To obtain the "high infrared radiation (IR) transmittance and low visible ray (VR) transmittance" necessary for a high-performance IR lens, cavity thickness and molding conditions were investigated experimentally for disk-shaped high-density polyethylene (HDPE). When injection molding and injection press molding were done using different cavity thicknesses, results showed that the molded part using the thinner cavity thickness maintained a semitransparent state. Simultaneously, they showed that higher IR transmittance was obtained. For high mold temperature and long cooling time, the increased core-layer thickness improved the IR transmittance. The VR transmittance decreased because crystallinity became higher. Furthermore, when injection press molding was conducted, the VR transmittance decreased because crystallinity became higher. IR transmittance and VR transmittance were obtained respectively as 65.4% and 6.4% when injection press molding was conducted at higher mold temperatures with longer cooling times.