Main factors affecting precision injection molds

Overview of Precision Injection Molding

Precision injection molding is based on the precision of injection molded products, that is, the dimensional tolerance, geometric tolerance and surface roughness of the product. To perform precision injection molding, there must be many related conditions, and 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, dimensional stability, good creep resistance, and environmental stress crack resistance. Secondly, a suitable injection molding machine should be selected according to the selected plastic material, finished product dimensional accuracy, piece weight, quality requirements and the expected mold structure. In the process of processing, the factors that affect the precision injection molding products mainly come from the precision of the mold, the injection shrinkage, and the changes in the ambient temperature and humidity of the product.

Influence of mold accuracy on precision injection molding

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 meet the requirements of product size, accuracy and shape. However, 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 rate of plastic products in precision injection molding technology.

Factors Affecting Injection 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 from the size of the plastic product plus the estimated shrinkage rate, and the shrinkage rate is recommended by the plastic manufacturer or engineering plastics manual It is not only related 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 plastic shrinkage are thermal shrinkage, phase change shrinkage, orientation shrinkage, compression shrinkage and elastic recovery, and these influencing factors are related to the molding conditions or operating conditions of precision injection molding products. Therefore, the relationship between these influencing factors and injection 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. , as well as the influence of gate cross-sectional area, product wall thickness, content of reinforcing fillers in plastic materials, crystallinity and orientation of plastic materials and other factors. The influence of the above factors is also different due to different plastic materials, other molding conditions such as temperature, humidity, continuous crystallization, internal stress after molding, and changes in the injection molding machine.

The effect of changes in ambient temperature and humidity of the product

Because the injection molding process is a process of transforming plastics from solid (powder or pellet) to liquid (melt) and back to solid (product). From the pellets to the melt, and then from the melt to the product, the temperature field, stress field, flow field, and density field are involved in the middle. Under the combined action of these fields, different plastics (thermosetting or thermoplastic, crystalline Or amorphous, reinforced or non-reinforced, etc.) with different polymer structure morphology and rheological properties. Any factors that affect the above "field" will definitely affect the physical and mechanical properties, size, shape, precision and appearance quality of plastic products.

In this way, the inherent link between process factors and polymer properties, structural morphology and plastic products will be manifested through plastic products. Analyzing 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 is also different from ordinary injection molding 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 molding shrinkage.
In view of the above reasons, in addition to the design elements of general molds, the following points must be considered when designing a precision injection mold: ①Appropriate mold dimensional tolerances; ②Prevent molding shrinkage errors; ③Prevent injection deformation; ④ Prevent mold release deformation; ⑤ Minimize mold manufacturing errors; ⑥ Prevent mold accuracy errors; ⑦ Maintain mold accuracy.

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