Effect of cavity surface coating on mold temperature variation and the quality of injection molded parts

Suiting for high gloss surface of injection molded parts free of painting is a great concern from both environment and cost effective considerations. As a result, variable mold temperature controls to achieve the mentioned goal have been paid great attentions. In this study, TiN and Teflon of various thicknesses were coated on the cavity surface of a tensile bar mold designed with double gate. During the injection molding process, melt-mold interface temperature was analyzed and simulated. In a regular injection molding of ABS resin using P20 as the mold material, the initial melt temperature may drop from 240 °C to about 65 °C after 0.01 s of contact with the cavity surface when the coolant temperature is 60 °C. For a TiN surface coating of 4 µm, the interface contact temperature was raised to 73.6 °C. For a Teflon coating of 22 µm, the contact surface temperature is as high as 100 °C initially (about 25 °C higher) and remains above 80 °C for about 0.4 s. Teflon coating on the cavity surface eliminates the weld-line marks, improves part surface smoothness and results in better tensile strength for weld line than TiN coating. Moreover, the cooling time was almost not affected. When surface coating is combined with infrared heating, not only the tensile strengths of the weld line were further enhanced but also the heating rate at mold surface is enhanced.