Introduction to Computers Laser Printers

Laser Printers Laser printers are fairly complex. They use lasers in a process of melting and cooling plastic in order to product a high quality image Laser printers are classified as page printers, because they print text and graphics simultaneously one complete page at a time Laser printers use a laser to charge a metal drum. The drum picks up plastic toner, and the toner is then fused onto the paper (using rollers and heat)

Laser Printer Components In addition to the laser, laser printers share the following components: Laser printers use a laser and electrical charges to transfer images to paper Laser printers move paper through the printer via motorized rollers Each laser printer has a high-voltage power supply to charge the drum. This power supply converts AC current into higher voltages required for the printing process Each laser printer has a DC power supply to operate most of the electronic components inside the printer Each laser printer has a controller, which is a circuit board that acts like a motherboard in the printer. This controller board makes it possible for the printer to have and add its own memory A duplexing assembly is required to print two-sided output on a laser printer. Duplexing assemblies are typically mounted on the back of a laser or inkjet paper A transfer belt is used on some high-end color laser printers. Colors are applied to the transfer belt and then to the paper. This step is repeated for different colors

Laser Printer Process The basic process for a laser printer is - Cleaning The cleaning phase prepares the drum by removing the previous image printed Processing The printer receives a document to be printed from the computer Charging The primary corona prepares the photosensitive drum for writing Exposing A laser beam changes the charge on the surface of the drum in a pattern Developing The developing roller applies toner to the drum, which sticks to the charged areas Transferring The transfer roller charges the paper to attract the toner Fusing The fusing assembly melts the toner to the paper

Cleaning Process The laser printing process begins with the cleaning phase The cleaning phase prepares the drum by removing the previous image printed and prepares the printer to start working It uses a rubber cleaning blade to remove any excess toner on the drum, and then it scrapes off the debris into a debris cavity Next, a heat roller is lubricated to ensure that enough heat will be evenly applied to transfer the next image printed Then an electrostatic erase lamp neutralizes the electrical charges that remain on the drum from the previous printed image

Image Processing In the laser printing process, the print job comes in from the PC and is stored in RAM on the printer itself. The print job is taking one page at a time out of the RAM and processed At that point, the print job from printer RAM is rasterized. This means that the document is transformed from data about text, fonts, spacing, images, etc, and converted into strips of pixels This is done because the printer only needs to know how to print the image it was given, it doesn’t need to know the formatting information for the Then the document is fed one strip at a time to the laser

Charging the Drum The heart of the laser printer is actually the component called the imaging drum, not the laser The first thing that happens after the print job is stored in the printers ram is that a uniform negative charge is applied to this drum by the primary charge roller The primary charge roller emits light that hits the surface of the drum while the drum rotates. This builds up a negative electrical charge uniformly all the way around the drum

Exposing the Image The laser then sends a pattern of pulses to an oscillating mirror. This mirror rotates side to side and reflects those pulses from the laser across the drum Wherever this laser hits the drum, it discharges the negative charge and that spot becomes neutral Wherever the laser did NOT hit, we still have a uniform negative charge, and wherever the laser did hit, the drum loses its negative charge and becomes neutral By doing this, we create a negative image of this document on the drum

Developing the Image At this point, the developer roller applies toner to the drum. The toner (that little black dust that you always see when you open up a laser printer) is basically chopped up, tiny little pieces of plastic, and has a negative charge Since it has the same charge as the areas where the laser did not hit on the drum, they are repelled. Therefore the toner will not stick to any areas of the drum that have a negative charge The only places that the toner will stick are the spots on the drum where the laser blinked its dots and discharged the drum Therefore, we create a negative image of the document on the drum using that plastic toner

Transferring the Paper At this point, our piece of paper actually comes into the picture. Up to this point, we've been dealing with the document electronically. Now we run a piece of paper through it The pickup rollers pick up the top of the paper from the paper tray and feed it through the printer. At the same time, separator pads spin in the opposite direction from under the paper and push back any extra paper. This prevents more than one piece of paper from being sent through the printer at a time

Transferring the Image At this point, we have to get the toner on the paper. Since the toner is negatively charged and it's stuck to the neutral areas of the drum, the toner isn't stuck very well To transfer that toner from the drum to the paper, we need to make the paper much more attractive to that negatively charged toner. To do that, we use the transfer roller. As the paper passes underneath the transfer roller it applies a positive charge to the paper As the paper moves though and the drum rotates, those toner particles in the neutral areas of the drum move to the positively charged paper, so the toner is essentially stripped off of the drum and laid down on the paper as the paper moves through the printer and underneath the drum. The drum rotates and transfers the toner down here onto the positively charged paper

Fusing the Toner We now have our piece of paper coming out of the printer, and its got all that toner stuck to it, but that toner will brush off easily if it isn't fused to the paper. Even though it is stuck on there with the positive to negative charge, it's not a very strong bond The fuser assembly will fit the toner to the paper. It is composed of two different rollers, a hot roller and a pressure roller The hot roller heats up and transfers that heat to the toner. The toner then melts and is pressed onto the paper by the pressure roller. This is why when you get a piece of paper out of a laser printer, it's hot As the paper cools, the plastic that makes up the toner adheres to the piece of paper

Laser Printer Pros and Cons Laser printers print very high quality documents, are fast and they are economical Not only is toner cheaper than inkjet cartridges over time, the printers themselves tend to be more durable and reliable than inkjet That being said, they are a heavier initial investment, are larger and bulkier, and take awhile to warm up The other major benefit inkjet printers have is that they are more versatile. Inkjet is way better at printing photos and images than laser printers, and inkjets can handle more material. Laser jets are best at printing monotone documents