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Design requirements for injection molds

date：2023/08/18

The characteristics of the injection mold are determined by the characteristics of the raw materials. There are two main points: one is the filling mold flow characteristics of the plastic melt during injection, and the other is the shrinkage of the plastic cavity in the mold cavity during cooling and solidification. These two points determine the injection mold. Particularity and difficulty of injection mold design. Because the plastic melt is a viscoelastic body, the viscosity changes with the shear stress and shear rate when the melt flows. During the flow process, the macromolecules are oriented in the flow direction. After the mold cavity is filled, the melt is partially compressed, and the cooling and solidification process The shrinkage of plastics is very complicated. The plastic shrinkage rates of different parts and directions of the mold cavity are different. Different types and types of plastic shrinkage rates are very different. When the same brand of resin or plastic is processed, due to different formulas, its mold flow Sex and shrinkage are also different. Based on the above characteristics, the design of the injection mold must fully understand the characteristics of the plastic raw materials processed, so that the designed injection mold is used reasonably, and the characteristics of the plastic are effectively used in the design process. Therefore, the issues to be considered in the injection mold design process are as follows.

1. In-depth analysis of plastic parts structure and its technical requirements. The structure of the plastic part determines the complexity of the injection mold structure, and the technical requirements of the plastic part (dimension accuracy, surface roughness, etc.) determine the difficulty of the injection mold manufacturing and molding process. Therefore, the special requirements for plastic injection molding that do not meet the requirements , Unreasonable structural shapes, etc., should be proposed to improve the design of injection parts, otherwise it will increase the difficulty of injection mold design and manufacturing and injection molding process;
2. Familiar with the technical specifications of the injection molding machine. The technical specifications of the injection molding machine restrict the size of the injection mold and the range of injection products that can be formed;
3. Understand the processing performance and technological performance of plastics. Including the flow position of the plastic melt, the larger flow distance ratio that the melt can reach: analysis of the flow resistance of the flow channel and the cavity, the discharge of the original air in the mold cavity, the plastic may occur in the injection mold Crystallization, orientation and internal stress caused by it, cooling shrinkage and compensation problems of plastics, plastics' requirements for injection mold temperature, etc .;
4. Consider the design and manufacture of injection molds, and mainly solve the following problems:
a. The injection mold structure should be reasonably selected. According to the drawings and technical requirements of plastic parts, research and select appropriate molding methods and equipment, combined with the factory's mechanical processing capabilities, put forward the injection mold structure plan, solicit opinions from relevant parties, analyze and discuss in order to design the injection mold structure Reasonable, reliable quality and easy operation. If necessary, the requirements for the modification of the drawings of plastic parts can be proposed according to the needs of the design and processing of the injection mold, but it must be approved by the user before implementation.
b. The dimensions of injection molded parts should be calculated correctly. Molded parts are the direct factors that determine the shape, size and surface quality of plastic parts. They are very important and require special attention. When calculating the dimensions of molded parts, the average shrinkage method can generally be used. For plastic parts with high precision and need to control the mold repair margin, it can be calculated according to the tolerance band method. For large precision plastic parts, the analog method can be used as much as possible to calculate the shrinkage of the geometric shape of the plastic parts in different directions. Make up for the influence of certain factors that are theoretically difficult to consider.
c. The designed injection mold should be easy to manufacture. When designing injection molds, try to make the designed injection molds easy to manufacture and cheap to manufacture. Especially for those more complex molded parts, we must consider whether to use the general machining method or the special machining method. If a special processing method is used, then how to assemble after processing, similar problems need to be considered and solved when designing the injection mold, at the same time, the repair of the mold after the trial should be considered, and sufficient mold repair margin should be left.
d. The designed injection mold should be highly efficient, safe and reliable. This requirement involves many aspects of the design of the injection mold, such as the filling of the casting system, the closed module, the temperature adjustment of the Xirong effect is good, and the demoulding mechanism is flexible and reliable.
e. Injection mold parts should be durable and durable. The durability of the injection mold parts affects the service life of the entire injection mold. Therefore, when designing such parts, not only should they make the necessary requirements for their materials, processing methods, heat treatment, etc., the pin parts like push rods are also easy to get stuck Bending and breaking, the resulting failures account for the majority of injection mold failures. Therefore, how to adjust and replace it should be considered, but the life of parts should be compatible with the injection mold.
f. The structure of the injection mold should be adapted to the molding characteristics of the plastic. When designing an injection mold, you should fully understand the molding characteristics of the plastic used and try to meet the requirements. This is also an important measure to obtain high-quality plastic parts.