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Precision injection molding is affected by many related factors and environmental conditions, and the most basic are the four basic factors of plastic materials, injection molds, injection molding processes and injection molding equipment. In the early stage of designing plastic products, engineering plastics corresponding to the performance requirements should be selected according to the application environment. Secondly, the appropriate injection molding machine should be selected according to the selected plastic material, the dimensional accuracy of the finished product, the weight of the part, the quality requirements and the expected mold structure.
Among the factors affecting precision injection molding, molds are the key to obtaining precision plastic products that meet quality requirements.
Mold design
Whether the mold design is reasonable will directly affect the quality of plastic products. Since the mold cavity size is derived from the required size of the plastic product plus the shrinkage of the material used, the shrinkage rate is often a value within the range recommended by the plastics manufacturer or engineering plastics manual, not only with the gate form of the mold. The position of the gate is related to the distribution, and is related to the crystal orientation (anisotropic) of the engineering plastic, the shape and size of the plastic product, and the distance and position of the gate. The main factors affecting the shrinkage of plastics include heat 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 articles. Therefore, the mold designer must have rich experience in design and injection molding, and must consider the relationship between these influencing factors and injection molding conditions and their 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 cross-sectional area of ​​the gate, the thickness of the product, the content of reinforcing filler in the plastic material, the crystallinity and orientation of the plastic material and other factors. The influence of the above factors also differs depending on the plastic material or other molding conditions such as temperature, humidity, continued crystallization, internal stress after molding, and changes in the injection molding machine.
Because the injection molding process is the process of converting plastic from solid (powder or pellet) to liquid (melt) to solid (product). From the pellet to the melt, and then from the melt to the product, the temperature field, stress field, flow field and density field are used. Under the combined action of these fields, different plastics (thermoset or thermoplastic, crystalline or amorphous, reinforced or unreinforced, etc.) have different polymer morphological and rheological properties. Any factors that affect the above-mentioned "field" will affect the physical and mechanical properties, size, shape, precision and appearance quality of plastic products.
Thus, the intrinsic link between process factors and polymer properties, structural morphology, and plastics is manifested by plastics. Analysis of these intrinsic links is of great significance for the rational preparation of injection molding process, rational design and manufacturing of molds according to drawings, and even the selection of injection molding equipment. Precision injection molding and ordinary injection molding also differ in injection pressure and injection rate. Precision injection molding often uses high pressure or ultra high pressure injection and high speed injection to obtain a small molding shrinkage. In view of the above various reasons, in addition to considering the design elements of general molds, the following points must be considered when designing precision injection molds:
1 Use appropriate mold dimensional tolerances;
2 to prevent the occurrence of molding shrinkage error;
3 to prevent injection molding deformation;
4 to prevent demoulding deformation;
5 to minimize mold manufacturing errors;
6 to prevent errors in mold accuracy;
7 Maintain mold accuracy.
Prevent molding shrinkage error
Since the shrinkage rate changes due to the injection pressure, the cavity pressure in the cavity should be as uniform as possible for a single cavity mold. As for the multi-cavity mold, the cavity pressure between the cavities should be small. In the case of single cavity multi-gate or multi-cavity multi-gate, it must be injected at the same injection pressure to make the cavity pressure uniform. To do this, you must ensure that the gates are balanced. In order to make the cavity pressure in the cavity uniform, it is preferable to keep the pressure at the gate entrance consistent. The equilibrium of the pressure at the gate is related to the flow resistance in the runner. Therefore, the flow path should be equalized before the gate pressure reaches equilibrium.
Since the melt temperature and the mold temperature have an effect on the actual shrinkage rate, in order to facilitate the determination of the molding conditions, it is necessary to pay attention to the arrangement of the cavities when designing the precision injection mold cavity. Because the molten plastic brings heat into the mold, the temperature gradient of the mold generally surrounds the cavity, forming a concentric shape centered on the main channel.
Therefore, design measures such as flow channel equalization, cavity arrangement, and concentric circular arrangement centered on the main channel are necessary to reduce the shrinkage error between the cavities, to extend the allowable range of molding conditions, and to reduce costs. . The cavity arrangement of the precision injection mold should meet the requirements of the flow channel equalization and the main channel as the center, and the cavity arrangement with the main line as the symmetry line must be adopted.
Since the mold temperature has a great influence on the molding shrinkage rate, it also directly affects the mechanical properties of the injection molded article, and also causes various molding defects such as fading on the surface of the product, so the mold must be kept within the specified temperature range, and The mold temperature does not change with time. The temperature difference between the cavities of the multi-cavity mold must also not change. For this reason, temperature control measures for heating or cooling the mold must be taken in the mold design, and in order to minimize the temperature difference between the mold cavities, the design of the temperature control-cooling circuit must be paid attention to. In the cavity and core temperature control loop, there are mainly 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 a parallel cooling circuit, the flow rate in one circuit that is split is smaller than the flow rate in the series cooling circuit, which may result in laminar flow, and the flow actually entering 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 also different, so that the temperature of each cavity is 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 inlet of the frontmost cavity is significantly different from the temperature of the cooling water at the inlet of the last cavity. The temperature difference between the cooling water inlet and outlet varies with the flow rate. For small precision injection molds, it is generally preferable to use a series cooling circuit from the viewpoint of reducing the cost of the mold.
The mold cavity and core should have their own cooling water circuit system. In the design of the cooling circuit, the thermal resistance of the circuit structure is different due to the difference in heat taken from the cavity and the core, and the temperature of the water at the inlet of the cavity and the core generates a large temperature difference. If the same system is used, the design of the cooling circuit is also difficult. In addition, when taking measures against warpage of the injection molded article, it is also desirable to maintain a certain temperature difference between the cavity and the core. Therefore, the design of the cavity and the cooling circuit of the core should be able to adjust and control the temperature separately.
Mold accuracy maintenance
In order to maintain the accuracy of the mold under the injection pressure and clamping force, the feasibility of grinding, grinding and polishing the cavity parts must be considered when designing the mold structure. Although the processing of the cavity and the core has reached the requirement of high precision, and the shrinkage rate is the same as expected, due to the center deviation during molding, the relevant dimensions of the inside and the outside of the formed product are difficult to reach the plastic. Design requirements for components. In order to maintain the dimensional accuracy of the dynamic and fixed model cavity on the parting surface, in addition to the guide column and guide sleeve centering commonly used in conventional molds, positioning points such as tapered positioning pins or wedge blocks must be added to ensure positioning. Accuracy is accurate and reliable.
The material for making precision injection molds should be high-quality alloy tool steel with high mechanical properties and small thermal creep. The mold materials for making cavities and runners should be selected with strict heat treatment, high hardness, good wear resistance and strong corrosion resistance. Materials resistant to heat deformation, while also considering the ease of machining and electrical processing and economics. In order to prevent the occurrence of aging changes and to change the dimensional accuracy of the mold, it is necessary to specify a tempering treatment or a low temperature treatment for reducing the retained austenite structure of the heat treatment of the mold material when designing the mold.
For vulnerable parts of precision injection molds, especially for cavities, cores and other consumables, the possibility of repair should be considered in the design to maintain high precision after mold repair.
Whether the mold exhaust design is reasonable or not is also a major factor in determining mold accuracy. Most of the precision products are formed by using engineering plastics. In order to ensure the good fluidity of plastics and the consistency of shrinkage of plastic molecules in the molding process, the mold temperature is generally required to be formed in a certain range. When the temperature of the mold is too high, the expansion of the mold parts will cause difficulty in the gas discharge in the mold, causing the product to be trapped, scorched, and lack of material. Therefore, the cavity part of the mold is designed to use more inlaid parts and ejector accessories. The venting groove should be designed on the rubbing surface to prevent the needle from burning.
Reasonable design of precision injection molds is the basis and necessary prerequisite for obtaining precision products. By reasonably determining the size and tolerance of the mold, taking measures to prevent shrinkage of the injection molded product, injection molding deformation, demoulding deformation, overflow, etc., as well as technical measures such as ensuring mold precision, and adopting the correct precision injection molding process, applicable engineering plastics Materials and precision injection molding equipment to achieve the best match is of great significance for improving the quality, reliability and performance of precision plastic parts, reducing production costs and improving production efficiency.
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