Image drum
The image drum (also known as a photoconductor or Photo Conductor Unit (PCU) according to the principle of operation ) is a basic component of laser printers and photocopiers on which electrophotography takes place.
construction
The image drum is an approx. 1 mm to 2 mm thick, precise piece of aluminum pipe with a device-specific diameter and slightly longer than the maximum intended printing width. The ends are provided with end pieces for suspension in open grinding bearings. One side has a form-fitting traction device (toothing) for the drive. A photoconductive layer is applied to the surface. The aluminum body is connected to the device chassis with a sliding contact.
function
The photoconductive coated surface of the image drum is electrically charged by a high voltage source via a charging corona . By means of focused light-emitting diodes , a laser beam or an exposure projected through an objective , an image is created as a pattern on the semiconducting drum surface, in that these are discharged in the light areas by photoconductivity . Upon further rotation of the imaging drum ink particles stay of the transported through the toner roller to the drum toner to the charged sites due to electrostatic attraction adhere. The previously latent image is thereby developed . As the rotation continues, the toner image is transferred to the paper on the separation corona or transfer roller. An erasure corona operated with alternating voltage discharges the drum, which leads to the erasure of the latent image. A wiper removes any remaining toner. The entire process takes place during one rotation of the drum.
The image is fixed on the paper in the so-called fixing unit, which is a further assembly behind the drum unit. A firm connection between the color particles of the toner and the paper is created by pressure and heat (hot fixation) or earlier only by pressure (cold fixation).
Types
A distinction can be made between three types of image drums: In laser printers, single-use image drums are usually used, which, depending on the model, have to be replaced after up to approx. 5,000 pages. So-called permanent image drums, on the other hand, can (depending on the model) leave up to approx. 300,000 printed pages in the printer. These drums are mainly installed by the Kyocera company . The third type of drum, the so-called semi - permanent drums, hold up to approx. 20,000 pages.
materials
Photosensitive materials are used to manufacture image drums. Classic copiers and laser printers have a coating of inorganic photoconductors such as arsenic tellurium or gallium arsenide, and until the 1980s, amorphous selenium as well. Modern devices use organic materials ( Organic Photo Conductor - OPC) , especially for environmental reasons .
The light-sensitive surface can degenerate irreversibly after prolonged exposure to UV . The result is spots or stripes on every printout, the repetition of which can be used to identify the drum circumference. OPCs react with most chemicals, including those found on human skin. Old drums with much thicker and selenium-based coatings could still be treated with suitable polishing agents, which removed the gray haze caused by the surface roughened by the paper sanding or hardened wiper.
Use in color systems
Revolver system
Different variants of the use of image drums have been developed for color laser printers. The simplest is a revolver system in which the paper is repeatedly fed past the same image drum once per color or the toner image is temporarily transferred on a transfer drum or a transfer belt, as these can be guided more precisely than paper and the color samples can be better synchronized. With the transfer drum or transfer belt, the image on the drum is not a mirror image, as it is only transferred to the paper in the second step. With each pass, the developer roller with the corresponding toner color is fed to the image drum. The image drum is exposed beforehand with the corresponding CMYK color partial image. This means that the revolver system is only about a quarter as fast in four-color printing as in monochrome printing.
Inline system
So-called inline printers have four independent sequential image drums for one color each, which make it possible to apply the colors to the sheet one after the other (on most models initially to a conveyor belt / transfer belt). These devices work considerably faster than laser printers with a revolver system. With some inline printers, the image drum of the respective color toner is also replaced because it is integrated into the toner cartridge.
Installation
Image drums are built into a drum unit with the wiper for the waste toner and its container or removal channel. The charging corona is partially integrated into this unit. Is the toner unit together with the drum unit, this is called a processing unit (from english processing unit ) . A developer unit is a toner unit, but it is sometimes imprecisely referred to as a process unit. In addition, toner units are either pre-filled with a single toner filling or can be refilled from a cartridge, which is also the case with process units.