Component and Function

laser printer

A laser printer is a popular type of personal computer printer that uses a non-impact (keys don't strike the paper), photocopier technology. When a document is sent to the printer, a laser beam "draws" the document on a selenium-coated drum using electrical charges. After the drum is charged, it is rolled in toner, a dry powder type of ink. The toner adheres to the charged image on the drum. The toner is transferred onto a piece of paper and fused to the paper with heat and pressure. After the document is printed, the electrical charge is removed from the drum and the excess toner is collected. Most laser printers print only in monochrome. A color laser printer is up to 10 times more expensive than a monochrome laser printer.

IBM introduced the first laser printer in 1975 for use with its mainframe computers. In 1984, Hewlett-Packard revolutionized laser-printing technology with its first LaserJet, a compact, fast, and reliable printer that personal computer users could afford. Since then, laser printers have decreased further in price and increased in quality. Hewlett Packard continues to be the leading manufacturer with competitors including Lexmark, Okidata, and Xerox.

The laser printer is different from an inkjet printer in a number of ways. The toner or ink in a laser printer is dry. In an inkjet, it is wet. Over time, an inkjet printer is about ten times more expensive to operate than a laser printer because ink needs replenishing more frequently. The printed paper from an inkjet printer will smear if wet, but a laser-printed document will not. Both types of printer operate quietly and allow fonts to be added by using 

2.Laser Printer Components:

A laser printer typically consists of 8 Field Replaceable:

1)Toner Cartridge

2)Laser Scanning Assembly

3)High Voltage Power Supply

4)DC Power Supply

5)Paper Transport Assembly

6)Transfer Corona Assembly

7)Fuser Assembly

8)Formatter Board

1) Toner Cartridge-A toner cartridge comprises of a toner, print drum, charge corona wire and the

cleaning blade. Toner consists of a carbon substance mixed with iron oxide and polyester resins.

2) Laser Scanning Assembly-A laser reflects off of a mirror onto the drum and discharges the

drum. Once the drum is discharged (to about -100 VDC), toner will stick to it.

3) High Voltage Power Supply (HVPS)-This converts the input AC voltage to DC and supplies

high voltages (-600 VDC) to the Charge corona wire and Transfer corona wire.

4) DC Power Supply (DCPS)-This applies +5 and -5 VDC to the formatter board and +24 VDC to

the paper transport motors.

5) Paper Transport Assembly-This is responsible for moving the paper through the printer. It

comprises of a motor and rollers. The pickup rollers pick up a single sheet of paper from the paper

tray, whereas the registration rollers synchronize the paper movement with the image on the print

drum.

6) Transfer Corona Assembly-This is either a wire or roller just beneath the printer cartridge,

which applies a high positive charge to the paper as it is pulled through the printer. The positively

charged paper attracts the negatively charged toner particles to form an image on the paper.

7) Fusing Assembly-The fuser contains 3 componentts, a halogen heating lamp, aTeflon coated

fusing roller and a pressure roller. The halogen lampheats the fusing roller to350 F before the toner

is melted onto the paper by the fuser unit.

8) Formatter Board

This is the main circuit board in the printer that is responsible for translating data received fromthe

computer into data that the printer can understand.

The four systems of a laser printer

1 .  The Formatter Systemreceives the print data from the computer and processes it to form an image and transfers it to the Engine Control S ystem.

2 .  The Engine Control Systemmonitors and controls all of the printer’s electrical and mechanical subsystems.

3.  The Image FormationSystemproduces the actual image on the paper .

4.  The Paper Pickup and Feed Systemmoves the paper from the paper trays through the printer and to the output bins for delivery to the user.

Formatter System
The formatter system receives data from the various printer interfaces and coordinates the image timing with the engine control system. The formatter system communicates with the user via the control panel. The control panel provides three status LEDs to inform the user of any potential problems and receives input from the user via two control panel buttons. One recent model in the 2000 line, the 2300, provides a character status display of error messages.

Engine Control System
The Engine Control System is comprised of the Engine Controller PCB and Intermediate PCB. The Engine Controller board contains the main controller logic, power supply, laser, and fuser control and coordinates all print engine activities. The Intermediate PCB serves as a connection point for various sensors and motor connections.

Image Formation System
Most of the “magic” of the laser printer is contained in the disposable toner cartridge, as demonstrated 

Since so much of the imaging process takes place here, replacing the toner cartridge is often the first step in troubleshooting a laser printer.

The image formation process can be broken down into six stages:

1.   Drum cleaning

2.   Primary charge

3.   Laser beam exposure

4.   Developing

5.   Transfer and separation

6.   Fusing

The toner cartridge contains a photosensitive drum. The properties of the drum allow an image to be written to the drum via a laser beam and later transfer that image to paper. The process starts by cleaning the drum. A special cleaning blade scraps any residual toner off the drum and moves it to a toner waste area. This keeps the waste toner from mixing with the new unused toner.

Next, a special charging roller applies a uniform negative charge to the drum’s surface. The charging roller erases any residual charge from the previous print job and maintains a constant potential on the drum’s surface.

The laser scanning assembly “writes” an image on the drum by discharging its negative potential. This creates a latent electrostatic image on the drum that is later developed into a visual image. The laser scanning assembly uses a rotating six-sided mirror to sweep the beam across the drum from left to right. A series of lenses and mirrors direct and focus the beam onto the drum’s surface. The drum rotates during the process allowing the entire surface to be discharged by the laser beam.

During the developing stage the electrostatic image is developed into a visible image. A developing cylinder inside the toner cartridge is responsible for depositing the toner onto the charged photosensitive drum. The toner material is made from black plastic resin bound to iron ore particles. The discharged areas of the drum attract the toner and the charged areas repel the toner.

The transfer stage moves the toner from the photosensitive drum to the paper. The transfer-charging roller applies a positive charge to the back of the paper, causing it to attract the toner particles from the drum. The stiffness of the paper and the roller’s small radius prevent the paper from sticking to the drum. A static charge eliminator also helps to keep the paper from sticking by weakening the attractive forces from the negatively charged drum to the positively charged paper. The paper is moved to the fusing stage and the drum returns to the cleaning and conditioning phase.

During the fusing phase, heat and pressure bond the image to the paper. The paper passes between a fusing film and pressure roller. The fusing film reaches a temperature of 195 degrees Celsius within 10 seconds. This fast warm up allows the printer to maintain low power consumption when not in use. The “fuser” is typically a field replaceable subsystem in the laser printer.

Paper Pickup and Feed System
The paper transport is where the bulk of the problems occur. Since a large portion of this part of the printer is mechanical, it is the most prone to failure. The transport system is comprised of several rubber rollers and parts that will all require replacement at some point in the printer’s life.

The paper transport system begins at the paper trays. Most office laser printers are equipped with one manual feed tray and one or more automatic feed trays. (The HP 2100 /2200 ships standard with a manual feed tray for envelopes and special paper and a 250-sheet tray for automatic paper feed.).

The process of removing the paper from the tray is the same regardless of the tray. A tray pickup roller rotates to “grab” a sheet of paper from the tray. A small separation pad at the front of the tray helps to assure that only one piece of paper is removed from the tray. When the separation pad wears out, the printer will begin to pull multiple sheets of paper through the printer, or cause the paper to jam or skew through the printer. When the pickup roller wears out the paper cannot be removed from the tray and the result is a paper jam in the paper tray.

From the tray, the paper is guided to the registration rollers and shutter. The shutter helps to correct any skewing of the paper. A series of sensors detect the paper’s progress and aid in synchronizing the printing process and detecting paper jams. The cassette paper sensor detects paper in the main 250-sheet cassette. A top of page sensor detects the top of the page and allows the image to be aligned with the start of the paper. The paper passes to the transfer-charging roller where the image is placed on the paper. It is then guided to the fusing assembly by a series of feed belts. The fuser rollers guide the paper to its final destination, either out the back of the printer or to another set of rollers that guide the paper facedown on top of the printer.