Injection molding nozzle material release mechanism
The injection molding sprue ejector is a mold device that automatically separates the plastic part from the runner condensate (sprue material). Its core function is to mechanically cut and eject the sprue material from the gate during the mold opening or ejection process, automating the separation of the plastic part and sprue material, reducing manual labor and improving production efficiency. In traditional injection molding, the sprue material is integral to the part and requires manual trimming, which is not only time-consuming and labor-intensive (accounting for approximately 30% of production time), but can also damage the part. The sprue ejector, however, uses cutting, ejection, and pulling actions to complete separation within 1-2 seconds after mold opening. It is particularly suitable for the high-volume production of small plastic parts (such as electronic components and toy parts). For example, in the production of USB connectors (16 cavities per mold), the implementation of the sprue ejector increased hourly output from 1,200 pieces (using manual separation) to 2,000 pieces. The remaining sprue material length is kept to within 0.5mm, far superior to the 2mm achieved with manual trimming.
Based on the cutting method, sprue material removal mechanisms can be divided into shear, pull-off, and twist-off types, each with its own application scenarios. The shearing mechanism uses the relative motion of a fixed and movable cutting edge to cut the sprue. It is suitable for parts with larger gate diameters (1-3mm). For example, a “fixed die cutting edge + movable die ejector” combination is used. During mold opening, the ejector pushes the sprue material into contact with the fixed die cutting edge, utilizing shear force to cut the sprue material, resulting in a smooth cut surface. The pull-off mechanism uses the tensile force during mold opening to cause the sprue material to naturally break away from the gate. It is suitable for brittle materials (such as PS and PMMA) or parts with point gates. A pull rod is provided to retain the sprue material in the movable mold. The mold opening force causes stress concentration at the gate, leading to fracture. While its structure is simple, the cut may be uneven. The twist-off mechanism uses a rotational motion to break the sprue material. It is suitable for sprues with circular runners. For example, a spiral groove pull rod is installed at the end of the runner. The pull rod rotates during mold opening, using torque to break the gate. It is suitable for tough materials (such as PE and PP). For example, when producing PP plastic buttons, a twist-off ejection mechanism is used. The spiral pull rod is rotated 30° to twist off the 1.5mm diameter gate, leaving a smooth cut without residue.
The structural design must be optimized based on the runner form and gate type to ensure reliable separation. For side-gated gates, the ejection mechanism typically consists of a cutting edge and an ejector pin. The cutting edge is located at the junction of the runner and the plastic part, with an angle of 30°-45° and a sharpness of Ra ≤ 0.4μm. The ejector pin is installed at the end of the runner, and the ejection stroke is 2-3mm longer than the part ejection stroke to ensure that the gate material is ejected first. For point-gated gates, a “pull rod + stripper” structure is often used. The pull rod has a spherical or inverted conical head. During mold opening, it pulls the gate material and moves with the moving mold. When it reaches the stripper, the stripper scrapes the gate material off the pull rod, achieving automatic removal. The ejection mechanism of hot runner gates must be coordinated with the nozzle design. A shutoff device should be installed at the junction of the runner plate and the nozzle. For example, the needle valve in a valve gate not only controls the feed but also shuts off the runner after the pressure holding period, preventing the generation of gate material. For example, a 32-cavity point gate mold uses an inverted tapered pull rod and a stripper plate, and the gate material removal rate reaches 100%, with no material sticking.
The performance parameters of the sprue ejection mechanism must be compatible with the molding process, primarily including shear force, ejection stroke, and actuation time. The shear force is calculated based on the gate diameter and material toughness using the following formula: Shear force (N) = Gate cross-sectional area (mm²) × Material shear strength (MPa) × Safety factor (1.5). For example, for a 2mm diameter ABS gate (shear strength 40MPa), the shear force must be ≥188N. The ejection stroke must ensure the sprue material is completely released from the mold. It is typically 1.2-1.5 times the runner length. For example, for a 100mm long runner, the ejection stroke is 120-150mm. The actuation time must be synchronized with the mold opening cycle. The shear action should be completed at the beginning of the mold opening (when the mold opening distance is 5-10mm), and the ejection action should be initiated 0.5 seconds before the part is ejected to avoid interference. For example, the sprue ejection mechanism of a certain mold controls the total time of cutting and ejection actions within 0.8 seconds by optimizing the cam transmission, adapting to the rapid molding cycle of 10 seconds/mold.
The commissioning and maintenance of the sprue ejection mechanism are critical to ensuring long-term stable operation. During commissioning, the cutting edge clearance (typically 0.01-0.03mm) must be adjusted. Excessive clearance can easily cause the sprue material to break rather than cut, while too small a clearance may damage the cutting edge. Through trial molds, observe the cutting edge quality and gradually optimize the clearance to the optimal level. The ejection speed must be stable to avoid splashing or jamming of the sprue material due to excessive speed. This can be achieved through segmented speed control (slow contact speed in the initial stage, rapid ejection in the middle stage, and slow separation in the final stage). Routine maintenance requires weekly cleaning of residual plastic from the cutting edge and runner, inspection of the pull rod for wear (wear exceeding 0.1mm requires replacement), and regular application of high-temperature grease (temperature resistance ≥150°C) to moving parts. For example, one company established a maintenance plan that included daily pre-shift inspection of cutting edge sharpness, weekly grease replacement, and monthly ejection stroke calibration, which reduced the failure rate of the sprue ejection mechanism from 5 to 0.5 times per month.
