Compression-Induced Solidification: A Novel Processing Technique for Precise Thermoplastic Optical Components with Negligible Internal Stresses

In the field of optical components, thermoplastics are replacing more and more glass mainly because of their better freedom of design and their cost-effective processing techniques. Nevertheless, especially lenses do not have an ideal design for plastic processing, because of their varying thickness from the centre to the edge. These lead to great differences in shrinkage due to the dif-ferent coefficients of thermal expansion of melt and solid state and, consequently, directly lead to warpage and residual stresses with state-of-the-art processing techniques. A promising solution is a new, innovative technique—compression-induced solidification (CIS)—where the melt is compressed at constant temperature until it solidifies. This results in isochronic solidification of the whole part even at high temperatures and reduces residual stresses and warpage due to the cooling of a body with homogenous shrinkage. In this paper, CIS integrated in the injection molding process is introduced, and the influence of process parameters on inner properties and dimensional accuracy of CIS polycarbonate parts are illustrated. Trials carried out indicate that an optimum level of compression pressure at the end of glass transition range and a sufficiently long period of holding time (hereinafter the adapting time) for reaching homogeneous temperatures within the melt until pressure is applied will generate parts with low residual stresses and high dimensional accuracy. 1. Introduction Optical components for high-precision applications, especially in the case of complex geometries, are mostly made of glass by expensive and time-consuming grinding and polishing techniques. For that purpose, plastics offer the possibility of cheap mass production and easily achievable complex geometries. The state of technology for the production of plastic parts are injection and injection compression molding. After mold filling in the injection molding process, cooling-dependent shrinkage is balanced by the packing pressure that is held until material in the the sprue freezes and the material flow in the cavity is inhibited. From that moment on, the molded part will shrink according to pressure- and to specific volume- and temperature- (pvT-) dependent behavior. Regions of accumulated material will freeze later than thinner parts and layers that are adjacent to the cold mold walls. The liquid state, with its thermal expansion coefficient generally at least two times higher than the solid state, will lead to inhomogeneous shrinkage over the whole part, as well as shrink marks and residual