Text 6 Gravure Cylinder Manufacture

With the exception of sheet-fed gravure printing which is now found only rarely, web-fed gravure printing requires a gapless gravure cylinder, onto which the image is applied directly, by means of etching or engraving. For this, the cylinder must be prepared in a costly mechanical and galvanic process.

In its basic design, the gravure cylinder consists of a thick-walled steel tube with flanged steel journals. To increase the stiffness of this hollow cylinder, some of the cylinder journals are drawn inwards and are supported inside the tube on additional steel discs. All of these joints are welded during the manufacture of the gravure cylinder so that a solid roller body is created, which still has to be balanced so that there are no vibrations when running at high speed (typically up to 15 m/s) in the printing press.

The cylinder receives a base copper layer on its surface, which, among other things, serves to achieve the specified diameter of the finished gravure cylinder.

For the application of another copper layer), which varies from print job to print job, there are several methods:

• The thin layer method:

The base copper layer is coated with an engravable copper layer (approximately 80 µm) in an electroplating process (fig. 2.2-3). 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 dechroming) 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.When the engraving copper is used up, a new copper layer (hard) is applied by means of electroplating. This method is employed in about 20% of cases.

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

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