Dan A. Hays, a senior fellow at the Xerox Corporation's Wilson Center for Research and Technology, explains.
The xerographic process, which was invented by Chester Carlson in 1938 and developed and commercialized by the Xerox Corporation, is widely used to produce high-quality text and graphic images on paper. Because Xerox marketed the first plain-paper copiers, photocopying machines made by other companies are sometimes referred to as "Xerox machines" by those who don¿t realize it¿s a misuse of the company¿s trademark. In addition, the xerographic process is actually used to make both copies and prints.
Carlson originally called the process electrophotography. It¿s based on two natural phenomena: that materials of opposite electrical charges attract and that some materials become better conductors of electricity when exposed to light. Carlson invented a six-step process to transfer an image from one surface to another using these phenomena. First, a photoconductive surface is given a positive electrical charge. The photoconductive surface is then exposed to the image of a document. Because the illuminated sections (the non-image areas) become more conductive, the charge dissipates in the exposed areas. Negatively charged powder spread over the surface adheres through electrostatic attraction to the positively charged image areas. A piece of paper is placed over the powder image and then given a positive charge. The negatively charged powder is attracted to the paper as it is separated from the photoconductor. Finally, heat fuses the powder image to the paper, producing a copy of the original image.
Carlson¿s first image, produced on October 22, 1938, was created with negatively charged yellowish moss spores (lycopodium) on a sulfur-coated zinc plate that had been positively charged by rubbing it with a handkerchief. Today¿s copiers and printers have automated and refined all the steps, moving paper at speeds of more than 250 feet a minute, digitally creating and exposing images, and producing images in a rainbow of colors.
Here¿s how the process works today:
- Charge. Inside every copier 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 ionize. 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 copier or printer, the image is exposed on the photoreceptor with a scanning modulated laser or a light-emitting-diode image bar. In older analog copiers, reflected light from an illuminated image is 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 colorant 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 magnetized 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 color 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-color 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 copiers and printers accomplish this with a rotating brush cleaner.
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 color. 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.