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| The basis of most Inkjet printers is a raster-scanning structure. This is built to a time honoured structure that dates back to Teletypes and golf-ball typewriters. A printhead rolls back and forth on a carriage which spans the page. Paper is moved forward after each sweep of the carriage. |  |
| Much the same design was used in daisy-wheel and dot-matrix printers. Some early dot matrix chassis were co-opted as the basis for inkjets. Paper feed differs because inkjet users generally want cut-sheet feeding. |  |
To get cost effective mass manufacturing inkjets use more plastic mouldings other than the steel inner chassis of the older and less mass-market dot-matrix machines.
Inkjets are novel because the price target is often mass production at very low price - more like that of a toy than a traditional printer. A mass market makes this possible even though printers are inherently fairly complicated objects.
Printheads are the heart of an inkjet printer design. 
Printheads are usually made by photolithography - the same process used in chip-making. The core of most printheads are effectively silicon chips.
| Thermal printheads are cheapest to make so that is the obvious choice in a low cost printer. Inkjet printers from HP, Canon and Lexmark use thermal heads. Thermal heads don't last very well, they tend to burn out drive elements particularly because of a problem called "kogation" - a buildup of scale. Low cost printers use a disposable cartridge so the printhead is kept in good order by being binned and replaced. |
Some printers have used a thermal printhead but in a separate disposable section that plugs in below the cartridge. The problem with this can be identifying printhead failure as distinct from just an air lock or temporary blockage. Users are unlikely to be keen on a messy replacement procedure, so heads may be an engineer changeable part - but this makes the procedure expensive.
| Piezoelectric printheads should last a long time because there is no part normally stressed to the point of failure. Epson and the Xerox Phaser use piezo-electric heads. Of course a piezo head can be blocked - so ink purity is vital. A particular point is that the ink must not contain dissolved air. Piezo heads are particularly vulnerable to air-locks so the manufacturers procedures for cartridge changing should be followed. |
Piezo printheads are much more costly to make so they are built into the printer and intended to last "for life". The cartridge is just a tank full of ink. Making a reliable contact between tank and head that isn't prone to air locks is problematic.
An inkjet cartridge is a plastic tank full of ink. Cartridges for thermal printhead inkjets usually have the head, its so called "intimate electronics" and electronic contact pads as well. Piezoelectric and long-life thermal heads are separate so in this case the cartridge may be nothing more than a tank of ink.
The first inkjet products were mono printers, but in the late 1980s interest in colour grew very quickly. Inkjet printers are the simplest way to produce colour quickly and effectively. Where a laser printer needs four toner / developer assemblies an inkjet just needs a different kind of cartridge - which is not so much an expense for the manufacturer as a sales opportunity.
The CMY-K Process.
The normal way to produce colour is called the CMY process because it uses three ink colours - Cyan, Magenta and Yellow. 
Three colours should be sufficient for any kind of printing. In practice there are problems. The three colours are supposed to add up to black, but they usually make a brown. Black is used very commonly - for text for instance, and using three lots of ink to make black writing would be expensive. Shuffling the head around to give three passes accurately might also slow the printer down. Almost all printers add a black ink to improve efficiency and give solid looking blacks.
Single cartridges would give the lowset cost build. This allows:
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Things are simpler for the logistics, dealership and tech support chain as well. A single cartridge even keeps things simple for the user - any problem, change the cartridge.
There are a few different ways to use just one cartridge -
 | make a mono printer - which isn't going to be popular with most users. Mono inkjets are only common in Fax machines and ATMs. |
 | put all the colours in one cartridge -subdividing the head to handle the CMYK colour system. |
With one subdivided cartridge and four inks one colour will exhaust first. For instance, someone mainly producing text will exhaust the black leaving the colours almost unused.
  | Have swappable mono and colour cartridges. Put the black cartridge in for text and the colour in for pictures. Of course this may be fiddly - a driver program might have to ask users to put pages through the printer twice. |
In the early 1990s single cartridge inkjets were normal but they aren't used these days; the attractions of colour are just too great.
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| Two cartridges allow one black and one colour cartridge, which gives a much better balance. With a pair of cartridges the user can print text and colour pictures in any proportion. There is a problem, of course, if the user wants to print a spreadsheet in black and red because the cyan will go unused. Pictures tend to include a lot of blue and green so the magenta isn't all used up. |   |
 | Tricolour cartridges tend to be a bit more complicated to make and expensive to buy. Very low cost printers tend to have expensive colour cartridges. |
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| Four cartridges give a much better situation - any colour can be used in any quantity and replaced as needed. Each cartridge can have an identical head. A development on this is to use one type of head for CMY and give the black cartridge a big, fast printhead because it is likely to be used most. |    |
Seven cartridges are sometimes used on photo- quality machines. Inkjet printing doesn't have a very good colour gamut (mixing only gives 8 colours including black and white). These primary colours are "dithered" with the white of the paper to produce light shades. One problem is that for very light areas - like the blue-whiteness of some skies - the dithering produces visible separate dots. To get brighter shades the printer can be equipped with a light version of the colours.
Six colour cartridges are also used. People see yellow as a light colour anyway and only a small part of the human eyes colour gamut is seen as yellow. Printer makers incorporate a light Cyan (skies) and a light magenta (skin tones) giving a six colour device.
Setting up a cartridge production line to make devices with built-in printheads is an expense to the manufacturer. This is nothing compared with the sales opportunity of selling ink in proprietary cartridges.
Eight cartridges are used in the Epson Stylus Photo R800. There are Cyan, Magenta and Yellow, separate Red and Blue to expand the colour gamut (it doesn't mix ink) a Black, a Matt Black and a Gloss optimiser.
Inkjet cartridges don't contain much ink - 5 to 10 millilitres is typical of small cartridges. Inkjet dots typically hold 1 to 10 picolitres - the dots of 1,200 dpi printing are around 10 picolitres. There are a billion picolitre dots per millilitre, so about a billion of these dots in a small cartridge.
At 1,200 dpi there are approximately 2300 dots per sq mm and the paper is 210 x 297 = 62,370 sq mm on a page so 143,451,000 dots on a page
Working through the same arithmetic in inches there are 1,440,000 dots per square inch and 96.525 square inches per page - so 138,996,000 dots on a page. In other words depending on how you measure the page and whether there is the slightest margin at 1200 dpi there are 140 million pixel positions per page. A 10 millilitre cartridge with 10 picolitre dots will cover 7 pages!
Colour pictures don't necessarily use just one colour to make up the pattern on the page - they can use more to make dark areas or less in light areas. Its probably fair to assume that this balances out so that a pixel remains at 10 picolitres. This suggests that the 40 millilitre content of a 4 tank printhead will print about 28 pages. If the cartridges are priced at £7 each then printing is costing about £1 per A4 page.
Inkjet printing for full-page photographic cover is not remarkably cheap - although it compares very well with the costs of traditional silver-halide photography.
Of course most users print some pictures and some text. Text uses much less ink - the page-cover is typically 5% - and it's predominantly black. To give a better price performance some manufacturers put the black ink in a bigger, cheaper cartridge (black pigment is genuinely cheaper and less troublesome). The manufacturer can now claim that one set of inks will print a thousand pages at a average price of 3p each! Claims like this are true but do rather depend on what is being printed. Run off a 50 page catalogue and they could easily cost £1 each - but then try to get 50 copies from a professional printer for £1 each!
Printer manufacturers could give the user a better deal by doubling the size of the cartridges. The cost of this is that there is now 80ml of ink sloshing around on top of the carriage - quite a lot of momentum to push back and forth. The greater the momentum of the printhead the bigger the motor and driver need to be - or the slower the printer works. Users tend to be focussed on low purchase price and fast printing; they are apparently careless about page price - the arguments are just too complicated.
Delivering liquid ink to the head can be done in two ways:
- Incorporate the ink cartridge into the carriage – possibly as a cartridge with both ink and head.
- Take flexible rubber tubes to the carriage from a static cartridge.
Cartridge on Carriage.
To cover the page the head has to move back and forth across the paper under the power of a motor. This process needs to be as fast as possible if the printer is to achieve high output speeds. Fast output therefore depends on a head that is as light as possible.
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 500- 1,000 pages of text output. As suggested above this is typically something of the order 10 millilitres per colour and perhaps more for black.
Cartridge in Base.
The best way to keep the head light is to move everything that is not essential into the base of the printer- control electronics - including the ink.
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 several thousand 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. Some printers with cartridges in the base use a simple pump to give flow to the head. The connecting tubes obviously impose some drag of their own on the carriage. In addition pipes have a tendency to develop air-locks, block, dry out or even split.
For people who want long print runs with low cost ink then putting the ink in the base is really the only option. As well as a few top-end consumer models that work this way there have also been continuous inking kits for printers like the Epson range that mount externally. Top end printers like the HP Designjet series usually have big ink cartridges at one side of the machine with an ink distribution system carrying the ink to the carriage and the heads.
Ink in the printheads would dry out without measures to protect them. This is much worse than it sounds because dry ink may not just be superficial, it may fill the chambers and prevent them firing. With no capillary connection it may not be possible to re-establish ink flow and wash hardened ink out. 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 whilst the control electronics fires the chambers. 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.
Despite the huge variety of designs one general thing can be said about service stations. The printer has to be powered up or the head won't move onto the service station, it will be caught in mid flight and unprotected. If the power goes off suddenly the printhead is unprotected. Don't just turn printers off at the wall - use the proper power cycle.
Paper Transport & Carriage Action.
Inkjet printers are almost always designed to handle cut-sheet paper. There have been just a few designs aimed at tractor-feed - and that is now a specialist market - if it exists at all. Plotters often take roll-feed paper driven by grit-wheels and trimmed by a stylus or guillotine.
The basic mechanism used to pick paper out of a tray and feed it is usually similar to that used in laser printers - D shaped kicker roller followed by a registration station and feed rollers. Where the laser printer maker can afford some elaboration, separate motors and clutches, opto detectors for paper presence and suchlike the inkjet maker normally cannot. Inkjet paper feed trays tend to be rather limited affairs on low cost printers taking 10 or so sheets at one time. In many cases this isn't really a problem to the user since they will have to select what kind of paper they are going to use - ordinary copy paper or photo paper.
There are several kinds of feed path - Rear fed gravity assisted with top tray output, favoured by Canon, Epson and Lexmark. Front tray in- front tray out favoured by HP. A point worth noting is that the tighter the bends in the paper path the less able the printer will be to handle stiff media. But then inkjets are very sensitive to media surface - so chances are you weren't going to print on card anyway. Some printers have a special tray that can be used to print CDs.
It might also just be worth pointing out that some printers are designed to look very chic, neat and compact with their paper trays folded away but they take quite a bit of desk-space with them open.
Larger capacity trays and the possibility of two trays - for different kinds of paper - appear on "SOHO" printers. These may be two or three times the price of basic home models but as well as the extra features - trays, duplex and network connections - they tend to use larger and more economical cartridges.
Duplex is sometimes an option on inkjet printers, built in to recent models particularly if they are aimed at business use.
The price of home printers is too low to allow the extra complexity of duplexing but it is also more difficult to duplex a page of inkjet material because in printing one side it will already have absorbed a lot of fluid that needs to dry.
The SOHO / Network printers like the HP Deskjet 6840 and Canon Pixma iP4000R have duplex built in. Duplex print can be slower. When using the duplex unit printers often stop at odd moments to give a drying interval.
Platen gap adjustment isn't usually provided on inkjet printers. Inkjets don't have any use for multipart forms so the media don't vary in thickness. The inkjet mechanism is also more flexible - by reputation the "St Helens" cartridge can shoot ink 100mm. More recent printers aren't likely to have quite such a long throw, but may manage a couple of millimetres. Printers which handle things like CDs and product labels might have an explicit platen gap adjustment.
Linefeed motors seem invariably to be stepper motors, usually with a planetary gear train to work both the feed rollers and the pickup roller. Running the motor backwards for several turns engages the pickup roller and puts the paper in the registration station. Running the motor forwards engages the paper feed rollers. There is sometimes a shuffling as the printer tries to exactly position the paper in on the registration point - it moves it back and forth and looks for an opto beam to make and break.
No one has yet made a practical page-width printhead for consumer printers, although they do exist on industrial machines. HP's CM8000 "Edgeline" series is one example of an office printer with a pagewidth head.
Most inkjet printers will therefore need a motor to drive the printhead(s) and cartridge(s) back and forth across the page. The printhead drive is normally called the "carriage motor" and it is at one side of the printer connected to the carriage by a drive belt. Chains, screw drive, rack and pinion have all been tried in dot matrix printers - a stepped rubber belt and a matching cog seem best.
Inkjet carriage drives are similar to those for dot-matrix printers but with the extra requirement to handle graphics accurately. The best solution achievable with a dot matrix was about 250 dots per inch so that translated into the head positioning accuracy. Inkjets with 5000 dpi horizontal resolution are working to a much finer resolution - a 200th of a millimetre.
Carriage drive on low cost printers has historically often been another stepper motor with an opto-sensor at one side of the frame, possibly opposite the service station. The printhead should be parked on the service station when the power is off, but of course there is always the chance it might not be. When the printer starts it disengages the service station then moves the motor slowly until the sensor changes - then it knows where the carriage is.
| Stepper motors aren't always excellent at responding accurately. The swift movements needed for something like a carriage are a challenge especially when its weight can vary. The printer's microprocessor either has to be rather conservative about the carriage drive speed or it can lose track of where the carriage is. Most larger printers use the alternative of a DC encoder motor. |
| DC encoder motors are conventional DC motors similar to those used in all sorts of household gadgets from cassette recorders to hair dryers. Normally the objection to a DC motor is that although it delivers quite a lot of power in a small package it isn't very controllable - it just provides mechanical force it doesn't accurately position to a fraction of a millimetre. |
Encoders are the extra component that give accuracy. The encoder is a slotted disk or striped tape that, together with some electronics, records precisely what the motor has achieved. The motor positioning circuitry can then change the polarity and drive power to achieve the precise effect equired.
Encoders are traditionally a slotted disk on the base of the motor. This gives a neat assembly that isn't vulnerable to damage and the measuring circuit is static.
A disadvantage with a slotted disk is that it isn't reporting precisely what the processor wants to know - which is actually the head position. Since dot positioning needs very fine positional accuracy slight give in the drive chain could misplace dots.
Hewlett Packard have tended to favour a graticule tape across the width of the printer. The encoder is in the carriage, and counts off the stripes as it passes over them. Printer built like this know precisely where the head is even if there is slack in the drive belt. The cost is that there is some encoder support circuitry in the carriage and extra connections in the carriage cable.
Motors cost money and manufacturers aim to achieve very low cost with home printers. One way to do this is to have one motor drive both the carriage and the paper feed. For instance a mechanism can be placed at the carriage left so that when the carriage is pushed against it the paper advances.
Driving the page through the printer is a rather different job and mechanisms to make the one motor do both tasks tend to add a lot of mechanical complexity and slow the printer down. There is generally a separate motor for paper feed and because it doesn't need much power a stepper is used.
The interface on recent printers is going to be USB. The USB communication protocol is bidirectional at three possible speeds 1.5bbps, 12mbps and USB 2.0 at 400mpbs. The figures above suggest that a 2400dpi fast printer may well need the full USB 2.0 speed - if it were passing pixel data uncompressed. A lot of older 300dpi devices didn't even load USB 1.
As the printhead travels across the page it will reach 19,200 positions and in each one it needs a new pattern transfering. There may be some advantage in the motor encoder pulses driving a custom chip to load the printhead with successive columns of data. Recent processors can probably handle this as an interrupt service at the same time as they are downloading data from the PC.
The bidirectional nature of USB means the printer can send a lot of control information back to the PC.
There are at least three or four different types of drop-on-demand inkjet printer on the market.
All micro manufacturing technologies improve with time and this is very evident in the twenty year improvement in inkjet specifications
Most of the inkjet printers on the market today are aimed at the home user and designed to have the lowest possible purchase cost to the buyer. Given three different products on the supermarket shelves at £49.99, £57.99 and £69.99 and no obvious distinction between them which would you buy? The VHS-Betamax war suggested people buy bargains rather than technological excellence.
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© Graham Huskinson 2010
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