Paul D. Fleming III, Paper Engineering, Chemical Engineering and Imaging
The evolution of photoengraving of gravure cylinders and plates followed the discovery of photographic reproduction. The etching of lines and dots through photo resists was the beginning of rotogravure. The carbon tissue method of image photo transfer to a gelatin resist was discovered and perfected in the latter quarter of the 19th century. This led to conventional carbon tissue and two positive engraving of copper plated cylinders.
Three methods of photoengraving have been used for engraving of gravure cylinders;
(a) Conventional, (b) Two Positive or Lateral hard dot and (c) Direct transfer.
Conventional gravure etching employs a combination of two glass photo positives exposed to carbon tissue. Carbon tissue is a water-sensitive fibrous paper coated with a smooth gelatin resist. The process allows the tissue to be etched by ferric chloride to a depth of about 45 microns. The resist is sensitized to light by submerging it is a potassium dichromate solution. The sensitized carbon tissue can be dried and refrigerated until used in the photo transfer process.
After conditioning back to operating temperature, the tissue is formed into sheet sizes for wrapping around a copper-plated cylinder. A glass positive was used to expose the light sensitive gelatin before Mylar films were available. One positive is a continuous tone variable density image. The second positive is a gravure screen of specified count 100-200 lines per inch. The two positives are exposed consecutively to UV light on the same sensitized surface.
After the double exposure, the carbon tissue is wrapped around the cylinder and all the paper and backing are removed. The exposed areas are hardened with alcohol and forced air drying. This is done in amber light to prevent UV exposure. In the areas exposed through the screen, membranes of variable thickness are formed, depending upon the amount of light passed through the continuous tone film. The membranes are thick in the highlights and thin in the solids.
This controls how quickly the etchant will break through and begin to etch the metal. This process forms cells of variable depth, but uniform cross sectional area. The variable depth controls the ink volume delivered. The etching process takes 25-30 minutes. The speed of etching is varied by changing the concentration of the Ferric chloride solution. Lower concentrations tend to break down the membranes and speed the process, while higher concentrations pass more slowly through the membranes.
Of course too low of FeCl3 concentrations will not etch at all, so there is an optimum value for fastest etch time. The Two positive process is similar to the conventional one. The main difference is that a halftone created from the original continuous tone image is used instead of the uniform screen. This allows the creation of cells of both variable depth and variable cross sectional area. The advantage of this process is that it is more controllable and the halftone dot structure gives more sharpness to the image.
The Direct transfer method employs only one positive. For this process a single halftone positive is used. This produces cells of uniform depth, but variable cross sectional area. This process became popular in the early 1950s when wrap around photographic film and photopolymer emulsions replaced carbon tissue. Chemical engraving methods have the advantage of being quick and providing a wider variety of cell configurations. They are, however, difficult to automate.