Mold control in the most popular precision injecti

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Mold control in precision injection molding

precision injection molding will be affected by many related factors and environmental conditions, and the most basic four basic factors are plastic materials, injection mold, injection process and injection equipment. In the early stage of designing plastic products, engineering plastics with corresponding performance requirements should be selected according to their application environment. Secondly, appropriate injection molding machines should be selected according to the selected plastic materials, dimensional accuracy of finished products, weight of finished parts, quality requirements and expected mold structure

among the relevant factors affecting precision injection molding, the mold is the key to obtain precision plastic products that meet the quality requirements

mold design

whether the mold design is reasonable will directly affect the quality of plastic products. Because the mold cavity size is obtained from the required size of plastic products plus the shrinkage rate of the materials used, and the shrinkage rate is often 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 related to the crystal orientation (anisotropy) of engineering plastics, the shape of plastic products, the distance and position from the size to the gate. The main factors affecting the shrinkage of plastics include thermal shrinkage, phase change shrinkage, orientation shrinkage, compression shrinkage and elastic recovery, which are related to the molding conditions or operating conditions of precision injection molded products. Therefore, the designer of the mold must have rich experience in design and injection molding, and must consider the relationship between these influencing factors and injection conditions and their apparent factors, such as injection pressure, mold cavity pressure and filling speed, injection melt temperature and mold temperature, mold structure and gate form and distribution, gate cross-sectional area, product wall thickness, 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 or other molding strips. 2. According to the use of the machine and the service life of the oil, such as temperature, humidity, continuous crystallization, internal stress after molding, and the change of the injection molding machine

because the injection molding process is a process that transforms plastics from solid (powder or granular) to liquid (melt) and then to solid (product). From the granular material to the melt, and then from the melt to the product, the temperature field, stress field, flow field and density field must pass through. Under the combined action of these fields, different plastics (thermosetting or thermoplastic, crystalline or non crystalline, reinforced or non reinforced, etc.) have different polymer structure morphology and rheological properties. All the factors that affect the above fields will certainly 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. The analysis of these internal relations is of great significance to the rational formulation of injection molding process, the rational design and manufacture of molds according to drawings, and even the rational selection of 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 small molding shrinkage. Based on the above reasons, in addition to the design elements of general molds, the following points must be considered in the design of precision injection molds:

1 adopt appropriate mold dimensional tolerances

2 prevent forming shrinkage error

3 prevent injection molding deformation

4 prevent demoulding deformation

5 to minimize die manufacturing error

6 prevent the error of die precision

7 maintain die accuracy

avoid forming shrinkage error

as the shrinkage will change due to injection pressure, for single cavity mold, the cavity pressure in the cavity should be as consistent as possible. For multi cavity mold, the cavity pressure difference between the 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. To do this, you must ensure 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 runner should be balanced before the gate pressure reaches equilibrium

he predicted that the project would achieve the goal of cost saving, weight reduction and carbon dioxide emission reduction. The melt temperature and mold temperature would have an impact on the actual shrinkage rate. Therefore, in order to facilitate the determination of molding conditions, attention must be paid to the arrangement of cavities when designing the cavity of precision injection mold. 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 forming conditions and reduce costs. The cavity arrangement of precision injection mold should meet the requirements of channel balance and arrangement centered on the main runner, and the cavity arrangement with the main runner as the symmetrical line must be adopted. Because the mold temperature has a great impact on the molding shrinkage, it also directly affects the mechanical properties of the injection molded products, and also causes various molding defects such as the surface blooming of the products. 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 shall not change. Therefore, the temperature control measures of heating the mold or cooling the friction and wear testing machine must be taken in the mold design. 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 two connection modes: 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 one 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 may not be 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 obviously 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 small-scale precision injection mold, it is suitable to use series cooling circuit to reduce mold cost

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. In addition, when taking measures to prevent warpage of injection molded products, it is also expected to maintain a certain temperature difference between the cavity and the core. Therefore, when designing the cooling circuit of cavity and core, it should be able to adjust and control the temperature respectively

maintenance of mold accuracy

in order to maintain the mold accuracy under injection pressure and clamping force, the feasibility of grinding, grinding and polishing of 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 rate in actual use 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 shall be selected as the material for manufacturing precision injection mold. The mold material for manufacturing cavity and sprue shall be the material with high hardness, good wear resistance, strong corrosion resistance and thermal deformation resistance after strict heat treatment. At the same time, the difficulty and economy of mechanical processing and electrical processing shall be considered. In order to prevent the dimensional accuracy of the die from changing due to aging change, the tempering treatment or low temperature treatment to reduce the residual austenite structure of the die material heat treatment 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 the high precision of the mold after maintenance

whether the mold exhaust design is reasonable is also a major factor to determine the mold accuracy. Most precision products are made of engineering plastics. In order to ensure the good fluidity of plastics and the consistency of plastic molecular shrinkage, the mold temperature is generally required to be within a certain range. When the mold temperature is too high, the gas in the mold will be difficult to be discharged due to the expansion of the mold accessories, resulting in gas trapping, scorching, lack of material and other adverse phenomena. Therefore, the design of the mold cavity should use more inlaid accessories, and the exhaust groove should be designed on the friction surface of the ejector accessories to prevent needle burning

rational design of precision injection mold 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 shrinkage error, injection molding deformation, demoulding deformation, edge overflow, etc. of injection molded products, and ensuring the precision of the mold, and adopting the correct precision injection molding process, applicable engineering plastic materials and precision injection molding equipment to achieve the best matching, it is important to improve the quality, reliability and performance of precision plastic parts and reduce the production cost, It is of great significance to improve production efficiency

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