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How The Laser Printer Works.

Updated: Oct 31, 2019

The Printerworks With The Help of Collection Of Mysterious Sub Systems

Laser printers are truly amazing machines. Think about it: They accept an input of electrical pulses through a wire with a fancy plug. Seconds, mere seconds, later they eject a perfectly printed document. That document can include any number of fonts, lines, illustrations, and photographs.

The Printerworks With The Help of Collection Of Mysterious Sub Systems
The Printerworks With The Help of Collection Of Mysterious Sub Systems

To the uninitiated, it looks like a miracle Demystifying the miracle is the first step toward becoming a qualified repair technician. While it may not be strictly necessary to understand something to fix it -you fix your headache by taking an aspirin, but you don't understand what's happening in your brain-true confidence and competence in your technical abilities will only come with a thorough understanding of the way the printer operates.

Often on a repair call, I can determine what is wrong and what I'm going to do just by listening to the printer go through its cycle. Invariably, I can form a detailed diagnosis after the machine has printed, or at least tried to print, a page. I can do this because I am intimately familiar with the way the machine operates, intuitively and intellectually.

THE XEROGRAPHIC PROCESS-AN "OLD" INNOVATION

Xerography has been around and popular for 30 years. The principles of using toner to develop an image are so well understood, it's surprising that laser printing wasn't developed earlier. The first HP LaserJet was developed based on the PC 25 copier.

The Nature of Selenium Xerography became possible when it was discovered that the element selenium had peculiar electrical characteristics. It had the capability of receiving and storing a localized charge when confronted with high voltage. Even more interesting, it was found that light would dissipate the accepted charge. Tests indicated that a charged sheet of selenium-coated aluminum (or any other suitable base metal) would release its charge when struck by light, only in the area exposed to the light and proportionate to the intensity of the exposure, providing there was a ground path.

This created the opportunity to use static charges in the photographic process, as the electrostatic image created by light on the charged field was analogous to a photographic negative.

The Action of Charged Particles and Charged Surfaces.


What was lacking was a method to develop the image and to establish the best way of creating it. Clearly, the electrostatic process could be used in cameras, but that application seemed impractical and inferior compared with processes using existing exposure materials. However, it was tailor-made for copying, a field previously dominated by mimeographs (producing low-quality output), thermal copiers (an undesirable process) and actual photocopiers (expensive to operate). Since one could create an electrostatic image of contrasting charges within a detained field, it became inevitable that a process would be found to develop the image visually using some form of opaque charged particle.


Static electricity had been well understood for some time; it had been used in parlor tricks back in the 1600s. There were, however, few practical applications using static electricity until the Twentieth Century The basis for developing an image on the selenium surface was a simple idea that like-charged particles repel one another and opposite-charged particles attract each other.


Fairly quickly, it was discovered that an image could be formed by shining an intense light on an object or paper and focusing the reflected light on the selenium-coated Organic Photoconductor (OPC). The result was that light areas in the original document were represented by discharged areas on the OPC, while the dark areas maintained their charge.

It was necessary to develop the darkened areas, the areas that still held a charge. In these machines, the initial charge placed on the OPC had one polarity, and the toner was charged with the opposite. This was called a "write white" system because the areas discharged by light, the written area, did not attract toner, while the areas not exposed to the light retained their charge and attracted toner.

THE IMAGE PROCESS IN THE LASER PRINTER


The first copiers were large, clumsy, heavy, failure-prone machines that made fair copies but were lousy at handling paper. By the end of the '60s, however, copiers had become common in American offices; the age of xerography had begun. By the late '70s, Canon brought the convenience of quick copying into small offices and homes with the PC copier. As personal computers gained power and popularity, it became apparent that some practical printer was needed that could readily merge text and graphics for office-prepared documents.

The PC copier looked like a fine platform for developing an electrostatic printer. The problem challenging early printer developers was designing an electrostatic imaging process that would take computer data and convert it to an electrostatic image.

Lasers were fairly well understood about this time, although they were far from being the logical choice for producing an image stream in an electrostatic printer In fact, though they were used in the earliest laser printer prototypes, true lasers have been discarded in favor of light-emitting diodes that offer similar properties.

Because we're talking about laser printers, however, we'll continue to call the light source a laser beam.

Since one could create an electrostatic image of contrasting charges within a defined field, it became inevitable that a process would be found to develop the image visually using some form of an opaque charged particle.

Static electricity had been well understood for some time; it had been used in parlor tricks back in the 1600s. There were, however, few practical applications using static electricity until the Twentieth Century.


The basis for developing an image on the selenium surface was a simple idea that like-charged particles repel one another and opposite-charged particles attract each other. Fairly quickly, it was discovered that an image could be formed by shining an intense light on an object or paper and focusing the reflected light on the selenium-coated Organic Photoconductor (OPC). The result was that light areas in the original document were represented by discharged areas on the OPC, while the dark areas maintained their charge.

While a specific area of the OPC or the paper for the final print is being subjected to only one part of the total process at a time, all six process steps occur simultaneously at different positions, once the paper feed is initiated. While one area of the OPC is being cleaned, another is being conditioned, while a third area is being written upon, and so on.


  1. Cleaning - In older printers, cleaning required two steps: (1) Toner left on the drum from the last transference was wiped off by a Cleaning Blade; and (2), the remaining electrostatic image had to be erased. Erasure was accomplished by focusing incandescent light through red filters onto the drum in the area just past the Cleaning Blade. This effectively grounded the drum surface in that area, discharging it uniformly and destroying any latent image. Later development introduced Primary Charge Rollers (PCRS) that combine electrostatic cleaning with conditioning and eliminate the need for Erase Lamps.

  2. Conditioning - Before any image can be produced, the OPC must be conditioned so that a uniform negative charge can be given to the drum. In older printers, this was done by a