Simply put, the developer unit manages the toner so a printer can use it. The developer unit is a reservoir of toner which is made available to the drum. It’s function is to develop the invisible latent image on the drum with toner.
Developer units are necessary for every laser printer. Color laser printers have 4 developer units, one for each color. B&W printers, also called monochrome printers, have one developer.
The developer unit contains one or two magnetic rollers with a layer of toner. The magnetic roller has a blade with a thin gap to maintain a uniform layer of toner. Augers keep the pool of toner circulating behind the magnetic roller.
While the magnetic rollers are close to the drum they are not touching the drum. The drum pulls toner from specific areas of the developing rollers to develop the image.
The drum unit and developer unit are two different components inside a laser printer. These two parts work together to form an image. Which is then transferred from the drum onto paper.
A developer unit is the specific unit with the magnetic roller, augers, and reservoir of mono/dual component toner. While a toner cartridge is a generic term which can refer to many parts. Toner cartridges can refer to simply a bottle of toner. Or it can mean a cartridge comprised of a drum unit, developer unit, and toner.
When toner and carrier are manufactured separately it’s called dual component toner. Mono component toner is where toner and carrier are manufactured together as one. Toner is the powdered pigment, carrier is the powdered part that holds the electrical charge.
The developer unit also forms a tribo electric charge by mixing it. Augers churn single or dual component toner and circulate it around inside the developer unit.
Common Problems
Developer units commonly cause print quality issues and dirty printers. They rarely cause noises or error codes.
In my experience developers are responsible for very odd print quality issues. Such as weird patterns, halo effects, fish eyes, light or dark areas. While drums are responsible for dark lines developers are responsible for light or void lines.
Apart from print quality issues developers cause a machine to run dirty. As they age the magnetic roller and charged toner gets weak. Instead of staying attached to the magnetic roller toner falls off it. Which then accumulates below and around the developer unit.
Stray toner can prematurely wear out drive gears and clutches. If toner piles up enough it can smear on passing paper causing more print quality issues.
There are a couple error codes are related to the developer unit. If the piezo sensor detects a lack of toner present it can cause errors. In my experience error codes from developer units are rare.
FAQ
The drum is a photo-conductive cylinder. While the developer unit contains all the toner.
A developer unit functions to make a uniform layer of toner available to the drum unit.
developer refers to a mix of carrier and toner. Toner is the pigment. Carrier is what holds the electrical charge.
Clean developer is a maintenance routine which can be executed on a copier or printer. It activates drive to the developer unit in an effort to “clean” it. As the magnetic roller rotates and augers spin small chunks of toner can be broken up.
The Copier Guy, aka Dave. I’ve worked on scanners, printers, copiers, and faxes since 1994. When I’m not fixing them I’m writing about them. Although, I’m probably better at fixing them. I’ve worked with every major brand. As well as several types of processes. If it uses paper I’ve probably worked on one.
