Mounting composition on film
Composition on film consists of a mounting operation carried out on a luminous table. Films of the text and titles and positives or negatives of the screened or unscreened photographs, depending on the printing method to be used, are arranged on a sheet of transparent plastic of the dimensions indicated on the layout, placed under the plastic sheet and lighted from beneath. The film is glued or fixed using pieces of transparent adhesive.
Conversion systems
One composition process can be converted to the other. A page composed on film, in negative, can be used for photogravure plates or for metal or engraved plates intended for letterpress printing. Contrariwise, a page composed in type can be converted to a positive or negative, direct or inverted transparency by any of a number of techniques.
The whole page, including the screened plates or photographic illustrations, can be subjected to conversion, or the text can be taken alone, leaving the positioning of illustrations until the positives or negatives of the illustrations, screened or not, according to the printing process to be used, are included in the makeup at a later stage.
Printing (press operation)
Printing in the second sense of the word—that is, press operation—is the technique by which ink or other colouring agent, transferred to paper or any other material, is localized on printing surfaces delimited by the composition of texts or the making of illustration material.
Colour printing
Juxtaposition of colours is achieved by submitting each sheet to successive impressions by typeforms each of which prints only on areas designed to carry a single colour and inked only in that colour.
There are sufficient wavelengths in the three primary colours, blue, red, and green, to reconstitute all the colours of the spectrum. The colour perceived from an inked impression results from the fact that the ink reflects some waves of the range of colours of the spectrum and absorbs the others—that is, prevents them from being seen. Thus, three colours of ink can reconstitute the visual effect of all the range of colours by combining them appropriately. Yellow totally absorbs all waves of colour that are basically blue and reflects waves of colour that are basically red and green; magenta (deep crimson) totally absorbs green and reflects red and blue; cyan (close to turquoise) absorbs red and reflects blue and green.
When two of these inks combine, each annuls in the other the capacity to reflect that one of the two primary colours that it cannot itself reflect; the eye then perceives only the one primary colour that both inks reflect; for example, red, by combining yellow and magenta inks. All three inks combined no longer reflect any of the three primary colours but appear as black.
In trichromatic printing the screened plates necessary for each of the three colours of ink are prepared by selecting the colours through filters. A fourth plate is usually used for making a print in black ink to accentuate the contours and modelling in the picture, making the process quadrichromatic.
Printing colours by superposition requires the exact positioning on top of one another of the successively printed constituent parts of the picture (usually printed in this order: magenta, yellow, cyan, black). The finer the definition of the screen, the more precise this positioning must be.
Letterpress printing
Letterpress printing consists of transferring a thin film of ink from the printing surface of the typeform to that of the paper by pressing the two together.
Letterpress presses are thus made up of two principal elements, one bearing the type form, the other exerting the pressure. These elements may be either flat or cylindrical. There are three principal types of presses, according to the way these elements are combined: (1) plane to plane; (2) cylinder to plane; and (3) cylinder to cylinder. In all types of letterpress presses the printing surface must be coated with a uniform layer of ink before receiving the paper. This inking is carried out by a mechanism composed of up to 20 rollers: take-up rollers carry the paste ink from the ink supply to the distributing and sliding rollers, which operate with a longitudinal to-and-fro movement to crush and spread the ink in an even layer on a level or cylindrical metal surface and finally contact rollers that transfer the ink to the printing surface of the typeform.
The first two kinds of presses are sheet fed. The third can be either sheet fed or roll fed (web fed), depending on the model and type of work. Sheets of paper must be fed into the press in a sequence synchronized with the movement of the press. Most modern presses are equipped with an automatic feeder that functions in conjunction with the movement of the press. The feeders operate by either friction or suction. In a friction-operated feeder, the sheets of a pile of stock on a slightly slanting surface are fanned out so that each sheet projects over the one beneath in such a way that the friction of a cylinder can dispatch them individually one after another toward the feedboard of the press, where three pegs guide each into position. In a suction operation the sheets remain piled up vertically; a wheel brushes a corner of the top sheet and separates it from the others, enabling a compressed-air blower to inject a cushion of air underneath the sheet. A system of vents connected to a suction pipe lifts the paper and carries it towards the press’s feedboard. An automatic device lifts the pile of sheets continuously into place. On all letterpress presses, slight irregularities of the surface of the typeform are compensated for by packing the platen with a material soft enough to absorb the irregularities.
As the sheets leave the press, a powder is sprayed onto their surface to form a separative coating that prevents the transfer of ink from one sheet to another. An alternative in modern high-speed presses is the incorporation in the ink of a special quick-drying agent.
Platen presses
Presses that operate plane to plane are called platen presses. A vertical clamping contrivance clamps the bed, which carries the form into which the composed type is locked, and the platen, which carries the sheet of paper while it is being printed. When this clamping contrivance is open, the typeform is inked by a series of rollers that descend and then reascend, and the printed sheet is removed and a new sheet placed in position on the platen.
The pressure exerted during impression is about 40 kilograms per square centimetre (570 pounds per square inch). A platen press can reach a production speed of 5,000 sheets per hour.