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Factors affect precision injection molding defects

The basis for judging precision injection molding is the precision of the injection molded product, that is, the dimensional tolerance, shape tolerance and surface roughness of the plastic 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 processes and injection molding equipment. When designing plastic products, engineering plastic materials should be selected first, and engineering plastics capable of precision injection molding must use materials with high mechanical properties, dimensional stability, good creep resistance and environmental stress cracking resistance. Secondly, the appropriate injection molding machine should be selected according to the selected plastic material, the dimensional accuracy of the finished product, the weight of the plastic part, the quality requirements and the expected mold structure. In the process of precision injection molding, the factors affecting precision injection molding products mainly come from the precision of the precision injection mold, the shrinkage of the injection molding, and the variation of 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 molds for precision injection molding should meet the requirements of product size, precision and shape. However, even if the precision and size of the mold are the same, the actual size of the injection molded plastic product may be inconsistent due to the difference in the amount of shrinkage. Therefore, effective control of shrinkage of plastic products is very important in precision injection molding technology.

Whether the injection mold design is reasonable or not will directly affect the shrinkage rate of plastic products. Since the mold cavity size is determined by the size of the plastic product plus the estimated shrinkage rate, the shrinkage rate is a range of values recommended by the plastic manufacturer or engineering plastics manual. It not only related to the gate form of the mold, the position and distribution of the gate, but also the crystal orientation (anisotropic) of the engineering plastic, the shape and size of the plastic product, the distance to the gate, and the location. The main factors affecting plastic shrinkage are heat shrinkage, phase change shrinkage, orientation shrinkage, compression shrinkage and elastic recovery. These factors are related to the molding conditions or operating conditions of precision injection molded products. Therefore, the relationship between these influencing factors and injection molding conditions and their apparent factors must be considered when designing the mold, such as injection pressure and cavity pressure and filling speed, injection melt temperature and mold temperature, mold structure and gate form and distribution. The influence of the above factors is also different due to different plastic materials, other molding conditions such as temperature, humidity, continued crystallization, internal stress after molding, and changes in the injection molding machine.
Plastic parts
Because the injection molding process is the process of converting plastic from solid (powder or pellet) to liquid (melt) to solid (product). From the pellets to the melt, from the melt to the product, through the temperature field, stress field, flow field and density field, under the joint action of these fields, different plastics (thermosetting or thermoplastic, crystalline) Or non-crystalline, reinforced or non-reinforced, etc.) have different polymer structural morphology and rheological properties. Any factors that affect the above "field" will affect the physical and mechanical properties, size, shape, precision and appearance quality of plastic products.

Thus, the intrinsic link between process factors and polymer properties, structural morphology, and plastics is manifested by plastics. Analysis of these intrinsic links is of great significance for the rational preparation of injection molding process, rational design and manufacturing of molds according to drawings, and even the selection of injection molding equipment. Precision injection molding and ordinary injection molding also differ 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. In view of the above various reasons, in addition to considering the design elements of general molds, the following points must be considered when designing precision injection molds:
1. Using appropriate mold dimensional tolerances
2. Preventing mold shrinkage error
3. Preventing injection molding deformation
4. Prevent demoulding from occurring
5. Minimize mold manufacturing errors
6 prevent mold accuracy errors
7 maintain mold accuracy.

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