HP color laserJet 4550 series - Page 109

C7085-90921 Chapter 5 Theory of operation 107
Transfer
During transfer, the toner image is transferred from the drum surface to the ITB (primary transfer) and
then from the ITB to the paper (secondary transfer).
Primary transfer
The ITB has a primary transfer roller that provides a positive bias that pulls the toner image from the
drum to the ITB. This process is repeated four times (one time for each toner cartridge). The positive
bias applied by the transfer roller increases with each pass to compensate for increasing layers of
toner. As shown in Figure 5-6, the toner particles still maintain their negative charge. Only the
transfer belt is charged positively.
Figure 5-6 Primary transfer
Secondary transfer
Once the entire print image is on the intermediate transfer belt, the secondary transfer roller is lifted
into position. At the appropriate time, a sheet of media is pulled into the transfer area. With the
secondary transfer roller biased at a positive potential, the toner on the ITB is pulled onto the media
as the ITB passes over the secondary transfer roller.