In the ink trapping using a cast-coated paper with black inks for offset proofing and synthetic paper, we detected many pinholes on printed surface. The numbers of the pinholes (N
pinhole) increased according to lapse of time after the trapping during initial 6 hrs. These N
pinhole values at 24 hrs decreased with increasing amount of transferred ink (y). The N
pinhole value of the printed matter with web offset ink at 24 hrs was independent of the y value. Using poly (ethyleneterephthalate) (PET) film, there are many pinholes on the surface. Clarifing ink transfer mechanism, the pinhole formations were studied by means of y-N
pinhole reactions, time-profiles of the N
pinhole value, effects of drying temperature, and drying atmosphere and so on.
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