Thirty years ago there was no home market for computer printers. Offices used

Typewriters from firms such as Brother, Smiths, Olympia, Olivetti and Remington.
Stencil and ink copiers from Roneo and Gestetner, Dyeline copiers and Xerox machines.
 
 

--

Xerox had special technology and is still a recogniseable name in computer printers and office copiers. Many of the other companies that once dominated the office equipment market have gone.

Xerography, which is the basis of toadys laser printers dates back to techniques developed by Carlson in the 1930s. An semiconductor surface is charged with static, then exposed to light so that some areas lose charge. The image is developed with a rosin or plastic powder, the powder adheres where there is a static charge. The powder image is transfered to paper and fixed by heat.

Xerox invented a way to scan a document with mirrors so that

Until the mid- '70s computer printers were either telegraph machines or band printers - the size of wardrobes with £10,000 price tags made by companies such as CDC.

Today's printer market is the culmination of several technological changes, only some of them directly related to printing.

Intel and TI both lay claim to "inventing" the microprocessor in 1971. It is probably fair to say that it was an idea whose time had come due to "Moore's Law" although that wasn't self evident at the time. It took a couple of years for the idea to catch on, then rapid adoption changed the computer industry. Printers could be small and have their own processor, making thermal, dot-matrix, laser and inkjet designs viable. Moores law is still operating vigorously, increasing the power of processors and the size of memory in printers and everything else. Older printers tend to be dumb

Personal computers emerged in the mid '70s. Apple marketed one in 1977 and IBM in 1981. There were enough enthusiasts to create an industry, but making much use of early machines needed dedication. Early dot matrix printers like the Anadex competed with daisy-wheel typewriters like the Diablo.
 
 
 

In 1983 Apple launched it's "Lisa" computer. Steve Jobs aimed to make a computer so simple his four year old daughter could use it, and used the idea of a graphical user interface, something Xerox had experimented with. Graphic trades like layout artists, architects and designers rapidly adopted the Apple Mac when it was launched a year later. Microsoft and IBM began to copy the idea with OS/2 and Windows. The ability to produce graphics on the screen meant the inkjet and laser printer would eventually dominate. Dot matrix designs did the job too slowly and crudely.
 

Before that, computer printers tended to be either modified typewriters, telegraph machines or big, expensive specialist devices like band-printers.

Most computer print technologies date back a long way. For instance, laser -printing sounds modern, but the electrostatic printing method was developed by Carlson in the 1930s. Impact printers are descendants of Scholes typewriter, first made practical in 1874.

Thermal printers

Inkjets are the disruptive new technology. Inkjet printing has two antecedents

Continuous inkjet printing
 
 

Making Printers

The most straightforward printer mechanisms are those in an x-y pen plotter, two motors move a pen around a page. Pen-plotters have dissapeared commercially, they were too slow to compete with inkjets. A dot-matrix printer is not too complex - it might just about be possible to build one as a school project. Twenty years ago, before printers became mass-market items, some designs really did look like they had been built in a garage. There are still dot-matrix printers made in short production runs of a few at a time.

Inkjet and laser printers look very different and really are much more complicated. Both printers are normally the products of research labs and design teams, then built in huge quantity - quit possibly by contract manufacturers.   The basic mechanism still holds. One motor drives horizontal movement, another drives vertical movement
//
Technically the inkjet and laser mechanisms are quite complex. It wouldn't be at all easy to make a one-off experimental inkjet head or a laser-printer scanner unit. Like silicon chips, however, they are readily enough made on production lines.

Each type of printer has a technological heart
//
Inkjets

There are two injet technologies

Continuous inkjet printing directs a steady stream of droplets at the item. Continuous inkjets are used in industrial printing, particularly to put small characters like sell-by dates onto products. Continuous inkjet devices tend to be "messy" and aren't used in office or home equipment.

Drop on demand inkjets have a head at heart. (mixed metaphor?). Inkjet heads are small - about the size of a postage stamp. The active part of the head are hair-thin capillary tubes which make the ink droplets. An ideal droplet is about one picolitres in volume. On the page a single drop will be just on the limits of human visual perception perhaps 0.05mm across -each being one picture element or "pixel". As with screens and a printed character is made by groups of pixels

If the head had just one capillary channel it might be easy to make, but printing would be chronically slow with perhaps 100 passes of the head to build up one row of text on the page. In fact inkjet heads typically have dozens or hundreds of capillary channels, but slow progress is still one of the main problems with this type of printer. Actually, manufacturers often turn this slowness to an advantage in reducing costs. Since the inkjet mechanism can't work particularly quickly it can rely on the users computer to do a lot of the work converting what is on the screen to what will appear on the page.

Inside the channels something has to pump the ink. There are two mecahnisms:

Heat is most frequently used. HP, Lexmark and Canon all use a similar technique. A tiny element vapourises some of the ink fluid and this forces a drop out onto the page, then an inrush of fresh ink cools things down ready for the next shot. The chambers in a thermal head can be small because all the ink they contain will move. This is both an advantage and a problem for thermal printheads:

The strong pumping action means thermal heads can fairly easily recover from air bubbles getting into the ink feed.

The action is not really controleable, all droplets are the same size.
 

Piezo-electric vibration can also be used to pump ink. Several materials (including silicon) distort when a voltage is applied, and change in the opposite direction if it is reversed. Epson use this technique in their printheads. Piezo-electric action is not so dramatic as boiling, so a piezo-electric heads chambers tend to be bigger to move the same volume of ink.

The piezo-electric effect can be varied by changing the voltage, so the head can produce small 1-2 picolitre drops for accuracy and big 10 picolitre drops for giving a solid fill to a large area.

Because the piezo-electric movement is relatively undramatic the heads are not very good at recovering from air bubbles. The only way to shift an air-lock is to run and re-run "cleaning" cycles which can be annoying wasteful.

Speed is another advantage for piezo-electric heads, the straight forward mechanical action can be done more rapidly than the heating and cooling in a thermal head.
 

Inventors can envisage micro-mechanical pumps using pistons, valves or cogs - its a matter of what is practical. A printhead has to give reliable performance at low cost when mass manufactured.

The basis for most heads is a silicon chip. Because of it's role in circuitry, silicon is the most studied material, there are lots of techniques and well developed equipment for micro-machining silicon using photolithography and etchants. An inkjet head needs a certain amount of support circuitry anyway -paths to the elements, drive transistors, and probably a shift-register so that serial data can drive a whole array of print channels.

Inkjet head design is advancing. Older print-heads have about 50 big channels each producing a few thousand drops per second. Recent heads have a couple of hundred smaller channels each capable of 50,000 drops per second.

An ideal inkjet printer might have page-width inkjet heads. A very wide head would do away with the need for hoizontal scanning of the page, so the printer could work very quickly - potentially matching laser-printer speeds.

The problem is that the capillary channels in a printhead are prone to blocking. If the droplets produced are too small to see then very small impurities introduced by ink drying, settling or by pigments flockulating will potentially block a channel, then there will be a white streak across the page. Flushing a lot of fresh ink through the head will often free it up, so printers offer users various cleaning cycles that will attempt this. Bigger and more complex heads with more and finer capillary channels are likely to be more prone to blocking; so there is some sort of upper limit on head size.
 

Inkjet heads are complex, neat little chips and their manufacture is towards the limits of a technology. Outside the printhead itself designers have more freedom.

Disposeable or fixed?

Printheads are fairly complicated to produce; but like silicon chips, the unit price goes down if a lot are made. This has lead to two quite distinct approaches - fixed and disposeable heads.

Disposeable heads are part of an ink cartridge. The head is expected to last as long as there is ink in the cartridge, then both are disposed of - preferably in a recycling box. One advantage is that it is unlikely that the user will suffer a head failure, it is always new. And if the head does fail the solution is simple - get another cartridge. This approach will clearly  suit a "corporate" environment, where computer users with problems are expected to phone IT for support. Support is reduced to "change the cartridge".

A potential design problem is that the head and it's cartridge load of ink both must be mounted on the carriage, which has to accellerate them back and forth across the page. The more ink there is in the cartridge the bigger the motor needed to do this, and the more the ink will slop about unless wadding or baffles are included in it's tank. A colour printer carriage with four 100 ml tanks would weigh about half a kilo - a lot of bulk if it is to move with pinpoint accuracy fifty to a hundred times in producing a page. Only plotters use such big and long lasting cartridges, lesser printers have little 10-20 ml cartridges.

Fixed heads are part of the printer carriage. The head is expected to take several successive cartridges - it may even be expected to last the whole life of the printer. One advantage is cost - the printer should be cheaper to run because it is not wasting heads. In fact, the heads can be coated with tough materials to make them last. Ultimately the head will fail, of course, and changing it may not be a simple user action - and because it is not a mass-market consumeable it may be expensive. The lower price of cartridges might be expected to appeal to home users, but the lower cost and the ecological benefit of not wasting printheads might appeal particualrly to public bodies.

There is a problem where an ink cartridge engages the head. Somehow the cartridge has to be opened and make a gas-tight seal without allowing any hardened old ink to enter the reservoir.

A new bit of design flexibility opens up, however. If the cartridge is separate there is no need to mount it on the carriage with the head. The head can be fed by flexible pipes from big cartridges in the base of the printer. To anyyone interested in fast, low cost printing this sounds like an attractive idea - but it does introduce problems with pumps and kinked pipes.

-

Making inkjet printheads can't be described as an environmentally friendly process - all sorts of coating and etchants to produce something which will have a brief life then be thrown away. But that is the fate of all silicon chips - most of them are obsolete at the end of five years. Because inkjet printer design is improving quite quickly printers don't have a long service life: first generation mono printers were replaced by those with colour capability, then by those offereing near-photographic quality and now by machines with true photographic abilities.
 

// Since the head must scan across the page it will be mounted in a carriage driven by a motor.  //
 

Inkjet printers are usually made in long production runs for the mass market - but not always. Only the head has to be made in a special fabrication facility. It is possible to make special cartridges with inks for coating plastic, fabric and metal. It is also possible to make special printer bodies. The simplest examples are the plotters based on more or less standard cartridges.
 

--

Laser Printers

Laser printer design involves a lot of mechanics. A substantial assembly of scanner, toner hopper, developer, drum, transfer station and fuser.  At a minimum this assembly of parts is going to take about a cubic foot of space - often rather larger. There are lots of cogs, driven by specially designed motors. The motor and cog assembly need support structures, and the cartridges have to be designed to slide in and out of this easily. The laser printer mechanism is called an "engine".

Efficient production of laser printer engines involves a lot of component parts - possibly several hundred, all needing to fit to within a couple of microns accuracy. Making the innards of a laser printer was once comparable to making a car engine, but manufacturers have been fairly succesful in reducing the parts count by using complex plastic forms and puched metal sheets.

Manufacturers know that the unit cost of a complex mechanical assembly is reduced by mass production. The idea has been refined since the first Ford cars were made. To keep costs down laser printer manufacturers tend to share engine designs. For years, the heart of many Hewlett Packard laser printers has been a Canon engine. The same engines have appeared with all sorts of manufacturers labels - there is often a sameness about their look.

Printers that look the same might be expected to work much the same, but this isn't so. In fact, there can be quite dramatic differences, especially in speed and in the way printers handle graphics.

There are obvious cosmetic differences that can be made using different shaped and coloured plastics around the same engine. The main reason printers can be so different despite using the same engine is electronic. The whole page to be printed by a laser printer has to be ready before the first drop of toner touches the page. The whole chain of laser - scanner - developer, drum and fuser must all move synchronously and without stopping, otherwise the page will smudge. An inkjet printer can do a line at a time and stop to think; a laser printer needs to prepare the whole page. To achieve this most laser printers have their own fast processors - a "RIP" or raster image processor - and quite a bit of memory to store the page the RIP compiles.

The "language" fed to the RIP, it's speed, and the size of it's memory are among the controlling factors in printer design - so adding a different board full of electronics alongside the engine can completely change it's behaviour.

Cartridge Construction

Cartridge design is less easily changed. The cartridge form factor and dimensions have to be the same for every manufacturer that uses the engine - the same set of plastic mouldings, cogs and drum metalwork. In general, the cartridges do all come from the same production line, so knowing it's the same engine allows you to use a Canon cartridge in an HP printer. However it is possible for manufacturers to run an engine in a very different way - use a different OPC coating on a drum, different grades of toner, and different control voltages. Using the wrong voltages in a machine that relies on electrostatic transfer will certainly produce a badly printed page, and may make a horrible mess inside the machine.

--

Laser printer consumeables can be arranged in very different ways. Many of the mechanical parts of the printer will wear out or be used up eventually. An ideal printer might be:

- a succession of exchangeable units - toner, developer, waste toner bottle, drum, and fuser. The problem is that it is difficult to keep a stock of so many consumeable bits, exhaustion of any one will bring the printer to a halt.

- one big cartridge, everything exchanged at once so users never have to deal with problems. The problem is that this is wasteful, different bits wear out at different rates.

Putting the devices in sequence:

Toner consumption obviously depends on page-cover. Toner cartridge ratings are normally for black text on white paper at 5% cover, white on a black background implies 95% cover and nearly 20 times the toner consumption. Pictures and big type in newsletters are somewhere in between. Size is a limit on how much toner can be built into a cartridge; printing 5,000 pages needs about a litre of fresh toner powder.

Developer life depends on page count. Old iron-filing developers wear out because the filings grind down finer and finer and the material is lost (some of it comes out in the print). Developer rollers can last to 100,000 pages. Iron filing developer material might last 20-50,000 pages. Modern Resin developers are less well documented, but the material seems to oxidise and dry out.

Drums are at the heart of a laser printer if anything is. Drums are normally coated with "OPC" organic photo-conductor, a thin plastic material that is an insulator in the dark and conductive when exposed to light. Light exposure degrades the OPC, but grinding away it's surface gives it more life. In the nature of things the drum surface gradually wears away, so there is a balance to be maintained between material thickness and wear. In the end anaylysis, drum life depends on design and page count but can typically be 10 to 20 thousand pages. Canon and Kyocera developed an amorphous silicon drum with a very long life. There is a lot of interest in organic semiconductors for screens, so better drum materials should be a spin-off.

Toner waste bottle use varies more or less with toner consumption. Not quite all the toner transfered onto a drum as an image later transfers to the paper; what is left is partly compressed, polluted with paper dust, OPC and developer and has to be wasted if print quality is to be maintained. Waste bottles are just collectors, but they do fill up.

Transfer stations are the point where the image is stripped off the drum onto the paper. Older printer and copier designs did this using a charged wire under the page; in newer machines it may use a conductive rubber roller. There is another charge wire or roller to give the drum its initial charge. These components don't wear out, but they do get dirty and damaged. Users might clean them - but usually not.

Fusers tend to last a long time 70,000 to 150,000 pages or more. Fusers often die more by accident than by wearing out. The printer gets bashed and blows the heater-lamp or some contamination goes through and wrecks the roller material.

Seperate Parts

Many early printers had all these separate parts. A new waste bottle came with a toner, and a new transfer wire with a developer. Users definitely did find it frustrating, particularly because long-lived parts like the developer typically had the really high price of spares, rather than the more humdrum prices of a consumeable. What was worse, users often got things wrong and wound up with toner spills, dirty print and an engineer call-out.

Combination Cartridges

Manufacturers have often adopted the solution that became widespread with the Canon LBP8 / HP-2. Put almost everything in one big cartridge that lasts between 5 and 10 thousand pages. It is potentially wasteful, because some of the components are perfectly good when it is removed. However it rarely goes wrong - (although the odd user will try to hammer a cartridge in the wrong way up! )

In practice the waste of cartridges is less serious than it used to be because one of the most active parts of the recycling industry is dealing with cartridges. In fact, when toners, developers and drums came as seperate parts they almost always went out as garbage. A lot of components are now refurbished and recycled. The very visible waste has inspired a new industry.

--
 
 

As a piece of machinery a printer is comparable in complexity to a video recorder - the mechanics are potentially a bit simpler, but it needs a computer interface. Not surprisingly, consumer printers fall in the same price range - £50 through £250.

Like any Fast colour laser printers intended for office use can cost a lot more.
 

There are two rather distinct arms to the printer market:

home office - inkjet

corporate office - laser
 

Laser printers

Relatively fast - typically 10 to 40 pages per minute depending mainly on printer price and a bit on page content.

Relatively expensive to buy
Colour is significantly more expensive

LCD control panel

Corporate models often have a built in network interface

Internal RIP and memory gives fast printing - with network models this has no impact on PC performance

Standard print languages such as PCL and/or PostScript. Some models do not rely on a Windows driver and can function entirely using Unix LPD and Postscript

Usually have a resolution up to 1200 dpi giving acceptable photos.

Solid black print. Colours don't smudge and don't usually fade. Doesn't require special paper for photos

Reliable - will often perform almost unnoticed for three or four years until pickup rollers are aged.

Long lasting cartridges

Typical print costs 1.5 - 2.5 p per page
 

-

Inkjet printers

Relatively slow - typically 1 to 10 pages per minute, depending rather a lot on page content.

Cheap to buy - the cheapest are under £50
Colour is little or no extra price

Fancy models have control panels, basic models don't

Usually have a USB connection to a nearby PC - some have a parallel connection

Most have negligible processors and memory. PCs may slow down significantly during printing.

Some support PCL standard language. Most models are heavily dependen't on MS-Windows driver and PC processor and memory. Some models only work with MS-Windows.

Some models have resolution well beyond 1200 dpi and 7 ink colours giving very good looking photos

Print quality often imperfect. Many inks are prone to smudging. Coloured inks are prone to rapid fading. Tends to require special paper for photos
 

Some fiddling around expected to get an acceptable print towards the end of cartridge life or when the printer is over three years old

Short-life cartridges

Typical print costs

--
 

For a start, some "manufacturers" don't really make things - they package them, provide the software and the brandname.

Actually, HP don't make printers in a direct sense. The engines for their laser printers are made by someone else - typically Canon. Their inkjet printers are made in contract assembly plants.