Laser Printer Mechanism

Inkjet Printers  

HP Photo A636 Photo Printer HP PhotoSmart D7560 HP BIJ 1000 HP Designjet Z3100
Printhead Swath

About three quarters of the printers made today use an inkjet mechanism. A chip with microscopic channels ejects droplets of liquid ink onto the page. Inkjet printheads are made in sophisticated and expensive production facilities but with a mass market the individual heads are sometimes cheap enough to be disposable.

To print a page the printer scans its printhead back and forth across a paper path. The head is mounted on a carriage and connected back to the electronics by a trailing cable. Low cost printers usually have a paper path that can handle sheets up to A4 / US letter sized - about 220mm wide.  Inkjet pagewidth doesn't really have any limits; with long carriage rails  printers like the Designjet series have widths up to 44 inches for A0 paper.

Inkjets use one or more of these essentially disposable printheads (perhaps four in a colour machine) and a couple of motors to move the printheads and the paper. An inkjet printer will have a microprocessor  for local control but most of the work translating the image in the users computers to the firing pattern for the nozzles can be done in the users computer. Given manufacturing economies of scale the cost of making a basic inkjet printer is comparable with that for a model train or a remote-control electronic toy.

Epson Stylus Photo PX820FWDMost of the inkjet printers bought today are combined with a scanner on top. This allows people to use the printer as a photocopier and stand-alone picture printer, as a fax and for email. Adding the scanner glass, camera and motor doesn't cost that much more and it makes the printer a lot more useful and desireable.

Inkjet printers are popular in the home and small business market. The inkjet mechanism can be made smaller than a laser printer, with fewer parts and most of them plastic. The inks for a personal printer can be very small and light enough to be built onto the printhead carriage, that allows the printer to be very low cost. As a result inkjet printers can be made very cheaply if there is a large enough market. The world market is thought to be around 80 million printers per year, largely divided between HP, Canon, Epson and Lexmark.

Inkjet printers have a reputation for being expensive to use and unreliable. This is partly deserved; printers sold for low prices in supermarkets aren't  intended for much use or to last a long time.Expense comes because ink is supplied in cartridges which keep it fresh and uncontaminated.  There are many kinds of cartridges partly because manufacturers are trying to find the best delivery mechanism but also because they tie customers to proprietary designs. Small proprietary cartridges tend to be expensive - sometimes dearer than fancy perfume or champagne.Link to page about the cost of print  Big commercial machines usually have cartridges that cost more to buy but are very much cheaper per millilitre of ink and page printed.Reliability is an issue because the nozzles that eject the ink need to be narrow if they are to provide fine line drawings and photographs. Nozzles in a modern printhead may be just 8-10 microns diameter, just about invisible to the eye. Ink needs to give strong colour, black needs to entirely mask the white of the paper for instance, so the ink contains a lot of colourant. The ink also needs to dry quickly when it hits the page. But the ink must not dry in the nozzles or they will block. Contaminants in the ink, dust from the page, grease from a finger-tip will also block the printhead nozzles. Blocked nozzles wouldn't greatly matter if printheads were disposable in the sense that ballpoint pens are - but they are often quite expensive. 
 
Inkjets can be made cheaper to run and more reliable. Making a high quality machine costs more and reducing the cost of ink reduces a revenue stream for the manufacturer. Printers intended for office use tend to be bigger and more expensive  have bigger cartridges which are built into the printer base and the ink is pumped out to the carriage. Inkjets aimed at commercial use are significantly more complicated and expensive than those intended purely for home use. 

Printhead Basics

Inkjets make an image directly on paper using individual droplets of liquid ink. Ink is held in a capillary tube so that surface tension prevents it running out. Near the tip of the tube is an ejection mechanism that will shoot a single drop out - its called drop on demand printing. The size of the drop is controlled by the size of the nozzle, the force applied, the surface tension and viscosity of the liquid and the Rayleigh instability. Some recent printers make drops a picolitre in size - a fine mist with droplets about 12 microns across.  
Inkjet head There are two broad classes of inkjet heads: thermal and piezo-electric.Sideshooter Inkjet HeadThermal printheads use a heater element in the ink tube to produce a steam bubble. The displacement and the shock wave from that ejects the ink droplet. The tube and heater can be very small, a few microns across, so the heater rapidly acts when power is on and that creates a vapour bubble. When the power goes off the vapour bubble almost immediately condenses on the cold ink, which rushes back and cools the heater. The rapid heating and cooling means a thermal printhead can fire thousands of drops per second.

Early thermal printheads weren't very sophisticated - HP and Canon's early machines both had just 12 nozzles which improved a bit on the 9 pin dot matrix machines they were then competing with. The simplest way to make very small resistors and align them with the plastic mouldings and the orifice plate containing the nozzles was to adopt the silicon chip making techniques of the semiconductor industry.

Semiconductor makers have constantly striven to make smaller and more densely packed devices so thermal printheads share the benefits of this research when it comes to getting higher resolution by making more nozzles firing smaller drops. When the number of nozzles clims into the 50s there is also a problem making all those contacts to the printhead and the thermal prinhead does it by having some local control circuits called the "intimate electronics" on the same chip as the heaters.

Thermal printhead development does have problems. To get thousands of droplets per second requires very rapid thermal and pressure changes. Heating the ink can cause salts to build up in the chamber and the pressure changes may cause cavitation. Thermal printheads tend to have a limited life and are sometimes made to be disposed of with the cartridge. Canon, HP and Lexmark use thermal printheads.
Piez-electric printheads
Piezo-electric printheads use an electric field to distort the shape of a crystal. Some materials generate a charge when they are put under pressure, the effect is used in gas lighters. The reverse also happens, put a charge across a material and it changes shape slightly. Quartz oscillators use the reversibility of the effect. Silicon has some piezoelectric properties but apparently not enough.

The material that shows the maximum change is lead zirconate titanate or PZT. Even with this material the distorting effect in the crystal is small so the actuators in piezo heads are quite large with respect to the nozzle.

Piezo printheads are more difficult to make than thermal devices. There isn't the same close relationship with chipmaking, heads and the manifolds feeding them might be built using laminated stainless steel. Piezo heads have difficulty packing large numbers of nozzles into a small space at a low cost.

A benefit is that piezo printheads can alter their drop-size to some extent and shooting larger dots at areas that need more cover can allow them to speed up. Piezo printheads can also handle just about any pure liquid - water or alcohol based inks with dyes or pigments -even hot wax.

A piezo printhead lasts indefinitely and is often intended to last for "the life of the printer" or to put it another way piezo heads last a long time but are sometimes uneconomic to replace. Epson use piezoelectric printheads even in relatively low cost inkjets. Large industrial inkjet machines also use piezoelectic heads made by companies such as Scailex and Xaar.

Other possible actuators include a variety of micromechanical devices. An inkjet could use a pin actuator not unlike a dot-matrix head. It could use a magnetic field more directly to drive a bearing, shaft or piston. A micromechanical actuator like the TI micromirror might be used, so might an acoustic effect. An ink with an electro-rheid effect would be interesting. A lot of companies tried to develop electrostatic inkjets in the 1970s. Xerox Phasers use a solid ink which is liquified before use through a piezoelectric head. Ricoh use a gel ink. As might be imagined there are a great many patents for such devices. However in practice most printheads are either thermal or piezoelectric.

Inkjet Printers

Inkjet Overview

Inkjet printers were originally designed to replace dot-matrix machines. They can be very similar in structure, the early machines often looked like dot matrix printers, used tractor feed paper and indeed the actual chassis was often very similar.

Early inkjet heads had just 12 nozzles so they were scanned back and forth across a page on a carriage forming a line of characters as they travelled. They were just like a dot-matrix head and if it weren't for details like the ribbon and shield it would almost have been possible to make a conversion kit.

Pagewidth Printhead

Recent inkjet printheads can have hundreds or even thousands of nozzles. The printhead still scans across the page but in each pass it makes a greater swath - several lines of text. Printheads with more nozzles allow the printer to work more quickly. If reliable printheads can be made with 10,000 nozzles in a line about 210mm (8.5 inches) wide then that single head could span the whole page and there would be no need for the complexity of the carriage and the scanning action.

Inkjets can be a bit more complicated than a dot matrix printer because they need a service station for the printhead. The service station has caps for the printhead nozzles, a scraper blade to clean waste ink of the head and a "spittoon" to spit droplets into when the printer cleans its printhead nozzles.

Some of the critical differences in quality between domestic and commercial inkjets are the elaboration of the ink feed and service station. Air pressure driven ink delivery, vacuum head cleaning and droplet detection cost more.

Although inkjets were originally competitors for dot matrix printers the printheads improved rapidly and they became competitors for laser printers. Users wanted to use cut sheet paper so manufacturers provided trays and pickup mechanisms.

Ink Supply 

There are at least five classes of ink supply system.  The type of ink supply system determines how fast the printer will work and how costly it will be to run.

Tricolour and Black:
Tricolour Cartridges

The cheapest way to build an inkjet printer is to put the inks into a tank on the carriage and share a single integrated printhead between them. Actually there is usually a separate black; most text is black.

Ink capacity is always limited with cartridges on the carriage because a large quantity would make the carriage too heavy for rapid scanning and accurate positioning.

With a tricolor cartridge the printhead is usually shared between the inks. Ink tanks may just be a couple of mililitres in capacity. Larger capacity probably wouldn't be welcome because when one ink is used up anything remaining in the others can't be used and the cartridge has to be replaced. Obviously tricolour cartridges aren't much use to someone printing block red letterheads - the cyan will be left.

Printers using tricolour cartridges aren't a great idea for photography where full colour cover of a page is wanted. They can suit things like student essays which can be very largely black text. The average home user prints less than 2 pages per day and at this rate the colour cartridge can last some time. A tricolour printer might be thought of as a black printer that happens to have a spot -colour capability.

Separate Cartridges:


Separate Cartridges

Separating the colours into 3 cartridges with integral heads makes the carriage slightly bigger. From the user point of view 4 separate cartridges are more economical - only those used up have to be replaced. Printers with disposable thermal printheads are still likely to be quite expensive to run - unless your refill the cartridges, of course. Because the heads are intended to be disposable they won't be the biggest and technically most advanced devices so the printer isn't likely to be the fastest on the market.

Separate Heads and Cartridges:


Separate cartridges and separate printheads

Typical thermal inkjet printheads don't have an infinite life but often outlast a single small ink supply. Separating the ink tank from the printhead allows each component to be used to its failure point. A typical thermal inkjet head will last through more than 4 ink changes before there is a persistent defect.

There can be an issue when the heads do fail, they are often quite expensive so users try to flush a lot of ink through them in hopes of making them work again.

Epson printers always have separate heads, the piezoelectric printhead is too expensive to be treated as a disposable part. Because the heads and tanks can be separated Epson printers can be converted to light industrial use with kits from people like Sawgrass.

Large Cartridge in the Printer Base


Inkjet with cartridges in the base

The most economical way to distribute ink is in large cartridges. (Well actually it's in large bottles but the printer manufacturers aren't going to do that). A large cartridge can't be on the printer carriage because accelerating the ink back and forth will fight against accurate positioning of the carriage.

Cartridges are in the printer base and connected to the printhead using flexible tubing. This does present some engineering problems; the tubes and printheads have to be primed with a liquid before they can be used so that implies a pressure feed of some kind and an ability to detect when liquid has made it to the printhead.

The implication is that there has to be a pressure feed for the ink and a more or less elaborate method for detecting and ensuring that the ink system is working. The printer is likely to be more elaborate.

Pagewidth Printheads


PageWidth Printheads

The ideal inket printhead doesn't have to be scanned back and forth across the page. Given more than 10,000 nozzles the head can hold still and the paper advance beneath. Inkjets that can do this can equal or exceed the fastest laser printers in performance.

Obviously the problem is that given a thousand inkjet nozzles and a few hundred pages there is a probability of a nozzle failing. The probability of failure can be reduced by improved printhead design or better service station mechanisms.

Printheads

HP Thinkjet printhead - Large View

The critical part of an inkjet is a print head, a micro-engineered device that borrows techniques from silicon chip making. As with chip making there has been a steady progress. Early printers that had just 12 nozzles and weren't a totally convincing replacement for dot-matrix printers. Recent printers have thousands of nozzles and photographic print quality.

To give the best quality print individual droplets might be as small as possible. It is always possible to build areas of colour from small drops and small drops will dry quickly leaving their colourant on the surface wheras large drops will sink in wasting colourant and saturating then cockling the paper.

Ideally droplets need to range from about 100 picolitres (about enough for a full stop in ordinary text) down to 1 picolitre. Thermal inkjet nozzles don't really allow for much variability in drop size although there are techniques using two or more resistors that allow it. Printheads can also have one row of small and another of large nozzles. Piezoelectric heads have greater drop size modulation.

Print speed will be proportional to the size of the droplets, the number of nozzles and their firing rate. Great numbers of nozzles are desireable. Ideally what is wanted is a single head the width of a page with nozzles at 1200 per inch or more. To span A4 with such a thing implies just over 10,000 nozzles.

Nozzles fine enough to deliver 1 picolitre drops need to be 10 -15 microns across - less than a quarter the diameter of a human hair. At this scale impurity in the ink will block a nozzle and so will floculation, coagulation or settling of pigments. Dust and fibres from the page might also contaminate nozzles. The main danger is ink coagulating and drying on the head and then getting pushed into nozzles.

With large numbers of nozzles the loss of one or two in any region would not normally be noticed.

Very big printheads made by chipmaking techniques will tend to have low yields and that would make them expensive.

Small printheads are a lot less costly but the printer is slower because fewer nozzles create a smaller swath which each pass.

Each manufacturer has their own nozzle design. For instance Canon originally used the sideshooter type drawn above but later changed to a roofshooter like that used by HP.  Nozzle shape might be used to vary the geometry of the dots whilst the printhead is travelling. In a perfect world to fill a page without overlap we want square dots but droplets naturally form circles. Dot formation can be controlled using papers with an absorbent coat, but that limits the use of the printers for ordinary office work. Ink coagulation can be limited by giving the  printhead a  hydrophobic fluorocarbon coating. 

Inkjet Development

Inkjets are an elegant idea, familiar liquid ink is turned to droplets and aimed at the page. Practical inkjets have tended to be a bit more messy; ink opaque enough to colour a page then dry quickly tends to coagulate in nozzles.

Early applications of inkjet printing dates back to the 1950s. IBM developed a continuous inkjet from the 1960s on as did many other companies. The idea is to pump ink through a nozzle where it breaks into droplets, the droplets are then either steered onto the page or into a recirculation gutter. Continuous machines tend to be much faster than drop on demand.  Continuous inkjet has found use as a production line datestamp printer and in some other high speed manufacturing applications. VideoJet is one of the leading makers today. At the moment continuous inkjet is not thought to have applications in the office and domestic sphere - it is too messy. Link to page about Continuous Inkjets

Drop on demand inkjet makes a drop only when needed; the two main methods are piezoelectric and thermal as outlined above. Siemens made a piezoelectric drop on demand 12 nozzle inkjet called the PT-80 in the 1970s but the follow-up 32 nozzle device failed, issues with the printhead blocking caused too much trouble and the idea was dropped. The Siliconics QuietypeHP Thinkjet Printhead

HP  (Frank Cloutier) and Canon (Endo and Hara) discovered the thermal inkjet around 1980    Early thermal printheads had just a few elements - 12 in the HP "St Helens" printhead (named after the volcano).

HP introduced the Thinkjet (thermal inkjet) on 1st February 1984. The marketing pitch was to sell it against dot-matrix printers:
HP Thinkjet
"Hewlett-Packard research has used inkjet technology to make the ThinkJet personal computer printer surprisingly quiet while printing 150 high quality, dot matrix characters per second for text or graphics. The ThinkJet printer weighs only 6¹/² pounds and it takes up just a bit more room than your telephone. So, it can work right on your desk. There's even a battery powered model that lets you print anywhere. And, the ThinkJet printer's ink supply and printhead are designed in one neat disposable unit that simply clicks out when it's time to change."

And all this for a starting price of just $495.00! That price was comparable to 9 pin dot matrix printers at the time. The HP and Canon printers form the baseline from which recent machines developed so its helpful to establish what they could do.

The Thinkjet printhead is a beauty in some ways (OK its appeal is to geeks). The big picture above more or less shows how it works with 13 electrical contacts on a circuit board at the front leading to resistors under the nozzles. The resistors are a thin film of a metal like tantalum which doesn't melt at low temperature and has some strength and acid resistance. The resistor momentarily vapourises the water based ink around it and the pressure and acoustic shock fire a jet through the orifice plate.  The volume of ink is controlled by surface tension. The Rayleigh effect almost immediately forms the jet into a spherical droplet which spreads to a small circle where it hits the paper. Droplets from this early head were quite large - around the size of a full stop.

The power transistors driving the resistors have meanwhile turned off so the vapour bubble condenses on the cold ink around it and rushes back cooling the resistor ready for the next shot. The Thinkjet head's 150 characters per second implies a top firing rate of just 1,500 dots per nozzle per second, not very fast but equal to keep up with most dot matrix printers.

The Thinkjet was designed to use tractor feed paper, most printers were at the time. The ink needed special paper at first, it tended to bleed into the page and turn faint. HP improved the ink and came up with plain paper printheads in black, blue and red.

In some ways the Thinkjet is a bit of a horror. Graphics are not it's forte; in a special graphics mode it achives 192 x 96 dots per inch - to be fair that's about what dot matrix was doing at the time - it shows how todays 2400 dpi inkjets have advanced.  There is no service station so when a nozzle blocks the user lifts the printhead out and primes it by pushing a paperclip gently through a pinhole in the back of the cartridge until ink oozes out of the nozzles, leaves it a minute then tamps off the mess on a tissue.

The printhead only contains 3ml of ink which HP reckoned to be enough for 500 pages - and probably is if you stick to correspondence and forms printing.

Inkjet print heads used to contain hundreds and now tend to contain a thousand or so  microscopic nozzles to actually deliver the ink droplets. The problem is that these tiny print nozzles block because of contamination in the ink supply or old ink drying in the wrong place. This wrecks the printhead. Inkjet reliability has been a problem. In the last couple of years several printer makers have produced inkjet models that they claim are cheaper to run than a laser printer suggesting a rising level of confidence.

Speed is the other problem for inkjets. A typical printer has one or more heads mounted in a carriage which is pushed back and forth across the page making a swath of print at each pass. Simple material might print quite quickly but complicated colour layouts often slow an inkjet printer down dramatically.

A big advantage with inkjets is that colour printing is just a minor engineering change - add some extra cartridges to the printhead carriage. In a thermal inkjet the cartridges are disposable so it costs the manufacturer very little to make a colour printer.

Inks

Inks are critical to what an inkjet can do - both in terms of how the material passes through the printhead and what it looks like on the page. An ink is predominantly a solvent carrying a colourant.

The most important part of an ink from the point of view of movement is the solvent. There are half a dozen different solvents:

Water or "aqueous" solvent - normally distilled and de-gassed rather than tapwater. Dries by absorbtion and evaporation

Alcohol - dries more quickly than water by evaporation and can act as a solvent for some things where water does not. Used particualraly for UV and weather resistant inks. Undersireable because of fumes.

Oils - dries largely by absorbtion

Hot-Melt - Liquid somewhere over 100 C. Dries by freezing at room temperatures (Xerox use it in phasers)

UV-curable - Solidifies on exposure to ultaviolet light. 

PigmentDye
ParticleMolecule
Size50 to 15 nm2nm
StateDispersed in suspensionDisolved in solution
AppearanceDull, Cant give glossVibrant, gives gloss with the right paper
Light fastnessExcellentCan be fair, often poor
Water fastnessFairpoor
Media reactionPhysical attachment to surfacePenetrartion and chemical bonding