Laser printers

Laser printers

Remember Laser printers are VIT (very important technology). Have a good understanding of general laser printer operations and the six steps of the laser printer's printing process.

Laser printers are considered page printers because they form and print all the text and graphics for one full sheet or page at a time. Three different printing processes are used in laser printers, each directly attributable to one or more manufacturer(s): EP, HP, and LED.

l Electrophotographic (EP) process: This process, which was the first laser printer technology, was developed by Xerox and Canon and is the technology used by all laser printers in one form or another. It uses a laser beam to produce an electrostatic charge and a dry toner to create the "printed" image.

l Hewlett-Packard (HP) process: The HP process is essentially the same as the EP process, with the exception of some minor operating procedures. It's similar enough to be considered the same process, yet different enough to get its own name.

l Light-emitting diode (LED) process: From outward appearances, you can't distinguish an LED printer from a laser printer. The difference boils down to the fact that an LED printer uses an array of about 2,500 light-emitting diodes instead of a laser to produce an electrostatic charge. An LED printer with a 600 dpi resolution has 600 LEDs per inch.

Inside the laser printer

Laser printers use toner to create the image on the printed page. Toner is a dry powder that consists of iron particles coated with a plastic resin that bonds to the paper during the print process. Toner is supplied to the printer in a removable cartridge that also contains many of the most important parts used in the printing process. The toner cartridge contains the photosensitive drum (a mechanism used to place a charge on the drum), a roller to develop the final image on the page, and, of course, the toner.

Plan on understanding just about everything there is to know about laser printers for the test, including the following list of major components. Expect questions about the overall laser printing process, as well as the role played by key components.

In addition to the toner cartridge, eight standard assemblies exist in a laser printer. These assemblies are

l The drum: The drum inside the toner cartridge is photosensitive, which means it reacts to light. The drum holds an electrostatic charge (except where it is exposed to light). The laser beam is reflected onto the surface of the drum to create a pattern of charged and not-so-charged spots, representing the image of the page to be printed.

l High-voltage power supply: The EP process uses very high voltage to charge the drum and transfer and hold the toner on the paper. The high-voltage power supply converts AC current into the higher voltages used by the printer.

l DC power supply: Like a computer, most of the electronic components in the laser printer use direct current. For example, logic circuits use +/–5V DC (volts direct current), and the paper transport motors use +24V DC. Also, like the computer's power supply, the laser printer DC power supply also contains the cooling system fan.

l Paper transport: Inside the laser printer are four types of rollers that move the paper through the printer. Each rubberized roller or set of rollers is driven by its own motor. The four roller types in the paper transport system are the feed roller (or the paper pickup roller), the registration roller, the fuser roller, and the exit roller.

If you're asked where most paper jams occur in a laser printer, the answer is the paper transport area.

l Primary corona: Also called the main corona or the primary grid, this device forms an electrical field that uniformly charges the photosensitive drum to –600V as a way to reset it prior to receiving the print image and toner.

l Transfer corona: This mechanism moves a page image from the drum to the paper. I cover how this happens a little later, but for now, know that the transfer corona charges the paper and the charge pulls the toner from the drum onto the paper. As the paper exits the transfer corona, a static charge eliminator strip reduces the charge on the paper so that it won't stick to the drum. Not all printers use a transfer corona; some use a transfer roller instead. When working on a printer with a transfer roller, be careful not to touch the roller with your bare hand or arm. The oils from your skin can spot the transfer roller and cause improperly charged paper, which shows up as defects in the printed image.

l Fusing rollers: The toner is melted permanently to the page by the fusing rollers that apply pressure and heat (between 165 and 180 degrees Celsius) to it. The fuser--not the laser--is what makes the printed pages hot.

l Controller: This is the motherboard of the laser printer, and it has similar architecture and components of a PC motherboard. The controller communicates with the PC, houses the memory in the printer, and forms the image printed on the page. Memory expansion is possible on virtually all laser printers. Adding memory allows the printer to reproduce larger documents or graphics in higher resolutions or to support additional soft fonts.

Tip A printer that experiences frequent memory overflow errors has a bad memory board, a memory board that was installed incorrectly, or a memory board that needs additional memory. Diagnose this problem by eliminating these conditions in this order.