HP LaserJet 4250 Service Manual - Page 172
Fusing the image, For the HP LaserJet 4250
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Fusing the image The image is held in place only by electrostatic attraction when it is transferred from the drum to the media. The lightest touch will smear it. During the fusing process, heat and pressure fuse the image to the media to produce a permanent image. The media passes between a heated fusing roller and a soft pressure roller. This melts the toner and presses it into the media. This printer utilizes an on-demand fusing method. This method has fast temperature-rising time, which shortens the wait time (the time it takes to heat the fuser to its operating temperature). No power is supplied to the fuser heater during the standby mode, which conserves energy. For the HP LaserJet 4200 and 4200L Series printers, a dc negative bias (charge) is applied to the fuser film. This stabilizes the toner so that it does not scatter over the media and produce a blurred image. For the HP LaserJet 4250, 4300, and 4350 Series printers, a dc negative bias is applied to the fuser film and a dc positive bias (charge) is applied to the pressure roller. This stabilizes the toner so that it does not scatter over the media and produce a blurred image. To prevent offset images, the surface of the fuser film is coated with fluorine. Figure 5-41 Fusing the image 158 5 Theory of operation
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