Tips & Tricks: Delamination in Sheetfed Offset Printing

The devil is in the details, as the saying goes. This is true for delamination - an error referring to the separation of layers in the printing stock, both paper and cardboard.

Possible causes
Printing paper often has to be cut to the right size in the print shop. One or multiple cuts are necessary. When the stack to be cut is inserted into the guillotine cutter, the bottommost sheet can get stuck on an incorrectly positioned cutter bar. This leads to a separation of layers and rolled-up strips during further processing.

On insertion into the printing press, the front or back of the sheets may be damaged from coming into contact with the cutting edges. This problem occurs particularly frequently when thick layers are involved. The layers get partially separated on the edges and roll up on the damaged spot when the layer is pushed onto the stack.

Delamination can sometimes be caused when wedges are inserted to level the height of the pile, or by the insertion of sword probes to measure humidity.

During paper manufacture, the paper reel is unrolled in order to cut it to size. Sometimes the layers become partially stuck together. As a result, when the paper is unrolled, the layers separate, tearing the surface and causing it to roll up.

Possible remedies
The aforementioned problems can almost always be prevented with particular vigilance and by working carefully. It is important that the bottommost sheet on the guillotine cutter is always thrown out. This significantly reduces the risk of rolled-up strips of paper being run through the printing press. In addition, wedges or indicator probes should be inserted with the utmost precaution. It is also highly advisable to have an air blast at the guillotine cutter table. Furthermore, it's absolutely necessary to make sure that a newly installed cutter bar does not protrude out from the table at any point.

A practical example
In one print shop, the delivered sheets had to be cut to the desired size. The bottommost sheet of the pile to be cut was always damaged at one edge that had been in contact with a falsely positioned cutter bar. The result was small rips and damage which led to multiple rolled-up strips during further processing. 

The Fogra Graphic Technology Research Association was brought in for expert assessment and was able to identify the cutter bar as the clear cause for the delamination. Multiple sheets were affected. They all exhibited damaged edges at the exact same spot. That was also precisely the point where the printing stock was delaminated and rolled up.

Once damage has been caused, the issue of liability comes up. For the processing of complaints, it is particularly helpful when the problem sheet can be established. Using this sheet, the cause of the rolled-up strips can often be identified. If the rolled-up strips are at the edge of the sheet and increase in width during processing, for example, the cause can be traced back to cutting or piling. If the delamination begins in the middle of the sheet, this indicates that the cause occurred in the paper mill. Fogra's processing of such complaints shows, however, that nine out of ten such cases are caused by faulty procedures in the print shop.

Layers can roll up in the pile as many as 15 times during various motional processes. This leads to rolled-up strips on the printing stock. If these are run through the printing press, the printing blankets inevitably get damaged and, in extreme cases, other units can also be damaged.