We all know the difference between an insulating and a conductor. Despite that there are also materials which, depending on their conditions, may change their main inner characteristics: photoconductors, for example, are insulating substances which, after absorbing light, turn into conductors. This feature is exploited by a procedure known as xerography, which is at the base of copying machines and laser printers. The working principle of photocopiers consists in creating an electric image of the to-be-copied document over the photoconductor. Some colored pigment particles (toner) are layered on the electric image in order to get stuck to a blank sheet of paper, reproducing the original document.
A thin layer of photoconducting material is applied to a grounded metallic belt.
- The free photoconducting surface is electron sprayed by a metallic wire, as if a nebulizer covered it with paint. The electrons polarize the photoconductor and get stuck to it: the effect of the polarization is to form a collection of negative charges on the opposite face of the photoconductor, in contact with the metallic belt. This negative charge attracts a correspondent positive charge which gathers on the upper face of the belt.
- At this point, the document is lightened up and its image is projected over the photoconductor. Its exposed areas become conductive and, as this surface has a smaller potential level compared to the ground, the electrons scattered over these regions are immediately grounded. The shadowed areas, instead, keep their charge: over the free photoconductor surface a negative charged copied image is created (whilst a correspondent positively charged picture is kept on the surface facing the metallic belt).
- It is now necessary to transfer onto the paper the charged image. By a special brush device, some toner particles (plastic insulating dust containing colored pigments) get positively charged and approached to the photoconductor. The toner, attracted by charged areas (negative), gets scattered according to the original picture.
- In order to let toner particles detach from the photoconductor and attach to the paper, it is necessary to get rid of the electrostatic attraction. The photoconductor gets lightened again and the charged image gets erased. The positive toner particles are kept on the photoconducting layer, in correspondence to the original picture.
- After that a previously negatively charged sheet of paper is pressed over the photoconducting layer. The toner particles gets attracted without modifying the original drawing.
- The final image gets eventually pressed whilst the toner melted by proper heating. Before a new copy the photoconductor is lightened for a third time to wipe the residual electrons and cleaned, in the very end, from the not transferred toner.
That’s it! Cool, isn’t it?Francesco Pochetti