Laser Printer Photoconductor

Printers > Laser > Photoconductor

Navigation Icons Guide
Printers - Main Index PagePrinters Index
Laser Printer Topic - Overview & IndexLaser Printer Topic Index 
Introductory Text on Laser Printing  & ElectrophotographyLaser Printer Intro Text
The core of a laser printer design is the photoconductive belt or drum. The life of these components is typically between 5,000 copies for a small drum in a cartridge to 500,000+ copies for a selenium drum on a large copier model.

Most photoconductive materials decay in light and on exposure to the atmosphere; so there will be a shelf-life as well, particularly when a drum is taken out of its packaging.

The price might be £25 to more than £300. At £25 for 5,000 copies a drum is costing  0.5p per copy - perhaps half the price of printing a page. Mono printers tend to have the OPC drum wrapped up with toner and developer in a short life cartridge. Older colour printers often share an OPC drum or belt cartridge between four colour developers. Recent colour printers have a drum for each colour giving an inline direct transfer design.

A laser printer relies on two principles, both centering on the OPC drum or belt
Opposite charges attract and like charges repel. Materials charged to opposite voltages attract one another quite strongly. A strong electrical attraction requires a high voltage. Laser printers use high voltages on the OPC surfaces to establish a strong grip on tiny particles of toner powder.
Light can change conductivity. Photo Conductors are insulators in the dark, conductors in the light. Exposing a laser printer drum to high voltage and then to a controlled scanning beam of light covers the OPC surface with a pattern made from static electricity.
Laser printer drum surfaces are first charged to a high voltage - typically about minus 600 volts. In photocopiers and older laser printers the charge device may be a "corotron"  and the voltage used is much higher  - more like 6,000 volts. These days a conductive foam roller in direct contact with the drum is often used instead. The advantage of the roller is that the direct contact allows it to use a lower voltage. The voltages are fairly substantial because the charge must be sufficient to grip toner.Drum Concept - Charged at top, Discarged where Lit. Toner Sticks, Toner Transfers to Paper

A laser or other light source then scans the photo-conductor - usually lighting it in areas to be printed.

Wherever light has hit the surface the charge is lost. The picture to be printed is formed in static electricity on the surface of the photo-conductor.

The photo-conductor is then presented to the developer / toner mechanism and the areas attractive to toner-particles turn into an actual image. Normally the developer is negatively charged as well, so the toner repels from areas of the photoconductor that retain a charge and only collects on the discharged areas.

In principle the charge handling can be either way round.

Photocopiers tend to use a "positive process" where dark areas in the original give dark areas on paper.

Laser printers have tended to use a negative process - areas struck by light tend to result in toner transfer and give dark paper.

The laser might leave the surface charged positive and this gives better detail. 

The precise methods  depend on the detailed electrochemistry of the materials. It might make sense to have the laser write in areas that are to print because only a small percentage of a text page carries writing, so this uses less light power and gives the laser a lower duty cycle.


Photostatic Materials.  Laser Printer Overview & Index - Photoconductor

There are lots of different photo-conductors, just as there have been a variety of photographic mediums (ranging from egg-albumin to Kodacolour). The original material used by Chester Carlson in experimental machines was sulphur on zinc.  Selenium drums have been used in photocopiers from the 1940s to date, it gives high sensitivity to green light. Selenium drums are long lasting with service life up to 500,000 pages but that isn't a particularly attractive property in the laser printer market where cartridges typically have a life of 10,000 pages or less.

  Manufacturers have also tried alloys of selenium and arsenic or tellurium which give better sensitivity across the optical spectrum. Photocopier manufacturers generally phased selenium out in the 1990s. OPC is more cost effective. 


Amorphous Silicon (aSi).  Laser Printer Overview & Index - Photoconductor

In the early 1990s Kyocera and Canon developed amorphous silicon drums - these are a hard material and should be very long lasting. The fact that they have not taken over the market may reflect a certain difficulty in use - the drums need to be heated. The cost of the patented technology may also be a disincentive. Possibly the main problem in the printer market is that the first printers using these techniques in the early 1990s got a poor reputation for reliability.


OPC.  Laser Printer Overview & Index - Photoconductor

The majority of recent drums use organic photo-conductors - specialised plastics.  Organic materials have been used for some tasks from the 1940s (Chester Carlson used anthracene in early devices) but the massive growth in OPC use happened in the 1980s. Peter Gregory says there are more than 3,000 patents, 600 of them appearing in 1983 and mainly filed by Canon, Ricoh and Konisiroku. Most copiers and laser printers now use some variety of OPC.
 
There are several varieties of OPC, usually made from several layers. There is a metal support which acts as a ground plain. Next to this is the Charge Generation Layer  which is ionized by light. On top is the Charge Transport layer which accepts a charge from a corotron or charge roller.OPC Drum Section - CTL and CGL

CTL

The outer layer is the Charge Transport Layer or CTL which takes up and holds the charge. The CTL is a polymer matrix that can hold a charge give by a high electric field, usually negative.

CGL

The inner layer is the Charge Generation Layer or CGL and this actually undergoes the photoelectric effect. A stream of photons from the laser are absorbed in the CGL and electrons there get enough energy to become mobile. Under the influence of the strong negative charge on the CTL the "holes" migrate towards the CTL cancelling its negative charge - or to look at it another way the electrons flow into the aluminium substrate and down to ground.

Exposure to light controls the charge.

The OPC is recognisable - usually a green, yellow or brown coloured drum. All OPC materials share several problems:
They are quite soft and can be easily scratched or chemically damaged by grease from finger-prints. Marks and grease would reduce effectiveness even if they did little chemical damage. Protecting the OPC from handling is one reason for enclosing the assembly in a cartridge.
OPC surfaces become progressively less sensitive over time, and so the copies begin to fade. OPC drums should be protected in an opaque black or metalised anti-static bag when not in use. For some materials degradation due to exposure can apparently be compensated for by polishing the surface of the CTL. Cartridges usually enclose the OPC in a box, with a shutter that closes and seals out light when it is out of the machine. 
OPCs don't have a particularly good linear response -brighter light isn't automatically reflected in losing just a bit of toner. Only recently have machines started to give a grey-scale. Modulating the level of the laser light should change the voltage in such a way that the amount of toner picked up is closely controlled. Photographic and colour reproduction are much easier if the colour levels of pixels can be controlled.

Drums are raster scanned - generally with alpha-numeric text


Mechanical Support. Laser Printer Overview & Index - Photoconductor

The photo-conductor needs some sort of conductive support and must be precisely positioned with respect to the optics, charge devices and developer. There are two common ways to provide this:
An aluminium or steel drum gives a very stable precision surface. The problem with a drum is that it gives a small surface area in a given space.
A belt made of metal or metallised plastic can give a large surface area, but needs a fairly elaborate set of rollers, tensioners and supports for accurate positioning.

The drum or belt has to rotate, but the motor driving it needs to position the image precisely. The cog-chain driving it should have little or no slack or the positioning accuracy will be poor. A resolution of 300 dpi implies significantly less than 0.08 mm jitter in the drive chain, recent 1200 dpi machines have to be better.

Most practical drums are part of a plastic carrier even if they are not bound up in a cartridge. Touching the drum spoils its qualities. The carrier often holds auxiliary components like the corona wires and sensors.

Most printers use one or more OPC drums. Drums are robust and pack into a small space. Mono printers generally have a single drum. Colour printers might have a drum for each colour then a belt, or toner-developers that rotate in a carousel over a drum followed by a belt or drum for transfer.

Belts are more complicated. The belt needs a conductive backing so it has a metal foil element as well as the OPC. The belt needs to be flat, so it is usually under spring tension. The belt will be carrying a static charge, so it is often supported only by rollers at the end.

Belts have a tendency to slip on their drive rollers. To get a belt position correct for a sequence of four colour images the belt usually has several opto-sensors and marks.

In earlier printers and copiers belts were often used rather than drums, perhaps to give a bigger surface area and longer life. In recent colour printers belts have been used as intermediate stores for the successive CMYK toner images - drums make the printer rather larger.

There is usually a brush made of carbon fibre or conductive foam to carry the electric charges released by the laser away from the drum or belt surface. Charges cannot reliably be carried through metal bearings because the lubricant in the bearing is an insulating layer. Contact brushes often look unimpressive, little bits of copper, tin-plate or conductive plastic but if they are contaminated with dirt they create imaging faults that are very difficult to diagnose. Perfectly good consumables are sometimes thrown away before they need be because the brushes need cleaning.


Charge Wires and Rollers.  Laser Printer Overview & Index - Photoconductor

Drum charge is usually negative and around minus 600 volts. The job of charging the surface used to be done by "Corotron" or corona wires. In recent times a contact roller is often used - in principle a carbon-fibre brush or tinsel might do the job.


 
Corotrons.  Laser Printer Overview & Index - PhotoconductorCorotron - Metal tray with a Charge Wire

The corotron is a fine wire carrying a voltage, sometimes moderated by a grid. The voltage is high - and the drum charge is usually negative. Charges are naturally on the surface of a conductor and where they meet a sharp point they tend to repel and stream off, so a fine wire charged to a high voltage is emitting a charge field.

One side of the corona enclosure is open and the others are a metal box. There are contours of charge between the wire and the metal, but at the open side the charge field reaches out and induces a field on whatever comes close.

Corotoron wires are often charged to about 3,000 volts or higher. The grid is usually regulated by a zener diode and controls how much charge arrives at the drum - as suggested around 600 is typical.

The corona wires are housed in a long tin tray with plastic endpieces, one containing a high voltage plug.

The wires are hair-thin,  usually in the centre of their tray and held in place by grooves in the plastic support. To keep the wire taught it is held under tension by a spring. The wires don’t wear out much but they do lose performance when they get dirty, and they attract dust out of the air because of their charge. Corona wires are so low-cost to make that they might be treated as disposable parts of a cartridge. On the other hand break a fixed wire and you could find you need an expensive spare.

Corona wires have become much less popular in the last few years.

Since they carry a high charge the corona assemblies attract dust, and this impairs their operation because it causes the charge to leak. Vertical streaks on laser print are usually caused by nodules of dirt or trapped hairs on corona wires. Corona wires and trays can be cleaned with a cotton bud.

Corotron - Section View


 
Ozone.  Laser Printer Overview & Index - PhotoconductorLaser Printers & Health

Another problem is that high voltages in the corona wires break the O2 bond on oxygen atoms in the atmosphere, which reforms making ozone (O3). The smell of ozone often wafts from photocopiers and laser printers when they are busy. Contrary to popular opinion, ozone is poisonous and carcinogenic. To reduce the risk of ozone pollution manufacturers generally fit activated carbon filters to machines. Unfortunately the carbon filters are often only changeable by engineers, who are pressed for time and may not bother. Filters also get filthy with atmospheric dust and toner attracted by the high voltages used by the corona wires and this can reduce their effectiveness prematurely. Users can be exposed to an unnecessary hazard if the ozone filter in a printer does not work. A smelly laser printer or photocopier is a health risk, and the machines should only be used in a well ventilated room!

Reducing the voltage below a certain point greatly reduces ozone production - newer designs try to achieve this. The limiting factor is that the electrostatic attraction between drum and toner still has to be sufficient to hold a pattern - or ideally a pattern with a greyscale.

 
Rollers.  Laser Printer Overview & Index - PhotoconductorCharge Roller - usually conductive foam

Many recent printers use charged conductive rubber rollers for the transfer voltage. This technique was used by Kyocera on the FS1500 both for transfer and waste toner pickup and is now common across most laser printers. Rollers make close contact so they allow the voltage used to be reduced significantly. Reducing the voltage avoids ozone production and reduces the design problems of distributing high voltages around the machine. Conductive rollers may not look as precise as corona wire due to the pore size in the foam-rubber, and their effectiveness can be impaired if they are dirty, scored, or greasy from fingerprints.

Measuring the high voltages can be a problem. Most standard test bench meters will be damaged if exposed to the voltages used and high-voltage resistors are needed to make a voltage reducer. The voltages are also intended to be delivered at very low current. If the voltages are shorted it is quite likely that the HT inverter circuits will be damaged.

HT Voltages.  Laser Printer Overview & Index - Photoconductor

A typical laser printer will make three or four High Tension voltages (HT). One to charge the drum, another to charge the developer, and a third for the transfer of the toner to paper. Voltages are generally made by several separate inverter circuits, then distributed around the machine by wires, strips of metal and springs. The contacts between different parts of the laser printer body and its consumables often look thoroughly unimpressive, they are nothing but tin strips. These strips get covered in toner or tarnish; they then make poor contact and voltages leak away. Periodic cleaning of the inside of the printer can improve print quality by preserving the voltages.

HT failure used to be fairly common but it no longer is. Reducing the voltages may have made it easier to build reliable inverters and transformers for the job.

Photoconductor Life.  Laser Printer Overview & Index - Photoconductor

The limitations on sensitivity mentioned above mean that in most laser printers the drum has a life of 5,000 - 50,000 copies after which it will be disposed of - or possibly recycled.

The life-limit on OPC seems to come partly from surface area - a bigger drum lasts longer  - and partly from contamination - perhaps oxidation. Other factors are exposure to light which exhausts the material and abrasion which may be good or bad.

One way to extend the life of the OPC material seems to be to make it rather thicker than necessary and then polish it. Many printers take this approach to some extent – the cleaning blade that primarily removes surplus toner abrades the OPC surface and the material is disposed in the waste toner. Some older photocopier designs seem to have arranged scraper blades to quite deliberately cut away at the spoiled surface of the drum (presumably when it was selenium) and this apparently gave it a longer life.

There is some manufacturing interest in giving OPCs a much longer life by providing a coating of diamond-like carbon (DLC) generated by a plasma in a hydrocarbon atmosphere. Problems are to:

  • retain the static carrying accuracy and transparency of the CTL
  • avoiding the hard DLC separating from the underlying soft OPC material


How much interest printer manufacturers will show in these developments remains to be seen. Manufacturers depend to some extent on a stream of cartridge sales and the trend seems to be towards smaller, cheaper cartridges that users don't notice buying rather than larger longer lasting designs.

Amorphous Silicon.  Laser Printer Overview & Index - Photoconductor

The Kyocera FS1500 uses an amorphous silicon drum (aSi). This material has the advantage that it is quite hard wearing -similar to glass - and does not degrade over time due to exposure to oxygen or light. On the other hand aSi is a brittle substance and manufacturing a perfect drum is quite difficult. The other slight problem is that to work effectively the drum has to be heated to about 70 centigrade – this is done by heater windings inside.

Obviously the choice of drum material has an impact on the polarity and voltage of the charge that needs to be applied to make up a satisfactory image. Insensitive photo-conductors will need higher voltages and a greater quanta of light. Leaky materials will lose some of the image definition – so might thick layers of material. Thin layers of material abraded by scraper blades and ceramic in the toner will have a short life.
 


Failure.  Laser Printer Overview & Index - Photoconductor

The most common causes of photoconductive drums and belts suffering premature failure are:
 
Scraper blade failure - if a printer is left idle for some time the scraper blade warps or perishes and no longer does its job properly. There may be a black mark horizontally across the page at the pitch of the drum. Prints look dirty.
Scratches - if a foreign object enters the printer it will quite likely score the OPC leaving a mark at the pitch of the drum.
Overexposure to light - being left out of the printer for any length of time. This leads to faint print and a dark background  with a series of faded bands down the page.
Bad contacts with the high voltage circuits can cause much the same sort of fault as overexposure  but probably without any banding down the page. Cleaning the contact pads may clear the problem. It is possible (but less usual) for the HT circuits themselves to decay and this will give similar symptoms. Outright failure of the HT will probably give a blank page or a very black page.
Marks vertically down the page - probably dirt or paper in the charging circuits - check that corotron wires are clean and/or look for damage on charge rollers.

The last two faults may be fixable without replacing the drum. In principle the scraper blade can be replaced as well - providing you don't mind opening up a toner-laden assembly.