Nowadays, almost everybody uses a printer on a daily basis without necessarily knowing the history behind digital printing. No, digital printing does not mean the production of electronic books or eBooks. On the contrary, digitally printing documents refer to the more method of producing print-outs through the use of laser printers and photocopiers. Laser printers and photocopiers use a process developed in the 1930s called xerography. Chester Carlson man developed this process which aptly means dry printing from the word’s Greek origins: xeros meaning dry, and graphia meaning writing. At the time that the Carlson developed the xenographic process, not many people were familiar with laser printer toner, or simply toners. The current process used by modern laser printers and copiers however is not exactly the same as the 1938 original. The initial version was devised for manual printing in small volumes and not to be used for mass printing. The main concept of combining electrostatic printing technology and photography has been maintained in modern printers but a breakthrough innovation in 1960 enabled the xerographic process to be used for bulk printing. The breakthrough was a simple exchange in component from a flat plate to a cylindrical drum coated with selenium. This switch enabled automation of Carlson’s process and led to the advent of laser printers, photocopiers, and different laser printer toner varieties as we know them today.
Laser Printing In Depth
The main process of laser printing or xerography follows seven basic steps, most of which are very similar to the steps in producing printed photographs. These seven basic dry writing processes include: charging, exposure, development, transfer, separation, fusing, and cleaning. In modern printers, an additional step for raster image processing is taken at the beginning prior to charging. Lasers, photoreceptors, laser printer toner, and paper are the main components of the whole xerographic process.
Preparation: Raster Image Processing
In the preparation phase of dry printing is raster image processing, the computer transmits instructions to the machine’s built-in image processor. The main task of this processor, often referred to as RIP (Raster Image Processor), is to store a map of the document or image that needs to be printed to the device’s raster memory. The map generated by the RIP is used to instruct the rest of the machine on what needs to be printed and what color of laser printer toner should be used when printing it.
The first official xerography step, charging, makes use of a drum or roller which emits electrostatic charges onto a photo conductor unit (PCU). The PCU is either a drum or a belt which is photosensitive. This means that electrostatic pulses can be stored by the PCU as long as it is kept in the dark. In this step, the PCU is charged with electrostatic pulses.
Exposure through Lasers
Once the PCU is charged, the lasers in the printer are pointed to the PCU indirectly via built-in mirrors and lenses. The laser is instructed by the RIP such that it is alternately switched on and off depending on the map stored in the RIP. Due to this exposure to laser beams, portions of the PCU get neutralized. This is the second step of the laser printing process.
Developing to Transferring on Paper
The portions that have been neutralized attract the fine particles of laser printer toner resulting in the development of solid images or text. This simple and quick step is called developing and heavily relies on the concept of opposite charge attraction. As a fourth step, the images are transferred from the PCU to a blank piece of paper by rolling the PCU over paper. Any excess toner is lifter or retracted from the paper and the PCU in a fifth step called the separation. The excess is returned to the cartridge for re-use. The laser printer toner, now transferred on paper, is then subject to fusing. In this step, the image is permanently fused to the paper through the use of extremely high heat (up to 200 degrees Celsius) and pressure. This is why freshly printed or copied documents feel warm when touched or held right after coming out of the a laser printer or photocopier. Once the whole map in the RIP has been printed on paper, the xerographic process is concluded through a cleaning step. In this step, small plastic blades run through the length of the PCU in order to ensure that no excess or stray laser printer toner that can ruin the document has been left behind.