Thin-wall injection molding of high-density polyethylene for infrared radiation system lenses

High-density polyethylene (HDPE) lenses are used for infrared radiation (IR) systems, such as radiation thermometers to transmit the IR of the 10 μm region. High IR transmittance and low visible ray (VR) transmittance are necessary for IR system lenses. This experimental investigation of thin-wall injection molding was conducted using 0.5 mm cavity thickness with a disk shape, finished to a mirror-like surface. As factors affecting transmittance, we evaluated the thickness, surface roughness, crystallinity, internal structure, and molecular chain orientation of molded parts, which were produced using four HDPE melt flow rates (MFRs). The changed molding conditions were mold temperature and holding pressure. Results showed that the thin-molded parts had higher IR transmittance. The thin-molded part was obtained with the smallest MFR of 5. Furthermore, the VR transmittance decreased when the molecular chain orientation in the molded parts was small and the crystallinity was high. The small orientation and high crystallinity were obtained simultaneously with the largest MFR of 42. Therefore, it was impossible to obtain high IR transmittance and low VR transmittance simultaneously by a change of MFR. This study confirmed that surface roughness and crystallinity do not affect transmittance.