The Car Plastic Parts of sedans such as bobbins, bases, fuse boxes, lamp holders, chip fuses, central power distribution boxes, sheathing, pushing frame, reed row components and covers are mostly injection molded, as these Car Plastic Parts Itself has a high design accuracy, making these Car Plastic Parts can not be used conventional injection molding, but must be used precision injection molding process technology. In order to ensure the performance, quality and reliability of the precision Car Plastic Parts of cars, injection molding of plastic products with high quality and conforming to product design requirements must be continuously improved on plastic materials, injection molding equipment, molds and injection molding processes.
1, the main factor affecting precision injection molding
The basis for judging precision injection molding is the precision of the injection products, namely the product size tolerance, the shape tolerance and the surface roughness. To carry out precision injection molding must have many relevant conditions, and the most essential is the plastic material, injection mold, injection molding process and injection molding equipment, these four basic factors. The design of plastic products, should first select the engineering plastic materials, and can be used for precision plastic injection molding engineering plastics and must choose those high mechanical properties, dimensional stability, good creep resistance, environmental stress cracking resistance materials. Second, should be based on the choice of plastic materials, product dimensional accuracy, weight, quality requirements and the expected mold structure selection of suitable injection molding machine. In the process of processing, the factors that affect the precision injection products mainly come from the accuracy of the mold, shrinkage of the injection molding, and the change range of the ambient temperature and the humidity of the product.
(1) mold accuracy
In precision injection molding, the mold is one of the keys to achieve precision plastic products that meet the quality requirements. The molds for precision injection molding should meet the requirements of product size, precision and shape. However, even if the accuracy of the mold, the same size, the actual size of the plastic molded products will vary due to shrinkage inconsistencies. Therefore, the effective control of shrinkage of plastic products in precision injection molding technology is very important.
(2) mold design rationality
Mold design is reasonable or not will have a direct impact on the shrinkage of plastic products, as the mold cavity size is calculated by the size of plastic products with the estimated shrinkage obtained, while the shrinkage is recommended by the plastic manufacturer or engineering plastics manual It is not only related to the mold gate, the location and distribution of the gate, but also the crystal orientation (anisotropy) of the plastic, the shape and size of the plastic, the distance from the gate and Location related. Affect the plastic shrinkage of the main thermal shrinkage, phase change shrinkage, orientation shrinkage, shrinkage and elastic recovery and other factors, and these factors and precision injection molding products forming conditions or operating conditions. Therefore, in the design of mold must take into account the relationship between these factors and injection molding 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 gate cross-sectional area, product wall thickness, reinforced plastic filler content, plastic material crystallinity and orientation and other factors. The impact of the above factors also vary with the plastic material, other molding conditions such as temperature, humidity, continue to crystallize, the internal stress after molding, injection molding machine changes.
(3) the environmental factors of products
As the injection molding process is the plastic from the solid (powder or pellets) to the liquid (melt) to solid (product) process. From the pellets to the melt, and then from the melt to the product, the temperature, stress field, flow field and density field in the middle pass through the action of these different fields of plastic (thermosetting or thermoplastic, crystalline Or noncrystalline, reinforced or non-reinforced, etc.) have different polymer morphologies and rheological properties. Any factors that affect the "field" will affect the physical and mechanical properties of plastic products, size, shape, precision and appearance quality.
In this way, the physical relationship between the process factors and the properties of the polymer, the morphology of the structure, and the plastic product is manifested by the plastic product. It is of great significance to analyze these internal relations clearly and rationally to formulate the injection molding process, to design the mold rationally and make the mold according to the drawings, and even to choose the injection molding and processing equipment reasonably. Precision injection molding and injection molding injection pressure and injection rate are also different, precision injection often use high pressure or high pressure injection, high-speed injection to obtain a smaller molding shrinkage.
In summary of the above reasons, the design precision injection mold in addition to considering the design elements of the general mold, but also to consider the following points:
① using the appropriate mold size tolerances;
② prevent the molding shrinkage error;
③ prevent injection molding deformation occurs;
④ to prevent the occurrence of demoulding deformation;
⑤ mold manufacturing error minimized;
⑥ to prevent the error of precision molds;
⑦ maintain mold accuracy.
2, to prevent molding shrinkage error
As the shrinkage rate due to injection pressure changes, therefore, for single-cavity mold cavity pressure within the cavity should be as uniform as possible; as for multi-cavity mold cavity pressure between the cavity should be a small difference. In the single-cavity multi-gate or multi-cavity multi-gate case, must be the same injection pressure injection, the cavity pressure is consistent. To do this, you must ensure that the gate position is balanced. In order to make the cavity cavity pressure uniform, it is best to make the inlet pressure at the gate consistent. The pressure equalization at the gate is related to the flow resistance in the runner. Therefore, before the gate pressure reaches equilibrium, the circulation should be balanced first.
As the melt temperature and mold temperature have an impact on the actual shrinkage rate, so in the design of precision injection mold cavity, in order to facilitate the determination of molding conditions, we must pay attention to the arrangement of cavities. Because molten plastic brings heat into the mold, the temperature gradient of the mold is typically distributed around the cavity, in the shape of a concentric circle centered on the main channel.
Therefore, design measures such as flow path equalization, cavity arrangement and concentric arrangement centered on the main flow path are necessary to reduce the shrinkage error between the respective cavities, to extend the allowable range of molding conditions, and to reduce the cost . Precise injection mold cavity arrangement should meet the flow path equalization and the main flow channel as the center of the arrangement of two requirements, and must be used to the main channel symmetrical line cavity arrangement, otherwise it will cause the contraction of the cavity difference .
As the mold temperature has a great influence on the molding shrinkage rate, but also directly affects the mechanical properties of the injection molded products, but also causes the surface of the product such as hair and other mold defects, it must keep the touch in the specified temperature range, but also To make the mold temperature does not change over time. Multi-cavity mold cavity between the temperature difference may not change. To this end, the mold design must take the mold heating or cooling temperature control measures, and in order to minimize the temperature difference between the mold cavity, we must pay attention to temperature control - cooling circuit design. In the cavity, core temperature control loop, the main series cooling and parallel cooling two connections. Reply "shrinkage" to see more
From the heat exchange efficiency point of view, the flow of cooling water should be turbulent. However, in a parallel cooling circuit, the flow rate in one circuit to be shunted is smaller than the flow rate in the series cooling circuit, which may result in a laminar flow, and the actual flow into each circuit is not necessarily the same. Since the temperature of the cooling water entering each circuit is the same, the temperature of each cavity should be the same, but in fact the flow rate in each circuit is different, and the cooling capacity of each circuit is not the same, so that the temperature of each cavity is also impossible Consistent. The disadvantage of using a series cooling circuit is that the flow resistance of the cooling water is large and the temperature of the cooling water at the front of the cavity at the entrance is significantly different from the temperature of the cooling water at the entrance of the last cavity. Cooling water inlet and outlet temperature changes due to the size of the flow rate. For the processing of Car Plastic Parts car small precision injection mold, generally from the lower mold cost considerations, the use of series cooling circuit is more appropriate. The temperature difference between the various cavities can be maintained at a maximum of 2 ° C if the performance of the used thermostat (machine) controls the flow of cooling water within 2 ° C.
Mold cavity and core should have their own cooling water circuit system. In the design of the cooling circuit, the thermal resistance of the loop structure is also different due to the different amount of heat absorbed from the cavity and the core, and the water temperature at the cavity and core inlet will have a great temperature difference. If using the same system, cooling circuit design is more difficult. Small injection mold cores for general sedan Car Plastic Parts are very small and there is great difficulty in using cooling water system. If possible, cores made of bronze can be used, while for solid beryllium copper cores insert cooling can be used. In addition, injection molded products to take to prevent warping measures, but also hope that the cavity and the core to maintain a certain temperature difference. Therefore, the design of cavity and core cooling circuit should be able to adjust the temperature and control.
3, the maintenance of mold accuracy
In order to maintain the accuracy of the mold under injection pressure and clamping force, the feasibility of machining, grinding and polishing the cavity parts must be considered when designing the mold structure. Although the cavity and core processing have reached the requirements of high precision, and the shrinkage rate is also the same as expected, but due to the center of the molding shift, the shape of its products inside and outside the relevant dimensions are difficult to reach the Car Plastic Parts design requirements. In order to maintain the dynamic and fixed mold cavity in the sub-surface of the dimensional accuracy, in addition to set the conventional mold used in the guide column, the guide set center, you must also install tapered positioning pins or wedge positioning positioning, etc., to ensure positioning Accuracy is accurate and reliable
Production of precision injection mold materials to choose high mechanical properties, low thermal creep of high quality alloy tool steel, cavity, runner mold material to be selected through rigorous heat treatment of high hardness, good wear resistance, corrosion resistance, Thermal deformation of the material, taking into account the mechanical processing, electrical processing of the easy and economical. In order to prevent the occurrence of aging changes to change the dimensional accuracy of the mold, the mold must be designed to reduce the mold material heat treatment of residual austenitic tissue tempering or low temperature treatment.
Vulnerable parts of precision injection molds, especially cavities, cores and other wearing parts, should be considered in the design of the possibility of repairs in order to maintain the mold after the repair is still high accuracy.