Basic Structure of Injection Mold
Injection molds consist of two main parts: the fixed mold and the moving mold. The moving mold is installed on the moving platen of the injection molding machine, and the fixed mold is installed on the fixed platen. During injection molding, the moving mold and the fixed mold close to form the gating system and cavity. When the mold opens, the moving mold and fixed mold separate to eject the plastic product.
Although the structure of the mold may vary due to different plastic varieties and properties, the shape and structure of plastic products, and the type of injection machine, the basic structure is consistent. The mold is mainly composed of a gating system, temperature control system, molding parts, and structural parts. Among them, the gating system and molding parts are the parts that directly contact the plastic and change with the plastic and the product. They are the most complex and most variable parts in the mold, requiring the highest processing finish and precision.
1. Gating System
The gating system refers to the runner part before the plastic enters the cavity from the nozzle, including the sprue, cold slug well, runner, and gate. The design of the gating system is directly related to the molding quality and production efficiency of plastic products.
1. Sprue: It is a channel in the mold connecting the injection machine nozzle to the runner or cavity. The top of the sprue is concave to connect with the nozzle. The inlet diameter of the sprue should be slightly larger than the nozzle diameter (0.8mm) to prevent overflow and blockage due to inaccurate connection. The inlet diameter depends on the size of the product, generally 4-8mm. The sprue diameter should expand inward at an angle of 3° to 5° to facilitate demolding of runner debris.
2. Cold Slug Well: It is a cavity set at the end of the sprue to trap the cold material produced between two injections at the nozzle tip, thereby preventing blockage of the runner or gate. If cold material enters the cavity, internal stress is easily generated in the product. The diameter of the cold slug well is about 8-10mm and the depth is 6mm. To facilitate demolding, the bottom is often borne by a demolding rod. The top of the demolding rod should be designed in a zigzag hook shape or with a recessed groove so that the sprue debris can be pulled out smoothly during demolding.
3. Runner: It is the channel connecting the sprue and each cavity in a multi-cavity mold. In order to fill each cavity with molten material at the same speed, the arrangement of runners on the mold should be symmetrical and equidistant. The cross-sectional shape and size of the runner affect the flow of plastic melt, product demolding, and the difficulty of mold manufacturing. If the flow of equal material volume is considered, the flow resistance of a circular cross-section runner is the smallest. However, because the specific surface area of the cylindrical runner is small, it is unfavorable for the cooling of runner debris, and this kind of runner must be opened on two halves of the mold, which is labor-intensive and easy to misalign. Therefore, trapezoidal or semi-circular cross-section runners are often used, and they are opened on the half of the mold with the demolding rod. The runner surface must be polished to reduce flow resistance and provide faster mold filling speed.
4. Gate: It is the channel connecting the runner (or sprue) and the cavity. The cross-sectional area of the channel can be equal to the sprue (or runner), 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 product quality.