Though it doesn't make a nice sound when struck, the laser printer drum is an important part of the band that is a laser printer. It is the metal roller that gets etched with the image of your print job by a laser. As the drum gets charged by a laser, it picks up toner, and then rolls that toner onto a piece of paper.
How a Laser Printer Drum Works
Within a laser printer, the drum picks up toner and deposits it on the paper. As it spins, a laser changes the charge on the outer coating of the drum in conjunction with the print item it's producing. These charged areas then attract the specially-made toner particles as they roll past the cartridge. The roller then deposits this toner on the paper, before the charge is reset in time for a new etching from the laser. Typically, a drum is separate from the toner cartridge, allowing the two to be replaced individually; however, some toner cartridges include a drum within, and are replaced simultaneously.
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Types of Drums
The key to a laser printer drum is its coating. The coating has to be responsive to light, allowing the image to be "etched" in electrical charge along its surface. However, it must also be rigid enough to retain the very tight fault tolerance necessary to create precise images. To that end, drums were originally produced with a selenium- or cadmium sulfide-based coating, which have a lifetime of 500,000 to 700,000 sheets. Organic PhotoConductors, a petroleum-based technology, can produce up to 1 million printed pages, but are more susceptible to damage than their harder counterparts. The latest technology in laser print drums is amorphous silicon, a hard substance which has a life of up to 5 million pages.
Advantages of Laser Drums Over Inkjets
A laser drum differs from an inkjet head in a number of ways. First off, the two technologies rely on different substances to produce images. Drums use small, plastic-like particles called "toner" which are then melted onto the surface of the paper, creating a semi-glossy finish that is resistant to smearing. The drums are also typically separate from toner, allowing the two items to be replaced separately as they break or age. By contrast, most inkjet printers integrate the head with the ink, requiring both be replaced simultaneously. The cost per printed page on laser printers is lower than inkjets, in spite of the higher upfront cost of laser printers.
Problems Drums Face
There is a very tight margin of error between the surface of the paper and the surface of a drum. Drums are susceptible to scratching, which will leave permanent marks on every print. Certain materials can also lose their ability to hold, or resist, a charge, resulting in the toner not being able to cling to the drum. If the toner is not electrostatically attracted to the drum, it will not print. Every drum has a life expectancy, after which point this loss of charge will begin to occur. Additionally, color laser printers require separate drums and toner cartridges for each color, increasing the potential for damage and increasing maintenance costs.