Rotogravure Pigmented Paper Making, Sensitizing Processing and use at Gravure Process
Gravure Cylinder Imaging
In addition to an image-carrying function, the screen structure of the gravure cylinder surface has the significant task of guiding and supporting the doctor blade. The blade supports itself on the cell walls, which demarcate the cells. The continuous-tone-like graduation in the image of conventional gravure (etching) is achieved through the various depths of the cells. However, there is a mixed form, the variable area and depth gravure process, in which the cell diameter and depth of the cells are altered for the continuous tone graduation.Variable area gravure printing without cell depth variation (corresponding to the dot size variation in offset and letterpress printing) has gained little significance. Electromechanical engraving with diamond stylus (variable area and depth gravure) is the dominant process. Only seldom is etching still used as an imaging process in gravure printshops. Despite this, and in the interests of thoroughness, a short description of this process is given below.
For transferring the image onto the gravure cylinder, conventional etching processes use a pigment paper, which is coated with a gelatin layer and sensitized (i. e., made light sensitive) with a chrome saline solution just before use. First the crossline screen and then the imaging film are exposed onto the pigment paper in a copying frame. The pigment paper is then “laminated” onto the gravure cylinder surface in a special pigmentpaper transfer machine. A stabilizing base paper,which is later removed, ensures that this transfer takes place in accurate register. The use of an “autofilm”where the photo emulsion is poured onto foil makes this base paper redundant. The process that follows, involving softening, base paper peeling, and washing out of the unexposed and therefore soluble gelatin in approximately 40 °C warm water (in this process the cylinder turns in a water bath) followed by a drying process, can be described as “developing.” All these operations are carried out in program-controlled automatic developing machines. A relief (corresponding to that of the etched cylinder) with gelatin layers of varying thickness forms between the raised cell walls.
Prior to etching, all non-printing sections are covered with an acid proof asphalt varnish – certain imperfections from the transfer can also still be corrected in this way. The actual etching procedure takes place in a program- driven single-bath etching machine. Here, the cylinder is bathed in or sprayed with a ferrous chloride solution until the required etching depth is achieved. The ferrous chloride solution “eats” its way through both the gelatin layer and into the copper of the imagecarrying layer. The deeper the gelatin cell is, the sooner the acid comes into contact with the copper and the deeper the cell created on the image-carrying layer.