Laser printing Process

Laser printing Process

In most cases, your PC talks with controller circuitry (1) in your laser printer to queue up and translate printing data; a raster image processor (RIP) converts images and text into a virtual matrix of tiny dots.

The main actor, however, is the photoconducting drum (2), a specially coated cylinder that receives a positive or negative charge from acharging roller (3) (or, in some printers, a corona wire). A laser beam(4), switching rapidly on and off and deflected off a rotating mirror (5), scans the charged drum horizontally in precise lines. When the beam flashes on, it reverses the charge of tiny spots on the drum, corresponding to dots that are to be printed black. After the laser scans a line, a stepper motor advances the drum, and the laser repeats the process—all, of course, blindingly fast.

Next, the drum's laser-kissed portion encounters the developer roller (6), which is coated in charged toner particles from the toner hopper (7), part of the toner cartridge. Charged toner clings to the discharged areas of the drum, reproducing, in reverse, your images and text.

Meanwhile, a belt or roller assembly (8) draws paper inside from thepaper tray (9), past a transfer roller or charging wire (10) that applies a charge opposite the toner's to the paper. As the paper sheet meets the drum, the drum-borne toner transfers to paper. A cleaning blade (11)then cleans the drum, and the process continues in a smooth, circular flow. (Color lasers work similarly, but the paper may require four passes by the drum for four toner colors. Alternately, the printer may transfer each color layer to an intermediate belt before applying it to the paper, or employ four drum/toner assemblies.)

Last, your page, with its imprint of tenuously anchored toner, reaches thefuser (12)—a heat roller and a pressure roller. It melts the toner, which contains resins and sometimes wax, onto the page. Voila, pages in your out tray

1.Cleaning:-

During the first stage of the image formation process, a photosensitive drum's surface is prepared to hold an image through a physical and electrical cleaning process. Before a new image is formed on the drum, leftover toner from the previous image must be removed. This is accomplished by a rubber-cleaning blade which scrapes toner off the drum into a debris cavity. The drum is also electro statically cleaned by erase lamps which illuminate the drum's photosensitive material to neutralize any electrical charges that remain on the drum.

2. Conditioning:- 

After the photosensitive drum has been physically and electrically cleaned, it must be conditioned. The conditioning process consists of the application of a uniform negative charge on the surface of the photosensitive drum by the primary corona wire. A high negative voltage is applied to the primary corona wire by the high voltage power supply. Negative charges from the wire migrate to the surface of the drum. The primary corona grid is positioned between the primary corona and the drum's surface to regulate the voltage applied to the drum so that a uniform negative charge is applied.

3. Writing:-

 After rotating past the conditioning station, the drum has a uniform high negative charge on its surface. At the writing station, a sweeping laser beam is used to discharge this potential in selected areas by focusing laser lighton selected portions of the photosensitive drum. This creates what is known as an electrostatic image. This image is later developed into a visible image. Portions of the drum not exposed to the laser still have a high negative charge.

4. Developing:-

 At the developing station, the invisible electrostatic image is developed into a visible image on the photosensitive drum when toner from a developer cylinder is transferred to discharged areas on the drum. The toner particles acquire a negative charge from the developer cylinder which is connected a negative DC power supply. This electrostatic charge causes the toner particles to be attracted to the areas of the drum which have been exposed to the laser light and repelled to the areas not exposed. An AC potential is applied to the developer cylinder to assist the attraction of the toner from the developer cylinder to the photosensitive drum.

5. Transfer:- 

At the transfer station, the toner image on the photosensitive drum is transferred to the paper. A corona wires is positioned behind the paper so that the paper, which is traveling the same speed as the drum's surface is rotating, contacts the drum. This corona produces a positive charge on the paper. Because the positive charges on the paper are stronger, they pull the negatively charged toner particles off the drum and on to the paper. As the paper and drum advance, a row of metal teeth with a high negative charge called the static charge eliminator weakens the attractive force between the negatively charged drum and the positively charged paper. This helps separate the photosensitive drum and paper.

6. Fusing:-

Until the paper reaches the fusing station the toner is held on the paper only by gravity and weak electrostatic forces. At the fusing station, toner is melted and forced into the paper by pressure and heat of the fusing roller to produce a permanent image. The heat of the fusing roller is maintained by a high intensity lamp inside the fusing roller. At this point the finished print is ejected and the process is restarted from stage one.