LASER PRINTER IMAGE FORMATION

0 CALIBRATION (COLOR). In a monochrome printer, precise control of image alignment and density is not critical – small variations are not even noticed. But in a color printer, since you are combining four separate color planes into a single composite image, it is vital that all four planes are the same density, and that they line up precisely with each other. This is why color printers need to calibrate frequently (on each power-up, and whenever a toner cartridge or other imaging element is replaced).

 

1 CONVERSION. The computer converts the print job into a page description language and sends it to the printer formatter board.

Page Description Languages include Adobe PostScript (PS), HP Printer Command Language (PCL), and Microsoft XML Page Specification (XPS).

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2 PROCESSING. The formatter separates the print job into image location information and print process instructions and stores the image information in memory. The DC controller synchronizes image formation with input and output and also signals the formatter to send data.

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3 CONDITIONING. The photoconductive drum is cleaned of toner by a rubber blade and receives an even, negative DC charge from the charge roller, as well as a dose of AC bias to remove residual images.

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4 WRITING. The laser beam discharges tiny areas of the photosensitive drum to form an invisible image.

The images on the drum and on the printed page consist of horizontal rows of closely spaced dots. On a 1,200 dpi printer, the dots would be 1/1,200 of an inch.

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Color Toner Cartridge and Laser/Scanner Orientation: Two Kinds Early Carousels. A single laser/scanner writes to four toner cartridges mounted in a carousel. After each cartridge drum is written upon, the image is transferred to the belt (see step 6), and the next cartridge is rotated into the active position, and so on.
Single Pass. Each cartridge is fixed and has its own laser. This allows all four color planes to be laid down in a single pass.

 

5 DEVELOPING. Negatively charged toner adheres to discharged areas of the drum.

The toner gets the negative charge from the developer roller. Since like-charged objects repel each other, the toner does not adhere to the negatively charged parts of the drum, but only to discharged areas.

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6 TRANSFERRING. The transfer roller adds a positive charge to the back of the paper, which attracts the toner from drum to page.

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Color Transfer Process: Two Kinds. Instead of a transfer roller, color laser printers have a transfer belt (or drum) that is large enough to hold a complete page-size image, or a full sheet of paper. There are two methods.

  • Multi-stage transfer process: toner is first transferred from the image drums to the transfer belt/drum (primary transfer). This process is repeated four times until we have a composite image (all four colors) on the drum/belt, and then that image is transferred to the paper (secondary transfer). Single-pass laser printers may use the multi-stage transfer process and carousel printers must use it.
  • Direct transfer, used by many newer printers, has the transfer belt carry a sheet of paper past the toner cartridges, with the toner transferred directly from the image drums to the paper.
 

7 FUSING. Lower roller pressure and upper roller heat bond the toner to the paper.

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