Use the pulling rod to pull off the nozzle material mechanism
The sprue slug-breaking mechanism, which utilizes a pull rod, is a crucial device in injection molds for automatically separating the plastic part from the sprue material. This significantly improves production automation, reduces manual labor, and lowers production costs. This mechanism uses the pull rod’s pulling force on the sprue material to pull it away from the plastic part during the mold opening process, automatically separating the two. This mechanism is suitable for molds employing small gates, such as point and side gates.
The structural design of the sprue puller is key to its effectiveness in breaking the sprue material. Common sprue pullers include spherical, conical, and hooked. The spherical puller has a hemispherical head. During injection, the melt enters the gate and wraps around the ball. Friction between the ball and the melt holds the sprue material in place during mold opening. The conical puller has a conical head, and its slope creates an axial force component, enhancing the pull on the sprue material. The hooked puller, which hooks the sprue material, is suitable for plastics with low viscosity and good fluidity. The puller’s diameter is determined by the gate size and plastic properties, generally 1.5-2 times the gate diameter. For example, if the gate diameter is 3mm, a sprue puller diameter of 5-6mm can be used to ensure sufficient strength to break the sprue material.
The coordination between the puller and sprue bushing significantly impacts the breakaway effect. The head of the puller must extend into the sprue bushing to a certain depth, typically 3-5mm, to allow the melt to fully envelop the head and generate reliable pulling force. A clearance of 0.2-0.3mm must also be maintained between the puller and sprue bushing to prevent friction and wear and facilitate smooth sprue material separation. When the mold is closed, the end face of the puller should be flush with or slightly lower than that of the sprue bushing to prevent obstruction of melt flow. Furthermore, the axis of the puller must align with that of the sprue bushing to avoid uneven pulling force due to eccentricity, which can result in incomplete breakaway of the sprue material or deformation of the part.
The mold opening stroke and ejection mechanism’s sequence of action must align with the process by which the pull rods break the sprue material. During the initial mold opening phase, the movable mold separates from the fixed mold, and the pull rods move with the movable mold, exerting tension on the sprue material. At this point, the plastic at the gate has not yet fully solidified and undergoes plastic deformation under the tensile force, ultimately breaking it at the base of the gate. The mold opening stroke must be sufficiently long to ensure the sprue material is completely pulled out of the sprue sleeve of the fixed mold, typically 5-10 times the gate length. The ejection mechanism should operate after the sprue material has broken through to prevent the plastic part from remaining separated from the sprue material during ejection, leading to part deformation or ejection difficulties. For molds with multiple gates, multiple pull rods are required, and the pulling force of each pull rod must be uniform to prevent inconsistent pulling force from partially breaking the sprue material.
The performance of the mechanism used by the pull rod to break the sprue material is significantly affected by the type of plastic and the molding process parameters. For crystalline plastics (such as PE and PP), due to their rapid solidification, appropriate insulation measures must be implemented at the gate to prolong their plastic deformation time and ensure that the sprue material can be smoothly broken. For amorphous plastics (such as PS and PC), the solidification rate is slower, and the breaking effect is better, but the mold opening speed must be controlled to avoid excessive tension that may damage the plastic part. In terms of molding process parameters, excessively high injection pressure and holding pressure will lead to excessive bonding between the sprue material and the pull rod, which may cause deformation of the plastic part during breaking and should be appropriately reduced. Excessively low melt temperature will reduce the plastic’s fluidity, making the gate more brittle and producing fragments during breaking, so the melt temperature should be appropriately increased.
Maintenance and improvement of the sprue-breaking mechanism using the sprue puller can further improve its reliability and efficiency. In daily use, the head of the sprue puller should be regularly checked for wear or deformation. If so, it should be repaired or replaced in a timely manner. Clean the residual plastic inside the sprue puller and the sprue sleeve to prevent material accumulation from affecting the transmission of tension. For molds with poor breaking effects, the sprue puller head structure can be improved. For example, an annular groove can be added to the surface of the spherical head to enhance the friction with the sprue material; or a serrated pattern can be provided on the inclined surface of the conical head to increase the pulling force. In addition, a guide groove can be provided on the path of the sprue material to prevent the sprue material from deflecting during the breaking process and ensure that it can be smoothly separated from the mold. By continuously optimizing the mechanism design and process parameters, the sprue-breaking mechanism using the sprue puller can play a greater role in injection molding production.
