What is Photo-Engraving of Printing Technology?

Photo-engravers make metal printing plates of illustrations, pictures, maps, charts, and other material that cannot be set in type. On such plates, and like the letters of type-set matter, the printing surfaces stand out in relief above the non-printing portion.

When the image (printing portion) is etched below the plate-surface, for use in gravure process, workers engaged on making such plates are called photogravure plate makers or roto-gravure photo-engravers.

In making a photo-engraved plate (i. e., blocks) for the letter­press process, the entire job may be one man or it may be divided among a number of skilled workers, each specialising in a particular operation. These specialised workers are known as photo-engravers, printers, etchers, finishers, routers, etc. In large printing jobs, work is almost always divided among a number of specialists.

(i) In Photo-engraving, the process is treated by a Cameraman. For a half-tone block, he photographs the material to be printed through a cross-lined screen which breaks down the copy into a multitude of tiny dots. For a line block, the screen is not required. He then develops the negative.

(ii) The Photo Printer prints the image on a metal plate. This is done by coating the metal plate with a sensitized chemi­cal solution; then exposing the plate to arc lights. The image (printing) areas are protected by chemicals, so that they are not etched away along with other non-printing areas of the plate when placed in an acid bath by the etcher.

(iii) The Etcher etches (cuts away) the background areas by means of an acid bath, leaving the image standing out in relief.

The printed plate is then finished (carefully inspected and touched up with hand-tools) and etched and re-etched (to bring out sharp details of the designs and illustrations).

(iv) A Router cuts away metal from the non-printing part of the plate to prevent it from touching the inking rollers during printing.

(v) A Mounter fixes the engraving etched type or image plate by cutting it to size, levelling its sides, drilling holes and mounting the plate on wooden block, by nails, to bring it to the right height for use in printing.

(vi) A Proof-man then prints a sample copy for checking the impression.

In roto-gravure photo-engraving, operations involved are the same as in letterpress photo-engraving, except that a positive (and not a negative) is used in making the plate and that it is the image (rather than the background) areas that are etched away.

There are some other workers also engaged in the photo­engraving section. These include:

(i) The Mounter who fixes etched type or image plate on wooden block for printing;

(ii) The Router who reduces level of non-type or non-image portions of the mounted plate to make it non-printable;

(iii) The Electro-type prepares the duplicate printing plate using electroplating process, using hand-tools, electro­plating equipment and various machines. This involves several steps. A mould of the type forms is made, a metallic coating is then deposited on the mould by coating it with a (copper sulphate or nickel) solution, if the mould is made of wax, or by depositing a film of metallic silver, if the mould is made of plastic.

The mould is then suspended in electrolytic solution, which forms a metallic shell deposit on the coated mould. Later on, the metallic shell is striped from the mould, and the shell is locked with metal and carefully finished.

(iv) The Stereotypes casts duplicate printing plates from a flung (specially prepared thick sheet of papier mache). The stereotyping process is much simpler, quicker and less expensive than electrotyping, though it does not make an equally durable fine plate.

In stereotyping, moulds are called mats (matrices) and are made of flung (papier mache, a material composed of paper pulp) instead of wax or plastic. For this, the moist mat (or a dry mat, as in the case of newspaper printing) is placed on the type-forms (typeset matter) and is covered with a padding-a cork blanket and a sheet of fiber board or even by extra sheets of thick paper (to protect the flung from damage). Then the covered form is pressed under heavy power-driven steel rolls which form impressions of the type and photo-engravings on the mat. Next, the paper mould is dried and placed within a stereotype casting box from which a stereo plate is cast.

In the larger presses, stereotype plates are cast automatically in a machine (called ‘autoplate’). In smaller shops, dectrotypers/ stereotypes are all-round workers, but in larger establishments, they may specialise in or perform only specific items of work, e. g. wax casting, moulding, finishing, block-making, etc.