What factors will affect the precision injection molding effect of the mold?


Author: MULAN -Plastic Molding Manufacturer

The basis for judging the precision injection molding of molds is the precision of injection molded products, that is, the dimensional tolerance, shape tolerance and surface roughness of the product. There must be many related conditions for precision injection molding, but the most essential are the four basic factors of plastic material, injection mold, injection molding process and injection molding equipment. When designing plastic products, engineering plastic materials should be selected first, and engineering plastics capable of precision injection molding must choose those materials with high mechanical properties, stable dimensions, 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 piece, the quality requirements and the expected mold structure. In the process of processing, the factors affecting the precision injection molding products mainly come from the precision of the mold, the shrinkage of injection molding, and the range of 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 used for precision injection molding should meet the requirements of product size, precision and shape.

But even if the precision and size of the mold are consistent, 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 rate of plastic products in precision injection molding technology. Whether the mold design is reasonable or not will directly affect the shrinkage rate of plastic products, because the size of the mold cavity is obtained by adding the estimated shrinkage rate to the size of the plastic product, and the shrinkage rate is determined by the plastic manufacturer or engineering plastics manual A value within a recommended range is not only related to the gate form, gate position and distribution of the mold, but also to the crystallographic orientation of engineering plastics.

The shape, size, distance and position of the plastic mold are related to the gate. The main factors affecting the shrinkage of plastics are heat shrinkage, phase change shrinkage, orientation shrinkage, compression shrinkage and elastic recovery, etc., and these influencing 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 its appearance 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 , and the influence of factors such as gate cross-sectional area, product wall thickness, content of reinforcing filler in plastic material, crystallinity and orientation of plastic material.

The influence of the above factors also varies with 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 transforming plastic from solid powder or pellets to liquid melt and then to solid products. From the pellets to the melt, and then from the melt to the product, there are temperature fields, stress fields, flow fields, and density fields in the middle. Under the combined action of these fields, different plastics are thermosetting or thermoplastic, crystalline or Amorphous, reinforced or non-reinforced, etc. have different polymer structures and rheological properties.

All factors that affect the above fields will definitely affect the physical and mechanical properties, size, shape, precision and appearance quality of plastic products. In this way, the intrinsic relationship between process factors and polymer performance, structural form and plastic products will be expressed through plastic products. Analyzing these internal connections is of great significance for rationally drawing up injection molding processing technology, rationally designing and manufacturing molds according to drawings, and even rationally selecting injection molding processing equipment.

Precision injection molding is also different from ordinary injection molding in terms of injection pressure and injection rate. Precision injection molding often uses high-pressure or ultra-high-pressure injection and high-speed injection to obtain a smaller molding shrinkage. Based on the above reasons, in addition to considering the design elements of general molds when designing precision injection molds, the following points must also be considered: ① Adopt appropriate mold size tolerances; ② Prevent molding shrinkage errors; ③ Prevent injection deformation; ④ Prevent demoulding deformation; ⑤ Minimize mold manufacturing errors; ⑥ Prevent errors in mold accuracy; ⑦ Maintain mold accuracy.

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