Design Essentials of Molds for Cars' Precision Plastic Parts (Part One)

Most of the plastic parts for cars are made by injection moldings. These plastic parts themselves have high design accuracy; they can not be made by the conventional injection moldings. Therefore, we must adopt precision injection molding technologies to manufacture plastic parts for cars. In order to ensure performance, quality and reliability of precision plastic parts for cars and to make plastic products have high quality and meet design requirements, it is necessary to improve the plastic materials, injection molding machines, molds and injection molding processes.

The main factors affecting precision injection moldings
Precision injections are determined by accuracy of injection molding products, that is, the products' size tolerance, form and position tolerance and surface roughness. There must be many related conditions for precision injection molding processes, and the most essential factors are plastic materials, injection mold, injection molding processes and injection molding machines these four basic factors. When we design plastic products, we should firstly select engineering plastic materials which can be processed by the precision injection molding and have features of good mechanical properties, dimensional stability, good creep resistance and environmental stress cracking resistance. Secondly, we should select suitable injection molding machines based on the chosen plastic materials, dimensional accuracy of the finished products, the products' weights, quality requirements and the expected mold structures. In the manufacturing process, factors affecting precision injection molding products are mainly mold accuracy, injection shrinkage as well as ambient temperature and humidity changes of the plastic products.

In precision injection molding, the mold is one of the key points which can meet quality requirements of precision plastic injection molding products; the mold which is used for precision injection molding should practically meet requirements of the precision injection molding products' sizes, accuracy and shapes. However, even if the mold has the same accuracy and sizes with the precision injection molding products, the actual sizes of the precision injection molding products may be inconsistent with the mold's due to shrinkage. Therefore, it is very important to effectively control shrinkage rates of injection molding products in precision injection molding technologies. 

Whether the mold design is reasonable or not will directly affect shrinkage rates of injection molding products. Sizes of the mold cavities are determined by sizes of the plastic products' sizes and the estimated shrinkage rates, and shrinkage rates are a range of values which are recommended by plastic manufacturers or engineering plastic handbooks. Therefore, shrinkage rates are not only related to forms, locations and distributions of pouring gates, but also related to crystal orientations of engineering plastic, shapes and sizes of injection molding products, locations of pouring gates as well as distances between injection molding products and pouring gates.

The factors affecting shrinkage rates of plastic are mainly heat shrinkage, phase change shrinkage, orientation shrinkage, compression shrinkage and elastic recovery, which are related to forming conditions or operating conditions of precision injection molding products. Therefore, we must consider relationships between these factors and injection conditions as well as apparent factors.