Variable Mold Temperature to Improve Surface Quality of Microcellular Injection Molded Parts Using Induction Heating Technology

Microcellular foam injection molding provides many advantages over conventional foams and their unfoamed counterparts, but its applications are limited by visible surface quality problems such as silver streaks and swirl marks. In this study, we propose a variable mold temperature method to improve the surface quality of molded parts. Electromagnetic induction heating is used in combination with water cooling to achieve rapid mold surface temperature control during the microcellular foam injection molding process. The effect of processing parameters, such as mold temperature, melt temperature, and injection velocity on the part surface quality, was investigated using surface roughness measurements and visual inspection of the molded parts. The results show that using induction heating to increase the mold surface temperature from 100 degrees C to 160 degrees C can decrease surface roughness of polycarbonate moldings from 25 mu m to 6.5 mu m. It was also found that the flow marks formed by gas bubbles on the part surface can be removed completely at a mold temperature of 160 degrees C. Further increases in the mold temperature show slight improvements in the surface roughness up to 180 degrees C, at which point the surface roughness starts to level off at 5 mu m. This surface roughness value reflects an 80% improvement without a significant increase in cycle time over parts molded at a mold temperature of 60 degrees C using water heating. Higher melt temperature and faster injection speed will also improve the surface quality of microcellular injection molded parts but not as significantly. The usefulness of a variable mold temperature in improving part surface quality during microcellular foam injection molding has been successfully demonstrated. (C) 2009 Wiley Periodicals, Inc. Adv Polym Techn 27: 224-232, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.20133