A polymer injection molding unit is disclosed having a mold wall which includes a portion made of an infrared transmissible material which is radiated by an external infrared radiation source. Due to the passage of the infrared radiation through the infrared transmissible material, the infrared radiation is not absorbed by the mold wall and the mold wall is not substantially heated. The infrared radiation passes through the mold wall and is absorbed by the molten polymer material near the mold wall. As a result, the temperature of the molten polymer material rises and its viscosity is lowered to thereby enhance the shape transcription of complicated or fine shapes present on the mold wall, without increasing the temperature of the mold wall.