The basis for the determination of precision injection molding is the precision of the injection molding product, namely the dimensional tolerance, shape tolerance and surface roughness of the product. To carry out precision injection molding must have many relevant conditions, and the most essential is 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. Engineering plastics capable of precision injection molding must use materials with high mechanical properties, dimensional stability, good creep resistance, and resistance to environmental stress cracking. Secondly, a suitable injection molding machine should be selected according to the selected plastic material, the dimensional accuracy of the finished product, the part weight, the quality requirements, and the expected mold structure. In the processing, the factors that affect the precision injection products mainly come from the precision of the mold, the shrinkage of the injection molding, and the changes in the ambient temperature and humidity of the products.
In precision injection molding, the mold is one of the keys to obtain precision plastic products that meet the quality requirements. The molds for precision injection molding should meet the requirements of product size, precision, 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 of plastic products in precision injection molding technology.
Whether the mold design is reasonable or not will directly affect the shrinkage rate of plastic products. The mold cavity size is determined by the plastic product size plus the estimated shrinkage rate, and the shrinkage rate is recommended by the plastic manufacturer or the engineering plastics manual. A range of values, not only related to the gate shape of the mold, location and distribution of the gate, but also the crystal orientation (anisotropic) of the engineering plastic, the shape, size, distance to the gate of the plastic product, and Location related. The main factors affecting plastic shrinkage are thermal shrinkage, phase change shrinkage, orientation shrinkage, compression shrinkage, and elastic recovery. These factors are related to the molding conditions or operating conditions of precision injection molding products. Therefore, the relationship between these influencing factors and the injection molding conditions and the appearance factors must be taken into consideration when designing the mold, such as injection pressure and cavity pressure and mold filling speed, injection melt temperature and mold temperature, mold structure and gate form and distribution. , And gate cross-sectional area, wall thickness of the product, the content of the reinforcing filler in the plastic material, the crystallinity and orientation of the plastic material and other factors. The above factors are also affected by different plastic materials and other molding conditions such as temperature, humidity, continued crystallization, internal stress after molding, and changes in the injection molding machine.
Since the injection molding process is the process of transforming the plastic from a solid (powder or pellet) to a liquid (melt) and a solid (article). From the pellets to the melt and then from the melt to the product, the temperature field, the stress field, the flow field, and the density field, etc., have to be used in the middle. Under the combined action of these fields, different plastics (thermoset or thermoplastic, crystalline) Or non-crystalline, reinforced or non-reinforced, etc.) have different polymer structural morphology and rheological properties. All factors that affect the above "field" will inevitably affect the physical and mechanical properties, dimensions, shape, accuracy and appearance quality of plastic products.
In this way, the intrinsic link between process factors and polymer properties, structural morphology and plastic products can be demonstrated through plastic products. The analysis of these intrinsic connections is of great significance for the rational development of injection molding processes, the rational design and manufacture of molds according to drawings, and even reasonable selection of injection molding equipment. Precision injection molding and ordinary injection molding also differ in injection pressure and injection rate. Precision injection molding often uses high-pressure or ultra-high pressure injection, high-speed injection to achieve a smaller molding shrinkage. Considering all of the above reasons, in addition to considering the design elements of a general mold, the following points must be considered when designing a precision injection mold: 1 using proper mold dimensional tolerances; 2 preventing mold shrinkage error; 3 preventing injection molding distortion; 4 To prevent the occurrence of mold release deformation; 5 to minimize the manufacturing error of the mold; 6 to prevent the accuracy of the mold error; 7 to maintain mold accuracy.
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