What are the components of an injection mold?

The injection mold consists of a movable mold and a fixed mold. The moving mold is installed on the moving template of the injection molding machine, while 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, and the plastic product is taken out. To ease the heavy lifting of mold design and manufacture, most injection molds use standard mold bases.

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

 
The main runner is the channel that connects the injection molding machine nozzle and the runner or mold cavity. The top of the sprue is concave and connected to the nozzle. The inlet diameter of the sprue should be slightly larger than the nozzle diameter (0.8 mm) to avoid overflow and prevent the two from being blocked due to improper connection. The diameter of the inlet depends on the size of the product, typically 4-8 mm. The diameter of the sprue should expand inward at an angle of 3-5 to facilitate demolding of the flow channel vegetation.

 
The cold material cavity is the cavity located at the end of the main channel, which is used to collect the cold material generated between the two injections at the end of the nozzle to prevent the branch channel or gate from being blocked. Once the cold material is mixed into the cavity, it will easily generate internal stress in the manufactured product. The diameter of the cold stock cavity is about 8-10 mm and the depth is 6 mm. In order to facilitate demolding, its bottom is usually carried by a demolding rod. The top of the ejector rod should be designed as a zigzag hook or a concave groove, so that the sprue can be pulled out smoothly during ejection.

 
A runner is a channel that connects the main runner and each cavity in a multi-slot mold. In order to fill each cavity with melt at an equal rate, the runners should be arranged symmetrically and equidistantly on the mold. The shape and size of the runner section affects the flow of the plastic melt, the release of the product and the difficulty of mold making.

 
If the flow rates are equal, the resistance of the circular cross-section flow path is the least. However, the specific surface area of the cylindrical runner is small, which is not conducive to the cooling of the split runner. In addition, the runners must be provided on both mold halves, which is labor-intensive and difficult to align.

Therefore, trapezoidal or semi-circular cross-section runners are usually used, which are provided on one half of the mold with ejector bars. The runner surface must be polished to reduce flow resistance and provide faster mold filling. The size of the runner depends on the type of plastic, size and thickness of the product. For most thermoplastics, the cross-sectional width of the runner is no more than 8 meters and can be as large as 10-12 meters and as small as 2-3 meters. On the premise of meeting the needs, the cross-sectional area should be reduced as much as possible to increase the runner and prolong the cooling time. door

 
It is the channel connecting the main channel (or branch channel) and the cavity. The cross-sectional area of the channel can be equal to the cross-sectional area of the main channel (or branch channel), but is usually reduced. So it is the smallest cross-sectional area in the entire runner system. The shape and size of the gate have a great impact on product quality.

The functions of the gate are:

a. Control the material flow rate:

b. During injection, backflow can be prevented due to premature solidification of the melt stored in this part:

c. Perform strong shear heating on the passing melt to reduce apparent viscosity and improve fluidity;

 d. Promote the separation of the product from the runner system. The design of gate shape, size and location depends on the properties of the plastic, size and structure of the product. Generally, the cross-sectional shape of the gate is rectangular or circular, 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 the small gate is easy to become larger, and the large gate is difficult to shrink. Generally, the gate position should be selected at the thickest part, which does not affect the appearance of the product.

The design of the gate size should take into account the properties of the plastic melt. A cavity is a space in a mold where a plastic product is formed. The parts used to form the cavity are collectively referred to as molded parts.

Each molded part usually has a special name. The moldings that make up the shape of the product are called female molds (also called female molds), and the moldings that make up the internal shape of the product (such as holes and grooves) are called cores or male molds (also called male molds). When designing a molded part, the overall structure of the cavity should be determined based on the properties of the plastic, product geometry, dimensional tolerances, and application requirements.

Secondly, according to the determined structure, select the parting surface, the position of the gate and the exhaust hole and the demoulding method.

Finally, design each part according to the size of the controlled product, and determine the combination mode of each part. The plastic melt has a high pressure when it enters the cavity, so it is necessary to select a reasonable material and check the strength and stiffness of the molded part.

In order to ensure that the surface of plastic products is smooth, beautiful and easy to demould, the surface roughness Ra in contact with the plastic should be greater than 0.32um, and should be resistant to corrosion. Generally, the hardness of the formed parts is improved by heat treatment, and corrosion-resistant steel is selected.

tempering system

In order to meet the mold temperature requirements of the injection molding process, a temperature adjustment system is required to adjust the mold temperature. For thermoplastic injection molds, the cooling system is mainly used to cool the mold. The common method of mold cooling is to open the cooling water channel in the mold, and use the circulating cooling water to take away the heat of the mold; in addition to the hot water or steam in the cooling water channel, electric heating elements can be installed in and around the mold.