Among the related factors that affect precision injection molding, molds are the key to obtaining precision plastic products that meet quality requirements.
Mold design
Whether the mold design is reasonable will directly affect the quality of plastic products. Since the mold cavity size is derived from the required size of the plastic product plus the shrinkage rate of the material used, the shrinkage rate is often a value within a range recommended by the plastic manufacturer or the engineering plastics manual. It is not only related to the mold gate type , The gate position is related to the distribution, but also related to the crystal orientation (anisotropy) of engineering plastics, the shape and size of the plastic product, and the distance and location from the gate. The main factors that affect plastic shrinkage include heat shrinkage, phase change shrinkage, orientation shrinkage, compression shrinkage and elastic recovery, etc. These influencing factors are related to the molding conditions or operating conditions of precision injection molded products. Therefore, the mold designer must have rich design and injection molding experience, and must consider the relationship between these influencing factors and the injection conditions and their apparent factors, 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, or 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 the pellet to the melt, and then from the melt to the product, the temperature field, the stress field, the flow field and the density field must be affected in the middle. Under the combined action of these fields, different plastics (thermosetting or thermoplastic, crystalline or amorphous, reinforced or non-reinforced, etc.) have different polymer structural morphologies and rheological properties. All 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 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 molding shrinkage rate. Based on the above reasons, in addition to the design elements of general molds, the following points must be considered when designing precision injection molds:
1 Use appropriate mold size tolerances;
2 Prevent the error of molding shrinkage rate;
3 Prevent injection deformation;
4 Prevent demoulding deformation;
5 Minimize mold manufacturing errors;
6 Prevent errors in mold accuracy;
7 Maintain mold accuracy.
Prevent errors in molding shrinkage
Since the shrinkage rate will change due to the injection pressure, for a single cavity mold, the cavity pressure in the cavity should be as consistent as possible. As for the multi-cavity mold, the cavity pressure between the cavities should be very small. In the case of single cavity with multiple gates or multiple cavity with multiple gates, injection must be made with the same injection pressure to make the cavity pressure consistent. For this reason, it is necessary to ensure that the gate position is balanced. In order to make the cavity pressure consistent in the cavity, it is best to keep the pressure at the gate entrance consistent. The pressure balance at the gate is related to the flow resistance in the runner. Therefore, before the gate pressure reaches equilibrium, the runner should be balanced.