The key of injection molding for precision plastic parts of car

Plastic parts of cars, For example, the coil frame, base, fuse box, lamp holder, chip fuse, central distribution box, sheath, push frame, reed bar assembly and outer cover are mostly injection molding. Because of their high design accuracy, they cannot be injected into conventional injection molding, but precision injection molding technology must be adopted. In order to ensure the performance, quality and reliability of the precision plastic parts of the car, the injection molding of plastic products with high quality and in accordance with the product design requirements must be improved continuously on the plastic materials, injection molding equipment, moulds and injection molding technology.

1、 The main factors affecting precision injection molding determine the precision of injection molding products, namely, the dimensional tolerance, shape and position tolerance and surface roughness of the products. There are many related conditions for precision injection molding, and the most essential is the four basic factors: plastic materials, injection molding, injection molding process and injection molding equipment. When designing plastic products, engineering plastics should be selected first, and those with high mechanical properties, stable size, good creep resistance and environmental stress cracking resistance must be selected for engineering plastics which can be used for precise injection molding. Secondly, the suitable injection molding machine should be selected according to the selected plastic material, the precision of finished product size, the weight of the parts, the quality requirements and the expected mold structure. In the process of processing, the factors affecting precision injection molding products mainly come from the precision of the mold, the shrinkage of injection molding, and the range of the changes of the environment temperature and humidity of the products. 7 X” |9 f- z0 G% p

In precision injection molding, mold is one of the key to obtain precision plastic products that meet the quality requirements. The mould for precision injection molding should meet the requirements of product size, precision and shape. But even if the precision and size of the die are consistent, the actual size of the plastic products molded will be different due to the difference of shrinkage. Therefore, it is very important to control the shrinkage of plastic products effectively in precision injection molding technology.

The reasonable design of the mould will directly affect the shrinkage rate of plastic products. Because the mold cavity size is obtained by the size of plastic products and the estimated shrinkage rate, the shrinkage rate is a value recommended by the plastic manufacturer or engineering plastic manual. It is not only related to the gate form, gate position and distribution of the mold, but also the crystallization of engineering plastics Orientation (anisotropy), shape, size, distance and location of plastic products to gate are related. The factors that affect the shrinkage rate of plastics are heat shrinkage, phase change shrinkage, orientation shrinkage, compression shrinkage and elastic recovery, which are related to the molding conditions or operation conditions of precision injection molding products. Therefore, the relationship between these factors and injection conditions and their apparent factors must be considered in the design of the mold, 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 gate sectional area, product wall thickness, content of reinforced filler in plastic materials, crystallinity and orientation of plastic materials, etc The impact of the. The influence of the above factors is also different due to the 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 the process of transforming plastics from solid (powder or granular) to liquid (melt) and solid (product). From granular to melt, and then from melt to product, the process is subject to the action 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) have different polymer structure and rheological properties. All factors that affect the above “field” will affect the physical and mechanical properties, dimensions, shape, accuracy and appearance quality of plastic products.

Thus, the internal relationship between process factors and polymer properties, structure and plastic products will be shown through plastic products. It is of great significance to make a reasonable injection molding process, design and manufacture the mold according to the drawings and even to choose the injection molding equipment reasonably. Precision injection and general injection molding also have different injection pressure and injection rate. High pressure or ultra high pressure injection and high-speed injection are often used to obtain a small shrinkage rate. In order to solve these problems, the following points should be considered in the design of precision injection mold, in addition to the design elements of general mold: ① proper die size tolerance; ② preventing the error of forming shrinkage; ③ preventing injection deformation; ④ preventing mold demoulding; ⑤ minimizing the manufacturing error of mould; ⑥ preventing the error of mold precision; ⑦ maintaining mold Accuracy.

2、 To prevent the error of forming shrinkage, the shrinkage rate will change due to injection pressure. Therefore, for single cavity mold, the cavity pressure in the cavity should be as consistent as possible; as for multi cavity mold, the cavity pressure between cavities should be very small. In the case of single cavity and multiple gates or multiple gates, the injection pressure must be the same to make the cavity pressure consistent. To do this, it is important to ensure that the gate position is balanced. In order to make the cavity pressure in the cavity consistent, it is better to keep the pressure at the gate entrance consistent. The equilibrium of pressure 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. Because the melt temperature and mold temperature have an impact on the actual shrinkage, in order to determine the molding conditions, the arrangement of the cavities must be paid attention to when designing the cavity of the precision injection mold. Because the molten plastic brings heat into the mold, the temperature gradient distribution of the mold is generally around the cavity, which is concentric circle with the main channel as the center.

Therefore, the design measures of channel equilibrium, cavity arrangement and concentric circular arrangement centering on the main channel are necessary to reduce the shrinkage error between the cavities, expand the allowable range of forming conditions and reduce the cost. The arrangement of the cavity of the precision injection mold should meet the requirements of the flow path balance and the main channel as the center arrangement, and the cavity arrangement with the mainstream channel as the symmetrical line must be adopted, otherwise the shrinkage rate of each cavity will be different.

Because the mold temperature has a great influence on the shrinkage rate of molding, it also directly affects the mechanical properties of injection molding products, and also causes various molding defects such as flower on the surface of the products. Therefore, the touch tool must be kept within the specified temperature range, and the mold temperature should not change with time. The temperature difference between the cavities of the multi cavity mold can not change. Therefore, the temperature control measures of heating or cooling of the mould must be taken in the design of the mold. In order to reduce the temperature difference between the mold cavities as much as possible, the design of temperature control cooling circuit must be paid attention to. In the cavity and core temperature control circuit, there are two connection modes: series cooling and parallel cooling.

From the view of heat exchange efficiency, the flow of cooling water should be turbulent. But in parallel cooling circuit, the flow rate in one circuit that becomes shunt is smaller than that in series cooling circuit, which may form laminar flow, and the flow actually entering each circuit is not necessarily the same. The cooling water temperature entering each circuit is the same, and the temperature of each cavity should be the same, but in fact, due to the different flow rate in each circuit and the cooling capacity of each circuit is different, the temperature of each cavity can not be consistent. The disadvantage of adopting series cooling circuit is that the flow resistance of cooling water is large, and the cooling water temperature at the front cavity entrance is obviously different from that at the entrance of the last cavity. The temperature difference at the inlet and outlet of cooling water varies with the flow rate. For small precision injection molding mold for processing car plastic parts, it is more suitable to adopt series cooling circuit in consideration of reducing the cost of the mold. If the performance of the temperature regulator can control the flow of cooling water within 2 ℃, the maximum temperature difference of each cavity can also be kept within 2 ℃.

The mold cavity and core shall have their own cooling water circuit system. In the design of cooling circuit, the thermal resistance of the circuit structure is different because of the different heat absorbed from the cavity and core, and the temperature difference between the cavity and the core inlet will be very large. If the same system is used, the design of cooling circuit is also difficult. Generally, the small injection mold core used in the plastic parts of cars is very small, so it is difficult to use cooling water system. If possible, core can be made of bronze material, and plug-in cooling can be used for solid beryllium bronze core. In addition, when the measures to prevent warping are taken for injection molding products, it is also hoped that the temperature difference between the cavity and the core will be maintained. Therefore, when setting cooling circuit of juice cavity and core, temperature regulation and control should be carried out respectively.

3、 In order to keep the precision of the mould under injection pressure and lock force, the feasibility of grinding, grinding and polishing of cavity parts must be considered in the design of die structure. Although the machining of cavity and core has reached the high precision and the shrinkage is the same as expected, due to the center deviation during forming, the relevant dimensions of the inner and outer sides of the products are difficult to meet the design requirements of plastic parts. In order to keep the dimension accuracy of the dynamic and fixed model cavity on the parting surface, in addition to setting the guide column and guide sleeve used in the conventional mold, the locating pairs such as tapered locating pin or wedge block must be added to ensure the accuracy and reliability of positioning.

The materials for making precision injection molding mould should be high-quality alloy tool steel with high mechanical properties and low thermal creep. The mold materials for cavity and runner should be made of materials with high hardness, good wear resistance, strong corrosion resistance and heat deformation after strict heat treatment, and the difficulty and economy of mechanical processing and electrical processing should be considered. In order to prevent the change of aging and change the size accuracy of the die, it is necessary to specify the tempering treatment or low temperature treatment to reduce the residual austenite structure of the die material heat treatment in the design of the die. For the vulnerable parts of precision injection molding, especially the vulnerable parts such as cavity and core, the possibility of repair should be considered in the design to keep the mould maintenance with high precision.

4、 Conclusion precision injection molding technology is the main and key production technology of car plastic parts, and the design of precision injection molding is the main part of this production technology. Reasonable design of precision injection molding mold is the basis and necessary prerequisite for obtaining precision products. By reasonably determining the size and tolerance of the mold, taking the technical measures to prevent shrinkage error, injection deformation, mold release deformation, overflow, etc., and ensuring the precision of the mold, and adopting the correct precision injection molding technology, applicable engineering plastic materials and precision injection equipment, the best matching can be achieved, which can improve the quality of the precision plastic parts of the car Reliability and performance, reducing production cost and improving production efficiency are of great significance.

About PTJMould Industrial Company Limited

PTJmould is a China mold maker of plastic molds- injection mold, die casting moulds, plastic blow molding, rotational molding, medical plastic injection molding, two shot plastic injection molding, insert molding, overmolding, metal injection molding, micro injection molding, powder injection molding, ceramic injection molding, liquid injection molding, husky injection molding, household mold, casting mold, die mold tool, custom molds, china moulds, rapid prototyping tooling, plastic prototyping tooling, punch press tooling, die and tooling for mobile/ cell phone parts, automotive parts, vacuum cleaners, rechargeable tools, telephones, copiers, computers, multimedia speakers, and many other electronic products and household appliances. And also a plastic product manufacturer, mold manufacturer China– plastic parts, plastic water tank, plastic balls, plastic containers, plastic buckle, plastic anchor, plastic hanger, plastic spoon, plastic pipe fitting, plastic tumble, plastic tableware, plastic cups, plastic bottles, plastic tray, plastic cosmetic container, plastic case, plastic food container, plastic chairs, plastic caps, plastic cap closure, plastic tubes, plastic water pipes, plastic knobs, plastic tubing, plastic utility boxes, plastic racks and so on.