Fine Lines in High Yield (Part XCVII): Size-Holding of Phototooling Films

摘要

Size-holding problems with artwork are quite common (Ref. 1) and can often be traced back to misunderstandings about the fundamental variables and their controls. My colleague, Bob Seyfert, recently reported on one such misunderstanding and offered suggestions on how to avoid the problem (Ref. 2) which I'd like to share with you. Silver halide phototooling films consist of 7-mil (178 μm) thick polyester base with gelatin coatings. The gelatin absorbs water, swells, and causes a size change of the film that is slightly larger than that of uncoated polyester. Film dimensions change with temperature and humidity. These are independent and additive responses. The response to a temperature change is fairly rapid while the response to a change in relative humidity may take hours. Polyester-based films have a CTE (coefficient of thermal expansion) of 18 ppm/℃ (i.e., a temperature change of 1℃) results in a size change of 18 μm over one meter or 9 μm over 50 cm (0.36 mils over 20″). The CHE (i.e., the coefficient of hygroscopic (humidity) expansion for silver films coated on 7-mil polyester) is in the range of 9-12 ppm per 1% change in relative humidity, depending on the type of gelatin binder being used. So the size change induced by a 1% change in RH will be about 5 μm over 50 cm (0.2 mils over 20″).