HP 4550 Service Manual - Page 114

Mechanical system This section discusses each of the imaging units involved in the image formation process. Photosensitive drum cartridge The drum drawer assembly is a user-replaceable assembly that contains the photosensitive drum, primary charging roller, drum memory, and waste toner storage. The drum motor (M4) drives the drum drive assembly, which in turn drives both the photosensitive drum and the ITB (discussed later in the mechanical system). Because the primary charging roller's function was discussed in the image formation section, only the drum memory and waste storage areas are covered here. Photosensitive drum life Located inside the drum cartridge is an EEPROM that stores an identification code along with drum life and waste toner information. A life threshold value is used as a comparison number to indicate when the drum has reached its end of life. During printer operation, the DC controller writes to the drum memory at specific times: when power is deactivated, when the drum drawer is opened, or whenever 20 pages are printed. Figure 5-13 Photosensitive drum cartridge Two messages appear as the drum cartridge reaches its end of life. The first is DRUM LIFE LOW, which appears on the control panel when about 85 percent of the drum's life has expired. When the drum is 100 percent used, the message DRUM LIFE OUT appears and the printer stops printing. A new drum cartridge must be installed at this time. Waste toner Inside the drum cartridge are three waste toner reservoirs that store the residual toner left behind during image formation. As discussed earlier in the image formation section, the residual toner from the ITB is transferred to the drum. A cleaning blade scrapes the residual toner off the drum's surface. The waste toner is then carried by toner feed plates into the waste toner reservoirs for storage. 112 Theory of operation C7085-90921