more closely represents the specific process performed in a laser printer, as opposed to
those carried out in photocopiers.
The HP print process also consists of six stages:
- Conditioning The drum is conditioned with a uniform negative charge
by a charge roller. - Writing The laser and mirrors are used to reduce the charge on the areas
of the drum that form the document to be printed. In the HP process, the
drum is discharged to ground in selected areas. In the EP process, the drum
is discharged to only –100 VDC. - Developing The drum rotates past the developing roller and toner is attracted
to its discharged areas. The drum now has toner stuck to it where the laser has
created the mirror image of the document to be printed. - Transfer A strong positive charge is applied to the corona wire and the
toner is pulled onto a sheet of paper. - Fusing The fusing rollers melt the toner onto the paper using a high
temperature (350 degrees Fahrenheit) halogen lamp and a pressure roller. - Cleaning A rubber blade inside the toner cartridge removes any toner left on
the drum into a used toner receptacle inside the cartridge. A bank of LEDs are
used to clear the drum, and the cycle starts over.
LED Printing
An LED printer replaces the laser and mirrors used to discharge the print drum with a
bank of light-emitting diodes (LEDs). The number of LEDs used is directly related to the
dots-per-inch capability of the printer. LEDs are physically very small and produce a
very bright light, which allows the printer to use one LED for each of the dots to be dis-
charged on the drum to form the image of the document. A printer rated at 600 dots per
inch (dpi) has 600 LEDs in each inch of its light source. As the drum rotates past the light
source, the LEDs are used to discharge the dots that form a single line of the image.
LCD Printing
The phases used in LCD printing are the same as the HP process except that in the De-
veloping phase light passing through an LCD (liquid crystal display) panel is used to
discharge the drum to receive toner. These printers are also called LCD shutter printers
because of the way liquid crystal elements work.
Liquid crystal is neither a liquid or a solid; it exists somewhere between those two states.
Each piece of the crystal is a long, thin shaft that has a twist between 90 to 270 degrees. When
electricity is applied to the crystal, it untwists in a predictable way. In its uncharged state,
liquid crystals do not block any light passing through its panel. However, after electricity
is applied and the crystal untwists, the crystal will not block the light. The amount of elec-
tricity applied determines the amount of light it blocks.
Chapter 17: Printers^427