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Chemical Based Process

Chemical Based Process

For the application of another copper layer, which varies from print job to print job, there are several methods that are described in the following sections (note: the top copper layer is twice as hard (Vickers hardness approximately HV 200) as the base copper, so that this copper layer has good cutting properties as regards the electromechanical engraving process):

The thin layer method:The base copper layer is coated with an engravable copper layer (approximately 80 μm) in an electroplating process  as shown in the figure above. This thin layer only allows a one-time engraving. The advantage of the thin layer technique is that all the gravure cylinders of one type have the same diameter dimensions and less mechanical surface treatment is required after the electroplating process than with thick layer processes (see below). The removal of the en - graving (after de-chroming) is achieved by dressing or milling the copper. After this, a new copper layer is applied. (In the special process known as copper recycling, the copper layer is removed in an electroplating reversal process. In this process, an additional nickel barrier layer of approximately 25 μm between the base copper and engraving copper is necessary). The thin layer technique is used in some 35% of cases, whereby the copper recycling method only accounts for some 5%.

The Ballard skin method : This method is also a thin layer process (one-time use of the engraving copper layer). The base cover is electrically covered with a removable copper skin (80–100 μm), whereby a special layer between base copper and Ballard skin ensures that the Ballard skin can be peeled off the gravure cylinder after printing. The Ballard skin method is employed in approximately 45% of cases.

Heavy copper plating (thick layer technique:An approximately 320 μm thick layer of engraving copper is applied onto the base copper in an electroplating process. This thickness of the layer permits engraving for approximately four print jobs. After each print job, a layer of approximately 80 μm is removed in a multi-stage mechanical process (milling, grinding). The former image is thus removed.

With all methods the cylinders are always hard chrome-plated after etching or engraving to reduce wear and tear. Therefore chemical chrome de-plating with hydrochloric acid must be undertaken prior to removal of the image carrying layer.

The process sequence for preparing an engraving cylinder is generally as follows:

  • Removing the used gravure cylinder from the gravure printing press
  • Washing the gravure cylinder to remove residual ink
  • Removing the copper image-carrying layer, either chemically, by means of electroplating, or mechanically
  • Preparing the copper plating process (degreasing and de-oxidizing, applying the barrier layer if the Ballard skin method was employed)
  • Electroplating
  • Surface finishing with a high-speed rotary diamond milling head and/or with a burnishing stone or a polishing band
  • Etching or engraving (producing the image on the gravure cylinder)
  • Test printing (proof print)
  • Correcting the cylinder, minus or plus (i.e., reducing or increasing the volume of cells)
  • Preparing the chrome-plating process (degreasing and de-oxidizing, preheating, and – if necessary – sometimes polishing)
  • Chrome-plating
  • Surface-finishing with a fine burnishing stone or abrasive paper
  • Storing the finished cylinder or installing it directly in the gravure printing press

Today, all these operations are performed, more or less fully automated, in production lines, whereby overhead traveling cranes and in some cases the transportation of the gravure cylinder from station to station is carried out by automated guided vehicle (AGV) systems
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Gravure Cylinder Imaging

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 print shops. Despite this, and in the interests of thoroughness, a short description of this process is given below.

Chemical Etching:

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 cross-line 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 pigment paper 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 image carrying 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.
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Pratik Bhai
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Mangalore, Karnataka
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Product Name : Electronically Engraved Cylinder
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Siddhi Vinayak Enterprises
Ahmedabad, Gujarat
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26-October-21

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