Laser printer work

For those curious how does a laser printer work, here’s a step by step of the process. The process is based on the principle like charges repel and opposite charges attract.

A laser printer works differently than ink printers or impact printers. Though all printers run on electricity, only a laser printer uses electrical charges to form the image.

What is a Laser Printer?

A laser printer is a printing device that uses an electrostatic process to form an image on paper out of toner.

While other printers use ink a laser printer uses toner. Other printers spray or press ink onto paper, whereas a laser printer uses electrical charges to attract toner onto the paper.

Laser printers and photocopiers use the same process to form an image on paper. Color and B & W printers use the same process, though it is duplicated for each color in any color printer.

The main steps of electrophotography:

  • Charge
  • Exposure
  • Develop
  • Transfer
  • Cleaning
  • Erase or conditioning
  • Fusing

Electrophotography was originally known as the Xerographic process since Xerox was the first to use it commercially. The original process used a lamp and mirrors to expose an original. This analog step has been upgraded to a digital process, typically involving a laser unit or occasionally a LED unit.

The main parts of a laser printer are:

  • Photoconductive drum
  • Toner
  • Developer unit (toner cartridge)
  • Fuser
laserjet enterprise m506

Charge

Many of the steps happen at the same time but charge is typically considered the starting point.

The laser process begins by applying an electrical charge to a photoconductive drum. By either a corona wire or a charge roller. Most high end color printers tend to use a corona wire.

The corona wire usually has a charge of 7000 VDC although this is very low amps. This makes the wire glow a purplish color. A scortron grid between the charge wire and drum regulates the charge applied to around 600-700 VDC.

A roller is in direct contact with the drum and applies a uniform charge of 600-700 VDC. A charge roller may get contaminated quicker than a wire. Any toner or paper dust build up can cause defects in the image.

The contacts for the various charges can be seen on many toner cartridges:

Exposure

In this step a class 1 laser will write an image on the photoconductive drum. The laser unit in a printer or copier works similar to the ones used in a laser light show.

A laser printer can be write to black or write to white. Meaning the laser can discharge the photoconductive drum to create a positive image or negative image depending on the manufacturer. Write to black is more common since it is less taxing on the laser unit.

The laser unit dictates whether a printer is 600 dpi (dots per inch) or 1200 dpi. Some high speed printers need 2 laser beams but most small printers only use one laser beam.

Laser Printer Works

Laser Unit

The laser beam or beams originate at the laser driver PCB (printed circuit board). Which is pointed at a polygon mirror rotating at 20,000 to 33,000 rpm (revolutions per minute) depending on the model.

Laser beam(s) turn on and off rapidly to write the image. The laser only writes in a horizontal plane one line at a time.

The photoconductive drum is rotating at a constant speed as the laser writes. This rotation combined with variances of the laser beam achieve writing in the vertical plane.

There’s a difference between the distance the laser beam travels to the center of the drum vs the edges of the drum. This difference is small but enough to require a corrective lens.

In order to save space the laser unit is mounted flat and uses a mirror to deflect the laser beam to the photoconductive drum below. The dichroic mirror ensures there is no light interference.

LED Unit

Some printers use a LED (light emitting diode) array instead of a laser. These units are smaller than a laser unit. They are as wide as the photoconductive drum yet are very thin.

They write an image one line at a time as the drum rotates as well.

Printer toner

Developer

In this step the image is developed into a visible image on the photoconductive drum. At this point on the drum the image is invisible electrical charges, aka a latent image. Whether the manufacturer decided to use a write to black or write to white laser unit, developing this image is the same.

As the drum rotates the latent image comes close enough to the developer roller that toner is attracted to it. Due to opposite charges attract and like charges repel, toner is only attracted to the image areas. While it’s repelled from the non image areas.

The developer roller serves several functions. It is magnetic, has a DC and an AC bias. The charges along with a regulating blade distribute a consistent, even layer of toner.

Once toner is introduced the image on the drum is visible. Although it is reversed, a mirror image. This image is held on by static electricity but can be easily wiped off the drum with a cloth.

The image is righted once transferred to the paper.

A more in depth breakdown of printer toner is covered elsewhere.

Transfer

In this step the developed image on the drum is transferred to the paper.

This works by bringing the paper into contact with the drum. Then an electrical charge is applied behind the paper. This attracts the toner from the drum and onto the paper.

The electrical charge is applied by another corona wire or transfer roller located behind the paper. Thicker paper requires an increased charge to get through the paper.

A final charge or discharge plate is used to assist the paper from separating with the drum.

Cleaning

While toner moves from the developer, to the drum, then the paper it does not transfer 100%. Some stray toner is left on the drum and needs cleaned off.

The residual toner is cleaned off with a blade or in some cases a brush. Depending on the model this small amount of toner is recycled back to the developer or sent to a waste toner container.

Also, during this step the drum is discharged with light to remove the latent image. This conditions the drum for the next charging.

Laser printer work

Fuser

Although the image is now on the paper, it is not permanent yet. In the final step the image is fused to the paper.

The fuser rollers work by applying heat and pressure which melts the toner into the paper. And that’s it. After passing through the fuser the process is complete and the paper is sent out of the printer.

Because of the heat an pressure the fuser is the most strained part in any laser printer. The fusers are designed to handle the stress and last 100,000 pages or more. The heavier weight paper it can handle the more durable the fuser.

Inkjet vs laser

Paper Handling

Paper starts in the cassette and takes a fairly straight path through the printer. It has to be synced up with the image on the drum before proceeding. This step also removes minor skew.

During a duplex operation the paper is flipped over so the second side can be printed on. The paper may partially exit the printer during this step.

Several of the steps are happening simultaneously to the drum as it rotates. While they can handle being charged and discharged tens of thousands of times, it only takes a few seconds of direct sunlight to ruin one. The photoreceptive drum is most commonly made from organic materials or Amorphous silicon. Years ago some were made with Cadmium Sulfide, which is toxic.

Toner is non-toxic and made from carbon. By itself it won’t hold a charge and needs a carrier. Iron filings was common but wax is quickly becoming the standard carrier. Because wax based toner is finer it can make better half-tones. More half-tones means a bigger color gamut. Wax based toner will also produce a semi-gloss appearance so fuser oil is no longer needed.

Print in Color

Up to this point how does a laser printer works applies to a B&W printer. A color laser printer has the same steps described above but multiplied for each color plus an intermediate transfer step.

Color printers typically have 4 drums, 4 developers, 4 charge units, 4 transfer units, and 4 cleaning units. The intermediate transfer belt transfers the developed image from each drum.

The timing of this step is critical in order to stack the 4 developed images into one image. Once that is accomplished the stacked image is transferred to the paper.

There are exceptions to the color process. Some print engines get by with only one photoconductive drum.


If you’ve ever wondered how does a laser printer work, hopefully this has provided an explanation.

Whether it’s at 10 pages per minute or 150 pages per minute those are the basics of how a laser printer works. If you think a laser printer is right for you check out the laser printer buying guide.

Laser Printer Works
copier guy

The Copier Guy, aka Dave. I’ve worked on scanners, printers, copiers, and faxes over 26 years. When I’m not fixing them I’m writing about them. Although, I’m probably better at fixing them. I have certificates from Canon, Xerox. Ricoh, Kyocera, Lexmark, HP, and Konica Minolta. My experience includes other brands as well as several types of processes. If it uses paper I’ve probably worked on one.

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