Study on the thermoforming of PC films used for in-mold decoration

Abstract

Preparation of thermoformed film is one of the critical steps for successful application of in-mold decoration (IMD) to parts of a complex shape. In the present study, a grid was printed on the surface of two PC films, 0.125 mm and 0.2 mm thick, and then the films were thermoformed into a cup shape by a drawing die under various process conditions. The effects of processing parameters including mold temperature, film pre-heating temperature, plug speed, plug holding time, and plug thermoforming depth on the dimensional variation and wall thickness distribution were investigated. From the dimensional variation of squared mark and thickness distribution of the formed film, one can understand the characteristics of thermoforming. From the experimental results, we can find that the variation of dimension and thickness at the sidewall of formed films increase with increasing mold temperature, pre-heating temperature, plug depth and holding time, whereas they decrease with increasing plug speed. The result indicates that mold temperature exhibits the most significant effect on the stretching ratio of the film and the variation rate of film thickness. Furthermore, the values of both maximum dimension and thickness variation rate are not sensitive to the film thickness under the molding conditions we selected, as values for the 0.2 mm film were only 0–8% higher than for the 0.125 mm film. In addition, we also observed that increases in the depth of thermoforming will cause wrinkling, especially in the thinner film. Multiple-step thermoforming can reduce the residual stress due to the partial relaxation of deformation in each forming stage, and was found to be useful in the reduction of warpage and wrinkle in the formed film.