INTRODUCTION
Lithographic platemaking provides an ink-receptive positive image and an ink-resistant (desensitized) nonimage area on a plane surface. Many chemical mixtures are used to provide and develop the photo-sensitive ink-receptive image areas and render nonimage areas resistant to adherence of ink. Most metals have a natural tendency to be ink receptive and are to varying degrees water repellent. All platemaking procedures have as their objective the preparation of a metal surface which will selectively and continuously receive a charge of ink from the press rollers, transfer this with maximum effect to a transfer blanket of rubber or synthetic composition surface, and maintain a clean non-image area by the use of a weak gum-acid solution continuously replenished by a press dampening system.
The main functions of platemaking are graining, imposition, plate-making, and proofing.
IMPOSITION
Purpose.To provide a “flat,” equal to the size of the plate to be processed, which will pass light through the image areas to a light-sensitive coating on a base plate and prevent passage of light to the nonimage areas. The arrangement of the negatives on the flat is dictated by subsequent folding, stitching, gathering, punching, and other binding operations.
Materials.Various rulers, straightedges, triangles, scribers, paper, knives, ruling pens, dividers, pencils, brushes for opaque, magnifiers, register punches, pens, pressure-sensitive tape, paperweights, and sheets of opaque masking paper.
Equipment required.Glass top light table so constructed that light will be provided underneath the entire area of the flat for visual inspection and positioning of negatives; lineup table, which consists of geared precision straightedges at right angles, for use in drawing accurate layouts and for verifying imposition and lineup of forms and pages after imposition.
Procedure.Following instruction on jacket, copy, or proofs, the imposer uses one of the several standard layouts or prepares a special layout to show position of pages for a particular signature. The lay-out will contain guidelines for the proper positioning of negatives with respect to page margins, point of fold if required, and the point of trim of the printed page. Also indicated are the space allowance for clamping the plate on the press and the gripper allowance requirement which is usually %6 or % inch. Negatives are inspected and trimmed to prevent overlap of one negative on another. Pages are placed in proper arrangement for correct folding, in accordance with imposition codes indicated on the jacket, face down on a golden-rod mask sheet. All negatives of register marks or guidelines required for positioning are correctly positioned on the flat. Page negatives are positioned on the flat according to the predetermined margins drawn on the layout. Because the negative is opaque and a line cannot be easily seen, it is advisable to place a straightedge across the entire flat at the position of margin requirement and move the negative image in alinement with the straightedge. A triangle which is longer than the right-angle dimension of the image being positioned may be used to place the negative in place at head or foot at the same time that side margin position is accomplished. Usually a visual placement of the negative with a verification of alinement with a straightedge will suffice for most positioning requirements. A determination that proper bleed allowance has been given can be verified visually from inspection of the trim line on the layout.
Halftones can be positioned in accordance with the guidelines on the layout because the lines are visible through the tone areas of the negative. After negatives have been placed in correct position for mar-gins, all variations such as bleed requirement, indentions of continuing text, special margin allowances for chapter heads, special alinement of illustrations across the fold, and correct position of other page elements must be carefully reviewed to be sure that all variances of standard page position are correctly treated. The negatives are attached to the goldenrod mask sheet with short pieces of pressure-sensitive tape. The opaque mask is cut away from those areas of the negative which are to be exposed to the sensitized plate. Minor scratches and prominent pinholes in the negative are opaqued with a brush in those areas from which the mask has been cut. The flat is ready for exposure to the sensitized metal sheet.
Step-and-repeat work.There are many occasions when it is desirable to print several of the same images on a single form. In the case of longrun work, many images on a sheet will materially reduce the amount of presswork. As an example, an 8 by 10 1/2-inch form calling for 10 million copies can be printed with 40,000 impressions by printing 25 images on a 4o by 52 1/2-inch sheet. If printed as a single image, 10 million impressions would be required. This plate can be produced by making 25 duplicate negatives or by using one negative and exposing this image to the plate 25 times. Special automatic machines, called step-and-repeat machines, can make as many exposures as necessary to a single plate from a single negative held in a special holder. This holder must be manually fixed into position in the machine. The movement of the special carrier and the exposure are all controlled by a punched tape and electronic controls without operator attention. The accuracy of this procedure for register work dictates that for all close register work of one or more colors this system would be used in preference to hand registering of multiple negatives on full-size flats. From information punched into the tape, the following steps are accomplished automatically: (1) Move the plate forward into position against negative holder, (2) apply vacuum, (3) switch arc lamp on, (4) expose for the correct time, (5) switch arc lamp off, (6) release vacuum, (7) move plate back from negative holder, (8) move holder and/or plate into next correct position as necessary, and (9) repeat previous steps as required. The plate must be manually placed and removed from the machine.
Deep-etch work.Imposition of positives for deep-etch platemaking is similar to that for negatives except that a positive flat is, clear in all of the nonprinting areas and the image is opaque. A transparent sheet must be used to position the positives instead of the opaque mask used for negatives. A positive is used instead of a negative because it is necessary to expose a coating in the nonprinting areas of the plate so that the base metal in the image areas can be uncovered. A very slight etch is given the image and a tough lacquer coating is inserted into the areas etched. When this is accomplished the original hardened nonimage area is dissolved by chemicals that do not affect the lacquer image. This step leaves the original imposed image as the printing image.