HOW THEY WORK : “How Does a Photocopying Machine Work (Cont’d)”

As seen in our last episode, a copier has a special device known as the drum. It is the drum, with the help of static electricity that does the transformation of the image from the reflecting mirror network to the hard copy on paper. Inside the copier there is also a very fine black powder called toner. The drum, charged with static electricity, can attract the toner particles. There are three things about the drum and the toner, that make the photocopier do the magic it does. The drum can be selectively charged, so that only parts of it attract toner. In a copier, you make an “image” by employing the static electricity accumulated onto the surface of its drum. Where the original sheet of paper is black, you create static electricity on the drum. Where the subject to be copied is white, you do not. What you want is for the white areas of the original sheet of paper to not attract toner. The way this selectivity is accomplished in a copier is with light; and that is why it is called a photocopier (copying using light)! Somehow, the toner has to get onto the drum and then onto a sheet of paper. The drum selectively attracts toner. Then the sheet of paper gets charged with static electricity and it attracts the toner off the drum. The toner being heat sensitive means that, the loose toner particles are attached (fused) to the paper with heat as soon as they come off the drum. The drum, or belt, is made out of photoconductive (sensitive to light) material. Below are the procedures involved in photocopying; the surface of the drum is charged. A very strong beam of light is shone across the paper that is placed on the copier’s glass surface. Light is reflected from white areas of the paper and strikes the drum below. This process may be termed as scanning the document. Wherever a photon of light hits, electrons are emitted from the photoconductive atoms in the drum and these neutralize the positive charges above. Dark areas on the original (such as pictures or text) do not reflect light onto the drum, leaving regions of positive charges on the drum’s surface. Negatively charged, dry, black pigment called toner is then spread over the surface of the drum, and the pigment particles are attracted only to the positive charges that remain. A positively charged sheet of paper then passes over the surface of the drum, attracting the beads of toner away from it. The paper is then heated and pressed to fuse the image formed by the toner sticking to the paper’s surface. The photoreceptor drum (or, in some photocopiers, belt) is the heart of the system. A drum is basically a metal roller covered by a layer of photoconductive material. This layer is made out of a semiconductor such as selenium, germanium or silicon. What makes elements like selenium so cool is that they can conduct electricity in some cases, but not in others. In the dark, the photoconductive layer on the drum acts as an insulator, resisting the flow of electrons from one atom to another; however, when the layer is hit by light, the energy of the photons liberates electrons and allows current to pass through! These newly freed electrons are what neutralize the positive charge coating the drum to form the latent image. Corona Wires are used in creating charges. For a photocopier to work, a field of positive charges must be generated on the surface of both the drum and the copy paper. These tasks are accomplished by the corona wires. These wires are subjected to a high voltage (about 2000Volts plus), which they subsequently transfer to the drum and paper in the form of static electricity. One of these wires is stretched parallel to the drum surface and charges the photoconductive surface with positive ions, and the other wire is positioned to coat the paper’s surface as the paper shoots by on its way to the drum. Lamps and Lenses are employed in collecting of the image from the subject matter being copied. Making a photocopy, requires a light source with enough energy to set into motion the photoconductive atoms. What wavelengths of light can do this? It turns out that most of the visible spectrum of light contains enough energy to drive the process, especially the green and blue end of the spectrum; anything lower than the red portion of the visible spectrum doesn’t have enough push to activate the photoconductor. Although Ultra Violet light has more than enough firepower to make a photocopy, it can be very damaging to our eyes and skin. This is why photocopiers use a plain old incandescent or fluorescent bulb to flash light onto the original document. The fuser provides the finishing touches that make the toner image on a sheet of paper permanent. The fuser has to do two things and these are; Melt and press the toner image into the paper, and prevent the melted toner and/or the paper from sticking to the fuser. All that’s required to accomplish these tasks is quartz tube lamps and Teflon-coated rollers. The sheet of paper is sent between two of the rollers. Then, the rollers gently press down on the page to embed the toner in the paper fibre. Meanwhile, inside the rollers, as the lamps are on, they generate enough heat to melt the toner. eddie@afrowebs.com