Precision injection molding of automotive plastic parts

Posted on : July 26 , 2022 By  GREFEE

Automotive plastic parts, such as coil frame, base, fuse box, lamp holder, chip fuse, central distribution box, sheath, push frame, reed row components and housing, are mostly produced by injection molding. Since these plastic parts have a higher requirement in terms of the designing accuracy, the automotive plastic parts suppliers cannot adopt ordinary plastic parts molding method, but to choose precision injection molding processing. To ensure the capabilities, quality, and reliability of the automotive injection molding plastic parts, as well as to obtain high quality parts that meet the designing requirements, the plastic material, injection molding equipments and molds, and the injection molding processing should be continually improved.  

Main factors affecting precision injection molded parts

The accuracy of the plastic parts determines whether an injection part is precise or not, which is the dimension tolerance of products, geometric tolerance and surface roughness. There are many related conditions to assess if it allows the precision injection molding, and the essential four basic factors are plastic material, injection molding mold, injection molding process and injection molding equipment.

The engineering plastic materials is the first choice for designing plastic products, and the materials with high mechanical properties, stable size, good creep resistance and environmental stress cracking resistance used for producing engineering plastics that support precision injection molding. Secondly, appropriate injection molding machines should use appropriate plastic materials, dimensional accuracy of finished products, weight, quality requirements and the decided mold structure of injection molding parts. Duding the processing, factors that affect the precision injection molded parts mainly include accuracy of the injection mold, the shrinkage of the injection molded parts, as well as the change range of the environmental temperature and humidity of the products.

In precision injection molded parts, the mold of injection molded parts is one of the keys to obtain precision plastic products that meet the quality requirements, especially that for precision injection molded parts should meet the requirements of product size, accuracy and shape. However, even if the precision and size of the injection mold are consistent, the actual size of the molded plastic products will be inconsistent due to the difference in shrinkage. Hence, it is very essential to control the shrinkage of plastic products in precision injection molding technology effectively.

The rationality of the design of injection mold directly affects the shrinkage rate of plastic products as the 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 plastic production injection parts manufacturers or engineering plastics manuals. This is not only associated with the gate form, gate position and distribution of injection mold, but also the crystal orientation (anisotropy) of engineering plastics, the shape and size of plastic products, and the distance and position to the gate. Factors affecting the shrinkage of plastics mainly include the thermal shrinkage, phase transformation shrinkage, orientation shrinkage, compression shrinkage and elastic recovery, which are closely associated with the molding conditions or operating conditions of precision injection molded parts.

Thus, the relationship between these factors and the conditions of the injection parts and the apparent factors, such as the injection pressure and cavity pressure and filling speed, the injection melt temperature and injection mold temperature, the injection 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 should be considered carefully when designing the injection mold.

The influence of above factors will vary from different plastic materials, and other molding conditions, such as temperature, humidity, continuous crystallization, internal stress after molding, injection molding machine. Due to the plastic is turned from the solid state into liquid (molten), then is the solid (product) during the injection molding process, it changes from granular material to melt, and then from melt to product, during which it must goes through a series of effects of temperature field, stress field, flow field and density field. Under the combined action of these fields, different plastics (thermosetting or thermoplastic, crystalline or non crystalline, reinforced or non reinforced, etc.) have different polymer structural morphology and rheological properties. Factors affecting the above will definitely affect the physical  mechanical properties, dimension, shapes, accuracy, and appearance quality of plastic products. By doing so, the internal links between process factors and polymer properties, structural morphology and plastic products will be presented through plastic products. As long as we finalize the internal links, it will be easy to properly organize the processing technology of injection molded parts and produce the injection parts properly according to the drawing. It is also of great significance to the selection of the equipments of the injection molding parts. There is also a difference in the pressure and injection rate of injection parts between precision injection parts and ordinary injection parts. The high premise injection molding parts often adopt high pressure or ultra high pressure injection, and high speed injection to obtain a small molding shrinkage rate. Based on above reasons, the following points must be considered when designing the injection mold of precision injection parts in addition to the design elements of general injection mold.

①adopt proper dimensional tolerance of injection mold;

②in case the generation of forming shrinkage error

③prevent the deformation of the parts

④prevent the deformation due to the mold release

⑤minimize the manufacturing error of injection mold

⑥the prevention of the error of the infection mold

⑦maintain the accuracy of the injection mold

 Prevent the formation of shrinkage error

Since the shrinkage rate will change due to the pressure of the injection molded part, the cavity pressure should be consistent for the single cavity injection mold. For the multi-cavity injection part mold, the pressure difference should be very less between the cavities. Under the conditions of single or multiple cavity gates, the injection pressure must be consistent to maintain the cavity pressure. For this, the position of the gate should be balance. To ensure the consistency of the cavity pressure, the pressure around the gate should be consistent.

The balance of the pressure around the gate is related to the flow resistance. Thus, before the pressure of the gate reaches its balance, a balanced flow should be maintained. Due to the impact of the molten temperature and mold temperature of the injection parts to the actual shrinkage rate, to determine the molding conditions, the arrangement of the caveat must be considered before designing the mold cavity of the precision injection molding parts. Since the molten plastic brings the heat into the mold cavity, but the temperature gradient distribution of the injection mold is generally around the cavity, in a concentric shape with the main runner as the center. Therefore, measures like balanced runners arrangement, cavity arrangement, and designing measures, such as concentric circular arrangement centered on the sprue, are necessary for reducing the shrinkage rate between each cavity, and expanding the allowable range of forming conditions and lowering the cost.

Since the mold temperature of the injection part has a huge influence to the molding shrinkage rate, and it also directly affects the mechanical properties of the injection molding parts. Besides, it also causes various forming defects like the surface of the product is scratched. Therefore, it is important to maintain the mold temperature within the standard range. What’s more, the temperature of the parts should not change according to the time.

The temperature of each cavity of the multiple cavity injection molding parts should be consistent. Thus, the temperature control measures for heating or cooling the injection mold must be taken in the design of injection mold. Moreover, to minimize the temperature difference between each cavity of the injection molding parts, the design of the temperature control—cooling circuits should be considered.

Two major connection modes in the temperature control circuit of cavity and core include series cooling and parallel cooling. In terms 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 create the laminar flow, and the actual flow into each circuit may not same. Since the temperature of the cooling water entering in each circuit is same, and either to the temperature in each cavity. However, the temperature in each cavity is inconsistent since the flow in each circuit is different, and the cooling capacity of each circuit is different, too.

A large flow resistance of cooling water is the disadvantage of using series cooling circuit. Moreover, there is a difference in the temperature of cooling water at the inlet of the most front cavity and the temperature 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, the cost of injection mold is required to consider. The series cooling circuit is more suitable. 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 of injection molded parts should 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 a large temperature difference will be produced at the inlet of the cavity and core. If the same system is used, the cooling circuit design is also difficult. Typically, small injection molded parts used for plastic parts of cars have small mold cores, so cooling water system is hard to install. 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, maintain a certain temperature difference between the cavity and the core. Is also important when taking measures to prevent warpage of injection molded parts. Hence, when setting the cooling circuit of juice cavity and core, it should be able to adjust and control the temperature respectively.

 Maintaining the accuracy of injection mold

To maintain the accuracy of the injection mold under the pressure and clamping force of the injection mold, the feasibility of milling, grinding and polishing the cavity parts must be considered when designing the injection mold structure. Even though the processing of the cavity and core has reached a high accuracy, and the shrinkage is the same as expected. However, due to the center offset during molding, it is hard for the relevant dimensions of the inner and outer sides of the molded products to meet the design requirements. To maintain the dimensional accuracy of the movable and fixed mold cavities on the parting surface, except for setting the guide posts and guide sleeves commonly used in the conventional injection mold, the else we can do is the positioning pairs such as conical positioning pins or adding the wedge blocks to ensure accurate and reliable positioning accuracy.

When producing the molds of high precision injection molding parts, the material should be fine alloys with high mechanical properties, and low thermal creep. The material for making the injection molding of cavity and sprue should has high hardness, good wear resistance, strong corrosion resistance and heat deformation resistance after rigorous heat treatment. Meanwhile, the difficulty and the economy of the mechanical processing and electrical processing should be taken into account.

To prevent the dimension accuracy of the injection molding parts changing according to the aging, tempering treatment or low temperature treatment to reduce the residual austenite structure of the heat treatment of the injection mold material must be specified when designing the injection mold. Vulnerable parts such as cavity and core, the feasibility of them should be considered to maintain a high accuracy after the repairing the mold of the injection molding parts.

Conclusion

The high precision injection molding technology is the key production technology for the plastic parts of the automotive. However, the design of it is the essential part for this technology. Properly designing the precision injection mold is the foundation to obtain precise products.  Through the appropriate dimensions of molds and tolerance, and take measures to prevent shrinkage error of injection molded parts, deformation of injection molded parts, demoulding deformation, edge overflow to ensure the accuracy of the mold. Moreover, It is of great significance for improving the quality, reliability and performance of precision plastic parts of cars, lowering costs and improving production efficiency to achieve the fit rate between applicable engineering plastic materials and precision injection molding equipment.

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