Printing Secrets from Print Acumen

Laser Printing

Every office has one, but how do they work?

Ever wondered how a laser printer gets the ink to stick onto the paper?

This article explains the theoretical and operational aspects of a monochrome (black and white) laser printers.

The Theory Behind It.

Laser printers operate on the xerographic process initially developed by Xerox in the 1970’s. This process consists of exposing a charged, light-sensitive drum to a carefully controlled light beam.

On the more expensive printers this is usually done using a laser beam, and on the less expensive units using several rows of light-emitting diodes. The light affects the charge on the drum, which then attracts the toner (coloured particles of iron oxide coated in plastic) to itself.

The toner is then transferred to a sheet of paper by using another static electrical charge then bonded it to the paper by using heat & pressure by passing it through a fixing or fusing unit.

The Practical

When you print something, several things happen:

The control panel indicates that it is printing, and the logic circuits in the printer creates a raster image (an image made of pixels) which is fed to the laser or LEDs during the print process.
The fixing unit heats up to operating temperature, usually around 200-205°C.

The motors in the unit start up and rotate the drum. At this point, the drum is given a uniform charge and excess toner is cleared. In addition, a fan usually turns on, and the laser motor spins up to operating speed. (This is the 'whirring' noise you may hear.) LED-based printers do not have laser motors, but you will still hear the noise from the drive motors and such.

The Printing Process

The feeding mechanism feeds a single sheet into the printer. Using sensors, the machine 'knows' where the paper is at any given time in the print cycle. The machine also starts laying an image on the drum at this point.
When the paper arrives, the drum already has part of the image on it, and begins to transfer the image to the paper.
By the time the image is fully transferred on the paper, the leading edge of the paper is usually in the fusing unit, where the image is fused to the paper using a combination of heat and pressure.
By the time the trailing edge of the page has left the fuser, the leading edge is usually already out of the machine.

Associated Problems

Environment: The early models where somewhat sensitive to the relative humidity of the printer's environment, which affects print quality. Modern printers generally do not have this problem, except for some of the colour units.
Expense: Although the price for parts has reduced somewhat, they are still relatively expensive so laser printers are generally used in a business office environment and not a home environment.
Maintenance: Laser printers do require routine maintenance following a set quantity if ‘clicks’ or impressions. They keep track of this via a page count, and the more expensive ones will let you know when they need it.
Mess: Sometimes the toner leaks out of the cartridge, and this can be very messy. If the toner damages the printer, it can also be very expensive to clean and repair.

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