This laser printer is ubiquitous in our daily lives, leading the pack as far as computer printer sales go. But they are also the great deceivers of the printing realm. Because although the word ‘laser’ suggests that a beam of light is what transfers ink to paper, the reality is quite different.
The Laser is Only a Small Part
Despite being included in the name of the process and printing machine, the laser’s role in printing is actually a small one. At its core, the printing process which involves the laser is known as electrophotography and is based on the same two physical principles: static electricity and photo-conductivity. The electrophotographic printing process was invented in 1971, with the first laser printers entering the market in 1984.
The principle of electrostatic attraction is that any material can carry a static electric charge, and therefore be attracted to or repelled by another material, dependent on strength and polarity of that charge. In the case of the laser printer, the toner carries a charge which attracts it to the image-carrying areas of the printer’s photoconductor.
The principle of photoconductivity is that some plastic and semiconductors will act in the dark as insulators, but act in the light as conductors. Therefore, if coated with a photoconductive material, any surface will charge in the dark to a high voltage. In the light, an image can be printed on that surface. Any place the light hits is where static electricity will be discharged. This effectively stores an image on the photoconductor in static.
The Photo-Conductive Surface
The surface of a laser printer that is photo-conductive is its drum. Made of metal, the outside of this drum contains one or more plastic layers. In addition to providing a photoconductive surface, the drum provides structural support, as well as temporary image memory during printing.
The surface of the drum serves as an insulator when in darkness, and so takes on a negative charge when it passes in darkness near a charged wire.
Laser Light Scanning
The role of the laser is to ‘paint’ the image to be printed onto the drum. Scanning from side to side, the laser works at the same time the drum is turning. This forms a raster pattern. This exposure of the drum to the laser’s light discharges its negative surface charge to the metal layers beneath, effectively transferring the image there in the form of static electricity.
Making the Image Visible
Once the static image has been transferred, the next step is to make it visible. This is done by introducing small toner particles to the drum’s static-charged surface. The vessel which holds the toner and iron filings is called the developer. This hopper is positioned facing the drum at very close proximity, with a charge equal to that of the drum. This ensures that the toner sticks only to areas where laser light has touched the drum and transferred the image in electro-static form.
As the developer rotates, a thin layer of filings coated with toner is sent toward the drum. The developer’s magnet causes the toner-coated filings to stand on their ends as they move past the drum and define the image in toner.
The Transfer Station
The now toner-defined image on the drum rotates until it is positioned over the paper, placing them both over what is known as a transfer station. Carrying a positive charge, the transfer station attracts the toner from the metal of the drum and transfers it to the paper. Following this action, the drum is cleaned by ‘eraser lights’ and a blade which removes excess toner, returning the drum to its clean state.
The result of the electrophotographic printing process is a printed image onto the paper’s top layer that is made of a very thin, melted plastic layer. This process is often employed where the printing of security labels is required, as it allows for label text to be protected against the ravages of light, weather and temperature.