The main factors affecting precision injection molding

The basis for judging precision injection molding is the accuracy of the injection molded product, that is, the product's dimensional tolerance, form and position tolerance and surface roughness. To carry out precision injection molding, there must be many relevant conditions, and the most essential are the four basic factors of plastic materials, injection molds, injection molding process and injection molding equipment. When designing plastic products, engineering plastic materials should be selected first, and engineering plastics that can be precision injection molded must choose those 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, finished product dimensional accuracy, part weight, quality requirements and expected mold structure. In the process of processing, the factors that affect precision injection molded products mainly come from the accuracy of the mold, injection shrinkage, and the range of changes in the environmental 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 truly 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 products 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 the plastic product, because the mold cavity size is calculated by adding the estimated shrinkage rate to the size of the plastic product, while the shrinkage rate is determined by the plastic manufacturer or engineering The value within a range recommended by the plastic manual is not only related to the gate form, gate position and distribution of the mold, but also to the crystal orientation of engineering plastics.

The shape, size, distance to the gate and position of the plastic product are related. The main factors that affect plastic shrinkage are thermal 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, the relationship between these influencing factors and injection conditions and their 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, reinforcing filler content in plastic materials, crystallinity and orientation of plastic materials. 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.

Because the injection molding process is a process of transforming plastic from solid powder or pellets to liquid melt and then to solid products. From pellets to melt, and then from melt to products, the temperature field, stress field, flow field and density field are in the middle. Under the combined action of these fields, different plastics have thermosetting or thermoplastic properties, crystallinity or Amorphous, reinforced or non-reinforced polymers have different polymer structures and rheological properties. All the factors that affect the above-mentioned fields will definitely affect the physical and mechanical properties, size, shape, precision and appearance quality of plastic products. In this way, the internal connection between process factors and polymer properties, structural morphology and plastic products will be manifested through plastic products. A clear analysis of these internal connections is of great significance to rationally formulating the injection molding process, rationally designing and manufacturing molds according to drawings, and even rationally selecting injection molding equipment. Precision injection molding and ordinary injection molding are also different 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 smaller mold shrinkage rate.

Combining the above reasons, in addition to the general mold design elements, the following points must be considered when designing precision injection molds: ①Use appropriate mold dimensional tolerances; ②Prevent molding shrinkage errors; ③Prevent injection molding Deformation; ④Prevent demoulding deformation; ⑤Minimize mold manufacturing errors; ⑥Prevent errors in mold accuracy; ⑦Maintain mold accuracy.