Nov 4, 2021 Viewed: 477 Tag: injection  

Principle of injection mold

Injection molds are parts that give plastics shape and size during molding. Although the structure of the mold may vary widely depending on the variety and performance of the plastic, the shape and structure of the plastic product, and the type of injection machine, the basic structure is the same. The mold is mainly composed of a pouring system, a molding part and a structural part. Among them, the casting system and molding parts are the parts that directly contact the plastic, and change with plastics and products. They are the most complicated and most variable parts of the mold, and require the highest processing finish and precision.

The gating system refers to the part of the flow channel before the plastic enters the cavity from the nozzle, including the main flow channel, the cold material cavity, the shunt channel, and the gate. Molded parts refer to various parts that make up the shape of the product, including moving molds, fixed molds and cavities, cores, molding rods, and exhaust ports.

1. Mainstream Road

It is a passage in the mold that connects the injection nozzle of the injection machine to the shunt or cavity. The top of the main flow channel is concave so as to engage with the nozzle. The diameter of the inlet of the main flow channel should be slightly larger than the diameter of the nozzle (0.8mm) to avoid overflow and prevent blockage of the two due to inaccurate connection. The inlet diameter depends on the size of the product, and is generally 4-8mm. The diameter of the main flow channel should be enlarged inward at an angle of 3 ° to 5 ° in order to facilitate the release of the flow path excess.

2.Split channel Z

It is the channel connecting the main flow channel and each cavity in the multi-slot mold. In order to fill the cavity with the molten material at the same speed, the arrangement of the runners on the mold should be symmetrically and equally spaced. The shape and size of the cross section of the shunt have an impact on the flow of the plastic melt, the release of the product, and the ease of mold manufacturing. If the flow is equal, the resistance of the flow channel with a circular cross section is the smallest. However, the cylindrical flow channel is smaller than the surface, which is not good for the cooling of the shunts. Moreover, this shunt must be set on two mold halves, which is labor-intensive and easy to align. Therefore, a trapezoidal or semi-circular cross-section runner is often used, and is opened on a half mold with a release rod. The runner surface must be polished to reduce the flow resistance and provide a faster filling speed. The size of the runner depends on the type of plastic, the size and thickness of the product. For most thermoplastics, the cross-section width of the shunt channel does not exceed 8mm, the largest can reach 10-12mm, and the smallest 2-3mm. Under the premise of meeting the requirements, the cross-sectional area should be reduced as much as possible, so as not to increase the shunts and extend the cooling time.


It is the channel connecting the main channel (or the shunt channel) and the cavity. The cross-sectional area of the channel can be equal to the mainstream channel (or the shunt channel), but it is usually reduced. Therefore, it is the smallest cross-sectional area in the entire flow channel system. The shape and size of the gate have a great impact on the quality of the product. The function of the gate is: A. Control the flow rate: B. During the injection, the melt that exists in this part can be prevented from flowing backward. C. The molten material passing through is subjected to strong shear to increase the temperature , Thereby reducing the apparent viscosity to improve fluidity: D, to facilitate the separation of the product from the flow channel system. The design of the gate shape, size and location depends on the nature of the plastic, the size and structure of the product. Generally, the cross-sectional shape of the gate is rectangular or circular, and the cross-sectional area should be small and the length should be short. This is not only based on the above-mentioned effects, but also because it is easier to make a small gate larger, and it is difficult to shrink a large gate. The gate location should generally be selected at the thickest part of the product without affecting the appearance. The design of the gate size should take into account the nature of the plastic melt.

4, cavity

It is the space for molding plastic products in the mold. The components used to form the cavity are collectively called shaped parts. Each molded part often has a special name. The forming parts that make up the shape of the product are called concave dies (also known as female dies), and the internal shapes (such as holes, slots, etc.) that make up the product are called cores or male dies (also called male dies). When designing a molded part, the overall structure of the cavity must first be determined according to the properties of the plastic, the geometry of the product, dimensional tolerances and use requirements. The second is to choose the parting surface, the position of the gate and the exhaust hole, and the demolding method according to the determined structure. Finally, the design of each part and the combination of each part are determined according to the size of the control product. Plastic melt has a high pressure when it enters the cavity, so the molded parts must be reasonably selected and checked for strength and stiffness. In order to ensure that the surface of plastic products is smooth, beautiful, and easy to be demolded, the surface contacting the plastic has a roughness Ra> 0.32um and must be resistant to corrosion. Molded parts are generally heat treated to increase hardness and are made of corrosion-resistant steel.

5.Exhaust port

It is a slot-shaped air outlet opened in the mold to exhaust the original and molten gas. When the melt is injected into the cavity, the air existing in the cavity and the gas brought in by the melt must be exhausted to the outside of the mold through the exhaust port at the end of the stream, otherwise the product will have air holes, poor welding, Dissatisfaction with filling, and even the accumulated air will burn the product due to the high temperature caused by compression. In general, the vent can be set at the end of the melt flow in the cavity, or on the parting surface of the mold. The latter is a shallow groove with a depth of 0.03-0.2mm and a width of 1.5-6mm on one side of the die. During injection, there will not be a lot of melt leakage from the vent, because the melt will cool and solidify there, blocking the passage. The opening position of the exhaust port must not face the operator, so as to prevent accidental discharge of the molten material to hurt people. In addition, the clearance between the ejection rod and the ejection hole, the clearance between the ejection block and the ejection plate and the core can be used for exhausting.

6. Structural parts D # i

It refers to various parts constituting the mold structure, including: guide, demolding, core pulling and parting. Such as front and rear splint, front and rear buckle template, pressure plate, pressure column, guide column, demoulding plate, demoulding rod and return rod.

7.Heating or cooling device

This is a device that solidifies and shapes the melt in the mold. For thermoplastics, generally the channels of the cooling medium in the male and female molds, the cooling medium is achieved by the circulating flow of the cooling medium. The cooling medium that passes in varies with the type of plastic and product structure, including cold water, hot water, hot oil, and steam. The key is high-efficiency uniform cooling. The uneven cooling will directly affect the quality and size of the product. The arrangement of cooling channels and the choice of cooling medium should be considered according to the thermal properties of the melt (including crystallization), the shape of the product and the mold structure.

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