HP Color LaserJet CM4730 Service Manual - Page 190

Transfer block The third part of the image formation process is the transfer block, in which the toner image is transferred from the photosensitive drum to the media. This block consists of three steps: ● Attraction ● Transfer ● Separation Step 5: Attraction To feed the media vertically, it is necessary to push the media against the ETB. The picked up media is fed as it is pushed against the ETB by the attraction roller. The positive DC bias is applied to the attraction roller, which charges the media positively to make the negatively charged toner easier to transfer onto the media during the transfer operation. ETB Attraction roller DC bias ETB engaged roller Paper Figure 4-21 Attaching the media to the ETB Step 6: Transfer A positive bias is applied to the transfer charging roller, which is directly opposite the photosensitive drum. As the ETB passes the transfer charging roller, it picks up a positive charge. The negatively charged toner on the photosensitive drum is transferred to the positively charged print media. Figure 4-22 Toner transfer on page 171 illustrates this step. This process is repeated for each color as the ETB carries the media from the bottom cartridge to the top (C, Y, M, K). In full-color printing, as four colors of toner are piled up on the media, the holding force of the toner weakens as the transfer process progresses. To counteract this effect, the DC controller increases the positive DC bias that is applied to the transfer charging roller for each successive color. 170 Chapter 4 Theory of operation ENWW