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The electrostatic printing process known as "laser printing" is very well established as one of the main computer printing processes. Laser printer technology has matured over the years since the HP Laserjet 1 and Apple LaserWriter appeared and become very reliable and quite cost effective.
The best performance available from today's ordinary laser printers is
- A3 paper handling - capable of producing wide spread-sheets and engineering drawings
- true 2400x2400 dpi printing - capable of resolving details and photographs
- 60 pages per minute output - equalling the fastest photocopiers
Faster Lasers.
The primary limiting factor is the laser.
An A3 page at 2,400 dpi contains approximately a billion pixels (1,013,760,000). If the scanning laser has a modulation signal at 100MHz then it will take about 10 seconds to scan the page, so the best possible performance from the printer with no other issues at all is 6 pages per minute. Reduce the esolution to 1200x1200 and the laser can map out 4 times as many pages in the same time. A more powerful laser or more sensitive OPC material would all allow higher speeds.
Lasers that can be modulated (turned on and off) very rapidly are needed for other technologies besides printing. Communications and data recording devices all need faster, more powerful lasers light sources. Communications lasers commonly achieve 10 GHz - but they possibly don't need to deliver as much power as a laser printer. Data recording lasers (in CDs and DVDs) need comparable power to laser printing and have modualtion rates in the tens of millions of bits per second.
The current generation of laser diodes emit light from an aperture in the side of the semiconductor junction. A lot of the photons created are lost as heat inside the structure of the laser, so they are rather inefficient.
Techniques to produce much more efficient semiconductor lasers are being developed (porous silicon etc). Brighter, more efficient, cheaper lasers will be applied to printers. A five to ten-fold increase in laser printer speed might be practical if the brightness of the laser light can be significantly increased. Of course one of the side effects is that precautions against laser-light contacting users or engineers eyesight might have to be reinforced. At the moment most printers have one or two interlock switches.
Faster Mirrors.
Even if a single scanning laser could be modulated more quickly this alone would not greatly help printer speed because it would just move the bottleneck in the process to the scanner's polygon mirror.
To produce an evenly printed page the polygon mirror needs to be optically near-perfect, as many sided as possible and/or to spin very rapidly at a perfectly controlled rate.
The scanner assembly can be improved. With current drive and bearing technology, and current optical engineering techniques a fifteen to twenty sided mirror rotating at about 15,000 rpm is a practical limit. New bearing technologies in development could quadruple the mirror speed. Magnetic bearings might push speeds to 250,000 rpm. With nano-engineering techniques currently emerging from the laboratory many more sides of optically perfect mirror might be achieved - but every smoke particle will have to be excluded from the mirror assembly.
Printer makers are already equalling or exceeding the 2,400 dpi expected of photo-typesetters. Two limits would remain, however.
Toner grain size would need to be reduced. Toner grains are already so fine that any escaped powder is very difficult to control. Nano-engineered toner particles are perfectly possible but the potential health hazards would have to be examined. There probably wouldn't be much use in toner particle sizes near the 1500nm wavelength of infra-red because it can't be resolved.
Laser printers can't have resolutions much better than about 500nm or 2000 dots per mm or 50,000 dots per inch. There might not be much point in anything approaching such a resolution because unaided human sight cant esolve it - whats the point of a document that can only be read with a microscope? On the other hand a laser printer that could directly print microfiche cards or even DVD disks would be an interesting mass storage device!
Multiple Lasers.
The laser and the scanner's polygon mirror serve much the same purpose as the print-head in a dot-matrix machine, so greater speeds could also be achieved with multiple lasers.
Some laser-printer engine-makers including OKI and Kyocera use LED optical heads. The head is a bar containing hundreds of LEDs per inch across the page - 2,400 LEDs for an 8" page at 300DPI. LEDs can be modulated less quickly than a semiconductor laser and give out less light; they make up for their deficiencies because they are so low-cost to make that thousands can be incorporated in a single head. They are also relatively efficient compared with today's lasers.
The obvious way to produce multiple lasers is by making a page-width bar of lasers like that used in LED machines. Given the low efficiency of current lasers that would presumably be a challenge - too much heat dissipation. Nevertheless if it were possible either for
| LEDs to produce modulation and light-quanta levels similar to those from lasers |
| or lasers to be made with the efficiency and cost of LEDS |
then a significant increase in print-engine speed could be achieved. Conceivably laser printers might be brought up to the speed of commercial presses.
Multiple lasers pointing onto the polygon mirror is another possibility - this may be used in some high speed printers (?).
Photoconductors are semiconductors with a high natural resistance in the dark. When light of a sufficient energy (frequency) is absorbed by a photoelectric material the bound electrons get enough energy to jump into the conduction band - the free electron and its partnering hole lower the resistance.
The properties of the photo-conductor material are very significant to laser printer operation. If the photo-conductor could take up a clear, sharp image from a very limited light source then electrostatic printers could function more quickly with light-sources such as LEDs.
Several photoconductor materials have been used in printers - selenium, silicon and organic photoconductors or "OPCs"
OPC (Organic Photo Conductor) - a plastic material usually green or brown in colour. OPC is commonly applied to either an aluminium drum or a metal belt. Drums are easier to drive but belts have a larger working surface. A colour printer will typically have one or more drums and a belt.
There are about a dozen types of OPC. They are usually made from several layers of compounds not particularly familiar in everyday life - for instance:
The Charge Transport Layer (CTL) is normally transparent - something like N,N-diphenyl-N-N-bis(3-methylphenyl)-(1-1-biphenyl_-4-4diamide (TPD).
The CTL is exposed to and holds a high voltage from a "corotron" or a charge roller. The charge is static usually at about minus 600 volts on the surface.
The Charge Generation Layer (CGL) is someting like oxotitanium phthalocyanine (TiOPC).
The laser which generates the images penetrates to the CGL and paints it with successive lines of the image in raster fashion. Photoelectric effects make the CGL locally conductive when exposed to the infra-red light of the laser and the resulting conductivity dissipates the charge of the transport layer into the metal substrate underneath. Nearby areas that emained dark retain their charge.
The aluminium cylinders used for drums are precision milled then sprayed, dipped and spin-coated in solutions of these materials. Uniformity is very important - the toner particles are just a few micrometers in size and a precise gap must be held between the OPC drum and the developer or transfer will be uneven.
More info wanted ??? - 4,4'-Diiododiphenyl - a yellow-white powder.
Health and safety information for an OPC from Lanier suggests
Aluminium substrate 99%
Stilbene derivative <1%
Polycarbonate <1%
Titanium dioxide <1%
Organic pigment <1%
Bisbenzylbenzene derivative <1%
there appear to be no hazardous materials or health issues . Other sources suggest that OPCs may pose a hazard however its worth pointing out that people don't normally have any direct contact with quantities of the material which is normally enclosed in a cartridge.
A potential issue with the OPC materials are that they are relatively soft and gradually eroded by the process and particularly by the wiper blade. This ability to shed old material seems to have been a design feature in some older copiers.
Manufacturers are aiming to improve OPC life by giving it a mechanically resistant coating - for instance of amorphous or "diamond like" carbon - DLC. The problem with hard coatings is that they have the potential to interfere with the charge carrying sensitivity - or to separate from the underlying material. The Journal of Electronic Materials Vol30 No2 2001 carried an article on creating a hard coating by Min-Sun Hwang, Eun-Teak Kim and Chongmu Lee of Inha University, Korea. US patent #20050175915 describes a process for building a protective amorphous carbon layer using successive mixtures of hydrogen and hyrocarbon gasses, electrical charges and plasmas.
Although DLC might extend the life of OPCs that doesn't mean manufacturers will readily use them. Most OPCs ship as part of a cartridge and it is usually the toner that exhausts first, leaving a part used drum. If anything the trend is towards integrated cartridges with shorter lives.
The electro-optical properties of organic materials have become an important field of research. Light Emitting Polymers (LEPs) are being developed for use as flat-screen displays. Presumably the research in to light emission applies to some extent to light detecting polymers such as OPCs.
Inorganic Photoconductors.
The most widely used inorganic photoconductor is selenium, which is a semiconductor and conducts better in the light than the dark. Selenium can be used on its own or alloyed with tellurium. It is also used in chemical compund form as arsenic triselenide. Cadmium sulphide is quite widely used as a photodetector in electronic devices. Dye-sensitized zinc oxide also works.
Silicon
Silicon - one of half a dozen semi-conductor metals. The silicon used in laser-printers is "amorphous" (glassy) silicon and the drum making process was developed by Kyocera and Canon and is patented.
The main advantage amorphous silicon seems to give over OPC is that it is a hard substance. Kyocera put a "lifetime" warranty on their drums. The production process apparently involves depositing the silicon on the drum using a plasma, which makes it expensive.
Amorphous silicon works better when warm so laser printers using it warm the drum. Unfortunately Kyocera's FS1500 series machines which used these drums got a reputation for unreliability. The drum would last a "lifetime" or actually 150,000 pages in principle but unfortunately the corona wires and other components around it did not, so engineers had to strip and clean the drum assembly quite often. Worse still spare drums were very difficult to get for a period.
Canon used amorphous silicon phototoconductor in the 7550 model.
Amorphous silicon can presumably be improved on - it is a less efficient photoelectric material than the crystaline silicon used in semiconductor electronics, and silicon is less fast-reacting than germanium.
Cadmium
Cadmium sulphide is widely used as a photodetector in automatic lightinging systems, camera light level sensors and so forth. In these uses the cadmium sulphide is sealed into a cell. The substance might not be suited to photoconductive drums although organic materials containing cadmium are used. Cadmium has been widely used as a colourant in plastics although its use in childrens toys has been discontinued.
Germanium
Germanium is reputedly hard to work with, but a 10% alloy of germanium in silicon apparently works well as a semiconductor.
Selenium
Photocopier drums have often used amorphous selenium and sometimes still do. The selenium is deposited as a single layer on a drum (using vacuum deposition). Selenium has good abrasion resistance giving it a service life of anything up to 500,000 copies. Photoconductive properties are good as well, it has good sensitivity in the red - green wavelenghts used by photocopiers although not in the red wavelengths that optical colour copiers would need. Selenium isn't very good in the infrared range that laser printer use. Selenium is brittle so it can't be used for flexible belts.
Apparently selenium is a health hazard. In small quantities selenium is essential to animal life being a key factor in the operation of several enzymes, so it can be taken as a dietary supplement. In large doses it is toxic causing the disease selenosis. Metallic selenium isn't very toxic because it isn't easily metabolised. However the manufacture and disposal of selenium and tellurium products needs some precautions.
In laser printer cartridges the long life of a selenium is not an advantage sinnce it would significantly outlast the other components.
Both the materials used as photoconductors seem capable of advance in terms of the precision with which a pixel is defined, speed of photoconductive action and sensitivity.
Toner is a solid powder made from a plastic or wax and a colourant. Sometimes there is an iron component as well to provide a built in developer (Canon and HP use "magnetic toners"). Kyocera "Ecosys" toners contain a ceramic powder to polish the amorphous silicon drum.
Most laser printers use a "negative" toner - that is the charge by which they are attracted to the drum.
A typical plastic component is styrene acrylate or polyester resin with carbon black colourant.
Particle size depends on print resolution. Finer resolutions suggest a smaller particle so
at 300 dpi there are 12 pixels per mm and the toner particle size is sbout 12 micrometers
at 600 dpi there are 24 pixels per mm and the toner particle size reduces to 8 micrometers
Toner powder has to be a consistent size otherwise some pixels will be misshaped
Toner is normally delivererd in cartridges and exposure to powder should be low. Recycled cartridges are notorious for being rather more inclined to have traces of toner on the case .
As a fine dust toner can be an irritant - as well as easily discolouring any material is accidentally contacts. Toner is best dislodged by shaking or lightly brushing - ideally outdoors to dilute any airborne exposure. Cold water can be effective as well, it may need a trace of washing-up liquid to overcome surface tension. On no account try to remove toner by using hot water or solvents - it will melt and fix to material.
Some of the components in toners may be carcinogenic (producing mesothelioma) in large quantities. Such a large exposure would be required that it is generally considered harmless.
The majority of printing is black and white - but most people would prefer colour.
Colour printing poses two problems: -
- mechanical elaboration needed for the colour process
- the need to vary the light levels to produce a grey-scale
The "intermediate store" process used in most of the current generation of printers seems to be the best compromise designers can come up with. The printer has four toner-developer mechanisms and the OPC generates a page from each in turn.
The laser - electrostatic process has historically been "digital" - a dot is present or absent from the paper and there is no grey-scale. The human eye can resolve about 1 million different colours, but simply turning the supply of the CMYK toners on and off naturally produces a colour gamut of just 8 (including white paper).
The colour gamut could be extended. Photoconductive materials can handle a grey-scale. The amount of charge can be varied by exposing the material to a greater or lesser amount of light. The amount of toner gripped by each pixel will then be proportional to the charge.
The light output from a laser or LED can be varied in two ways, either by varying the current supplied or by pulse modulation.
The toner dose delivered to the paper not only has to vary, but it's absorbtion of light has to vary in a controlled fashion over a wide range to reproduce colour accurately. To produce over a million colours the level of the CMYK processes have to be controlable to at least 256 levels each. This has proved difficult to achieve because the photoconductive effect in most materials has a strong hysterisis.
To achieve a grey-scale laser printers commonly set dither-patterns in a group of pixels. A 1200 dpi printer might use groups of 4x4 pixels to achieve what looks like a grey-scale at 300dpi.
Combining some grey-scale capability with high resolution dither-patterns could be relatively straight-forward, but obviously printer designers want to get the benefits of both high resolution and accurate colour rendering. Software can examine the image looking for how colour areas are made up and using alternative combinations of grey scale and dithering to achieve the best results.
All computer printing has traditionally been quite expensive compared with the mass production systems used for newspapers, books and periodicals. There are a few extra costs:
| Intelligent steering of ink onto the paper. It is only quite recently that the cost of gigabytes of memory needed to hold page images and the computer power to create them have fallen to trivial levels. |
| Colourant costs more. Traditional printers ink formulations don't have to meet the stringent purity levels wanted for inkjets or the peculiar electrostatic requirements for laser printing. |
| Machinery costs more - an inkjet printhead with 500 microscopic channels or a laser printers, scanner, OPC and developer mechanism cost more than a traditional printers plate. However this is changing. The completely general purpose nature of the inkjet and laser mechanism suggest they could cost less than preparing a plate. |
Laser Printing
The lowest cost laser-printing is probably the 0.7p per page claimed by Kyocera. (But I need to look at the costs for IBM and Xerox's big machines) No manufacturer claims to be most expensive (for obvious reasons) but some mono laser printers that cost very little to buy seem to cost 5p or more per page to operate.
It can be quite difficult to establish a real cost for some printers. The manufacturer may well publish a retail price and an expected life for cartridges - but:
| The real street price of consumables can be rather different to any recommended price. The most popular printer engines will tend to have the lowest retail prices for consumables if only because the distribution chain are willing to sacrifice margin on high turnover items. |
| Where printer manufacturers quote a page-price they usually state that this is at 5% cover. This level is probably realistic for correspondence text. Users with graphics and colour capability tend to use a lot of ink. Remember that if a 5% page costs 2p then a page of black with white writing costs 38p. We would not be surprised if some solidly-printed colour CMYK laser printer pages cost over £1 to produce - about 30p for each colour. Solidly printed inkjet pages on photographic quality paper can cost more than £2. |
| Cartridges for inkjets and lasers can behave in odd fashions. Epson inkjet printers are notorious for leaving a quarter of the ink unused - but it is vital that the printer does not exhaust the ink to prevent air-locks in the head. Laser printers can usually un their cartridges right down to the point where they stop printing without harm - but usually the printers controls won't let you do this. |
The price of laser-printing is made up of the following:
Initial purchase of the machinery - a £1,000 printer with a half-million page life is costing 0.2p per page. Many printers get nowhere near their ated life, so purchase is a larger proportion.
Toner is the innevitable consumable. The printer has to consume toner to colour the paper.
The remaining problem would then be the toner/developer system - delivering a finer toner to allow finer print. There doesn’t seem to be a particular problem with speeding up the developer or reducing the size of the toner powder.
It isn’t clear whether there is an easy way to reduce the cost of toner.
There probably is a downward limit on page cost somewhere near the 0.7p per page currently attained by the cheapest printers such as the Kyocera FS1650. The limit is set by the cost of the toner powder, the plastic containers needed to move it, and by wear and tear on developer, drum and fuser.
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The process used by most laser printers is virtually identical to photocopying. The laser’s only role is to act as a fairly powerful and controllable light source, and as mentioned it is replaced by a projector lamp in the Qume Crystalprint. The laser in the average printer has an output of 5mW - not enough to do much harm if pointed at skin. However the laser light is generally infra-red, so it is invisible. There is a potential danger because even at this low power lasers can damage the retina of the human eye - so it is important never to over-ride the safety switches in a futile attempt to actually see the beam.
Direct Laser.
It is possible to build a laser printer that simply burns the image onto the paper by modulating and steering the beam with mirrors. In a machine of this kind the laser obviously has to be up-rated -perhaps a couple of kilowatts might be sufficient. Laser printers of this kind were tried by Siemens, who had three installed at DSS Longbenton in the mid ‘80s. Obviously it would be possible to build a machine capable of hundreds of pages per minute if the mirror steering can act quickly enough. Such a device might eplace conventional newsprint presses.
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© Graham Huskinson 2010
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