Most of the plastic parts of the car are injection molded, because these plastic parts themselves have a high design accuracy, making these plastic parts can not use conventional injection molding, and must use precision injection molding process technology. In order to ensure the performance, quality and reliability of the precision plastic parts of the car, it is necessary to improve the plastic materials, injection molding equipment and molds and injection molding processes with high quality and meet the requirements of the product design.
The main factors affecting precision injection molding
Determine the precision injection is based on the accuracy of injection molded products, that is, the product size tolerance, shape and position tolerance and surface roughness. There must be many relevant conditions for precision injection molding, and the most essential is the four basic factors of plastic material, injection mold, injection molding process and injection molding equipment. Design of plastic products, should first select the engineering plastics materials, and can be precision injection molding engineering plastics and must use those mechanical properties, size, stability, creep resistance, resistance to environmental stress cracking material. Second, should be based on the choice of plastic materials, finished product size accuracy, weight, quality requirements and the expected mold structure selection of suitable injection molding machine. In the process of processing, the impact of precision injection molding products mainly from the mold accuracy, injection shrinkage, and the product of the ambient temperature and humidity changes and so on.
In precision injection molding, mold is used to meet the quality requirements of precision plastic products, one of the key, precision injection mold should be consistent with the product size, accuracy and shape requirements. However, even if the mold accuracy, size consistent, the actual size of its molded plastic products will be inconsistent due to differences in shrinkage. Therefore, the effective control of the shrinkage of plastic products in precision injection molding technology is very important.
Whether the mold design is reasonable or not will directly affect the shrinkage of plastic products, because the mold cavity size is determined by the size of the plastic product with the estimated shrinkage rate, and shrinkage is recommended by the plastic manufacturer or engineering plastic manual Of a range of values, it is not only with the mold of the gate form, gate location and distribution, but also with the engineering plastics crystal orientation.
The shape and size of plastic products, to the distance and location of the gate. The factors that affect the plastic shrinkage are heat shrinkage, phase transition shrinkage, orientation shrinkage, compression shrinkage and elastic recovery and other factors, and these factors and precision injection molding products, molding conditions or operating conditions. Therefore, in the design of molds must consider the relationship between these factors and injection conditions and its 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 , As well as the cross-sectional area of the gate, the wall thickness of the product, the content of the reinforcing filler in the plastic material, the crystallinity and orientation of the plastic material. The impact of these factors is also due to different plastic materials, other molding conditions such as temperature, humidity, continue to crystallize, after forming the internal stress, injection molding machine changes and different.
As the injection process is the plastic from the solid powder or pellets to the liquid melt and the process of transition to solid products. From the pellets to the melt, and then from the melt to the product, the middle through the temperature field, stress field, flow field and the role of density field, under the combined action of these fields, different plastic thermosetting or thermoplastic, crystalline or Non-crystalline, reinforced or non-reinforced, etc. have different polymer structure and rheological properties. Any factors that affect the above-mentioned fields will certainly affect the physical and mechanical properties, size, shape, accuracy and appearance quality of plastic products. In this way, the intrinsic relationship between process factors and polymer properties, structural morphology and plastic products is manifested through plastic products. Analysis of these internal links, the rational development of injection molding process, a reasonable design and drawings according to the drawings, and even a reasonable choice of injection molding equipment are of great significance. Precision injection molding and general injection in the injection pressure and injection rate is also different, precision injection often used high pressure or ultra high pressure injection, high speed injection to obtain a smaller molding shrinkage. In addition to the above reasons, the design of precision injection molds in addition to considering the general design elements of the mold, we must also consider the following: ① the use of appropriate mold size tolerance; ② to prevent the formation of shrinkage shrinkage; ③ to prevent injection deformation; To prevent the occurrence of mold deformation; ⑤ mold manufacturing error to a minimum; ⑥ to prevent the accuracy of the mold error; ⑦ to maintain the accuracy of the mold.
To prevent the occurrence of molding shrinkage error.
As the shrinkage rate will change due to injection pressure, therefore, for single-cavity mold, cavity cavity cavity pressure should be consistent; as for multi-cavity mold, cavity cavity between the cavity pressure should be very small. In the case of single-chamber multi-gate or multi-cavity multi-gate, must be the same injection pressure injection, so that the cavity pressure. For this reason, it is necessary to ensure that the gate position is balanced. In order to make the cavity cavity pressure consistent, it is best to keep the pressure at the gate entrance consistent. The pressure at the gate is related to the flow resistance in the flow channel. Therefore, before the gate pressure reaches equilibrium, the circulation should be balanced. Since the melt temperature and the mold temperature have an effect on the actual shrinkage rate, it is necessary to pay attention to the arrangement of the cavities in order to facilitate the determination of the molding conditions when designing the precision injection mold cavity.
The above is the car design of precision plastic parts of the mold design points.