Although it may be referred to as an imaging/drum unit or photoreceptor drum, an image drum is primarily responsible for transferring the toner along with the image or text, onto the paper.
In order for this transfer to take place, an initial positive charge is received from the corona wire (a charged wire inside the printer). From there, the laser will then write on the drum and a negative charge appears where the image/text should appear.
As the paper is fed through the rollers, the negative charge pulls the toner from the cartridge to the drum before transferring onto the paper. Once the toner hits the drum, the oppositely charged areas are attracted to it in order to create the text and images. After this has taken place, the paper goes through a ‘fuser unit’ which effectively melts the toner to the page holding the image on the page.
The imaging unit itself can go through a lot of wear and tear. This is due to the drum being repeatedly hit by a laser and this will eventually see performance drop. A way in which you can identify when the drum is ready to be replaced is if black spots on the page begin to look lighter.
It’s worth noting that a brand new toner cartridge will fail to resolve the issue of faded text and pictures when imaging/drum unit needs replacing. The number of pages an image drum can print before it needs replacing can vary from anywhere between 10,000 to 25,000 pages. For this reason, it’s not uncommon for manufacturers to include drum units with cartridges meaning that customers can combine replacing the toner cartridge with the image drum itself.
We would recommend looking through your printer manual in order to determine the best course of action should you encounter an image drum issue.