Sprue bushing for hot runner injection mold
The sprue bushing of a hot runner injection mold is a key component connecting the injection molding machine nozzle to the hot runner system. Its function is to smoothly guide the melt ejected from the injection molding machine into the hot runner plate while isolating heat transfer between the nozzle and the mold to ensure stable melt temperature. Unlike traditional cold runner sprue bushings, hot runner sprue bushings must simultaneously meet the requirements of high-temperature sealing, precise positioning, and low heat loss. Their structural design directly affects the melt flow efficiency and the overall performance of the hot runner system. Improper design can lead to melt leakage, temperature fluctuations, or nozzle wear. Therefore, a customized design is required based on the injection molding machine specifications, plastic properties, and hot runner type.
The sprue bushing’s structure must match the injection molding machine’s nozzle and hot runner plate. Common types include straight, tapered, and combined. Straight sprue bushings are suitable for small and medium-sized molds used for low-pressure injection. They have a cylindrical inner bore and a clearance of 0.02-0.04mm with the nozzle. While simple in structure, their sealing performance is average. Tapered sprue bushings , with a 15°-30° taper, mate with the nozzle and self-seal under injection pressure. They are suitable for high-pressure injection applications (such as engineering plastics molding). They offer reliable sealing but require high precision. Combined sprue bushings integrate guiding, sealing, and thermal insulation functions, featuring a guiding cone at the front, a sealing ring in the middle, and a thermal pad at the rear. They are suitable for large, high-precision molds. The sprue bushing’s inlet diameter should be 0.5-1mm larger than the injection molding machine’s nozzle diameter, and its outlet diameter should align with the hot runner plate’s sprue channel diameter to ensure smooth melt flow and avoid steps or dead spots.
The sprue bushing’s material selection and heat treatment must meet high-temperature resistance, wear resistance, and thermal insulation requirements. Due to its prolonged contact with the high-temperature melt (typically 180-300°C) and the nozzle, the sprue bushing must be made of heat-resistant steel (such as H13 or SKD61). After quenching and tempering, the hardness must reach HRC 45-50, and the surface roughness must be below Ra 0.4μm to reduce melt adhesion and wear. For processing corrosive plastics (such as PVC and POM), the inner surface of the sprue bushing should be chrome-plated (0.01-0.03mm thick) to improve corrosion resistance. A thermal insulation ring (made of polytetrafluoroethylene or ceramic, 2-3mm thick) should be installed at the contact point between the sprue bushing and the hot runner plate to reduce heat transfer to the mold. Air slots should be provided on the mating surface with the fixed platen for thermal insulation, reducing the increased cooling energy consumption caused by increased platen temperature.
The positioning and fixing design of the sprue bushing must ensure coaxiality with the injection molding machine nozzle to prevent wear and leakage caused by eccentricity. The sprue bushing is positioned within the fixed platen using a stepped or stopper structure, with a radial clearance of 0.01-0.02mm. It is secured axially with bolts or a pressure plate to prevent displacement under injection pressure. The locating ring and the injection molding machine locating hole adopt an H7/H6 tolerance to ensure that the center of the sprue bushing does not deviate from the center of the injection molding machine nozzle by more than 0.05mm. For large molds, a support block should be installed at the bottom of the sprue bushing to enhance overall rigidity. The support block should be made of the same material as the sprue bushing and be at least 20mm thick. In addition, the sprue bushing should have a cooling water channel. An annular water channel should be machined 5-8mm from the inner hole to provide cooling water (temperature 20-30°C) to prevent nozzle-sprue bushing adhesion due to excessive temperatures. However, the distance between the water channel and the inner hole should be carefully considered to avoid compromising the strength of the sprue bushing.
The sprue bushing’s sealing design is crucial for preventing melt leakage, ensuring an effective seal at all contact points. Sealing with the injection molding machine nozzle can be achieved using either a conical or spherical seal. The conical seal utilizes a 60° cone that mates with the nozzle. The sealing pressure increases with injection pressure, making it suitable for most plastics. The spherical seal utilizes a spherical contact surface with a radius of R10-R20, making it suitable for high-pressure injection (pressures exceeding 150 bar) or low-viscosity plastics, providing a more reliable seal. The connection to the hot runner plate requires an O-ring (made of fluororubber, temperature resistant above 200°C). This O-ring should be installed in the sprue bushing’s sealing groove, with a groove depth 0.1-0.2mm smaller than the ring’s diameter to ensure sufficient compression. Furthermore, the sprue bushing inlet should have a chamfered corner (R1-R2) to prevent scratching during nozzle insertion. The inner bore should be polished to reduce melt flow resistance and retention.
Sprue bushing maintenance and replacement must comply with regulations to extend service life and ensure stable production. Before each production run, inspect the inner bore of the sprue bushing for plastic residue or scratches. Clean with a copper brush and apply high-temperature grease. If nozzle leakage is detected during production, inspect the sealing surface for wear. If the wear depth exceeds 0.1mm, regrind or replace it. For sprue bushings used for a long time, regularly measure the inner bore dimensions and replace them when the diameter wear exceeds 0.2mm to avoid affecting melt flow. When replacing the sprue bushing, be sure to clean the locating holes and mating surfaces to ensure they are free of impurities. After installation, check the coaxiality. Use a dial indicator to check radial runout, which should be kept within 0.03mm. Through proper design and maintenance, the sprue bushing effectively connects the injection molding machine to the hot runner system, ensuring stable melt flow and reducing downtime.
