Printer and Photocopier Troubleshooting and Repair Collection : Laser printer and photocopier operation 
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Laser printer and photocopier operation

Copiers and laser printers have a lot in common. The major difference is in how the image is formed on a photosensitive drum:

Beyond this, copiers and laser printers are nearly identical (at least in principle) except that copiers use a positive process (dark areas in the original result in marks on the paper) and laser printers commonly use a negative process (a spot of light results in a dark mark on the paper).

The most sophisticated machines are now actually scanner-laser printer combinations with buffer memory so that multiple copies can be made without rescanning the original, sorting and collating is more flexible, scaling and rotation can be done digitally, and other features not possible with simple copiers.

(Portions from: Copenhagen Cowboy (cowboy@fastlane.net).)

The photosensitive drum is the heart of the laser printer or copier. In larger machines, it may be a separately replaceable unit. In most laser printers and smaller copiers, it is part of the 'toner cartridge' and is a throw-away (or may be recycled).

The drum is coated with a photosensitive material which has an extremely high resistance when in darkness. It's resistance drops to a low value when illuminated.

All of the following takes place as a continuous process as the drum rotates. Note that the actual photosensitive drum in most copiers and laser printers has a circumference that is much smaller than the length of the printed page. Therefore, only a portion fits at any given time and the charging, exposure, transfer to the paper, cleaning, and erasing is a continuous process:

That is the basic process. Many variations are possible and depending upon the machine and manufacturer, some of this may be a little different. Where a (disposable) toner cartridge is used, many of these components are replaced with the cartridge - typically the drum, toner itself and developer (usually combined into a single powder), developer magnet (really neat!), cleaning blades, some of the corona wires.

There is also some photocopier information at:

Laser printer operation summary

(Portions from: Zaki (zg@ix.netcom.com).)

In general the principle of electrostatic laser printing is as follows:

  1. Charging a photoconductive selenium (or other) coated drum.

  2. Discharging the drum with the laser steering engine in accordance with the input image rasterized pattern. (the laser is modulated to generate a predefined pixel pattern on the face of the drum - the focal plane).

  3. The rotating drum attracts toner to the charged pattern (latent image) generated by the laser.

  4. The toner is transfered from the drum to the moving papaer to generate a full image.

  5. The paper carrying the toner moves through the heater to fuse the toner to a fine non-erasable image.

The laser steering engine is combined of the following components:

Cleaning and Handling of Photosensitive Drums

Where the drum is located inside a replaceable toner cartridge, there is no need for special handling. However, where the drum is a separate unit, the following applies. Or, if for some reason, you need to disassemble (gasp!) a cartridge:

(From: David Kuhajda (dkuhajda@locl.net).)

Whatever you do, do NOT use alcohol on an organically based drum, it will ruin it. The alcohol causes the material to crystalize. I use to do copier service and this was stressed a lot by the manufacture as they switched from the old selenium drums to the new opc drums. Direct sunlight will immediately destroy the drum. A couple of minutes under normal lighting is no problem, just place it in a dark area and put a black cloth over the top of the drum while it is out. If you are replacing the drum cleaning blade or cleaning the crud off the blade, make sure you powder up the drum completely and the blade before reapplying power. The toner actually is a slight lubricant and the rubber cleaning blade directly on the drum will also ruin it. Just print a few low text copies after reassembling to allow the blade to reseat properly.

(From: hapticz@email.msn.com.)

Short periods (less than 5 min) under fluorescent lighting is safe.

Direct sunlight kills them immediately.

Just have a clean brown paper bag to shove it into while it sits on the table outside the machine.

Often more damage is done to them physically during insertion/removal. just be careful.

Xerox used to clean the 10" diameter drums with 90% isopropyl alcohol and some kind of "Kim Wipes" in our office, that was years ago though.

Book on laser printer maintenance and rapair

(From: Michael (ncacaver@aol.com).)

Get the book: "Easy Laser Printer Maintenance and Repair by Stephen J. Bigelow".

Your local library should have it or be able to get it. Stephen J. Bigelow has several other books on printer repair, both laser and non laser types. All are very good.

Discussion on laser diodes in laser printers

"I just acquired the optics from a dead laser printer and have been trying to understand it. There are two functions I have yet to grasp. One is something which it has but for which I see no need. There seems to be a heater (Contains mica) and a thermometer, with PCB markings like "T1" and H2" or something similar. If these the laser is temperature controlled, why? There seems to be a control photodetector to monitor the laser diode so temperature control appears like overkill unless the photodiode itself has too much temperature dependence and the drum exposure is very critical."

(From: Jonathan M. Elson (jmelson@artsci.wustl.edu).)

There is a heater inside the fuser roller. This is what melts the toner into the paper. It is thermostatically controlled, and then has a safety thermostat in case the control fails.

There are two photodetectors for the laser. One compensates for dimming of the laser over years of use, the other picks up the beam at a particular angle of the polygon mirror, and synchronizes the raster electronics to the polygon rotation.

"The other thing is something I cannot find, the aperture defining the nice well-formed pixel. So far I must admit the study has been a bit superficial but the aperture ought to be pretty obvious if there is one!"

The laser is the aperture. With an optical path of 0.5 m or so, the laser is a pretty good approximation of a true point source. A simple lens makes it look like a very good point source.

"Finally, how are the correction lens made? They look like slices out of the middle of some fair sized lenses, but that would be a very wasteful way to make them. Can they be diamond formed to nearly the final shape and with such good finish so only a simple polish completes them. Grinding the old-fashioned way on a sliver of glass looks doomed to generating all sorts of defective approximations to a sphere. (As far as I can tell they are glass, or some wonderfully hard plastic I would like to know more about!) Can they be molded to sufficient precision? (The sides are ground or sawn.) Thanks to anyone who can bring me up to date on lens fabrication technique."

I think they mold these lenses to near correct shape, then grind and polish to the desired aspheric shape with specialty machines for that purpose. (Note that almost all eyeglasses are aspheric for astigmatism correction.) Yes, these lenses are glass, I've had a few printers apart myself.

Types of toner

(From: Lionel Wagner (ck508@FreeNet.Carleton.CA).)

There are two basic kinds of toner: magnetic and non-magnetic. If your laser printer has a Cannon 'engine' it most likely uses magnetic. NEVER use the wrong type. The imaging process is extremely delicate and specific toners are important. Use of toner that is slightly different could result in all black or all white copies.

So you put in the wrong type of toner?

"I have a 3M Model 6312 copier. I believe it is a re-badged Lanier. I didn't pay much for it but it worked well. When the toner warning light came on, I made the mistake of adding the wrong kind of toner. I removed the wrong toner as much as I could by vacuum. Is there anything I should do before adding the right type of toner? Did I do serious damage to the system? What to do if the warning light remained on even with the right type of toner added? Any suggestion will be greatly appreciated."

(From: Lionel Wagner (ck508@FreeNet.Carleton.CA).)

If your copier uses non-magnetic toner, it is mixed with iron powder, called the Developer. Both have to be removed and all residue vacuumed out. If the copier uses magnetic toner, less of it will remain in the machine. Try to get as much as possible out. Do not scratch the roller on which the toner sits.

WARNING: See the section: Warnings about vacuuming laser printer toner before using a household vacuum cleaner to do this!

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