The developer unit is a device in a laser printer that draws the toner powder through the system and eventually to the paper. Unlike ink-based printers which directly deposit ink onto the page, laser printers must affix the dry toner powder to the page. The developer unit is an integral part of the process -- without it, the toner never gets to the page in the first place.

How It Works

The developer unit is a collection of extremely small beads attached to a rotating metal cylinder or roller. The beads carry a small negative electric charge, which attracts toner when passed through the toner hopper. This attraction happens because the toner powder has a positive electrical charge. The developer drum then passes the toner-coated beads past the drum assembly, which has an electrostatic version of the image to be printed. The electrostatic image has a stronger negative charge than the developer unit, which draws the toner away from the developer and onto the drum to form the image. The developer then moves back into the toner hopper to gather more toner powder.