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January 25, 2011

Can you explain a little about Plate distortion & stretch factors?

All plates are prone to distortion and shrinkage, in particular traditional molded rubber plates that will shrink and stretch both in their length, width and thickness, usually in the range of 1.5 to 2%. This is very much determined by what materials are included in the plate construction, the compounds themselves, the plate thickness, and the size of cylinder or sleeve it is to be mounted onto.

Now, with photopolymer, shrinkage issues are generally disregarded as in theory they should not shrink with the stabilization (polyester sheet) that is bonded to the back of the photopolymer. This said, anyone that makes plates knows not to expose and washout plates from the same job running around the plate length and the across the plate width as a difference in tightly registered plates will be seen as a slight mis-registration due in part to the different alignment of the polymer strings which can cause a slightly different stretch or apparent shrinkage difference.

This can even apply between plates that consist of heavy solids and other plates that have little or no print areas, in particular with thicker plates and deeper etched plates. This is why it is critical to maintain a uniform depth when plates are washed out if photopolymer and applies the same to metal molds if chemically etched. All plates will distort( stretch) when wrapped around a curved surface and will elongate on the outer surface and contract on the inner surface of the plate. For modern photopolymer plates the standard calculation is 3.1416 x 2 times plate thickness over its repeat length. Plates mounted on stickyback prior to mounting on the print cylinder have much more surface stretch and distortion than when mounting a plate onto the stickyback that has already been mounted on the print cylinder or sleeve.

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