Gating system of injection mold
The gating system of an injection mold is the key channel that guides the plastic melt from the injection molding machine nozzle into the mold cavity. Its rational design directly determines the molding quality, production efficiency, and material utilization of the plastic part. The basic components of the gating system include the main runner, branch runners, gate, and cold well. These four components work together to form a complete melt delivery network. The main runner is the first channel connecting the injection molding machine nozzle to the mold. Its shape is usually conical, ensuring smooth melt flow and facilitating the release of the slurry. The branch runner is responsible for distributing the melt conveyed by the main runner to the various cavities. Its cross-section is usually circular, trapezoidal, or U-shaped. The circular cross-section offers the lowest flow resistance but is more difficult to process. The trapezoidal cross-section is widely used in actual production because it is easier to process and remove the slurry. The gate is a small channel connecting the branch runner to the mold cavity. Its function is to control the melt filling rate and holding time, while also facilitating the separation of the slurry from the gate and the plastic part. The cold slug well is used to store the cold slug at the front of the melt to prevent the cold slug from entering the cavity and affecting the quality of the plastic part. It is usually set at the end of the main channel or the turning point of the branch channel.
The design of the main channel needs to match the parameters of the injection molding machine to ensure stable delivery of the melt. The inlet diameter of the main channel should be 0.5-1mm larger than the aperture of the injection molding machine nozzle to avoid obstruction or leakage of the melt flow due to size mismatch. The spherical radius at the inlet should be 1-2mm larger than the spherical radius of the nozzle to ensure a close fit between the two and reduce the risk of melt leakage under high pressure. The length of the main channel should be as short as possible, generally not exceeding 100mm. Excessive length will cause the melt temperature to drop too much during the flow process, increase flow resistance, and increase the amount of cold material and material waste. In addition, the main channel usually needs to be set in the center of the mold and equipped with a main channel bushing (i.e., gate sleeve). The bushing is made of high-strength and wear-resistant material to withstand repeated impacts of the nozzle and erosion of the melt, thereby extending the service life of the mold.
The design of runners must consider the melt flow characteristics and cavity distribution to ensure uniform melt distribution. The cross-sectional dimensions of the runners should be determined based on the part size, wall thickness, and plastic fluidity. For plastics with good fluidity (such as polyethylene and polypropylene), the runners can be thinner; for plastics with poor fluidity (such as polycarbonate and polyoxymethylene), larger cross-sectional dimensions are required to reduce flow resistance. The layout of the runners should be determined by the number and distribution of cavities. Common types include balanced and unbalanced. A balanced layout ensures that all runners have the same length and cross-sectional dimensions, ensuring that the melt reaches all cavities simultaneously. This is suitable for high-performance multi-cavity molds. An unbalanced layout adjusts the runner length and cross-sectional dimensions based on cavity location, achieving simultaneous melt filling by varying flow resistance. This is suitable for molds with a large number of cavities or irregularly distributed cavities. Furthermore, the runner surfaces should be polished to reduce surface roughness, thereby reducing friction during melt flow and facilitating mold release.
The design of the gate is the most critical link in the casting system, and the choice of its type and size has a significant impact on the quality of the plastic part. Common gate types include direct gates, side gates, point gates, and latent gates. Direct gates are suitable for large plastic parts or medium-thick walled plastic parts. They have a larger cross-sectional size and low flow resistance, but the gate marks are obvious and require subsequent processing. Side gates are suitable for small and medium-sized plastic parts. They are opened on the edge or side of the plastic part, with smaller gate marks, which facilitates automated production. Point gates are suitable for plastic parts with higher appearance requirements. Their diameter is usually 0.5-2mm, and the gate marks are tiny, but a dedicated demolding mechanism is required to remove the condensate. Subtle gates are hidden on the inside or parting surface of the plastic part and automatically separate from the plastic part during demolding, making them suitable for molds used in automated production. The selection of gate location is also crucial. It should be set in the thick-walled part of the plastic part to facilitate melt filling and pressure holding and shrinkage compensation; avoid setting it on important appearance surfaces or stress-bearing parts of the plastic part to reduce the impact of gate marks on the performance of the plastic part; at the same time, the gate position should enable the melt to flow smoothly in the mold cavity to avoid eddy currents, jets or air entrapment.
The design of the slug well and venting system are crucial supporting measures to ensure the proper functioning of the gating system and should not be overlooked. The size of the slug well should be determined based on the amount of slug, typically with a depth of 1-1.5 times its diameter. A slug puller can be installed at its base to facilitate the removal of slug solids from the main runner during mold opening. For point-gated molds, the slug well is typically located at the end of the runner and, in conjunction with the slug puller, ensures smooth demolding. While not a core component of the gating system, the venting system is closely related to its design. Its function is to expel air and volatiles from the mold cavity and gating system. Vents are typically located at the cavity edge opposite the gate, at the parting surface, or at the clearance between the ejector and the mold plate. Their depth should be less than the plastic’s flash (typically 0.02-0.05mm) and their width should be 5-10mm. This ensures effective venting without causing flash. For large or complex cavities, venting slots can also be installed at the end of the runner to facilitate the removal of air and ensure smooth melt filling. In short, the design of the pouring system needs to comprehensively consider multiple factors such as the plastic part structure, plastic properties, molding process, and production requirements, and achieve high-quality and efficient injection molding by optimizing the parameters of each component.
