Process stability in five-colour pad printing of contact lenses

This paper describes an experimental investigation into process stability and the effects of temperature on changes in wet ink transfer onto a contact lens. A production printing press was instrumented with thermocouples and thermal imaging camera to record strategic temperatures. The overall result from the thermal audit of the press shows that the excursions of the temperature are small and are not likely to have a significant effect on print quality. The changes in product quality were measured using spectrophotometry. The main finding from colour measurements is that the process is stable. However, during the pre-production print runs most of the ?E values lie outside the acceptable limit. There are a number of reasons for this, the most likely being that the viscosity of the ink lies outside the correct operating band. The application of a cone on plate rheometer was suitable for measuring viscosity variation during the trial. The results showed a significant increase in ink viscosity. This was attributed to solvent evaporation and polymerization reaction.