photo conductor unit (PCU)
Laser printers and color copiers work with an image drum, Photo Conductor Unit ( PCU) or Optical Photo Conductor( OPC), which is electrostatically charged and from which the paper to be printed is printed.
An image drum is a metallic cylinder with a light-sensitive semiconductor surface. This coating, whose conductivity changes with exposure, works on the principle of photoconduction, Organic Photo Conductor (OPC). In the OPC process, a charge image is created on the light-sensitive image drum. For this purpose, the drum surface is precharged with high voltage and then exposed. When exposed by light-emitting diodes or with a laser beam, the exposed particles discharge and become conductive; the unexposed ones retain their charge state and are non-conductive.
The image drum exposed in this way is sprinkled with negatively charged toner, which attaches itself to the discharged areas. Subsequently, this toner charge image of the image drum is transferred via a toner roller to the medium to be printed.
In four-color printing according to CMYK - Cyan, Magenta, Yellow, Key (Black) - a charge image is created successively for each individual color printout, sprinkled with the corresponding colored toner and transferred to the paper. The excess toner is stripped off and returned to the toner container. After the toner images have been applied, they are burned into the paper with pressure under heat. The charge image on the image drum is discharged with high voltage.