Monday, December 22, 2008

Doubling

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  1. Another source of doubling between units occurring only on sheet-feed presses is a badly set "double diameter" transfer drum with one of it's two gripper block settings set substantially out of symmetry with it's counterpart.

    This type of doubling is easily located, because the doubling only occurs with inks printed prior to the problematic "Double Diameter" drum and not with the inks printed afterwards. Additionally, the doubling pattern is typically very regular, with every other sheet looking much the same.

    Being that the "Double Diameter" drum (to the best of my knowledge) transfers the sheet without any interaction by the squeeze between the blanket and impression cylinder. It can really only be remedied by a press mechanic. With a couple of caveats.

    The gripper bite settings at the "Head Stops", the gripper bite margin may influence the doubling. You can try larger or smaller gripper bite margins without cocking the sheet by moving the Head Stops in synchrony. Then if needed you can roll the plate cylinders to compensate for the head stop changes.

    Another thing you might be able to do to fix the problem without a press mechanic is to inspect and clear any material that may be stuck on one side of the Double Drum and not the other. Especially near the grippers.

    otherthoughts

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  2. I would clarify. All blankets pick up the ink from previous units. This is not backtrapping. The final blanket will have all inks in the image on it. This is normal. Dots should be in register with themselves. When they are not in register the result is doubling.

    From how I understand it, backtrapping happens only when inks are overprinted and the ink splits in the previous ink film on the substrate instead of the current ink film that is over printed on the previous ink film. That backtrapped ink goes up into the roller train and contaminates it.

    The previous inks that are printed on the blanket and that are not overprinted, can not normally go up into the roller train because they are located in the non image area and the plate will not accept that ink and therefore it stays on the blanket.

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