Printers

Understand the inner workings of laser printers for better troubleshooting

Nearly every business has them and troubleshooting them can be a drain on an IT professional's time. Don't let laser printers tie you up. Understand a laser printer's underlying systems and how they function.


When we think of hardware the first thing that comes to mind are PCs and servers. However, for most of us there is another device we have to contend with: the laser printer. The laser printer has been a common fixture in the business world for years, but many IT professionals still have difficulties in troubleshooting them and diagnosing problems.

As a result, I will introduce you to the inner workings of a laser printer, using the Hewlett Packard 2000 series printers as an example. However, the information and techniques in this article will apply to most laser printers regardless of the brand.

The four systems of a laser printer
The HP 2000 series printer is functionally divided into four systems:
  1. The Formatter System receives the print data from the computer and processes it to form an image and transfers it to the Engine Control System.
  2. The Engine Control System monitors and controls all of the printer’s electrical and mechanical subsystems.
  3. The Image FormationSystem produces the actual image on the paper.
  4. The Paper Pickup and Feed System moves the paper from the paper trays through the printer and to the output bins for delivery to the user.

Figure A provides a graphical illustration of this functionality.

Figure A


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 in Figure B.

Figure B


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.

This entire process is illustrated in Figure C.

Figure C


Summary
Now that you know how a laser printer works, next time you can learn how to perform repairs.

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