We detected many holes on wet printed surface immediately after ink trapping using polyethylene terephthalate (PET) film with a black ink for offset proofing. Number of the holes increased with decreasing the amount of ink trapped (y). The holes were divided roughly to two types, A and B. Characteristics of the type A holes are followings; variable shapes, variable hole size, interference stripes around the holes, etc. The latter ones were round and small (φ < 10μm). The number of the small size holes (φ < 20μm) increased with decreasing the y value. The number of middle size ones (20 < φ < 50μm) has a maximum around y=2.5g m
-2. The number of the largest size ones (φ > 50μm) increased with increasing the y value. We obtained similar results in the ink trappings using the PET film with a black ink for synthetic paper. By using the papers, the number of the type A holes decreased and the number of the type B holes increased according to decrease of air permeability of the paper. Therefore, the type A holes are hollow holes yielded by fixed air bubbles between the printing substrate and the ink layer on a roll, and the type B holes must be pinholes yielded both by suspended air bubbles in the ink and by air bubbles appeared from the paper surface.
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