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Author: MULAN -Plastic Molding Manufacturer
The injection mold consists of a moving mold and a fixed mold. The moving 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, and the movable mold and the fixed mold are separated to take out the plastic product when the mold is opened. In order to reduce the heavy workload of mold design and manufacturing, most injection molds use standard mold bases.
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1. Gating system The gating system refers to the part of the runner before the plastic enters the cavity from the nozzle, including the main flow channel, cold material cavity, runner and gate. 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 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.
2. The main channel is a channel in the mold that connects the nozzle of the injection molding machine to the runner or cavity. The top of the sprue is concave to connect with the nozzle. The inlet diameter of the main channel should be slightly larger than the nozzle diameter (0.8mm) to avoid overflow and prevent the two from being blocked due to inaccurate connection.
The inlet diameter depends on the size of the product, generally 4-8mm. The diameter of the sprue should expand inward at an angle of 3° to 5° to facilitate the demoulding of the excess in the flow path. 3. Cold slug cavity It is a cavity at the end of the main flow channel, which is used to catch the cold material produced between the two injections at the end of the nozzle, so as to prevent the blockage of the runner or gate.
Once the cold material is mixed into the cavity, internal stress is easily generated in the manufactured product. The diameter of the cold slug hole is about 8-10mm, and the depth is 6mm. In order to facilitate demoulding, its bottom is often borne by a demoulding rod.
The top of the demoulding rod should be designed as a zigzag hook or a sunken groove, so that the main channel can be pulled out smoothly when demoulding. Fourth, the runner is the channel connecting the main channel and each cavity in the multi-slot mold. In order to make the molten material fill each cavity at an equal speed, the arrangement of the runners on the mold should be symmetrical and equidistant.
The shape and size of the cross-section of the runner have an impact on the flow of the plastic melt, the demoulding of the product and the difficulty of the mold manufacturer. If the flow of the same amount of material is concerned, the flow channel with a circular cross-section has the least resistance. But because the specific surface of the cylindrical runner is small, it is unfavorable to the cooling of the debris in the runner, and the runner must be opened on the two halves of the mold, which is labor-intensive and difficult to align.
Therefore, trapezoidal or semicircular cross-section runners are often used, and they are opened on half of the mold with ejector pins. The surface of the runner 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 8m, the extra-large one can reach 10-12m, and the extra-small one can reach 2-3m. Under the premise of meeting the needs, the cross-sectional area should be reduced as much as possible to increase the debris in the shunt channel and prolong the cooling time. 5. The gate is the channel connecting the main channel (or runner) and the cavity.
The cross-sectional area of the channel can be equal to that of the main channel (or branch channel), but it is usually reduced. So 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: A. Control the flow rate of the material: B. During the injection, it can prevent the backflow due to the early solidification of the melt stored in this part: C. Make the passing melt receive strong shear and increase the temperature , so as to reduce the apparent viscosity to improve fluidity: D, to facilitate the separation of products and runner systems. The design of the shape, size and position of the gate 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 small gates to become larger, but it is difficult for large gates to shrink.
The gate position should generally be selected at the place where the product is the thickest without affecting the appearance. The design of the gate size should take into account the nature of the plastic melt. Cavity It is the space in which plastic products are formed in the mold.
The components used to form the cavity are collectively referred to as molded parts. Each molded part often has a special name. The forming part that constitutes the shape of the product is called the die (also known as the female die), and the part that constitutes the internal shape of the product (such as holes, grooves, etc.) is called the core or punch (also known as the male die).
When designing a molded part, the overall structure of the cavity must first be determined according to the properties of the plastic, the geometric shape of the product, the dimensional tolerance and the use requirements. The second is to select the parting surface, the position of the gate and the vent hole and the demoulding method according to the determined structure. Finally, the design of each part is carried out according to the size of the control product and the combination of each part is determined.
When the plastic melt enters the cavity, there is a high pressure, so the molded parts should be selected reasonably and checked for strength and stiffness. In order to ensure the smooth and beautiful surface of plastic products and easy demoulding, the roughness of the surface in contact with plastic should be Ra>0.32um, and corrosion resistance. Formed parts are generally heat-treated to increase hardness and made of corrosion-resistant steel.
6. Temperature adjustment system In order to meet the mold temperature requirements of the injection process, 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 set up a cooling water channel in the mold, and use the circulating cooling water to take away the heat of the mold; in addition to using hot water or steam in the cooling water channel, the heating of the mold can also be installed inside and around the mold. Heating element.