The characteristics of the injection mold are usually determined by the characteristics of the raw materials, there are two main points: one is the filling flow characteristics of the melt plastic pellet during injection, and the other is the shrinkage position of the plastic in the mold cavity when it is cooled and solidified.
These two points determine the particularity of the injection mold and the difficulty of mold design. Because the melt plastic belongs to a viscoelastic thing, the viscosity of the fusant changes with the shear stress and the shear rate when the melt flows, the macromolecule generates orientation along the flow direction in the process of full flow, the melt is partially compressed after the mold cavity is filled, the plastic shrinkage in the cooling and solidification process is very complex, the plastic shrinkage rate in each part and direction in the mold cavity is different, and the plastic shrinkage rate of different types and grades of material is very different. The filling fluidity and shrinkage are also different. Based on the above characteristics, when doing the injection mold design, we should fully understand the characteristics of the plastic raw materials, make the design of the injection mold reasonable use, and effectively utilize the characteristics of the plastic in the design process. Therefore, the issues to be considered in the injection mold design process are as follows.
- In-depth analysis of the structure of plastic parts and its technical requirements. The structure of a plastic part determines the complexity of the injection mold structure. The technical requirements (dimensional accuracy, Surface roughness, etc.) of a plastic part determine the difficulty of the injection mold manufacturing and molding process, therefore, for injection molding does not meet the special requirements of the unreasonable structure shape, etc. , we should give our suggestions to improve the design of injection molding, otherwise it will increase the difficulty of injection mold design and manufacture and injection molding process.
- Familiar with the technical specifications of injection molding machine. The technical specifications of the injection molding machine restrict the size of the injection mold and the range of injection molding products.
- Understand the manufacturing performance and processing properties of plastic material. It Includes: 1) the flow path of melt plastic, the larger flow distance ratio that the fusant can achieve; 2) analyzing the flow resistance of the flow path and the every positions of the mold cavity; 3) the venting of the original air in the mold cavity; 4) the possible crystallization, orientation, and internal stress caused of the plastic in the injection mold; 5) the cooling shrinkage and compensation of the plastic; 6) the requirements of the plastic on the mold temperature, etc.
- The injection mold structure should be selected reasonably. According to the drawings and technical requirements of plastic parts, do some research and choose appropriate molding methods and equipment, based on the manufacturing capacity of the factory, put forward the injection mold structure scheme. Also, we need to fully solicit the opinions of relevant departments, analyze and discuss, so that the designed injection mold structure is reasonable, reliable and easy to operate. If necessary, according to the needs of injection mold design and processing, the requirements for modifying the drawings of plastic parts can be proposed, but the implementation can be carried out only after obtaining the consent of the user.5
- The size of the injection molded parts should be calculated correctly. Molding parts are a direct factor in determining the shape, size and surface quality of plastic parts. When calculating the size of molded parts, the average shrinkage method can generally be used. For plastic parts with high precision and need to control the mold repair allowance, it can be calculated according to the tolerance zone method. For large precise plastic parts, the analogy method can be used if possible.
- The injection mold that we design should be easy to manufacture. When designing the molds, try to make it easy to manufacture and cheap. Especially for those complex parts, it must be considered whether to use general machining methods or special processes. If a special processing method is used, then how to assemble after processing, similar problems need to be considered and solved when designing, and should also take the mold repairing into account after the test, to leave enough mold repair allowance.
In one word, when designing an injection mold, the plastic molder should understand the structure of plastic parts and its technical requirements, as well as the properties of plastic material that will be used. Besides, make it easier to manufacture and make the cost as low as possible based on good quality are also very important.