Printing Techniques-III

ELECTRONIC PRINTING PROCESSES

All the processes previously discussed employ a fixed printing surface that transfers the same pattern of ink during each cycle of the press. Simple physical ink-transfer mechanisms allow these processes to operate at high speed. Because of the high cost of making a set of plates, mounting them on the press, and running the press until the printing is in register (properly aligned) and colors are correct, these processes require fairly long press runs to be economically feasible. For short-run printing—especially of highly variable information, electronic processes are more economical. These processes do not use printing plates, and they produce good reproductions without wasting paper.

Electrophotographic Printing.

Modern electrostatic office copiers have a printing surface that can be instantaneously formed by photographing or scanning an original. The surface is coated with a photoconductive material such as selenium or cadmium sulfide. In the dark, a photoconductor acts as an insulator, retaining a charge of static electricity. Areas of the surface illuminated in a camera or by a laser beam become conductive and lose their charge. The remaining areas retain their charge, attracting oppositely charged particles of colorant called toner. The toner is then transferred to a piece of paper or plastic using electrostatic forces rather than pressure. This cycle is repeated for each copy, making the process far too slow and complicated for mass printing applications. For small quantities, however, some color electrophotographic printers can reproduce color originals with image quality that approaches that of offset lithography.

Ink-jet Printing.

A computer-controlled array of ink nozzles can produce images on a moving sheet or a web of paper. Simple ink-jet printers are used routinely to print variable information such as the expiration dates on food packages or address labels on direct mail pieces, and are sometimes installed on the end of a conventional printing press. Sophisticated color ink-jet printers are able to produce lithographic-quality reproductions in extremely short runs.

Microcapsule Printing.

This technology uses paper impregnated with billions of microscopic capsules of liquid photopolymer-based dye. The paper is exposed to light reflected from an original image, and the dyes inside the capsules harden in proportion to the amount of light they receive. The exposed paper is then pressed through steel rollers against a receiver paper, and varying amounts of unhardened dye are deposited on the receiver to form an image. The process can be used to make high-quality color reproductions in small quantities.