HP 4600 Service Manual - Page 162

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 paper. This block consists of three steps: ■ Attaching ■ Transfer ■ Separation Step 6: Attaching After the media is picked up from the input tray, the attaching roller pushes it against the ETB. The attaching roller has a positive dc bias, and it creates a positive charge on the print media. Figure 5-24 Attaching the paper to the ETB illustrates this step. Figure 5-24 Attaching the paper to the ETB Step 7: 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 5-25 Toner transfer 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 four-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. 146 Chapter 5 Theory of operation ENWW