( Originally Published 1963 )
Regular and copperized.—The regular method is applicable to both zinc and aluminum. The copperized plate is restricted to aluminum. The purpose of deep etch is to provide an image which is slightly below the general plane of the nonimage area. Although this depth is considered to be 0.0002 or 0.0003 of an inch deep, the life of the plate is materially extended on press. It is not unusual to obtain 200,000 impressions from a deep-etch plate. The reason for the in-creased life is considered to be due to the fact that the image is protected to a considerable degree from normal pressure and frictional forces of press rollers, ink, and blanket because it is recessed in the plate.
The coating and developer materials are different than those used for surface plates. The following are additional: aluminum and zinc developers and etches, anhydrous alcohol, and special etching pads for each metal.
Procedure.—Preparation and exposure of the plate is the same as for surface plates, although the length of exposure for deep etch is usually greater. After exposure, the plate is processed in a downdraft table which removes volatile residue downward and prevents contamination of the room air. Two applications of developer are applied to the plate. Excess is removed with a squeegee. At this time the image areas are completely developed down to the base metal.
The etch required for a particular metal is applied to the plate and permitted to act on the image for a minute or two and then squeegeed off. After etching, the image is recessed into the grained surface of the plate and any residue must be thoroughly cleaned and removed. This is accomplished with two or more applications of anhydrous alcohol. The special need for anhydrous alcohol arises from the fact that the stencil coating of the nonimage area is attacked by any moisture present. Each application is wiped clean with fresh paper towels to avoid any possible contamination of the image areas. If the plate is to be copperized, a copper solution is applied to an aluminum plate at this time and a very thin film of copper will plate out of the solution onto the bare aluminum image.
After copperizing is completed, this solution is washed off with one or two applications of anhydrous alcohol. Copperizing is advantageous because it is a very ink-receptive metal and provides an ideal base for an ink-receptive lacquer image. The plate is fanned dry and lacquer and developing ink are successfully applied to the plate. Each in turn is rubbed down thoroughly and dried. The image area is now complete, but the nonprinting area contains the original exposed coating which must also be removed. This is done by soaking in a tank of water for several minutes and scrubbing with a soft-bristle brush. This leaves the deep-etched image alone on the plate. The plate is given a final etch to desensitize the nonprinting area and a final gum application and is then ready for production.
OTHER PLATE PROCESSES
Deep-etch stainless steel.—The procedure is the same as for aluminum, including the use of the same deep-etching solution or an iron perchloride solution. The life of a stainless steel plate with its high tensile strength is much greater than either aluminum or zinc, which have a tendency to crack at the point of clamping on the press cylinder.
Presensitized.—These plates are received from the manufacturer with a diazo sensitized coating already applied and are immediately ready for exposure. The development of these plates is usually accomplished with one or two applications of a specially prepared developer material supplied by the manufacturer. A final gum application is also required. Bath sides of these plates are coated and can be used for two images.
The original images produced on the plate are the only images that can be obtained from one piece of metal. After printing both images, the metal cannot be used again, but can be salvaged as scrap. The cost of removing the various layers of base coatings in addition to the image would be more costly than a manufactured presensitized plate.