. CMYK cartridges.
Speed
Characters per second - usually dot matrix
Lines per minute - usually band and shuttle printer
Pages per minute - laser and inkjet printers
Inches per second - thermal printers
Paper Size
80, 132 or 136 column - dot matrix
A4, A3 - lasers and inkjets
A0 - plotters
4 inch - thermal
Also Tractor or Cut Sheet