Automotive Plastic Parts Manufacturer in china

Automotive plastic parts, such as coil skeletons, bases, fuse boxes, lamp holders, blade fuses, central distribution boxes, sheaths, push racks, center console injection molding and outer covers, are mostly injection molded. Since these plastic parts have high design accuracy, conventional injection molding cannot be used for these plastic parts, but precision injection molding technology must be used. In order to ensure the performance, quality and reliability of automotive precision plastic parts, and to produce high-quality plastic products that meet product design requirements, plastic materials, injection molding equipment, molds and injection molding processes must be continuously improved.

1. Main factors affecting precision injection molding The basis for determining precision injection molding is the accuracy of the injection molded product, that is, the dimensional tolerance, shape and position tolerance and surface roughness of the product. There must be many related conditions for precision injection molding, and the most essential ones 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-injected must be selected from those with high mechanical properties, dimensional stability, good creep resistance and resistance to environmental stress cracking. Secondly, the appropriate injection molding machine should be selected according to the selected plastic material, finished product size accuracy, piece weight, quality requirements and expected mold structure. In the processing process, the factors affecting precision injection molded products mainly come from the accuracy of the mold, injection shrinkage, and the environmental temperature and humidity variation range of the product. In precision injection molding, the mold is one of the keys to obtain precision plastic products that meet quality requirements. The mold used for precision injection molding should meet the requirements of product size, accuracy and shape. However, even if the accuracy and size of the mold are consistent, 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 rate 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. Since the mold cavity size is obtained by adding the estimated shrinkage rate to the size of the plastic product, and the shrinkage rate is a value within a range recommended by the plastic manufacturer or the engineering plastics manual, it is not only related to the gate form, gate position and distribution of the mold, but also to the crystal orientation (anisotropy) of the engineering plastic, the shape, size, distance and position of the plastic product to the gate. The main factors that affect the shrinkage rate of plastics are heat shrinkage, phase change shrinkage, orientation shrinkage, compression shrinkage and elastic recovery, and these factors are related to the molding conditions or operating conditions of precision injection molded products. Therefore, when designing the mold, it is necessary to consider the relationship between these factors and the injection molding 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, as well as the influence of factors such as gate cross-sectional area, product wall thickness, content of reinforcing fillers in plastic materials, crystallinity and orientation of plastic materials. The influence of the above factors also varies depending on the plastic material, 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 plastic from solid (powder or granular) to liquid (melt) and then to solid (product). From pellets to melt, and then from melt to product, the process must go through the effects of temperature field, stress field, flow field and density field. Under the joint action of these fields, different plastics (thermosetting or thermoplastic, crystalline or non-crystalline, reinforced or non-reinforced, etc.) have different polymer structural forms and rheological properties. Any factors that affect the above “fields” will inevitably affect the physical and mechanical properties, size, shape, precision and appearance quality of plastic products. In this way, the inherent connection between process factors and polymer performance, structural form and plastic products will be manifested through plastic products. Analyzing these inherent connections clearly is of great significance for rationally formulating injection molding process, rationally designing and manufacturing molds according to drawings, and even rationally selecting injection molding equipment. Precision injection molding is also different from ordinary injection molding 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. In view of the above reasons, in addition to considering the design elements of general molds, the following points must also be considered when designing precision injection molds: ① Use appropriate mold size tolerances; ② Prevent molding shrinkage errors; ③ Prevent injection deformation; ④ Prevent demolding deformation; ⑤ Minimize mold manufacturing errors; ⑥ Prevent mold precision errors; ⑦ Maintain mold precision.

2. Prevent molding shrinkage errors Since the shrinkage rate will change due to injection pressure, for single-cavity molds, the cavity pressure in the cavity should be as consistent as possible; as for multi-cavity molds, the cavity pressure between cavities should be very small. In the case of a single cavity with multiple gates or multiple cavities with multiple gates, 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 at the gate entrance consistent. The balance of pressure at the gate is related to the flow resistance in the flow channel. Therefore, before the gate pressure is balanced, the flow should be balanced. Since the melt temperature and mold temperature affect the actual shrinkage rate, when designing the cavity of the precision injection mold, in order to facilitate the determination of the molding conditions, attention must be paid to the arrangement of the cavity. Because the molten plastic brings heat into the mold, and the temperature gradient distribution of the mold is generally around the cavity, in the shape of concentric circles centered on the main channel. Therefore, design measures such as flow channel balance, cavity arrangement, and concentric arrangement centered on the main channel are necessary to reduce the shrinkage error between cavities, expand the allowable range of molding conditions, and reduce costs. The cavity arrangement of the precision injection mold should meet the requirements of flow channel balance and arrangement centered on the main channel, and the cavity arrangement method with the main channel as the symmetry line must be adopted, otherwise it will cause differences in shrinkage rates of various cavities. Since the mold temperature has a great influence on the molding shrinkage rate, it also directly affects the mechanical properties of the injection molded product and causes various molding defects such as flowery surface of the product. Therefore, the mold must be kept within the specified temperature range, and the mold temperature must not change with time. The temperature difference between the cavities of the multi-cavity mold must not change. For this reason, temperature control measures for heating or cooling the mold must be taken in mold design, and in order to minimize the temperature difference between the mold cavities, attention must be paid to the design of the temperature control-cooling circuit. In the cavity and core temperature control circuit, there are mainly two connection methods: series cooling and parallel cooling. From the perspective of heat exchange efficiency, the flow of cooling water should be turbulent. However, in the parallel cooling circuit, the flow in a circuit that becomes a diversion is smaller than the flow in the series cooling circuit, which may form a laminar flow, and the actual flow entering each circuit may not be the same. Since the cooling water temperature entering each circuit is the same, the temperature of each cavity should also be the same, but in fact, due to the different flow rates in each circuit and the different cooling capacity of each circuit, the temperature of each cavity cannot be consistent. The disadvantage of using a series cooling circuit is that the flow resistance of the cooling water is large, and the cooling water temperature at the inlet of the front cavity is significantly different from the cooling water temperature at the inlet of the last cavity. The temperature difference between the cooling water inlet and outlet varies with the size of the flow rate. For small precision injection molds for processing car plastic parts, it is generally more appropriate to use a series cooling circuit in order to reduce mold costs. If the performance of the mold temperature control instrument (machine) used can control the flow of cooling water within 2°C, the maximum temperature difference of each cavity can also be maintained within 2°C. The mold cavity and core should have their own cooling water circuit system. In the design of the cooling circuit, due to the different heat intake from the cavity and the core, the thermal resistance of the circuit structure is also different, and the water temperature at the entrance of the cavity and the core will have a large temperature difference. If the same system is used, the design of the cooling circuit is also difficult. The cores of small injection molds used for general car plastic parts are very small, and it is very difficult to use cooling water systems. If possible, the core can be made of bronze material, and the solid beryllium bronze core can be cooled by insert cooling. In addition, when taking measures to prevent warping of injection molded products, it is also hoped that a certain temperature difference between the cavity and the core can be maintained. Therefore, when designing the cooling circuits of the juice cavity and the core, the temperature should be adjusted and controlled separately.

3. Maintaining mold accuracy of automotive plastic parts manufacturer In order to maintain the mold accuracy under injection pressure and clamping force, the feasibility of grinding, grinding and polishing the cavity parts must be considered when designing the mold structure. Although the processing of the cavity and core has reached the high-precision requirements, and the shrinkage rate is the same as expected, due to the center offset during molding, the relevant dimensions of the inside and outside of the molded products are difficult to meet the design requirements of plastic parts. In order to maintain the dimensional accuracy of the moving and fixed model cavities on the parting surface, in addition to setting the guide column and guide sleeve centering commonly used in conventional molds, it is also necessary to add positioning pairs such as tapered positioning pins or wedge blocks to ensure accurate and reliable positioning accuracy. The material for making precision injection molds should be high-quality alloy tool steel with high mechanical properties and low thermal creep. The mold material for making cavities and runners should be selected with high hardness, good wear resistance, strong corrosion resistance, and resistance to thermal deformation after strict heat treatment. At the same time, the difficulty and economy of mechanical processing and electrical processing should also be considered. In order to prevent the dimensional accuracy of the mold from changing due to aging, it is necessary to specify the tempering treatment or low temperature treatment of the residual austenite structure of the mold material during heat treatment when designing the mold. For the vulnerable parts of precision injection molds, especially the cavity, core and other vulnerable parts, the possibility of repair should be considered during design to maintain the high accuracy of the mold after repair.

IV. Conclusion Precision injection molding technology is the main and key production technology for automobile plastic parts, and the design of precision injection molds is the main part of this production technology. Reasonable design of precision injection molds is the basis and necessary prerequisite for obtaining precision products. By reasonably determining the size and tolerance of the mold, taking technical measures to prevent the shrinkage error, injection deformation, demolding deformation, overflow, etc. of the injection molded products, and ensuring the accuracy of the mold, and using the correct precision injection molding process, applicable engineering plastic materials and precision injection molding equipment to achieve the best match, it is of great significance to improve the quality, reliability and performance of precision plastic parts of automobiles, reduce production costs, and improve production efficiency.