Basic structure and types of injection molds
The basic structure of an injection mold is the foundation for molten plastic to be molded into a finished product. It consists of multiple components with different functions working together to complete the molding process, including feeding, plasticizing, injection, pressure holding, cooling, and demolding. Although the structure of an injection mold varies depending on factors such as the shape, size, and material of the product, it can generally be divided into two major parts: the movable mold and the fixed mold. The fixed mold is fixed to the fixed platen of the injection molding machine and connected to the injection molding machine’s nozzle, responsible for introducing the molten material into the mold cavity. The movable mold is mounted on the movable platen of the injection molding machine and moves with the movable platen in opening and closing. During mold opening, it is responsible for ejecting the product from the cavity. The parting surface between the movable and fixed molds is the interface between the mold opening and closing, and also serves as the sealing surface for the molten material to fill the cavity. Its accuracy directly affects the flash generation and dimensional accuracy of the product.
The basic structure of an injection mold can be further divided into functional components such as the molding components, gating system, guide mechanism, ejection mechanism, temperature control system, and exhaust system. Molding components, including the cavity, core, and inserts, come into direct contact with the molten material and determine the shape and size of the finished product. These components are typically made of high-strength, wear-resistant alloy tool steels, such as P20 and 718H, and undergo heat treatment to enhance hardness and service life. The gating system is the channel that guides the molten material from the nozzle into the mold cavity. It consists of main runners, branch runners, gates, and slug wells. Its design must ensure smooth melt flow and uniform filling, avoiding eddy currents and stagnation. The guide mechanism, consisting of guide pins and guide bushings, ensures accurate alignment between the movable and fixed molds during mold opening and closing, preventing damage to mold components due to misalignment. The ejection mechanism is responsible for ejecting the product from the core or cavity after mold opening. Common ejection mechanisms include push rods, push tubes, and ejector plates.
The temperature control system is an important component to ensure the stability of product quality. Its function is to control the mold temperature within an appropriate range through heating or cooling to meet the requirements of melt plasticization, filling, cooling and solidification. For crystalline plastics, such as polyethylene and polypropylene, the mold usually needs to be heated to promote the crystallization of plastic molecules and increase the density and strength of the product; while for non-crystalline plastics, such as polystyrene and polyvinyl chloride, the cooling system is mainly used to accelerate the cooling and solidification of the product and shorten the molding cycle. The temperature control system usually consists of cooling water channels, heating pipes, thermostats, etc. The layout of the cooling water channels must be uniform and reasonable to avoid excessive temperature differences in different parts of the mold, which may cause uneven shrinkage or deformation of the product. For large and complex molds, zoned temperature control can also be used to set different temperature parameters according to the heat dissipation conditions of various parts of the mold.
Injection molds can be divided into different types according to different classification standards. Based on the material of the molded product, they can be divided into thermoplastic molds and thermosetting plastic molds; based on the number of cavities in the mold, they can be divided into single-cavity molds and multi-cavity molds; and based on the mold’s structural characteristics, they can be divided into two-plate molds, three-plate molds, hot runner molds, etc. The two-plate mold is the most basic and widely used type. Its structure is simple, with only one parting surface, and it is suitable for products with simple molding structures. The three-plate mold has two parting surfaces, which allows for point gate feeding and is suitable for products with high surface quality requirements and where side gates are not suitable. Hot runner molds use a heating device to keep the melt in the runner in a constant molten state, eliminating the need to remove the runner solids, improving raw material utilization and production efficiency, and are suitable for products produced in large quantities.
Different types of injection molds have their own scope of application and advantages and disadvantages. When selecting a mold type, it is necessary to comprehensively consider factors such as the structural characteristics of the product, production batch, quality requirements, and cost budget. For example, for small products produced in large quantities, multi-cavity hot runner molds can significantly improve production efficiency and reduce costs; while for large products with complex structures and high dimensional accuracy requirements, single-cavity precision molds should be used to ensure molding quality. With the development of injection molding technology, the structure and type of molds are constantly innovating. For example, the emergence of new molds such as stacked molds, two-color molds, and gas-assisted molding molds has further expanded the application areas of injection molding and met the needs of different industries for diversified and high-quality products. Mold designers need to reasonably select the mold type and optimize the structural design based on specific production needs to achieve high-quality, efficient, and low-cost production goals.
