Purpose.By means of chemical and mechanical action, graining provides a matte finish to a flat sheet of metal. Actions of the abrasive with the driving force of the oscillating marbles provides an increased surface area Of 2 to 6 times the nominal dimensional area of the flat metal sheet. Graining also provides a capillarity which serves as thou-sands of miniature reservoirs for receiving and holding the various chemicals required for plate processing and production.
Materials.Marbles (steel, glass, plastic, or wood of various diameters) provide the weight and driving action on the abrasive; abrasive (aluminum oxide, silica, garnet, and pumice) of selected fineness of particle size causes the abrasion of the smooth metal surface to the desired depth and determines the character of the matte finish; chemicals (water, a lubricant for abrasive and marbles; trisodium phosphate, a cleaner; ammonium bichromate, an antioxidant) insure proper control of the action of abrasive and marbles on the plate surface; metal sheets (zinc, aluminum, and steel) of 10 to 20 thousandths of an inch in thickness which form the carrier for the image to be wrapped around and fastened on the plate cylinder of the offset press.
Equipment.Graining tubs are shallow metal trays which hold the metal sheets by fixed clamps and serve as a container for abrasive and marbles. The power-driven tray oscillates at various speeds to accomplish selected abrasion of metal surface.
Procedures.Sheets of the appropriate metal are clamped to the bed of the graining tub and covered with a layer of marbles of the particular type which will produce the fineness and depth of grain desired. Water in sufficient amount is run into the tub to provide easy movement of the marbles and abrasive as the tub oscillates. The amount and kind of abrasive, the time of grainer motion, and the speed in revolutions per minute will provide a very wide selection of grain results to be obtained. Rigid adherence to quantity of materials and graining time intervals are required to produce uniform-finish plates.
VARIATION IN GRAINING PROCEDURES
In addition to the above procedure of tub or ball graining, the following methods are briefly described: Brush grain.This method uses large round brushes which rotate in an abrasive slurry and create an abraded surface on the sheet metal. Various kinds of wire and various sizes of wire in combination with varying sizes of abrasive particle will provide an infinite variety of finish grain. Very fine grain characteristics can be achieved quickly with this method. Vapor blast grain.A very fine abrasive is forced under air pressure through nozzles at right angles to a plate rotating on a cylinder. Being a dry process, the abrasive can be reclaimed for reuse if properly screened to eliminate metal particles and worn abrasive. This method is capable of producing a wider range of grain characteristics than other methods. In addition, this method can produce a deeper grain trough than techniques which act horizontally on the surface. Chemical grain.Mixtures of chemicals, very carefully formulated for precise action on a particular metal, are sponged evenly over the surface and permitted to react a sufficient length of time to achieve the desired grain characteristic. A bath may also be prepared and metal sheets immersed in the liquid to complete the action. No abrasive is required or needed with this technique. The plate is washed with water and is ready for coating.
PLATEMAKING
Purpose.To produce from a flat, a printing plate which will re-produce in full fidelity all of the image present in the flatthe image area of the plate to be fully ink receptive and the nonimage area to be ink repellent. Both of these conditions to be replenishable on the offset press by the inking and dampening systems for many thousands of impressions.
SURFACE PLATEMAKING
Materials required.For zinc: dilute hydrochloric acid, ammonium bichromate, sensitized casein coating, lacquer, developing ink, ammonium hydroxide, final plate etch, and gum arabic; felt cleaning pads, cotton, cheesecloth, and sponges.
Equipment required.Whirlers to apply and to dry coating on grained surfaces, contact printing frames with arc lamps, wash tanks for application and removal of chemicals, tables, fans for drying chemicals on plates, downdraft tables, used for deep-etch processing (this table has an air-exhaust system which exhausts volatile vapors down-ward instead of permitting them to distribute in the room air), stepand-repeat machine to cast multiple images on a single plate, and A frames to hold plates in upright position in various stages of processing.
The thickness of the coating will have a noticeable effect on the amount of exposure required to produce the plate. A heavy coating will require more exposure than a thin coating to achieve a given hardness of image.
CLEANLINESS AND UNIFORMITY
It cannot be overemphasized that thorough and completely uniform procedures in every minute step of platemaking have an effective result in producing uniform quality of the final image and trouble-free reliability in press production. Surface platemaking is a chemical process and the failure to observe elementary standards of cleanliness and avoidance of any contamination of solutions designed for a particular step will result in numerous failures and breakdowns which become almost impossible to trace in origin. As an example of annoying difficulties which seemingly have no cause, a series of plates were being produced with minute scratches that appeared only after several dozen sheets were run on press. The cause of this was finally traced to developing cloths which were laid on the table after zinc plates had been processed and the graining residues were being picked up on the developing cloths and carried to the image surface of subsequently processed plates.
PROCEDURES
Surface plates.To clean residue from the grained plate and prepare the surface for the sensitized coating, the plate is placed in a tank, flushed with water, and given a brush cleaning with a hydrochloric acid solution and ammonium bichromate (phosphoric acid for aluminum plates). The plate is then thoroughly flushed with water and is ready to be coated. The plate is clamped in a whirler spindle carriage, whirler speed is set at approximately 6o revolutions per minute, and a measured amount of coating is poured onto the center of the whirling plate. The coating is thus evenly distributed across the plate. The whirler is closed, heating elements turned on, and the plate dried as it continues to whirl. The speed of the whirler and the Baume of the coating will determine the thickness of the coating applied to the surface. The plate is ready for exposure after a few minutes of drying in the whirler.
The coated plate is placed face up on a vacuum frame and a pre-pared flat is placed in proper position on the plate with the emulsion side of the negative in contact with the coating. If sensitivity guides are called for to test the plate performance, they are inserted into the flat. The proper exposure is made and the plate is separated from the flat and placed on a table for application of lacquer and developing ink. A pool of lacquer in the center of the plate is spread evenly over the entire plate and dried with a fan. This lacquer provides a tough image in the exposed areas and gives added press life to the plate. Similarly, a pool of ink in the center of the plate is spread evenly over the entire plate and dried with a fan. The addition of ink to the image provides a black color for visual inspection and an ink-receptive image for easy rollup on the press.
After the ink coating is dried, the plate is immersed for a minute in a tank containing a weak solution of ammonium hydroxide. Unexposed casein coatings are not soluble in water but are soluble in a weak ammonia solution. This action loosens the coating from the nonimage areas which are then flushed from the plate by the use of sprays of water over the plate in a separate wash tank. At this time the plate is inspected for faulty image result. If the plate is satisfactory for production, a final protective etch which prevents oxidation of the non-printing areas is applied over the entire plate and flushed off. Excess water is removed with a squeegee and a solution of gum arabic is wiped smoothly over the entire plate to maintain a stable condition of surface chemistry now established. The plate is now ready for press production.