Main factors affecting precision injection molding

Precision injection molding differs from ordinary injection molding in that it has higher requirements for plastic products, smaller tolerances, and a more beautiful appearance.
Overview of Precision Injection Molding
Precision injection molding is based on the accuracy of the injection molded product, that is, the dimensional tolerance, geometric tolerance, and surface roughness of the product. There must be many related conditions for precise injection molding, and the most essential are the four basic factors: plastic material, injection mold, injection molding process, and injection molding equipment. When designing plastic products, engineering plastics materials should be selected first, and for engineering plastics capable of precision injection molding, materials with high mechanical properties, stable dimensions, good creep resistance, and environmental stress cracking resistance must be selected. Secondly, suitable injection molding machines should be selected based on the selected plastic material, finished product dimensional accuracy, piece weight, quality requirements, and expected mold structure. During the processing process, the factors that affect precision injection molded products mainly come from the accuracy of the mold, injection molding shrinkage, and the range of environmental temperature and humidity changes of the product.
The Effect of Mold Precision on Precision Injection Molding
In precision injection molding, molds are one of the keys to obtaining precision plastic products that meet quality requirements. The molds used for precision injection molding should meet the requirements for product size, accuracy, and shape. However, even if the accuracy and size of the mold are consistent, the actual size of the molded plastic products will vary due to differences in shrinkage. Therefore, effectively controlling the shrinkage of plastic products is very important in precision injection molding technology.
Factors Affecting Injection Molding Shrinkage
Whether the mold design is reasonable or not will directly affect the shrinkage rate of plastic products. Because the mold cavity size is obtained by adding the estimated shrinkage rate to the plastic product size, and the shrinkage rate is a range of values recommended by the plastic manufacturer or engineering plastics manual. It is not only related to the mold gate form, gate location, and distribution, but also related to the crystalline orientation (anisotropy) of engineering plastics, the shape of plastic products Size, distance to the gate, and location. The main factors that affect the shrinkage of plastics include thermal shrinkage, phase change shrinkage, orientation shrinkage, compression shrinkage, and elastic recovery, and these factors are related to the molding or operating conditions of precision injection molded products. Therefore, when designing a mold, it is necessary to consider the relationship between these influencing factors and injection molding conditions and their apparent factors, such as injection molding pressure and cavity pressure and mold filling speed, injection melt temperature and mold temperature, mold structure, gate form and distribution, as well as the impact of factors such as gate cross-sectional area, product wall thickness, the content of reinforcing filler in plastic materials, and the crystallinity and orientation of plastic materials. The influence of the above factors also varies depending on the plastic material, other molding conditions such as temperature, humidity, continued crystallization, internal stress after molding, and changes in the injection molding machine.
Effects of changes in ambient temperature and humidity of products
Because the injection molding process is the process of transforming plastics from solid (powder or granular) to liquid (melt) and then to solid (product). From granular materials to melts, and then from melts to products, there are temperature fields, stress fields, flow fields, and density fields in the process. Under the combined action of these fields, different plastics (thermosetting or thermoplastic, crystalline or amorphous, reinforced or non reinforced, etc.) have different polymer structures and rheological properties. Any factors that affect the above "field" will inevitably affect the physical and mechanical properties, size, shape, accuracy, and appearance quality of plastic products.
Summary
In this way, the inherent relationship between process factors and the performance, structural morphology, and plastic products of polymers will be manifested through plastic products. Analyzing and clarifying these internal relationships is of great significance for rationally formulating injection molding processes, designing and manufacturing molds according to drawings, and even selecting injection molding equipment. Precision injection molding is also different from conventional injection molding in terms of injection pressure and injection rate. Precision injection molding often uses high-pressure or ultra-high-pressure injection, high-speed injection to achieve a small molding shrinkage. Based on the above reasons, in addition to considering the design elements of general molds, the following points must also be considered when designing precision injection molds: ① adopting appropriate mold dimensional tolerances; ② Prevent molding shrinkage error; ③ Prevent injection molding deformation; ④ Prevent mold release deformation; ⑤ Minimize mold manufacturing errors; ⑥ Prevent mold accuracy errors; ⑦ Maintain mold accuracy.