Injection Mold Essential Knowledge

Injection Mold

Injection mold is composed of two parts: moving mold and fixed bolor. Moving mold is mounted on the moving template of injection molding machine, while fixed mold is mounted on fixed template of injection molding machine. During injection molding process, moving mold and fixed mold are closed to form pouring system and cavity; while during mold opening, moving mold and fixed mold are separated in order to take out plastic products.


Structure of mold may vary in different situations, depending on factors such as plastic variety, properties, shape, structure, and type of injection machine. However, the basic structure includes a pouring system, a temperature regulating system, molding parts and structural parts. Of these, Pouring system and molding parts are the parts that come into direct contact with plastic, and are most complex, vary most, and require highest level of machining accuracy.


The pouring system includes the main flow channel, cold material cavity, manifold and gate. Main role is to guide plastic melt from injector nozzle to cavity, which directly affects molding quality and production efficiency of plastic products. Molding parts include moving mold, fixed mold, cavity, core, molding rod and exhaust port.

Pouring system

  1. Main runner

Main flow channel is channel connecting nozzle of injection molding machine to manifold or cavity. Its design should be considered to avoid overflow and ensure smooth connection. The diameter of main runner depends on size of product, and diameter of inlet should be slightly larger than diameter of nozzle to avoid blocking. In order to release the mold for convenience, diameter of main flow channel inward at an angle of 3 ° to 5 ° to expand.

  1. Cold material hole

Cold material hole is located at end of main runner, used to capture cold material generated at the end of nozzle between two injections, to prevent manifold or gate from clogging. The diameter of the cavity is about 8-10 mm and depth is 6 mm, bottom of cavity is supported by a mold release lever for easy mold release.

  1. Shunt channel

The manifold connects the main flow channel to individual cavities and needs to be symmetrically and equidistantly distributed to ensure that melt fills the cavities at same speed. The cross-sectional shape and size of manifold affect melt flow, release and ease of mold making. Trapezoidal or semi-circular cross sections are commonly used, polished to minimize flow resistance. 

  1. Gates

Gate is channel that connects the main flow channel (or manifold) to mold cavity, and its cross-sectional area is usually small. The shape and size of the gate has an important effect on quality of product, including controlling material flow rate, preventing back-flow, improving fluidity and facilitating separation.

Temperature control system

In order to meet the requirements of injection process on mold temperature, there is a need for a temperature control system to regulate mold temperature. For injection mold, main design of cooling system to ensure that mold cooling. This can be achieved by setting up a cooling water channel inside mold, the circulating flow of cooling water to take away the heat of mold. In addition, electric heating elements can be installed inside and around mold for heating.

Molding parts

Molding parts are mainly composed of core and concave mold. The core is responsible for forming inner surface of product, while concave mold forms the outer surface shape of product. After closing the mold, core and cavity form cavity of mold. According to the process and manufacturing requirements, sometimes, core and cavity are combined by a number of pieces, and sometimes they are made as a whole, with inserts only in the vulnerable parts. The exhaust port is used to discharge the original and the gas brought in by the melt. When the melt is injected into the cavity, the air and gas must be discharged through exhaust port to avoid air holes, poor seams and other problems in the product. Exhaust port can be located at the end of the flow in the cavity or on the parting surface.

Structural parts

Structural parts include guiding, demolding, core pulling and various parts of parting, such as front and rear clamps, front and rear buckling templates, pressure plates, pressure columns, guiding columns, demolding templates, demolding rods, return rods and so on. The guiding parts are used to ensure that the movable and fixed molds are accurately centered when the molds are closed, usually using four sets of guide pillars and guide bushings to form a guiding structure. The push-out mechanism pushes out plastic products and condensate in process of mold opening, which includes push-out fixed plate, push plate and push rod. Side core pulling mechanism is suitable for plastic products with side concave or side holes, which require side parting first and then pulling out the side cores. Standard mold holders are often used for injection molds to reduce design and manufacturing effort.

Please note that the exhaust port should be positioned so that it is not facing the operator to prevent injuries caused by molten material splashing. Material selection and strength and stiffness checking of molded parts should be reasonable to ensure the molding quality and mold life.

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