Laser printer and photocopier operation

• A copier uses a bright light and lens to focus an image of the original (actually, a strip at a time which is scanned in most modern low to medium performance copiers) onto the drum. Adjusting the lens-to-original and lens-to-drum distance is used to vary the reduction or magnification.

• A laser printer uses a low power sharply focused laser beam to scan one line at a time on the drum. Modern laser printers use infra-red solid state laserdiodes similar to those used in CD players and optical disk drives while older ones used helium neon lasers.

  The digital image is generated from a bit map stored in the printer's memory and modulates the laser beam. Scanning is mechanical - a high speed motor spins a multifaceted deflection mirror to get the X-axis and the paper moves to get the Y axis.

  LED printers use a large array of LEDs as the image source but are otherwise similar to laser printers.

  Plain paper fax machines use similar techniques in their printing mechanism.

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:

• The drum's surface is charged to a high positive voltage (typically 5 to 6 kV) by a set of charging corona wires in close proximity to the drum.

• The exposure process differs for copiers and laser printers:
  • For copiers, a swath of the original is focused onto the drum. As the drum turns, a quartz lamp and strip mirror moves along the original and second strip turning mirror moves at half this speed. The result is that the entire original's image is kind of 'peeled' onto the rotating drum. (Look through the glass platform that supports the original of a copier as it is copying and you will see what I mean.)

  • For laser printers, the negative image of the page stored in the printer's buffer memory (the laser is turned on where the print is to be black) is read out and scanned onto the drum one line (i.e., 1/300th or 1/600th of an inch) at a time.

  Where the light hits the drum's surface, its resistance drops dramatically and the charge in these areas is dissipated.

  At this point, a swath of the image of your ultimate copied or printed page resides as areas of electrostatic charge on the drum. This is a 'latent' image and must be 'developed'.

• As the drum continues to turn, the latent image rotates past the 'developer unit' which contains a mixture of developer and toner. For the most part, developer is not really used up during the printing process but some is lost and may need to be replenished from time-to-time (depends on design).
  • Developer is a material which includes powdered iron or other powder which is attracted by a magnet.

  • Toner is the actual 'ink' and consists of very finely powdered thermo plastic particles. These are 'fixed' in the fuser by literally melting the image onto the paper.

  Depending on design, the developer material may be separate or actually combined with the toner.

  A magnet in the developer unit which is as long as the page is wide causes the developer along with trapped toner to stand out following its lines of force off of its long N-S pole pieces. This forms a kind of brush of toner and developer material which is in contact with the drum as it rotates with its latent image. Normally, the developer material brush is C-shaped, and toner particles are carried in the C-shape (the back of the 'C' is against the drum).

  Here is where the developing processes of copiers and laser printers differ:

  • For copiers, the relative charges of the drum and toner are set up so that toner is drawn to the unexposed (dark parts of the original) portions of the drum resulting in a positive image on the paper.

  • For laser printers, the relative charges of the drum and toner are set up so that toner is drawn to the exposed (where the laser beam was turned on) portions of the drum resulting in a negative image on the paper.

• The drum continues to rotate around and comes in contact with the paper.

  Below the paper is another corona, the 'transfer corona'. Another high voltage is applied to the back of the paper (once again, around 7 or 8 kV DC) to draw the toner from the drum to the paper. (Remember, all this is going on in a continual cycle and it is all in motion).

• Depending on the manufacturer of the machine, you may or may not have a third corona, the 'separation corona'. This is needed to separate the paper from the drum, but not disturb the toner on the paper (the separation corona is usually 4 or 5 kV AC (if it was DC, you would separate the paper, but have VERY smeared toner all over the page as to make it unreadable). The separation corona usually has guides over it to keep the paper from 'dipping' down too far into the corona shell.

• Paper is then transported to the fuser which 'fixes' the toner to the paper via heat (to soften the toner particles) and pressure (to embed them in the paper fiber). There are parts in the fuser which also keep the paper from sticking to the hot rollers. A thermostatically controlled quartz tube lamp provides the heat inside the anti-stick (Teflon coated) fuser roller.

• Finally, your copy or printed page is ready!

• However, we are not done as there is still some toner on the drum - it is not possible to get it all off electrically) so there is usually a rubber or plastic blade which rubs in direct contact with the drum. This 'drum blade' scrapes the toner off the drum, and the 'recovery blade' catches it to keep it from falling back into the machine. A 'used toner auger' transports the used toner (which is now changed both physically and electrically and is also contaminated with paper dust (don't reuse your used toner) because it can eventually damage the developer unit, cleaning blades, fuser sections and other parts of the mechanism.

• Now that all the toner has been scraped off the drum, there is still some residual charge on the drum from the previous exposure process. You can't scrape the static charge off the drum, so the cleaned drum is now fully exposed to a bright light to discharge the drum surface and prepare it again for a new charge, which comes right after the discharge lamps.

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:

• Diverse Devices

Laser printer operation summary

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:

• Infra-red diode laser, 3 to 4 mW in basic units, up to 30 mW or more for high performance printers.

• Beam expander to form the required size of the collimated input beam which generates the beam spot size in the focal plan.

• Cylindrical lens to reshape the laser elliptical beam to a round one.

• Spinning polygonal mirror to deflect the laser over the focal plan.

• F-Theta lens to flatten the inherent circular plan of a rotating mirror. This lens is a very special lens which only few in the optical community know how to design and fabricate. The one that you own is particularly special because it is a Sectioned F-Theta lens which are typicaly more expensive (most of them are spherical).

  If you need to scan or to print in high resolution 500 dpi or higher, you end up using a glass F-Theta lens.

Cleaning and Handling of Photosensitive Drums

(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

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

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!