Printing

The use of photographs in book and magazine production has long since supplanted hand-engraving, for which a considerable amount of copper was used. Nevertheless the use of copper has increased. Today, by far the larger part of book illustration, except in the case of line blocks which are generally zinc, is by means of 'half-tone blocks' or their copies, 'electros'. These are produced by photo-engraving, which is also known as process engraving.

A half-tone printed picture is made up, as a magnifying glass will show, of thousands of dots of ink large and small, large dots in the lightest areas, grading down into fine dots in the shadows. A copper plate with the polished surface sensitized by an emulsion, usually bichromated fish glue, has the image of the photograph impressed upon it in a camera; and a glass screen, ruled to a suitable mesh, is inserted between the negative and the plate, so as to regulate the sizes of the dots and thereby the quality of the half-tone. The sensitized plate is then heated or 'burnt in' until the glue becomes a hard enamel. To avoid softening the copper on heating, process plates always contain a little silver or arsenic, which raises its softening temperature. The plate is now etched, so as to make the dots stand up in relief and thereby provide a printing surface.

One interesting application of copper in printing is in the making of matrices for 'Linotype' and other automatic type-setting machines. The compositor presses a keyboard on the machine, rather like a typewriter keyboard, and as each key is depressed a small piece of brass called a matrix is released from an inclined storage rack or magazine. The matrices are each impressed with a letter or figure; leaded brass is usually employed, although copper or bronze may also be used. A belt conveyor carries the released matrix to a box, where it joins others to form all the words in a line. The operator then presses a small lever which moves the line opposite the mould wheel, and the whole line is cast in one piece or slug by pumping in molten type-metal. After trimming, the finished slug is delivered on to the galley; meanwhile, the matrices are automatically returned to their magazines.

Copper Printed Circuit Figure 46. In recent years the introduction of the copper printed circuit has revolutionized wiring in the electrical and electronics industries.
Another very interesting use for copper which has come to the fore during recent years is the printed circuit, now so widely employed by electrical engineers everywhere, especially in the electronics industry. This shows the value of applying the techniques established in one industry to another completely different in structure and outlook. Wherever elaborate wiring layouts are involved, as in radio and television sets, computers, etc., an immense amount of labour in wiring and fixing the circuits may be saved by simply photographing the wiring diagram on to a copper sheet, which can then be etched so as to leave the diagram in relief (Fig. 46).

By punching holes through the points of connexion, through which terminal wires or tags from valves or other parts pass, and soldering these tags to the plate, the circuits are established; thus, by the aid of dip-soldering, all the joints can be fixed in a single operation. For the circuits themselves a laminated sheet is employed, having a face and sometimes also a base of copper foil which is bonded to a sheet of plastic.