Injection molding main channel design
The injection molding sprue is a critical channel connecting the injection molding machine nozzle and the runners. Its design directly impacts melt flow characteristics, pressure loss, and part quality. The sprue’s primary function is to smoothly guide the molten plastic from the nozzle into the mold while simultaneously transmitting injection and holding pressure to ensure the melt fills the cavity. Improper design can increase melt flow resistance, lead to excessively rapid cooling, or generate vortices, causing defects such as material shortages, flash, and bubbles. For example, in the production of ABS parts, an undersized sprue can result in pressure loss exceeding 30%, leading to underfill. Improper angles can also generate vortices, entraining air and forming bubbles. Therefore, a thorough understanding of the key design elements of the sprue and a sound determination of its structural parameters are crucial for improving the stability and economic efficiency of injection molding production.
The cross-sectional shape and dimensions of the sprue are key design parameters and must be compatible with the injection molding machine nozzle and melt characteristics. The sprue typically adopts a conical cross-section, which reduces melt flow resistance and facilitates demolding. The cone angle α is a critical parameter. A too small angle (less than 2°) makes demolding difficult, while a too large angle (greater than 6°) increases the contact area between the melt and the wall, leading to rapid cooling and increased pressure loss. Common angles range from 3° to 5°, with 3° to 4° being recommended for plastics with good flowability (such as PE and PP) and 4° to 5° for plastics with poor flowability (such as PC and PMMA). The sprue’s large end diameter D should be 0.5-1mm larger than the injection molding machine nozzle diameter to prevent melt leakage. Since nozzle diameters are typically 4-8mm, the sprue’s large end diameter is typically 5-9mm. The small end diameter d (where it contacts the nozzle) should be slightly larger than the nozzle outlet diameter (approximately 0.5mm), and a fillet radius R = 1-3mm should be used to prevent stress concentration and melt vortexes. For example, when the nozzle diameter is 6mm, the small end diameter of the main channel is 6.5mm, and the radius R=2mm to ensure smooth transition of the melt.
The sprue length should be designed to be as short as possible to minimize pressure loss and cooling time. Excessively long sprues can cause excessive melt temperature drops, poor fluidity, increased aggregate weight, and material waste. The sprue length, L, is typically controlled between 60 and 150 mm, depending on mold thickness and nozzle extension. The calculation formula is: L = mold platen thickness + sprue bushing protrusion from the platen – nozzle entry depth. For example, if the platen thickness is 80 mm, the sprue bushing protrusion is 10 mm, and the nozzle entry depth is 20 mm, the sprue length, L, equals 80 mm + 10 mm – 20 mm = 70 mm. For large molds, if the sprue length exceeds 150 mm, a hot runner system should be used to prevent excessive melt cooling. Furthermore, the sprue axis must coincide with the injection molding machine nozzle axis, with a coaxiality error of no more than 0.1 mm. Otherwise, melt flow will deviate, resulting in pressure loss and localized wear.
The design and material selection of the sprue bushing are crucial to extending mold life. The sprue bushing must be made of a high-strength, highly wear-resistant material, such as SKD61 or Cr12MoV, with a hardness of HRC50-55 after heat treatment to withstand repeated impact from the nozzle and friction from the melt. The sprue bushing must be easily replaceable, positioned with a step to the fixed platen, and the clearance must be controlled at 0.01-0.02mm to prevent melt leakage. The radius R1 of the sphere in contact with the nozzle must be 1-2mm larger than the spherical radius R of the nozzle (e.g., nozzle R=10mm, sprue bushing R1=11-12mm) to ensure good contact and avoid crushing. The cooling system for the sprue bushing must be designed based on the material’s properties. For high-melting-point plastics (such as PC), cooling channels must be installed around the sprue bushing, 15-20mm from the outer surface, to prevent premature solidification of the melt in the sprue. For example, the cooling water channel of the gate sleeve of a PC mold has a diameter of 8mm and a water temperature of 80-90°C, which can keep the melt temperature in the main channel above 260°C and ensure good fluidity.
The mold release and venting design of the sprue are crucial for ensuring smooth production. A cold slug well should be located at the end of the sprue to collect cold slugs from the leading edge of the melt, preventing them from entering the mold cavity and impacting part quality. The diameter of the cold slug well is typically 1.5-2 times the diameter of the sprue’s larger end, with a depth of 5-15mm. For large molds, a puller can be installed at the bottom of the cold slug well to facilitate slug release. For example, if the sprue’s larger end diameter is 8mm, the cold slug well should have a diameter of 12-16mm and a depth of 10mm. Combined with a Z-shaped puller, this ensures smooth slug removal. Venting of the sprue can be achieved through venting slots connected to the branch runners. These slots, located near the cold slug well and 0.02-0.05mm deep and 5-10mm wide, discharge gases introduced by the melt. Furthermore, the sprue’s surface roughness should be controlled to Ra ≤ 0.8μm, and fine grinding and polishing should be performed to reduce melt flow resistance and slug adhesion. For example, a surface-polished main channel can increase the melt flow rate by 10%-15% and reduce the pressure loss by 5%-10%.
Common problems and optimization strategies for sprue design require continuous review in practice. If the sprue has difficulty releasing the solidified material from the mold, it may be due to a too-small angle or a rough surface. Increase the angle to 4°-5° and improve the surface finish. If leakage occurs at the nozzle-sprue bushing interface, check whether the spherical radius matches or replace a worn sprue bushing. If the melt cools too quickly within the sprue, shorten its length or increase cooling (for high-melting-point materials). For multi-cavity molds, the sprue should be located in the mold center to ensure uniform runner lengths. For automated production molds, the sprue design should facilitate robotic removal of solidified material. Large-diameter cold wells and rigid pull rods can be used. Using CAE mold flow analysis software to simulate melt flow and pressure distribution within the sprue can identify design flaws and optimize parameters. For example, if simulations reveal excessive pressure loss in a particular area of the sprue, the diameter of that area can be increased or the angle optimized to achieve more uniform pressure distribution.
