Effect of mold surface antistiction treatment on microinjection replication quality using Cr-N/Zr-DLC thin-layer coating

Sticking in microinjection molds is a significant problem in controlling the qualities of molded parts such as replication rate and appearance. To resolve the problem, this study investigates the effect of chromium nitride (Cr-N) and diamond-like carbon that contains zirconium (Zr-DLC) layers deposited on mold inserts on the antisticking properties of molds. The injection-molded flat plate used is 0.4 mm thick and has 5, 10, and 20 μm high v-grooved microfeatures. To evaluate the importance of the major control factors in determining the microinjection molding quality and to obtain the optimal settings of those factors, the Taguchi method and analysis of variance were utilized. The sticking behaviors of the melt plastics on the cavities that were deposited Cr-N/Zr-DLC layers using an unbalanced magnetron sputtering process were studied with scanning electron microscopy. The experimental results indicate that an antistiction coating most strongly affects replication quality and is followed in that respect by the mold temperature and the barrel temperature. As the dimensions of the microfeatures decrease, the effect of the mold temperature on replication rate becomes more significant. In particular, a Zr-DLC coating on the mold surface yields a low coefficient of friction and a low resistance to the flow of polymer during cavity filling, improving the replication of the heights of the microfeatures. Nevertheless, a Cr-N coating provides better antisticking. This improvement follows from the lower stripping force, which results in the lower surface free energy on the Cr-N layers.