The injection mold is composed of a movable mold and a […]
The injection mold is composed of a movable mold and a fixed mold. The movable 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 a pouring system and cavity. When the mold is opened, the movable mold and the fixed mold are separated to take out the plastic products. In order to reduce the heavy workload of mold design and manufacturing, most injection molds use standard mold bases.
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The gating system refers to the part of the runner before the plastic enters the cavity from the nozzle, including the main runner, cold slug, runner and gate, etc. The pouring system is also called the runner system. It is a set of feed channels that lead the plastic melt from the nozzle of the injection machine to the cavity. It is usually composed of a main runner, a runner, a gate and a cold material cavity. It is directly related to the molding quality and production efficiency of plastic products.
Main runner It is a passage in the mold that connects the nozzle of the injection molding machine to the runner or cavity. The top of the main runner is concave in order to connect with the nozzle. The inlet diameter of the main runner 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 main runner should be expanded inward at an angle of 3° to 5° to facilitate the demolding of the runner.
Cold slug hole It is a cavity at the end of the main runner to trap the cold slug generated between two injections at the end of the nozzle, so as to prevent blockage of the runner or gate. Once the cold material is mixed into the cavity, internal stress is likely to occur in the manufactured product. The diameter of the cold slug hole is about 8-10mm and the depth is 6mm. In order to facilitate demolding, the bottom is often borne by the demolding rod. The top of the stripping rod should be designed into a zigzag hook shape or set with a recessed groove, so that the sprue waste can be pulled out smoothly during demolding.
Runner It is the channel connecting the main runner and each cavity in the multi-slot mold. In order to fill the cavities with the melt at 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 product demolding and the difficulty of mold manufacturing.
If the flow of the same amount of material is used, 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 unfavorable for the cooling of the runner redundant, and the runner must be opened on the two mold halves, which is laborious and difficult to align.
Therefore, trapezoidal or semicircular cross-section runners are often used, and they are opened on half of the mold with a stripping rod. The surface of the runner 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 runners does not exceed 8m, the extra-large ones can reach 10-12m, and the extra-small ones 2-3m. On the premise of meeting the needs, the cross-sectional area should be reduced as much as possible to increase the debris of the runner and extend the cooling time.
It is a channel connecting the main runner (or branch runner) and the cavity. The cross-sectional area of the channel can be equal to the main flow channel (or branch channel), but it 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 influence on the quality of the product.
The role of the gate is:
A. Control the material flow speed:
B. It can prevent backflow due to the early solidification of the melt stored in this part during injection:
C. The passing melt is subjected to strong shear to increase the temperature, thereby reducing the apparent viscosity and improving the fluidity:
D. It is convenient to separate the product from the runner system. The design of gate shape, size and position 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, and it is difficult for large gates to shrink. The gate location should generally be selected where the product is thickest without affecting the appearance.
The design of the gate size should take into account the properties of the plastic melt. The cavity 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 molded part often has a special name. The molded parts that make up the shape of the product are called concave molds (also called female molds), and the ones that make up the internal shape of the product (such as holes, grooves, etc.) are called cores or punches (also called male molds). When designing molded parts, first determine the overall structure of the cavity according to the properties of the plastic, the geometry of the product, the dimensional tolerances and the requirements for use.
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, according to the size of the control product, the design of each part and the combination of each part are determined. The plastic melt has a high pressure when it enters the cavity, so the molded parts must be selected reasonably and checked for strength and rigidity.
In order to ensure that the surface of plastic products is smooth and beautiful and easy to demold, the roughness of the surface in contact with the plastic should be Ra>0.32um, and it should be corrosion resistant. Formed parts are generally heat treated to increase the hardness, and are made of corrosion-resistant steel.
Temperature control system
In order to meet the mold temperature requirements of the injection process, a temperature adjustment system is required to adjust the mold temperature. For injection molds for thermoplastics, a 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 use the cooling water channel hot water or steam, and also install electricity in and around the mold. Heating element.