- 學位論文
冷卻速率與模內氣體反壓技術應用於聚丙烯超臨界微細發泡產品結晶度與發泡品質影響之研究
Effect of Cooling Rate and Mold Counter Pressure on The Crystallinity and Foaming Control in Microcellular Injection Molded Polypropylene Parts
摘要
超臨界流體微細發泡射出成型製程可降低熔融塑膠黏度、減低產品變形量、提升尺寸穩定性、節能省料等優點，具高度應用潛力。此製程是由麻省理工學院Nam P. Suh博士於1980年代所研究開發，原始的理論構想是將超臨界流體注入高分子熔膠內混鍊後充填至模具，冷卻階段則藉由壓力釋放後成核形成微細結構氣孔。MuCell的應用也因其優點多與適用性廣，目前領域涵蓋3C家電、汽車、半導體、工業用零組件。雖然此技術已經廣泛成功地使用非結晶、半結晶材料，但非結晶高分子材料並非為的新挑戰，而是在半結晶材料成型過程中的冷卻階段、高分子鏈會形成有序排列的晶體組織，結晶行為在超臨界微細發泡成型中會有兩個重要的問題產生。首先是晶體的形成過程中釋放多餘的熱量，會影響微細孔泡生成時無法預期的控制，造成不規則的氣孔尺寸。其次是高分子熔膠內的晶體結構成長時會推擠並迫使超臨界流體在產品邊緣部分發泡，而無法均勻控制在產品的核心。本研究發展如何控制與解決這些問題的方法，第一為有效地控制冷卻速率， 增加冷卻速度可以減少在結晶過程中的結晶度，此可凍結高分子微結構於欲結晶前的行為，此方法以有效被證實。第二是採用模內氣體反壓來觀測對發泡與結晶成長的影響，模內氣體反壓的建立可以有效控制微細孔泡尺寸以及分布於非結晶超臨界發泡射出成型，因此它具有效於控制半結晶材料氣泡成長程度的優點。論文中設計一系列實驗，並以差示掃描量熱法 (DSC)與掃描式電子顯微鏡(SEM)，研究結果顯示，兩種控制準則可以有效增進產品表面品質並強化技術未來應用性。
並列摘要
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.
參考文獻
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