So how does a Colour Photocopier Work?
- Charge. Inside every photocopier and laser printer is a light-sensitive surface called a photoreceptor. It consists of a thin layer of photoconductive material that is applied to a flexible belt or drum. The photoreceptor is insulating in the dark, but becomes conducting when it is exposed to light. It is charged in the dark by applying a high DC voltage to adjacent wires, which produces an intense electric field near the wires that causes the air molecules to ionise. Ions of the same polarity as the voltage on the wires deposit on the photoreceptor's surface, creating an electric field across it.
- Expose. In a digital photocopier or printer, the image is exposed on the photoreceptor with a scanning modulated laser or a light-emitting-diode image bar. In older analogue photocopiers, reflected light from an illuminated image would have projected onto the photoreceptor. In either case, the areas of the photoreceptor exposed to light are selectively discharged, causing a reduction in the electric field. The darker areas retain their charge.
- Develop. Pigmented powder used to develop the image is called toner. Toner particles made of colourant and plastic resin have precisely controlled electrostatic properties and range from about five to 10 micrometers in diameter. They are mixed with and charged by magnetised carrier beads that transport them to the development zone. The particles are charged by the phenomenon of triboelectricity (often referred to as static electricity). The electric field associated with the charge pattern of the image on the photoreceptor exerts an electrostatic force on the charged toner, which adheres to the image. A colour document is formed by a printer with four separate xerographic units that create and develop separate cyan, magenta, yellow and black images. The superposition of these powder images produces full-colour documents.
- Transfer. The powder image is transferred from the photoreceptor onto paper by bringing the paper in contact with the toner and then applying a charge with polarity opposite to that of the toner. The charge must be strong enough to overcome the powder's adhesion to the photoreceptor. A second precisely controlled charge releases the paper, now containing the image, from the photoreceptor.
- Fuse. In the fusing process, the toner comprising the image is melted and bonded to the paper. This is accomplished by passing the paper through a pair of rollers. A heated roll melts the toner, which is fused to the paper with the aid of pressure from the second roll.
- Clean. Toner transfer from the photoreceptor to the paper is not 100 percent efficient, and residual toner must be removed from the photoreceptor before the next print cycle. Most medium- and high-speed photocopiers and printers accomplish this with a rotating brush cleaner.
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Xerography is a unique process that depends on chemical, electrical, mechanical and software know-how. The rapid and economical digital printing process easily produces either one print or hundreds of identical prints in black and white or colour. More importantly, the capability of page-to-page variable imaging with xerography enables on-demand printing of complete documents such as brochures and books. Such on-demand printing can save time, reduce cost, and eliminate document obsolescence, overruns and warehousing.
Colours and Toner in Colour Photocopiers
The secret in going from black and white to colour is to separate the coloured image into its 3 "process colour" components. By colour mixing using toner (ink) of just 3 colours: cyan, magenta, and yellow any colour image can be reproduced. To compensate for some limitations of the system, black is usually added as a forth component to render dark colours. So the colour photocopier first creates 4 separate component pictures by filtering the original picture through appropriate (red, green, blue, and no filter for the black) filters. The latent images (charge patterns) can be on different photoconductor drums or on adjacent sectors of the same drum or sequentially on the same drum. The charge image created by the red partial image is then contacted with cyan toner, the one created by the green light with the magenta toner, etc., and the toners are sequentially transferred to the substrate, possibly via an intermediate carrier (rubber sheet). This is the story for the analog process. In the digital case, the image is first digitised and stored by scanning using wavelength-sensitive elements. In a second step, computer-controlled coloured lasers then "write" the image on the photoconductor drum, followed by transfer to