Photocopying

The next step in copying involves the addition of a toner to the photoconducting surface. A toner is a positively charged material that is added to the photoconducting surface. Since it carries an electrical charge opposite that of the negatively-charged photoconducting surface, the toner sticks to the surface. The photoconducting surface now carries toner on its surface that matches regions of negative electrical charge which, in turn, matches dark regions on the original document, such as the "a" mentioned above.

Finally, paper carrying a negative electrical charge is brought into contact with the photoconducting surface. The negative charge on the paper is made great enough to pull the positively-charge toner away from the photoconducting surface and onto itself. The letter "a" formed by toner on the photoconducting surface, for example, has now been transferred to the paper. The paper passes through a pair of rollers that fuses (squeezes) the toner into the paper, forming a positive image that exactly corresponds to the image on the original document.

As the final copy is delivered to a tray outside the machine, the photoconducting surface continues on its way. Any remaining electrical charge is removed and the surface is cleaned. It then passes on to the charger, where the whole cycle is ready to be repeated.

Many kinds of toners have been developed for use in this process. As an example, one kind of toner consists of a thermoplastic resin (one that melts when it is heated) mixed with finely divided carbon. When the copy paper is passed through the rollers at the end of the copying process, the resin melts and then forms a permanent mixture with the carbon when it re-cools. Another kind of toner consists of finely divided carbon suspended in a petroleum-like liquid. The toner is sprayed on the photoconducting surface and, when the liquid evaporates, the carbon is left behind.