Introduction to the key points of mold design for

2022-08-09
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Key points of molding mold design for automotive precision plastic parts

plastic parts of cars, such as coil frame, base, fuse box, lamp holder, chip fuse, central distribution box, sheath, push frame, reed row components, at the same time, most of the automation system and housing are injection molded. Due to the high design accuracy of these plastic parts, conventional injection molding cannot be used for these plastic parts, Precision injection molding technology must be adopted. In order to ensure the performance, quality and reliability of precision plastic parts of cars and produce plastic products with high quality and meeting the requirements of product design, plastic materials, injection equipment and molds and injection molding process must be continuously improved

1 main factors affecting precision injection molding

the basis for judging precision injection molding is the accuracy of injection molded products, that is, the dimensional tolerance, geometric tolerance and surface roughness of products. To carry out precision injection molding, there must be many related conditions, and the most essential four basic factors are plastic materials, injection molds, injection molding technology and injection molding equipment. When designing plastic products, engineering plastic materials should be selected first, and engineering plastics that can be accurately injected must choose those materials with high mechanical properties, stable size, good creep resistance and environmental stress cracking resistance. Secondly, suitable injection molding machines should be selected according to the selected plastic materials, dimensional accuracy of finished products, piece weight, quality requirements and expected mold structure. In the processing process, the factors that affect the precision injection molding products mainly come from the accuracy of the mold, the injection shrinkage, and the environmental temperature and humidity change range 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 mold for precision injection molding should actually 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 products will be inconsistent due to the difference in shrinkage. Therefore, it is very important to effectively control the shrinkage of plastic products in the precision injection technology of stretching, tightening, bending, tearing, shearing, 180 degree peeling and 90 degree peeling

whether the mold design is reasonable or not will directly affect the shrinkage rate of plastic products, because the mold cavity size is obtained from the size of plastic products plus the estimated shrinkage rate, and the shrinkage rate is a value within a range recommended by the plastic manufacturer or engineering plastics manual, which is not only related to the gate form, gate position and distribution of the mold, but also related to the crystal orientation (anisotropy) of engineering plastics The shape and size of plastic products are related to the distance and position to the gate. The main factors affecting the shrinkage of plastics are thermal shrinkage, phase transformation shrinkage, orientation shrinkage, compression shrinkage and elastic recovery, which are related to the molding conditions or operating conditions of precision injection molded products. Therefore, when designing the mold, we must consider the relationship between these influencing factors and injection conditions and their apparent factors, such as injection pressure, cavity pressure and filling speed, injection melt temperature and mold temperature, mold structure and gate form and distribution, as well as the influence of gate cross-sectional area, product wall thickness, the content of reinforcing filler in plastic materials, the crystallinity and orientation of plastic materials. The influence of the above factors is also different due to different plastic materials, other molding conditions such as temperature, humidity, continuous crystallization, internal stress after molding, and the change of injection molding machine

because the injection molding process is a process of transforming plastic from solid (powder or pellet) to liquid (melt) and then to solid (product). From pellets to melts, and then from melts to products, there are temperature fields, stress fields, flow fields and density fields. Under the combined action of these fields, different plastics (thermosetting or thermoplastic, crystalline or amorphous, reinforced or unreinforced, etc.) have different polymer structural morphology and rheological properties. All factors that affect the above "field" will inevitably affect the physical and mechanical properties, size, shape, accuracy and appearance quality of plastic products

in this way, the internal relationship between process factors and polymer properties, structural morphology and plastic products will be shown through plastic products. Analyzing and clarifying these internal relations is of great significance to reasonably formulate the injection molding process, reasonably design and manufacture the mold according to the drawing, and even reasonably select the injection molding equipment. Precision injection molding is also different from ordinary injection molding in injection pressure and injection rate. Precision injection molding often adopts high-pressure or ultra-high pressure injection and high-speed injection to obtain small molding shrinkage. 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 adopt appropriate mold dimensional tolerances; ② Prevent forming shrinkage error; ③ Prevent injection molding deformation; ④ Prevent more advanced hair technology from demoulding deformation; ⑤ Minimize mold manufacturing error; ⑥ Prevent the error of mold accuracy; ⑦ Maintain mold accuracy

2 prevent forming shrinkage error

since the shrinkage will change due to injection pressure, for single cavity mold, the cavity pressure in the cavity should be as consistent as possible; As for multi cavity mold, the cavity pressure difference between cavities should be very small. In the case of single cavity with multiple gates or multiple cavities with multiple gates, the injection pressure must be the same to make the cavity pressure consistent. For this reason, it must be ensured that the gate position is balanced. In order to make the cavity pressure in the mold cavity consistent, it is best to keep the pressure at the gate inlet consistent. The pressure balance at the gate is related to the flow resistance in the runner. Therefore, the flow should be balanced before the gate pressure reaches equilibrium

due to the influence of melt temperature and mold temperature on the actual shrinkage rate, when designing the cavity of precision injection mold, in order to facilitate the determination of molding conditions, we must pay attention 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 a concentric shape with the sprue as the center

therefore, design measures such as channel equalization, cavity arrangement and concentric circular arrangement centered on the main runner are necessary to reduce the shrinkage error between cavities, expand the allowable range of molding conditions and reduce costs. The cavity arrangement of precision injection mold should meet the requirements of channel balance and the arrangement centered on the sprue, and the cavity arrangement with the sprue as the symmetrical line must be adopted, otherwise the shrinkage of each cavity will be different

because the mold temperature has a great influence on the molding shrinkage rate, it also directly affects the mechanical properties of injection molded products, and also causes various molding defects such as the surface of the products, so the mold must be kept within the specified temperature range, and the mold temperature must not change at any time. The temperature difference between the cavities of the multi cavity mold shall not change. Therefore, the material properties produced by the mold design are also very high. In the design, the temperature control measures of heating or cooling the mold must be taken, and in order to minimize the temperature difference between the mold cavities, attention must be paid to the design of temperature control cooling circuit. In the temperature control circuit of cavity and core, there are two connection modes: 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 the circuit that becomes a shunt is smaller than that in the series cooling circuit, which may form laminar flow, and the actual flow into each circuit is not necessarily the same. Since the temperature of cooling water entering each circuit is the same, the temperature of each mold cavity should also be the same, but in fact, due to the different flow in each circuit and the different cooling capacity of each circuit, the temperature of each mold cavity can not be the same. The disadvantage of using series cooling circuit is that the flow resistance of cooling water is large, and the temperature of cooling water at the inlet of the most front cavity is significantly different from that at the inlet of the last cavity. The temperature difference between the inlet and outlet of cooling water varies with the flow. For the small-scale precision injection mold for processing plastic parts of cars, it is generally suitable to adopt series cooling circuit in order to reduce the mold cost. If the performance of the mold temperature regulating controller (machine) used can control the flow of cooling water within 2 ℃, the maximum temperature difference of each cavity can also be maintained within 2 ℃

the mold cavity and core shall have their own cooling water circuit system. In the design of the cooling circuit, due to the different heat absorbed from the cavity and core, the thermal resistance of the circuit structure is also different, and the water temperature at the inlet of the cavity and core will have a large temperature difference. If the same system is used, the cooling circuit design is also difficult. Generally, the core of small injection mold used for plastic parts of cars is very small, so it is very difficult to adopt cooling water system. If possible, the core can be made of bronze material, and the solid beryllium bronze core can be cooled by plug-in method. In addition, when taking measures to prevent warpage of injection molded products, we also hope to maintain a certain temperature difference between the cavity and the core. Therefore, when setting the cooling circuit of juice cavity and core, it should be able to adjust and control the temperature respectively

3 maintenance of mold accuracy

in order to maintain the mold accuracy under injection pressure and clamping force, the feasibility of grinding, grinding and polishing cavity parts must be considered when designing the mold structure. Although the processing of the cavity and core has reached the requirements of high precision, and the shrinkage is the same as expected, due to the center offset during molding, the relevant dimensions of the inner and outer sides of the molded products are difficult to meet the design requirements of plastic parts. In order to maintain the dimensional accuracy of the movable and fixed mold cavities on the parting surface, in addition to setting the guide posts and guide sleeves commonly used in conventional molds, positioning pairs such as conical positioning pins or wedge blocks must be added to ensure accurate and reliable positioning accuracy

high quality alloy tool steel with high mechanical properties and low thermal creep should be selected as the material for making precision injection molds. The mold material for making cavities and sprues should be materials with high hardness, good wear resistance, strong corrosion resistance and heat deformation resistance after strict heat treatment. At the same time, the difficulty and economy of mechanical processing and electrical processing should be considered. In order to prevent the dimensional accuracy of the die from changing due to aging changes, the tempering treatment or low temperature treatment that reduces the residual austenite structure of the heat treatment of the die material must be specified in the design of the die

for vulnerable parts of precision injection mold, especially vulnerable parts such as cavity and core, the possibility of repair should be considered in the design, so as to maintain high accuracy after mold maintenance

4 conclusion

precision injection molding technology is the main and key production technology of car plastic parts, and the design of precision injection mold is the main part of this production technology. The reasonable design of precision injection mold is the foundation and necessary premise to obtain precision products. By reasonably determining the size and tolerance of the mold, taking technical measures to prevent shrinkage error, injection molding deformation, demoulding deformation, overflow of injection molded products, and ensuring the accuracy of the mold, and adopting the correct precision injection molding process, applicable engineering plastic materials, and precision injection molding equipment to achieve the best match, it can improve the quality of precision plastic parts of cars

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