How is the injection mold composed?


Although the structure of the mold may vary due to the […]

Although the structure of the mold may vary due to the variety and performance of plastics, the shape and structure of plastic products, and the type of injection machine, the basic structure is the same. The mold is mainly composed of pouring system, temperature regulation system, forming parts and structural parts. Among them, the gating system and the molding parts are the parts that are in direct contact with the plastic, and change with the plastic and the product. They are the most complex and changeable parts in the mold, and require the highest processing finish and precision.

The injection mold consists of a movable mold and a fixed mold. The movable mold is installed on the moving template of the injection molding machine, and the fixed mold is installed on the fixed template of the injection molding machine. During injection molding, the movable mold and the fixed mold are closed to form a gating system and a cavity. When the mold is opened, the movable mold and the fixed mold are separated to take out the plastic product. In order to reduce the heavy workload of mold design and manufacturing, most of the injection molds use standard mold bases.

Gating system

The gating system refers to the part of the runner before the plastic enters the cavity from the nozzle, including the main runner, the cold material cavity, the runner and the gate.

The gating system, also known as the runner system, is a set of feeding channels that lead the plastic melt from the nozzle of the injection machine to the cavity. It is directly related to the molding quality and production efficiency of plastic products.


It is a passage in the mold that connects the injection nozzle of the injection molding machine to the runner or cavity. The top of the sprue is concave for engagement with the nozzle. The inlet diameter of the main channel should be slightly larger than the diameter of the nozzle (0.8mm) to avoid overflow and prevent the two from being blocked due to inaccurate connection. The diameter of the inlet depends on the size of the product, generally 4-8mm. The diameter of the sprue should be expanded inward at an angle of 3° to 5° to facilitate the release of the runner debris.

cold slug

It is a cavity at the end of the main channel to capture the cold material generated between two injections at the end of the nozzle, thereby preventing the blockage of the branch channel or gate. If the cold material is mixed into the cavity, internal stress is easily generated in the manufactured product. The diameter of the cold material hole is about 8-10mm and the depth is 6mm. In order to facilitate demoulding, the bottom is often borne by the demoulding rod. The top of the demoulding rod should be designed as a zigzag hook or a sunken groove, so that the sprue can be pulled out smoothly when demoulding.


It is the channel connecting the main channel and each cavity in the multi-slot mold. In order to make the melt fill each cavity at the same speed, the arrangement of the runners on the mold should be symmetrical and equidistant. The shape and size of the runner section have an impact on the flow of the plastic melt, the demolding of the product and the ease of mold manufacturing. If the flow of the same amount of material is considered, the flow channel resistance with a circular cross-section is the smallest. However, because the specific surface of the cylindrical runner is small, it is not good for the cooling of the runner excess, and the runner must be opened on the two halves of the mold, which is labor-intensive and difficult to align. Therefore, trapezoidal or semi-circular cross-section runners are often used, and are opened on half of the mold with ejector bars. The runner surface must be polished to reduce flow resistance and provide 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-sectional width of the runner does not exceed 8mm, the extra large can reach 10-12mm, and the extra small is 2-3mm. Under the premise of meeting the needs, the cross-sectional area should be reduced as much as possible to increase the shunting duct debris and prolong the cooling time.


It is the channel that connects the main channel (or shunt channel) and the cavity. The cross-sectional area of ​​the channel can be equal to the main channel (or branch channel), but it is usually reduced. Therefore, it is the part with the smallest cross-sectional area in the entire runner system. The shape and size of the gate have a great influence on the quality of the product.

The function of the gate is to:

A. Control the flow rate:

B. During injection, the melt stored in this part can be prematurely solidified to prevent backflow:

C. The passing melt is subjected to strong shear to increase the temperature, thereby reducing the apparent viscosity to improve fluidity:

D. It is convenient to separate the product from the runner system. The design of 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 effects, but also because it is easier for a small gate to become larger, while it is difficult to shrink a large gate. The gate position 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 properties of the plastic melt. Cavity It is the space in the mold for molding plastic products. The components used to form the cavity are collectively referred to as molded parts. Each formed part often has a special name. The molding parts that constitute the shape of the product are called concave molds (also known as female molds), and those that constitute the internal shapes of the products (such as holes, grooves, etc.) are called cores or punches (also known as male molds). When designing a molded part, the overall structure of the cavity should be determined first according to the performance of the plastic, the geometry of the product, dimensional tolerances and usage requirements. The second is to select the parting surface, the position of the gate and the exhaust hole and the demoulding method according to the determined structure. Finally, according to the size of the control product, the design of each part and the combination of the parts are determined. The plastic melt has a high pressure when it enters the cavity, so the molding parts should be reasonably selected and checked for strength and stiffness. In order to ensure the smooth and beautiful surface of plastic products and easy demoulding, all surfaces in contact with plastics should have a roughness Ra>0.32um and be corrosion-resistant. Formed parts are generally heat treated to increase hardness and made of corrosion-resistant steel.

temperature control system

In order to meet the requirements of the injection process for the mold temperature, a temperature adjustment system is required to adjust the temperature of the mold. For injection molds for thermoplastics, the cooling system is mainly designed to cool the mold. The common method of mold cooling is to open a cooling water channel in the mold, and use the circulating cooling water to take away the heat of the mold; the heating of the mold can not only use hot water or steam in the cooling water channel, but also install electricity in and around the mold. Heating element.

Molded parts

Molded parts refer to various parts that make up the shape of the product, including movable molds, fixed molds and cavities, cores, molding rods, and exhaust ports. The molded part consists of a core and a die. The core forms the inner surface of the article, and the die forms the outer surface shape of the article. After the mold is closed, the core and the cavity constitute the cavity of the mold. According to the process and manufacturing requirements, sometimes the core and the die are composed of several blocks, and sometimes they are made as a whole, and inserts are only used in the parts that are easily damaged and difficult to process.

exhaust vent

It is a groove-shaped air outlet opened in the mold to discharge the original gas and the gas brought in by the melt. When the melt is injected into the cavity, the air originally stored in the cavity and the gas brought in by the melt must be discharged out of the mold through the exhaust port at the end of the material flow, otherwise the product will have pores, poor connection, Filling the mold is not satisfied, and even the accumulated air will burn the product due to the high temperature caused by compression. Under normal circumstances, the vent hole can be set either at the end of the flow of the melt in the cavity, or at 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 molten material oozing out of the vent hole, because the molten material will cool and solidify there and block the channel. The opening position of the exhaust port should not face the operator to prevent accidental ejection of molten material and injury. In addition, the matching gap between the ejector rod and the ejector hole, the matching gap between the ejector block and the stripper plate and the core can also be used to exhaust air.

structural parts

It refers to various parts that make up the mold structure, including: guiding, demoulding, core pulling and parting various parts. Such as front and rear splints, front and rear buckle templates, bearing plates, bearing columns, guide columns, stripping plates, stripping rods and return rods, etc.

1. Guide parts

In order to ensure that the movable mold and the fixed mold can be accurately aligned when the mold is closed, guide parts must be provided in the mold. In the injection mold, four sets of guide posts and guide sleeves are usually used to form the guide parts, and sometimes it is necessary to set the inner and outer cone surfaces that match each other on the movable mold and the fixed mold to assist positioning.

2. Launch institutions

During the mold opening process, a push-out mechanism is required to push or pull out the plastic product and its aggregate in the runner. Push out the fixed plate and the push plate to clamp the push rod. A reset rod is generally fixed in the push rod, and the reset rod resets the push plate when the movable and fixed molds are closed.

3. Side core pulling mechanism

Some plastic products with side concave or side holes must be laterally parted before being pushed out, and the lateral core can be pulled out before they can be demolded smoothly. At this time, a side core pulling mechanism needs to be installed in the mold.