Panasonic UF 885 - Panafax B/W Laser, Service Manual

Page: 196 / 342

System Description
196
6.1.3 Printing Process Operation
Charge
In the dark, the Bias Charge Roller (BCR) applies a high, uniform negative charge to the surface of the OPC Drum. The surface
potential is approximately -650 VDC and remains because the drum has a high electric resistance in the dark.
Exposure
A portion of the laser beam is deflected to the timing sensor [Beam Detection (BD) Sensor], which controls the start timing of
scanning on the OPC Drum. The CPU also uses the timing sensor to detect abnormal signals. The light beam from the laser
diode is modulated by the digital signal (nVIDEO) and converted to parallel light waves by the collimator lens. The beam is then
directed to the rotating polygon mirror, where it is reflected to the f-
θ
lens and then focused onto the OPC Drum surface. The
laser beam moves across the surface of the OPC Drum in the scanning direction. Where the laser beam is applied, the negative
charge on the drum dissipates, and where the laser is not applied, the negative charge remains. This action forms a latent,
electrostatic image on the OPC Drum, corresponding to the original image.
Development
This development process uses a conventional method, where toner coats a Development Roller and transfers to the latent
image on the OPC Drum. In the Toner Cartridge, the (mono-component) toner is negatively charged by the friction between the
rotating Development Roller (Mag Roller) and the Developing Blade. This combination and the rotation of the Mixing Blade
transfers the toner from the reservoir and forms a brush effect on the Mag roller. Where the magnetic brush lightly touches the
OPC Drum, the negatively charged toner is attracted to the latent image on the drum, forming a mirror image of the original on
the drum. Any remaining toner is removed from the Mag Roller by the Developing Blade and is recycled back into the toner
reservoir. A bias voltage of approximately 1.7 kVACp-p at 1.7 kHz, riding on a -500 VDC bias is applied to the magnetic brush to
achieve maximum print quality.
Transfer and Separation
As the paper is fed between the OPC Drum and the Bias Transfer Roller (BTR) (630), a positive charge of approxi600
VDC (+3
µ
A steady current) is applied to the backside of the paper by the BTR. The toner particles are attracted away from the
drum towards the surface of the paper. During cleaning, the BTR is charged to approximately -800 VDC to repel toner on the
OPC Drum and prevent toner from being attracted to the BTR. After transfer has occurred, the paper passes over the Discharge
Plate (617) in the Plate Discharge Guide (616), reducing the difference of potential between the OPC Drum and the paper. The
stiffness of the paper causes the paper to separate from the drum.