The basis for determining precision injection molding is the accuracy of the injection molded product, that is, the dimensional tolerance, shape tolerance and surface roughness of the product. There are many relevant conditions for precision injection molding, and the most essential are the four basic factors of plastic materials, injection molds, injection molding process and injection equipment. When designing plastic products, engineering plastic materials should be selected first, and engineering plastics capable of precision injection molding must select materials with high mechanical properties, stable dimensions, good creep resistance, and resistance to environmental stress cracking. Secondly, the appropriate injection molding machine should be selected according to the selected plastic material, finished product dimensional accuracy, part weight, quality requirements and the expected mold structure. In the processing process, the factors that affect precision injection molded products mainly come from the precision of the mold, the shrinkage of the injection molding, and the changes in the ambient temperature and humidity of the product.
In precision injection molding, the mold is one of the keys to obtain precision plastic products that meet the quality requirements. The mold for precision injection molding should actually meet the requirements of product size, accuracy, and shape. But even if the precision and size of the mold are the same, the actual size of the molded plastic product will be inconsistent due to the difference in shrinkage. Therefore, it is very important to effectively control the shrinkage of plastic products in precision injection molding technology.
Whether the mold design is reasonable or not will directly affect the shrinkage rate of the plastic product, because the mold cavity size is obtained from the plastic product size plus the estimated shrinkage rate, and the shrinkage rate is recommended by the plastic manufacturer or engineering plastics manual The value in a range of not only relates to the gate form, gate position and distribution of the mold, but also to the crystal orientation (anisotropy) of engineering plastics, the shape and size of plastic products, the distance to the gate and Location related. The main factors affecting the shrinkage rate of plastics are heat shrinkage, phase change shrinkage, orientation shrinkage, compression shrinkage and elastic recovery. These influencing factors are related to the molding conditions or operating conditions of precision injection molded products. Therefore, when designing the mold, the relationship between these influencing factors and the injection conditions and their apparent factors must be considered, such as injection pressure and cavity pressure and filling speed, injection melt temperature and mold temperature, mold structure and gate form and distribution , And the influence of factors such as gate cross-sectional area, product wall thickness, content of reinforcing filler in plastic materials, crystallinity and orientation of plastic materials. The effects of the above factors are also different due to different plastic materials, other molding conditions such as temperature, humidity, continued crystallization, internal stress after molding, and changes in injection molding machines.
Because the injection molding process is the process of changing plastics from solid (powder or pellet) to liquid (melt) to solid (product). From the pellet to the melt, and then from the melt to the product, there is a temperature field, a stress field, a flow field, and a density field in the middle. Under the joint action of these fields, different plastics (thermosetting or thermoplastic, crystallinity) Or non-crystalline, reinforced or non-reinforced, etc.) have different polymer structure morphology and rheological properties. All factors that affect the above-mentioned “field” will definitely affect the physical and mechanical properties, size, shape, accuracy and appearance quality of plastic products.
In this way, the inherent relationship between process factors and polymer properties, structural morphology and plastic products will be expressed through plastic products. It is of great significance to analyze these intrinsic connections to reasonably formulate injection molding processing technology, rationally design and manufacture molds according to drawings, and even rationally select injection molding processing equipment. Precision injection molding and ordinary injection molding also have differences in injection pressure and injection rate. Precision injection molding often uses high-pressure or ultra-high-pressure injection and high-speed injection to obtain a small molding shrinkage rate. Based on the above reasons, in addition to the design elements of general molds, the following points must be considered when designing precision injection molds: ① adopt appropriate mold dimensional tolerances; ② prevent molding shrinkage error; ③ prevent injection deformation; ④ Prevent demolding deformation; ⑤ Minimize mold manufacturing errors; ⑥ Prevent mold accuracy errors; ⑦ Maintain mold accuracy.