Author: MULAN -Plastic Molding Manufacturer
The injection mold consists of a moving 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 gating system and a cavity. When the mold is opened, the movable mold and the fixed mold are separated, and the plastic mold is taken out.
In order to reduce the heavy workload of mold design and manufacturing, injection molds mostly use standard mold bases. Gating system The gating system refers to the part of the flow path before the plastic enters the cavity from the nozzle, including the main flow path, 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 injector nozzle to the cavity, usually consisting 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 molds. The sprue is the channel in the mold that connects the nozzle of the injection molding machine to the runner or 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 (0.8mm) to avoid overflow and prevent the two from being blocked due to inaccurate connection. According to the size of the product, the inlet diameter is generally 4-8mm. The diameter of the sprue should expand inwards to 3-5°Angle, in order to demould the excess in the runner.
The cold slug hole is a cavity at the end of the sprue, which is used to trap the cold slug generated between two injections at the nozzle end, so as to prevent the runner or gate from being blocked. Once the cold material is mixed into the cavity, it is easy to generate internal stress. The diameter of the cold slug 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 sinking groove, so that the main channel can be pulled out smoothly during demoulding. In the splitter slot mold, the splitter channel is the channel connecting the sprue and each cavity.
In order to make the molten material fill various cavities at a constant speed, the arrangement of the runners on the mold should be symmetrical and equidistant. The shape and size of the runner section affect the flow of plastic melt, the difficulty of product demoulding and mold manufacturing. If the flow is equal to the amount of material, the flow channel resistance of the circular cross section is the least.
However, since the specific surface of the cylindrical runner is small, it is not conducive to the cooling of the debris in the runner, and the runner must be arranged on the two halves of the mold, which is labor-intensive and difficult to align. For this reason, splitter channels of trapezoidal or semicircular cross-section are often used and placed on half molds with ejector pins. Runner surfaces must be polished to reduce flow resistance and provide faster mold filling.
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.
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 main channel (or runner), but usually it will be 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 product quality. The function of the gate is to: Control the flow rate of the material: During the injection process, the melt stored in this part can solidify early to prevent backflow: Increase the temperature of the melt through strong shear, thereby reducing the apparent viscosity and improving fluidity : It is convenient to separate the product from the runner system. 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, the cross-sectional area is small, and the length is short, not only based on the above functions, but also because the small gate is easy to become larger, and the large gate is difficult to shrink. The gate position should generally be selected at the place where the product is the thickest and does not affect the appearance. The design of the gate size should consider the properties of the plastic melt.
The cavity is the space in the mold where the plastic mold is formed. The components that make up the cavity are collectively called the molded part. Each molded part often has a special name.
The forming parts that form the shape of the product are called dies (also known as female dies), and the forming parts that form the internal shape of the product (such as holes, grooves, etc.) are called cores or punches (also known as positive dies). When designing molded parts, the overall structure of the cavity must first be determined according to the properties of the plastic, the geometry of the product, the dimensional tolerances and the requirements for use. Secondly, according to the determined structure, select the position of the parting surface, gate and vent hole and the demoulding method.
Finally, design each component according to the size of the control product, and determine the combination of each component. When the plastic melt enters the cavity, the pressure is very high, so the material of the molded part should be selected reasonably, and its strength and rigidity should be checked. In order to ensure that the surface of the plastic mold is clean and beautiful, and easy to release the mold, the surface roughness in contact with the plastic is Ra>0.32um, corrosion resistance.
Formed parts are generally heat-treated to increase hardness and made of corrosion-resistant steel. Temperature regulation system In order to meet the mold temperature requirements of the injection process, a temperature regulation system is required to adjust the mold temperature. The main design of the thermoplastic injection mold is the cooling system to cool the mold.
The common method of mold cooling is to set cooling water channels in the mold, and use circulating cooling water to take away the heat of the mold; in addition to the hot water or steam in the cooling water channels, electric heating elements can also be installed inside and around the mold. For more information about injection molds, please visit the official website of Xingye: http://www.dgxingyesj.com/.