Shia-chung Chen (Committee Co-chair), George Huang (Committee Chair), Wen Ren Jong (Committee Member)
Master of Science in Engineering (MSEgr)
Microcellular polymer injection molding is a growing industry technique due to its ability to produce dimensionally stable stress free parts while reducing cycle time, material usage, and energy costs. The process was invented by Dr. Nam P. Suh at the Massachusetts Institute of Technology in the early 1980's. The basic idea is to dissolve a supercritical fluid into the polymer melt which will nucleate and expand within the part core after injection during the cooling stage. Microcellular polymer injection molding is becoming increasingly popular in automotive, semiconductor, and industrial applications. While the technique has been widely successful using amorphous polymers, semi-crystalline polymers present new challenges not encountered during processing with their amorphous counterparts. The polymer chains in a semi-crystalline material develop an organized crystal structure during the cooling stage. Crystal development generates two main issues for microcellular processing. The first being that the excess heat released during the crystal formation affects the expansion of the microcellular bubble causing unpredictable non-uniform growth. The second is that the growth of the crystal structure within the polymer melt expels and displaces the supercritical fluid forcing the foaming to occur out at the edges of the part rather than uniformly through its core. This paper develops and explores strategies to control and overcome these problems. The first strategy is to effectively control the cooling rate. It is well know and has been proven, that increasing the cooling rate during the crystallization process can decrease crystallinity effectively freezing the polymer microstructure in place before the polymer chains can become completely organized. The second strategy is to utilize in mold counter pressure to observe its effect of the development of the foaming and crystallization. In mold counter pressure has been found to be an effective means of controlling bubble size and distribution during amorphous microcellular injection molding therefore it has merit for being an effective method to control foaming with semi-crystalline polymers. These two strategies have been implemented on a set of experiments and the results measured and observed by differential scanning calorimetry and scanning electron microscopy. The results of the experiment indicate the strategies implied are effective methods for improving part quality and also impose confidence in further development.
Department or Program
Department of Mechanical and Materials Engineering
Year Degree Awarded
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