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What are the steps in the injection molding process?


Author: MULAN -Plastic Molding Manufacturer

The injection mold consists of a moving mold and a fixed mold. The mobile mold is installed on the movable 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 the gating system and the 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 injection molds use standard mold bases. The injection mold factory Xingye Xiaobian will introduce the process of injection molds: Gating system The gating system refers to the part of the runner before the plastic enters the cavity from the nozzle, including the main channel, cold cavity, runner and gate wait. The gating system, also known as the runner system, is a set of feed channels that guide the plastic melt from the nozzle of the injection molding machine to the cavity. It usually consists of a main channel, a runner, a gate and a cold cavity.

It is directly related to the molding quality and production efficiency of plastic products. The sprue is the channel connecting the nozzle of the injection molding machine and the runner or mold cavity. The top of the main channel is concave and connected with the nozzle.

The inlet diameter of the main channel 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, generally 4-8 mm. The diameter of the sprue should expand inward at an angle of 3-5 to facilitate demoulding of flow channel vegetations.

The cold material cavity is located at the end of the main flow channel and is used to collect the cold material generated between two injections at the end of the nozzle to prevent the runner 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 cavity is about 8-10 mm and the depth is 6 mm.

In order to facilitate demoulding, its bottom is usually supported by a demoulding rod. The top of the demoulding rod should be designed as a zigzag hook or a concave groove, so that it can be pulled out of the main channel smoothly during demoulding. A runner is a channel that connects the sprue to each cavity in a multi-groove mold.

In order to fill each cavity with melt at an equal speed, the runners should be arranged symmetrically and equidistantly on the mold. The shape and size of the cross-section of the runner affect the flow of the plastic melt, the demoulding of the product and the difficulty of mold manufacturing. If the flow rate is equal, the resistance of the circular cross-section flow channel is the least.

However, the specific surface area of ​​the cylindrical runner is small, which is not conducive to the cooling of the sub-runner. Furthermore, the runners must be placed on the two mold halves, which is labor-intensive and difficult to align. For this reason, it is common to use runners of trapezoidal or semicircular cross-section, which are placed on one half of the mold with ejector pins.

Runner surfaces must be polished to reduce flow resistance and provide faster mold filling. The size of the sprue 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 8 meters, and can be as large as 10-12 meters and as small as 2-3 meters.

Under 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. Straight gate It is a channel connecting the main channel (or runner) 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 usually it will be reduced.

So it is the smallest cross-sectional area in the entire runner system. The shape and size of the gate have a great influence on the product quality. The function of the gate is: 1. Control the flow rate of the material: b. During the injection process, due to the premature solidification of the melt stored in the part, it can prevent backflow: c. Perform strong shearing and heating of the passing melt to reduce Apparent viscosity, improve fluidity; d. Promote the separation of product and runner system.

The design of the shape, size and position of the gate depends on the performance of the plastic, the 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 small gates are easy to become larger, while large gates are difficult to shrink. Generally, the gate position should be selected at the thickest point, without affecting the appearance of the product.

The design of the gate size should consider the properties of the plastic melt. The cavity is the space in which the plastic product is formed in the mold. The parts used to form the cavity are collectively referred to as molded parts.

Each molded part usually has a special name. The molded parts that make up the shape of the product are called female molds (also known as female molds), and the molded parts that make up the internal shape of products (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 according to the properties of the plastic, the geometry of the product, dimensional tolerances and application requirements.

Secondly, according to the determined structure, select the parting surface, the position of the gate and the vent hole, and the demoulding method. Finally, design each part according to the size of the accused product, and determine the combination of each part. The plastic melt has high pressure when it enters the cavity, so it is necessary to choose a reasonable material and check the strength and rigidity of the molded part.

In order to ensure that the surface of the plastic product is smooth and beautiful, and easy to release the mold, the surface roughness Ra in contact with the plastic is Ra0.32um, which should be corrosion-resistant. Generally, the hardness of the formed parts is improved through 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 regulation system is needed 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 inside and around the mold. For more information about plastic molds in Shanghai, please visit the official website of Xingye:

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