Among the related factors affecting precision injection molding, molds are the key to obtaining precision plastic products that meet quality requirements. The design of the mold, whether the mold design is reasonable will directly affect the quality of plastic products. Because the mold cavity size is obtained by the required size of the plastic product plus the shrinkage rate of the material used, and the shrinkage rate is often recommended by the plastic manufacturer or engineering plastics manual within a range of values, it is not only related to the gate form, gate position and distribution of the mold, but also related to the crystalline orientation (anisotropy) of the engineering plastic, the shape, size and distance and position of the plastic product to the gate. The main factors affecting the shrinkage rate of plastics include heat shrinkage, phase change shrinkage, orientation shrinkage, compression shrinkage and elastic recovery, and these influencing factors are related to the molding conditions or operating conditions of precision injection molded products. Therefore, the designer of the mold must have rich experience in design and injection molding, and must consider the relationship between these influencing factors and injection molding conditions and their apparent factors, such as injection pressure and cavity pressure and filling speed, injection melt temperature and mold temperature, mold structure and gate form and distribution, as well as gate cross-sectional area, product wall thickness, reinforced filler content in plastic materials, crystallinity and orientation of plastic materials and other factors. The influence of the above factors also varies depending on the plastic material, or other molding conditions such as temperature, humidity, continued crystallization, and changes in the injection molding machine after molding.
Because the injection molding process is the process of transforming plastics from solid (powder or granules) to liquid (melt) and then to solid (products). From pellets to melt, and then from melt to products, the middle must pass through the action of temperature field, flow field and density field. Under the combined action of these fields, different plastics (thermoset or thermoplastic, crystalline or amorphous, reinforced or unreinforced, etc.) have different polymer structure morphology and rheological properties. All the factors that affect the above "field" will inevitably affect the physical and mechanical properties, size, shape, accuracy and appearance quality of plastic products.
In this way, the intrinsic relationship between process factors and the properties, structural morphology and plastic products of the polymer will be expressed through plastic products. Analyzing these internal relationships is of great significance for rationally drawing up injection molding processing technology, rationally designing and manufacturing molds according to drawings, and even reasonably selecting injection molding processing equipment. Precision injection molding and ordinary injection molding are also different in injection pressure and injection rate, precision injection often uses high pressure or ultra-high pressure injection, high-speed injection to obtain a small molding shrinkage.
