Lithographic Presswork


Lithographic presswork transfers a continuously inked image from a plane surface by means of a cylinder-mounted metal plate containing the image to a rubber blanket and then to a sheet or roll of paper. Many varieties and thicknesses of paper stock and numerous different colors of ink are needed and used to produce the many kinds of printed jobs which are produced on offset presses. The phases of presswork are preparatory steps or makeready, starting adjustments, and running adjustments.


Procedure.-(1) Determine that press is oiled, that feeder, inking, dampening, printing, and delivery systems are complete and in good operating condition. Particular attention should be given to safety devices, controls, and cylinders. Remove old plates, waste stock, and leftover sheets to prevent possible mixing with new job. Obtain stock required. Be sure that proper ink is presently in the ink fountain or replace if necessary. Check to see that fountain etch reservoir is clean and has the proper kind of solution—a mixture of gum arabic, water, and some acid—and that the pH or acid value is proper. The mini-mum concentration of acid necessary for a particular ink, paper, and plate is always best. Additional acid may cause difficulty in ink drying and emulsification. The water fountain etch controls the non-printing areas of the plate and prevents ink from being transferred from the inking system to these areas of the plate and then to the blanket. Mike the thickness of the plate and obtain sufficient packing sheets to bring the plate surface level with the plate cylinder bearers. Insert the plate into leading edge clamps, place the packing sheet behind the plate, inch the press with pressure on to roll the cylinder around until tail clamps are in position, insert back of plate in tail edge clamps, and tighten. Pack and clamp blanket on the blanket cylinder in a similar fashion as the plate, but, in order to obtain the recommended pressure of 0.003 inch between plate and blanket, it is necessary to pack the blanket with 0.003 additional packing paper than that required to bring it up to the blanket cylinder bearer height. The 0.003 pressure or squeeze is required to transfer ink properly to the blanket. Set the impression cylinder so that there is 0.003 squeeze between the sheet of paper and the blanket. The impression cylinder is a smooth-surfaced steel cylinder which presses the sheet against the image on the blanket. Place stock in feeder of press, raise the pile of stock to proper height to engage sheet-feeding mechanisms, adjust air-actuated sheet controls, and test action. Run several sheets through the press and adjust delivery unit to insure that delivered sheets will properly and uniformly jog into delivery pile. If all is satisfactory, some test sheets should be run through the press. In order to print, wash the plate with clean water and a sponge, start fountain mechanism, and engage plate dampener rollers to contact plate, roll a few times, examine plate for even dampening across entire plate, engage ink rollers to plate, and feed sheets through press with impression. Inspect sheets for proper position and color. Obtain approval from supervisor. If satisfactory, start press for continuous production.


The first few sheets printed should be removed from the delivery and discarded, until proper color is restored to equal approved press sheet. For the first several hundred sheets, very frequent examination to insure that the proper amount of ink transfer and a minimum required amount of fountain solution are being distributed to the plate. The appearance of the plate is often an aid in determining if fountain solution is correct. If an excess amount of moisture is on the plate, it may appear wet or bright in comparison to correct control. The fountain control should first be established and then proper adjustment of ink can be more easily regulated. During the first few hundred sheets, also, the performance of the plate can be evaluated for any tendency for the background to scum or to lose parts of the image. If the plate seems not to be ink receptive, an application of an asphaltum solution may overcome this difficulty. If background areas are not clean, special plate etches may be of assistance. If these measures are of no aid, the plate must be removed from the press and a new plate made. If plate is performing satisfactorily, the press can be placed on full operating speed and full running operation commenced.


The same observations should be followed as were observed on starting. Frequent inspections of the sheet should be made for both ink density and sharpness of printing. If many impressions are required from the plate, the following additional items must be observed and dealt with if they occur.

Variations of moisture content of paper.—If normal paper is permitted to stand under humid conditions, moisture from the air will be absorbed by the edges of the paper. Since the inside portions of the paper are not affected, the edges form waves and when fed through the press may wrinkle under pressure of the blanket and impression cylinders. If this condition is not too far advanced, banks of heat lamps can be placed on each side of the feeder and this will tend to remove this moisture as the sheets are removed from the pile.

Ink emulsification.—Each revolution of the press causes some of the moisture from the fountain system to be taken up by the ink rollers. This moisture is continually being distributed through the numerous rollers of the inking system. Eventually the ink will be so contaminated that poor distribution and loss of good ink density will result. This condition must be observed and occasionally this contaminated ink must be removed and new ink placed in the inking system. For this reason, the fountain settings should be kept at a point which supplies the absolute minimum required moisture to the plate. Some new designs of dampening systems avoid part of this problem by providing solutions that evaporate rapidly in air or which do not permit the units of the dampening system to touch the plate.

Roller stripping.—The use of gum arabic in the fountain solution, because of its reaction with an acid, is the best means of keeping the nonprinting areas of a plate clean. However, gum which is transferred to the inking system also prevents proper distribution of ink on the metal rollers of the inking system; and if an excess of gum is permitted to enter the system, the metal rollers will not accept and distribute ink, and they will “strip.” A change to the proper fountain solution and fresh ink will usually overcome this problem.

Blanket stretch.—When a blanket is first installed in the cylinder clamps, all of the possible stretch is not overcome. After several thou-sand flexings under pressure, a blanket will yield or relax and may become slack on the cylinder. This condition should be observed and the blanket retightened as needed.

Image blinding.—Some materials used to form the image on a plate are sensitive to gum arabic, and if not protected by a sufficient charge of ink, may lose their ability to accept a charge of ink from the ink rollers. This must be watched carefully and corrected by prompt washing with water or special etches or the condition may be permanent and the plate not suitable for further use. The normal abrasion of the rolling pressure of a blanket will eventually wear the image from the plate, and this is, also, a cause of a “blind” or non-ink-receptive image.