I. Intrduction
Plastics are widely used in industry as one of the basic materials. Comparing with other materials, plastics have many advantages, i.e., lighter weight, easier processing, and better mechanical/chemical properties. Since 1976 plastics have surpassed steel, copper, and aluminum in combined volume and become the most widely used materials in the US. Injection molding is one of the most versatile and important manufacturing processes which are capable of mass-producing complicated plastics parts with excellent precision. In the modern plastics industry approximately 80% plastic merchandise are produced by the injection molding system especially in recent years during which more and more plastic products are in great demand due to the expansion of domestic and international markets for automobiles, electronic appliances, building materials, food and medicine industries.