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The heart of a laser printer is the photoconductor, which carries a charge on its surface in the dark but loses it where the surface is exposed to light. Most photoconductors are made of plastics and known as "OPCs" - organic photoconductors. The name is a contrast with an older generation of semiconductor metal photoconductors mainly used in photocopiers.
The OPC itself is usually a coating on a drum or belt. Drums tend to be prefered because they are more mechanically stable and the process needs parts positioned to fine tollerances to work.
The laser scanner (or sometimes an LED array) produces a "latent image" on the OPC. The OPC surface then travels to a developer, which allows a layer of toner powder to migrate to the OPC wherever the charge is the correct polarity.
Laser printer drums usually turn several times to produce one page. As the drum passes the transfer station the toner image is pulled onto the paper – or at least most of it is. In practice the transfer is rarely complete.
The cleaning mechanism removes any residual toner from the surface of the photo-conductor.
The cleaning mechanism usually has three main parts
- Eraser lamps – a row of LEDs that bathe a horizontal strip of the drum in light to remove any static charge
- Scraper blade that forces any toner off the drum
- Waste transport mechanism that removes the waste to a storage place
Eraser Lamps.
The eraser lamps are usually nothing more than Red LEDs mounted on a board – usually about one per inch of drum width. Bathing the OPC material in light releases it's static charge. The OPC backing material in turn needs to be discharged - this is usually achieved by conductive foam or carbon-fibre brushes.
Blades.
The scraper blade usually seems to be made of a fairly stiff transparent silicon-rubber. A stiff mylar might seem more appropriate to the scraper function – perhaps it is too abrasive. The idea seems to be that the relatively soft "conformant" material makes better contact.
The LEDs might ideally be mounted so that they illuminate the exact area at the tip of the blade - some printer designs do seem to be aiming at this
The combination of light and scraping action separates waste toner from the OPC and leaves it in a state ready for the next image.
The blade is usually the only point of contact that resists the drum turning. If you push the drum round with finger pressure the blade applies a surprising amount of pressure.
Large and fast printers sometimes use a rotating brush to polish the waste off the drum. This is probably more effective with a fast turning drum and less abrasive inproving drum life.
Waste Transport.
The waste transport mechanism often seems to be a plastic "archimedes" screw-feed. The screw-feed extends across the page and runs relatively slowly - the volume of material to be moved is very small. Some printer cartridges just dump the waste into a compartment at the front - which emoves the need for any transport.
Waste Bottle.
The total amount of waste produced seems to vary with printer designs, but to be 10 – 20% of the toner input. In single cartridge printer designs the waste is just packed at one end and is thrown away with the cartridge. In some printers the waste storage bottle is separate.
If the waste storage bottle is separate there is often some device to sense when it is full. The Kyocera FS1500 series count how many copies have been done since the last bottle was changed – they sense this using a silver label which reflects light onto an opto-detector when a new bottle is put in. Photocopiers often shine an optical sensor across the top of the bottle and when it is blocked they report that the bottle is full. This latter approach which actually measures a critical state seems rather more sensible.
Waste Re-Use.
Photo-copier users sometimes tip the waste toner back into the toner supply hopper. This practice is more difficult with laser printers because there are usually interlocks on the cartridges that make the toner difficult to get at. Even if it is possible to get to the toner never pour waste toner back into a copier or printer.
- The composition of waste toner will have been degraded.
- The waste scraper blade and screw compress the material and produces lumps.
- Waste toner has been exposed to oxidation which may change its chemical composition,
- There will be dust from the OPC polishing process, kaolin from the paper and broken down filings from the developer present in the waste
Some laser printers actually demand consumables after a given page-count and trying to refill hoppers will not change the count – it will just degrade the image quality.
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© Graham Huskinson 2010
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