Electrostatic transfer of toner particles onto paper
With the first machines, transfer of toner onto paper was principally obtained from the mechanical pressure exerted by a transfer roll placed on the back of the paper sheet. Such purely mechanical solution offered the advantage of providing perfect synchronization between drum and paper, but suffered from relatively low transfer rate.
An electrostatic assistance had therefore to be introduced. A DC voltage applied between the internal metallic core of the transfer roll and the grounded drum surface, creates a DC electric field in the transfer zone. The toner particles, of resistive type, are electrically charged by a DC corotron device located upstream the transfer zone. These charged particles now interact with the transfer field that tend to take them off from the drum and fly to the paper.
Finally, a second corotron, of AC type, dissipates the electrostatic charges in the post-transfer zone.