Abstract
Ink trappings were studied using polyethylene terephthalate (PET) film with black inks for offset proofing and synthetic paper. By observing printed matter from reverse side through the PET film, we detected many air bubbles between the ink layer and the PET film. They are classified roughly to two groups, small number of large ones (φ=2-5μm) and many small ones (φ=0.5-1.0μm). Number of small air bubbles(Nair bubble, measured immediately after the trapping)decreased with increasing the amount of ink trapped (y) and increased with increasing the ink distribution time. Therefore, the small air bubbles have been yielded by the ink distribution. We also measured many ink peaks (immediately after the trapping) and pinholes (at 24h) on the printed surface. Numbers of ink peaks and pinholes (Nink peak and Npinhole, respectively) increased with increasing the ink distribution time. Next, we studied effects of nip width on these values (distribution time=2min.; nip width=2, 3 and 4mm). The Nair bubble value decreased with increasing nip width contrary to increase the Nink peak and Npinhole values. The latter effect can be represented by differences in the values of 2 and 4mm nip widths. At y=2gm-2, the difference in the Nair bubble values is about one third (synthetic paper ink) or a half (offset proofing ink) of the difference in the Nink peak values.
