I. Working Principle of Laser Printers
A laser printer is a printing device that combines laser scanning technology and electronic imaging technology. The imaging process of a laser printer involves seven steps: paper feeding, charging, laser irradiation, developing, transfer, fixing, and cleaning, before the printer can complete the output.
01 Paper feeding
The printer's paper feed rollers feed the paper onto the paper path, and other devices ensure that the paper moves normally in the paper path, allowing the paper to be accurately transferred and finally fixed.
02 Charging
When the printer receives a print job, it charges the charging roller in the drum. After the charging roller comes into contact with the photosensitive drum, it eliminates the residual charge on the surface of the photosensitive drum and evenly applies a negative charge to the surface of the photosensitive drum.
03 Laser Irradiation
When a printer receives a print job, the laser emitter emits a laser beam that illuminates the photosensitive drum. The illuminated portion of the drum surface becomes a conductor, carrying away the negative charge and acquiring a positive charge, thus forming a latent image of the print content.
04 Development
The developing roller absorbs toner from the toner hopper, and the developing blade spreads the toner on the developing roller into a thin, even layer.
05 Transfer
In the toner cartridge, the transfer roller carries a positive charge.
06 Fixing
The toner on the paper is heated and melted by the heating element, and then fixed to the paper by the resin in the toner, making the toner more firmly attached and less likely to fall off.
07 Cleaning
After printing, residual toner and charge remain on the photosensitive drum.
The cleaning scraper will clean the photosensitive drum to prevent it from affecting the next print.
II. Precautions for charging laser printers
The photoconductor material on the surface of the photosensitive drum is an insulator and neutral in the absence of light, carrying no charge. To achieve an "electrostatic latent image" on the photoconductor surface, it must be charged. Only then, when a laser beam scans the photoconductor, the exposed points become conductive, forming a beam array. The charge in the array interacts with the substrate to form a "potential difference latent image." When the photosensitive drum rotates to a position tangent to the developing magnetic roller, it attracts toner, which carries a charge opposite to that on the photoconductor surface, to the photosensitive drum surface, thus displaying a toner image on the drum.
To enable the photosensitive drum to adsorb toner according to the image information, the drum must first be charged. The charging electrode is a tungsten filament parallel to the axis of the photosensitive drum, carrying a DC high voltage of 5-7kV. When the surface of the drum is very close to the tungsten filament, the surrounding air is ionized, generating a corona discharge that charges the photosensitive drum. The positive or negative voltage is determined by the voltage carried by the tungsten filament. If the photoconductive material is a selenium-tellurium alloy, it is positively charged. After one rotation of the photosensitive drum, the entire surface is charged.
The method of charging the photosensitive drum of a laser printer varies depending on the model, but the basic charging principle is the same: using DC high-voltage corona discharge to charge the surface of the photosensitive drum.
Early laser printers often used a composite structure of electrode wires and grids for charging, while most newer laser printers use a charging roller (FCR) to charge the photosensitive drum. When a high-voltage generator delivers a high voltage to the electrode wires, a strong electric field is formed between the electrode wires and the grid, releasing a corona discharge. This ionizes the air between the electrode wires and the photosensitive drum, causing air ions to migrate to the surface of the photosensitive drum and charge the photoconductor (drum). This method ensures a uniform charge on the photoconductor (drum) surface, but it also generates a large number of negative ions (ozone). When ozone accumulates to a certain level, it is harmful to the human body. Early Canon LBP-SX and ST models, early HP HP2 and 3 models, Panasonic KX6500 (manufactured in Japan), and Lenovo LJ6L and LJ6P models all used this charging method.
Most modern laser printers use a charging roller for charging. Because it uses a contact charging method, it does not require a high charging voltage and does not produce ozone. However, the accumulation of ionized dust increases wear on the photosensitive drum and can also cause uneven charging.
