Saturday, October 05, 2013

CANON LBP1120 - LASER PRINTER - WORKING PRINCIPLE


PRIMARY CHARGING
For preparation of latent image formation, a uniform negative potential is applied to the photosensitive drum surface.  Primary charging of this printer is performed by the method that directly charges the photosensitive drum.  The primary charging roller consists of semi conductive rubber. In addition to DC bias, AC bias is applied to the primary charging roller to maintain the potential on the drum surface uniform. The DC bias changes with the developing DC bias that changes according to the IMAGE DENSITY DATA signal sent from the interface controller.  This direct charging method has advantages such as the lower applied voltage and almost no ozone generation over the conventional corona charging method.
SCANNING EXPOSURE
When the laser beam scans onto the photosensitive drum surface, charges on the light area are neutralized to form an electrostatic latent image.
DEVELOPING STAGE
Toner is applied to the electrostatic latent image on photosensitive drum surface to form a visible image. This printer performs the toner projection development with a single-component toner.
## Charges in the exposed areas of the photosensitive drum are shown as positive in this figure, even though they are actually negative. It means that the photosensitive drum potential is higher as compared with that of cylinder.
The developing unit consists of a developing cylinder and rubber blade. The developing cylinder consists of a fixed magnet and a cylinder which rotates around the magnet.  The single-component toner mainly consists of magnetite and resin binder and is held to the cylinder by magnetic
attraction. The toner functions as an insulator, and acquires a negative charge by friction due to the rotating cylinder.  The areas on the drum that were exposed to the laser beam have a higher potential (are less negative) than the negatively charged toner particles on the developing cylinder. When these areas approach the toner layer of the cylinder (with negative charges), the potential difference (higher on the drum) projects the toner particles to them.  This is called toner projection development, which develops the latent image on the drum into a visible image.
An AC bias is applied to the developing cylinder to help project the toner particles to the drum surface and improve the contrast of the printed image. The centre voltage of the AC bias (1460 Vp-p) varies with the developing DC bias.  This printer changes the developing DC bias value according to the IMAGE DENSITY DATA signal sent from the interface controller. The changes of the developing DC bias value makes the potential difference between the cylinder and drum so that the density of the print can be adjusted.
In the transfer stage, the toner image is transferred from the drum surface onto the paper.
Positive charges are applied to the back of the paper to attract the negatively charged toner particles to the paper.
The elasticity of the paper causes it to separate from the drum (curvature separation).  To stabilize the paper feed system and to prevent droplets on the printed image at low temperature and low humidity, the charge on the back of the paper is reduced by the static charge eliminator.
FIXING STAGE
As the toner image transferred onto the paper in the transfer stage is held to the paper only by the static electricity, even a light touch will smear the image.  The toner particles are fused to the paper to make a permanent image by applying pressure and heat to the paper and toner.
As the toner particles transferred onto the paper are held to the paper by the positive charge applied to the back of the paper, they may be dispersed at fixing.  This printer applies the negative DC voltage to the fixing film via the pressure roller shaft. The negative charge is applied to the paper surface by the fixing film before the paper reaches the fixing heater. This improves the adhesion force of the toner particles to the paper and prevents dispersion of the toner particles.  This printer utilizes an on-demand fixing method which uses the fixing film that requires small heat capacity. This method can heat the roller quickly and does not require the power supply to the fixing heater at the standby mode. Therefore, the wait time is reduced to eight seconds or less, and the energy can be saved.  The fixing film is a cylindrical polyamide film. The surface is coated with fluorine to prevent offset.
At the photosensitive drum cleaning stage, the photosensitive drum surface is cleaned in preparation for the next print operation.
Prior to the next printing, the residual toner on the drum surface is scraped away by the cleaning blade to clean the drum surface. The removed toner is collected into the cleaner container.