If you've ever wondered how a laser printer gets all that ink onto the paper, then you've come to the right place. In this entry, both the theoretical and operational aspects of a 'modern' monochrome (black and white) laser printer are discussed.

Theory

All modern laser printers operate on the xerographic process initally developed by Xerox in the 1970s. This consists of exposing a charged, light-sensitive drum to a carefully controlled light beam. This can be done using a laser beam on the more expensive units, or using several rows of light-emitting diodes on the less expensive ones. The light affects the charge on the surface of the drum, which then attracts coloured particles of iron oxide coated in plastic (also known as toner) to itself. These particles are transferred to a sheet of paper by using another static electrical charge. The paper is then passed to a fixing or fusing unit that bonds the particles permanently to the paper using heat and pressure.

Practical

When you print a document, several things happen:

1. The control panel indicates that it is printing, and the logic circuits in the printer form what is termed a raster image (ie one made of pixels) which is fed to the laser or LEDs during the print process.

2. The fixing unit heats up to operating temperature, usually around 200-205°C (390-400°F). This can take a while, especially if the printer has been in power-save mode for some time.

3. The motors in the unit start up and rotate the drum. At this point, the drum is given a uniform charge, and any excess toner that might be on it is cleared. In addition, a fan usually turns on, and the laser motor spins up to operating speed. (This is the 'jet' 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 Print Process

1. The paper pick-up rollers activate and feed a single sheet into the printer. The printer 'knows' where the paper is from several sensors and a programmed idea of where the paper should be at any given time in the print cycle. The printer also starts putting an image on the drum at this point.

2. By the time the paper arrives, the drum already has part of the image on it, and begins to transfer the image to the paper.

3. 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.

4. By the time the trailing edge of the page has left the fuser, the leading edge is usually already out of the printer.

Problems

• Environment: The early models where somewhat sensitive to the relative humidity of the printer's environment, which affects print quality somewhat. Modern printers generally do not have this problem, except for some of the colour units, which are much more expensive than the black and white units.

• Expense: Although the price for most of the parts has come down, they are still (at the time of writng) a bit expensive. Hence, you normally only see them in a business office environment.

• Maintenance: Laser printers do require routine maintenance. 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.

Popular Brands

These are brands that a printer technician might recommend to people if asked. Any printer in the market today is fine for personal and occasional use, but specific printers are capable of delivering specific features for specific needs.

• Hewlett-Packard is the de facto standard for most businesses. They are widely available, relatively easy to fix, and quite reliable. They have a wide varity of models for a wide variety of uses, ranging from a home office to a departmental printer. They also have several models of colour laser printers on the market.

• Xerox are more known for their copy machines (which are based on the same process), but they do make a solid, quality printer. They are more oriented towards the business market, and have everything from a executive printer to a machine that can double as a departmental copier.

• Lexmark is a former division of IBM and has a wide range of printers aimed at the entire market.

• Okidata, known for its rugged dot matrix printers, makes a good, affordable laser printer based on LED technology. Instead of using a laser beam to create the image, their printers use two rows of densely placed light-emitting diodes above the imaging drum. This makes the printers less expensive to manufacture. Okidata is aimed more at home users and small businesses.