How to perform precision injection molding?

Update:16-10-2020
Summary:

We usually say that precision injection molding means t […]

We usually say that precision injection molding means that the appearance accuracy of injection molded products should meet strict dimensional tolerances, geometric tolerances and surface roughness. 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 that can be precision injection molded must choose those 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, 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 temperature of the mold, the control of the injection molding process, the range of environmental temperature and humidity changes in the production site, and the annealing treatment of the acquired product.

In terms of 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, and the mold material should be strictly selected. 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 of plastic products in precision injection molding technology.

Whether the injection mold is designed properly 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 plastic manual The recommended value within a range 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 the plastic product, and the distance to the gate It is related to the location and also closely related to the mold cooling distribution system. 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 injection molding machines.

Because the injection molding process is a process of transforming plastic from solid (powder or pellets) to liquid (melt) to solid (product). From pellets to melt, and then from melt to product, the temperature field, stress field, flow field, and density field are in the middle. Under the combined action of these fields, different plastics (thermosetting or thermoplastic, crystalline Or non-crystalline, reinforced or non-reinforced, etc.) have different polymer structures and rheological properties. All the factors that affect the above-mentioned "field" 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 expressed through plastic products. A clear analysis of these internal connections is of great significance to rationally formulating injection molding processes, 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 molding shrinkage rate. Considering the above-mentioned various reasons, in addition to the general mold design elements, the following points must be considered when designing precision injection molds: ① Use appropriate mold dimensional tolerances; ② Prevent molding shrinkage errors; ③ Prevent injection deformation; Prevent demolding deformation; ⑤ minimize mold manufacturing errors; ⑥ prevent errors in mold accuracy; ⑦ maintain mold accuracy.

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