An experimental investigation of injection press molding (IPM) was conducted to assess 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. The changed conditions were the cavity open distance and delay time considering the polymer melt flowability. Other molding conditions were held constant. Mold surface roughnesses of two kinds were used. Data for IR and VR transmittance were evaluated using measurements or observation results obtained for surface roughness, thickness, differential scanning calorimetry (DSC), crystallinity, and the internal structure. Results show that the surface roughness and thickness of molded parts did not influence IR or VR transmittance. For thin skin layers with low orientation of molecular chains, the IR transmittance was higher for longer delay times. For low molecular chain orientation in the shear-core layer, the VR transmittance was also low. The molecular chain orientation differed depending on IPM conditions. Setting a longer delay time produced a uniform distribution of the molded part thickness. Furthermore, thickness became a constant value when a mold with high surface roughness was used. POLYM. ENG. SCI., 2017. (c) 2017 Society of Plastics Engineers