The Laser Printer Process
- Cleaning - Cleans the drum
- Charging - Charges the drum
- Writing - Writes an image to the drum
- Fusing - Fuses the ink permanently to the paper
The electrophotographic (EP) laser printing process
1. Cleaning - The EP drum must be cleaned, erased, and desensitized of any electronic charge it may have as a result of a previous process. A rubber blade is used to remove any toner or particles from the drum. The used toner is disposed of into a cleaning unit or bucket. A fluorescent lamp is used to remove any electronic charge retained by the EP drum from a previous process. This preparation stage is vital; the drum must be properly prepared in order to produce a sharp image. Think about it: if your camera lens is dirty, you are probably not going to get a clear picture.
2. Charging - At this point, the EP drum cannot hold an image; it needs to be conditioned to do so. This is accomplished with a charge of -600V applied to the EP drum by the primary corona wire. The charge is evenly distributed across the entire drum, creating an electronic field. This process enables the drum to become photoconductive and prepares it for the writing phase.
3. Writing - At this stage of the process, the printer's laser beam writing unit and a series of mirrors are used to draw tiny dots on the EP drum, which represent the final image to be produced. The area of the drum that the laser beam comes in contact with loses some of its negative charge (by approximately -100V) and becomes relatively more positive (the charge is still considered negative, just not as negative as the areas not hit by the laser beam). When the laser beam has finished creating the image on the relatively positive EP drum, the printer's controller starts the paper sheet-feed process by pulling a sheet of paper into the printer. The paper stands ready at the printer's registration rollers until the controller directs it farther into the printer.
Note Paper-feed rollers have sensors to control the proper flow of paper out of the paper tray. It is a common occurrence to receive a "paper jam" error on the printer LED display, but on investigation, you find that there is no paper in the tray. Chances are that there is a particle of foreign matter in the way, or the sensor is dirty. While troubleshooting, always check the paper-feed sensors first in this situation.
4. Developing - At this point in the process, the EP drum is ready to accept toner on the areas or dots that have a more positive charge. The toner cartridge houses a toner-developing roller that is magnetized and constantly turning. The magnetized roller attracts the toner particles located near it and dispenses the toner to the positively charged areas (dots) on the rotating EP drum. The EP drum now has a "picture", or mirror duplicate of the image, to be placed on the paper.
5. Transferring - It's time to get the image, drawn in toner, from the EP drum to the paper. Keep in mind that the toner is being held on the EP drum with a relative negative charge. At this point, the paper has been pulled into the printer. The paper passes by the transfer corona wire, or in some printers a transfer roller, where it receives a highly positive charge on its back side. The paper then passes under the negatively charged EP drum, and the toner is transferred onto the highly charged paper. A static charge eliminator, otherwise known as an eliminator comb, is used to keep the paper from wrapping itself around the EP drum.
6. Fusing - The toner must now be fused, or bonded onto the paper. A fuser assembly, which is a quartz heating lamp inside a roller tube, is situated above a rubber roller pressure assembly. The paper and its toner are fed between the two devices. The toner is heated (melted) by the fuser assembly and pressed onto the paper permanently by the pressure rollers. It is important to note that there is a built-in temperature sensor on the heated rollers. If the temperature during this process rises above 180 F, the sensor will shut down the printer.
Note: Silicon oil is used to lubricate the fusing rollers during the fusing process to keep the paper from sticking to the rollers.
7. End of cycle - A cleaning pad is used to remove excess toner and residue from the heated rollers. The paper containing the final image is rollled out of the printer.