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Optical Properties of HDPE in Injection Molding and Injection Press Molding for IR System Lenses Part II: Mold Temperature and Surface Roughness Effects on Injection Molding

Abstract An experimental investigation of injection molding was conducted to assess the high infrared radiation (IR) transmittance with an opaque state (low visibility ray (VR) transmittance) necessary for IR system lenses as a target high-density polyethylene (HDPE) IR transmission material. In our earlier investigation, experimental investigations of injection molding and injection press molding were conducted using the mold cavity of a disk-shaped finished mirror-like surface. For high mold temperature and long cooling time, the increased core-layer thickness improved the IR transmittance. Simultaneously, the VR transmittance decreased because the crystallinity became higher. Furthermore, when injection press molding was done, the VR transmittance decreased because the crystallinity increased. Using a 0.5-mm-thick mold cavity with disk shapes having different surface roughness, an injection molding experiment was conducted for this study while changing only the mold temperature of 20 to 80°C. Results show that when the mold surface roughness became high, the surface roughness of molded parts became high, too. Thereby the IR and VR transmittance were decreased. However, when the mold temperature was high, the influences of the so-called skin-core structure and crystallinity were stronger than the influence of the surface roughness of molded parts. The IR transmittance increased because of the decrease of the skin layer. Furthermore, the VR transmittance decreased because of a simultaneous rise of the crystallinity. The minimum value of obtained VR transmittance was 9.3% at mold temperature of 80°C using a mold of the highest surface roughness. This molded part reached an opaque state. Furthermore, a higher IR transmittance of 64.9% was obtained. When injection molding was conducted using a mold of a mirror-like surface with the same conditions as those in this investigation in our earlier investigation, the obtained IR and VR transmittances were 65.4% and 0.6%, respectively. Consequently, when the mold temperature was high, a few differences were found with transmittance of the molded parts using the mold of the mirror-like surface. Results showed that no difficulty arises when a mold having higher surface roughness is used at a higher mold temperature.