Characterization of Injection Compression Molding (ICM) in Fresnel Lenses Production Using Amorphous Polymer Materials

Injection compression molding (ICM) is the leading process technology for the mass manufacturing of high precision polymer optics. To characterize the process, the effects of process factors on the part geometrical accuracy must be investigated. This study aims at quantifying the effect of the injection compression molding process on the replication accuracy in Fresnel lenses production. The replication fidelity was assessed at different distances from the gate using step height measurements of the Fresnel lens structured profile as indicator. Two different optical grade polymer materials were compared: Polymethil-Metacrylate (PMMA) and Cyclo-Olephin Polymer (COP). Three different process parameters, namely compression starting point, compression gap and packing pressure, were tested for the COP grade while only two for the PMMA grade. The influence of packing and compression was experimentally demonstrated, highlighting the importance of the packing phase to ensure full replication of the part. A significant interaction was detected between packing pressure and compression gap. The replication fidelity showed a great dependence on the distance from the gate. In particular, worse replication was observed far from the gate due to the presence of entrapped air, which was also influenced by the compression gap size.