Most of the plastic parts of cars are made by injection molding. Because of their high design accuracy, these plastic parts cannot be routinely injected, but precise injection molding technology is necessary. In order to ensure the performance, quality and reliability of the precise plastic parts of the car, the injection molded products with high quality and conformity with the design requirements of plastics must be continuously improved for plastic materials, injection equipment, mould and injection molding technology.
The main factors affecting the precision injection molding
The precision injection molding is based on the precision of the injection products, that is, the dimensional tolerance, the geometric tolerance and the surface roughness of the products. There must be many related conditions for precision injection, and the four essential factors are plastic material, injection mold, injection process and injection equipment. When designing plastic products, engineering plastics should be selected first, while those for precision injection must be chosen with high mechanical properties, stable size, good creep resistance and environmental stress cracking. Secondly, the suitable injection molding machine should be selected according to the selected plastic material, the precision of the finished product, the weight, the quality requirements and the expected die structure. During the processing, the factors that influence the precision injection products mainly come from the accuracy of mold, injection shrinkage, and the environmental temperature and humidity range of products.
In precision injection molding, mold is one of the key points to achieve quality requirements of precision plastic products. The mould for precision injection should conform to the requirements of product size, accuracy and shape. But even if the precision and size of the mold are consistent, the actual size of the molded plastic products will not be consistent with the difference in the shrinkage. Therefore, it is very important to effectively control the shrinkage of plastic products in the precision injection molding technology.
The mold design is reasonable or not will directly affect the plastic shrinkage, the mold cavity size is obtained by the plastic product size plus the estimated shrinkage rate, and the rate of contraction is a numerical range recommended by the manufacturer or plastic engineering plastics in the manual, it not only with the mold gate and the gate location and distribution, crystal orientation and engineering plastics.
The shape and size of the plastic products are related to the distance and position of the gate. The main factors that influence the shrinkage of plastics include thermal shrinkage, phase transformation shrinkage, oriented shrinkage, compression shrinkage and elastic recovery, which are related to the forming conditions or operation conditions of precision injection products. Therefore, in the mold design must consider these factors and the relationship between the injection conditions and epigenetic factors, such as injection pressure and cavity pressure and filling speed, melt temperature and mold temperature, injection mold structure and gate form and distribution, and the influence area of the gate, the wall thickness, the plastic material reinforced filler content, crystallinity and orientation factors of plastic materials. The above factors are also affected by different plastic materials and other molding conditions such as temperature, humidity, continuous crystallization, internal stress after forming, and injection molding machine.
Because the injection process is the process of transforming plastic from solid powder or granular to liquid melt to solid product. From pellets to melt, the melt to the products, to go through the middle of the temperature field, stress field, velocity field and density field effect in the interaction of these fields, different polymer structure morphology and rheological properties of different plastics thermosetting or thermoplastic, crystalline or non crystalline, enhanced or has enhanced. All factors affecting the above field will affect the physical and mechanical properties, size, shape, precision and appearance quality of the plastic products. In this way, the intrinsic link between the process factors and the properties of the polymer, the structure form and the plastic products will be shown by the plastic products. Analyzing and understanding these internal relations is very important for making reasonable injection molding process, designing rationally, making moulds according to drawings, and even rationally selecting injection processing equipment. Precision injection and ordinary injection are also different in injection pressure and injection rate. Precision injection often uses high pressure or ultra-high pressure injection and high-speed injection to achieve smaller molding shrinkage. According to the above reasons, the design of precision injection mold with considering general mold design elements, we must consider the following points: the mold size tolerance appropriate; the prevent shrinkage error; to prevent the occurrence of injection to prevent the occurrence of deformation; the demoulding deformation; and to minimize the error of mold manufacturing; prevent the error of the die precision mold to maintain accuracy.
Prevent the formation of shrinkage error.
Because the shrinkage rate will be changed due to the injection pressure, therefore, for the single cavity mold, the cavity pressure of the mold cavity should be the same as possible. As for the multi cavity mold, the cavity pressure between the mold cavity should be very small. In the case of a single cavity multiple gate or multi cavity multiple gate, the same injection pressure must be injected to make the cavity pressure consistent. To this end, it is necessary to ensure that the gate position is balanced. In order to make the cavity pressure in the cavity consistent, it is best to keep the pressure of the gate at the entrance. The balance of the pressure at the gate is related to the flow resistance in the channel. Therefore, before the gate pressure is balanced, the circulation should be balanced. Due to the influence of melt temperature and mold temperature on the actual shrinkage rate, when designing the cavity of precision injection mold, we must pay attention to the arrangement of the cavity in order to facilitate the determination of the molding conditions.
The above is the key point of the design of the molding mold for the car precision plastic parts.
