Abstract
Keywords - Miceocellular Foaming, Injection Molding, Shrinkage, Interior Parts, polypropylene
Abstract - When a product is hardened under the influence of external constraints and the constraints are removed from the product after solidification, stress is distributed differently in parts of the product, so the size of the product becomes different from that of the mold or the die. As a result, the manufacturer comes to have a product, the size of which is different from that he designed. Because parts used in precise appliances generally have an extremely small tolerance, the size of molded products often causes serious problems. What is more, parts or final products often fail to function properly when they are assembled or used because of different shrinkage rates caused by the change of size.
Microcellular foaming technology has been developed originally with the object of preserving the mechanical property of polymer resin as well as saving the raw materials of polymer resin. The basic concept of microcellular foaming is forming cells sized 10-50 micrometer in diameter inside polymer product. The cells have influences on several physical properties of polymer product. One of the influences is reducing shrinkage problematic in polymer forming. If the technology is effective in lowering the cost of materials for polymer resin as well as shrinkage rate, it may be widely applicable.
This study is purposed to explain the relationship between shrinkage rate and cell morphology by identifying the effects of the quantity of gas input and the content of talc on cell morphology, which affects shrinkage rate, using ASTM standard sample pieces molded through the microcellular foaming technology.
All the materials used for this work were commercial products and they were used as received without further treatments. Polymer resins used in the experiment were polypropylene (PP) used as a car interior material (for rear door side trim, switch panel, etc.), one non-talc-containing resin (JI-370) and the other 20% talc filled resin (HT-340). The reason for using the two types of resin is to examine the effect of talc on shrinkage. In addition, highly purified N2 was used as a blowing agent. The injection molding machine used in the experiment is one adapted for microcellular foaming process, and the high pressure gas supply system is a device designed to compress N2 at a high pressure and supply it at a fixed flow rate.