Paul asks,
I would like to know about washboarding in corrugated – how much variation in surface flatness (in microns) is typically seen for the types of corrugated board used in applications like shelf ready packaging?
I don’t know that there is a published standard on how smooth the surface of a corrugated sheet should be for the type of printing used for display or shelf ready packaging, but I think we are safe to say pretty darned smooth.
One of the key elements of printing the type of quality high-graphics usually found in display work and shelf ready packaging is minimizing the impression (pressure) between the plate and the substrate. The greater the impression pressure the more the printing plates distort and that distortion results in growth and distortion of the dots that create your image. Typically in high-graphics printing we are looking for a kiss impression, just enough that the printing plate just touches or “kisses” the surface of the board and transfers the ink. For the best print results we want .0015” to .003” (~38 to 76 microns) impression when printing. Oh course the lightest impression we can get away with.
Now, if the high and low points of the board surface exceed this kiss impression depth then additional pressure will be necessary in order to obtain coverage in the low points caused by fluting. Then, as stated above, as we add more impression our printing starts to suffer. With large block print you might think you can get away with over impression and perhaps you can a tad more than you can with process or fine lines and text, but excess impression on solid coverage will result in color variation through striping of the print.
We should also note that typically washboarding is less prevalent on small flute (such as a E, F or N) because there are more support points and they are closer together than a C or B flute.
Digital printing may be a little more forgiving than offset, but we still have to remember that the smoothness of the surface contributes to the overall aesthetics of the packaging and not just the print quality.
—Ralph
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