Injection molding uses push plate to cut off the gate material mechanism
The push-plate mechanism used in injection molding to cut off the sprue material is a key feature for automated production, particularly in three-plate molds or hot runner molds. The push-plate’s axial movement separates the sprue material from the molded part. The cut is typically made at the junction of the gate and the part, requiring a smooth cut (residual length < 0.5mm). A 16-cavity connector mold utilizes this mechanism. The push-plate is guided by four 16mm diameter guide pins. After mold opening, it is driven by a hydraulic cylinder (30mm stroke) to cut the gate at a speed of 50mm/s. This increases the sprue material separation success rate from 92% (using manual separation) to 99.8%, and reduces the processing time per part from 3 seconds to 0.5 seconds. The key to this mechanism is the design of the cutting edge, which must be matched to the gate dimensions. The cutting edge typically has an angle of 30°-45° and a hardness of HRC55 or higher (e.g., Cr12MoV) to ensure long-term wear resistance.
The guiding accuracy of the push plate and the mold directly impacts the cutting effect. The parallelism and perpendicularity of the push plate must be controlled within 0.02mm/m to avoid skewed cuts or damage to the plastic part due to deflection. In a mobile phone casing mold, excessive push plate guide clearance (0.1mm) caused the push plate to tilt during cutting, resulting in cracks in 3% of the plastic parts. By replacing precision guide pins and bushings (with a clearance of 0.01-0.02mm) and adding a wear-resistant plate (5mm thick, made of copper alloy) to the bottom of the push plate, the parallelism error was controlled to 0.01mm/m, and the cracking defect was eliminated. The push plate’s movement must be smooth and impact-free. Hydraulic or spring buffering can be used. In one case, a 0.5mm buffer distance at the end of the push plate’s stroke, combined with a nitrogen spring (5kN force), reduced the cutting impact force from 150N to 50N, eliminating the surface indentation defect in the plastic part.
The timing and stroke of the cut-off must be coordinated with the mold opening. Typically, the push plate is activated after the mold has opened to a safe distance (>50mm), with a stroke slightly greater than the gate height (e.g., for a 2mm gate height, a stroke of 3-4mm is appropriate). In a hot runner mold, the push plate cut-off timing was too early (activating at 30mm mold opening), resulting in incomplete separation of the plastic part and the sprue material, and stringing after cut-off. Adjusting the start time to 60mm after the mold opening and using a 4mm stroke eliminated the stringing. Stroke control can be achieved using stoppers or electronic rulers. On one production line, an electronic ruler precisely controls the stroke (with an accuracy of ±0.05mm), reducing the gate residual length deviation from ±0.3mm to ±0.1mm, meeting subsequent assembly requirements. For gates of varying thicknesses, an adjustable stroke mechanism is required. On one mold, by replacing stoppers of varying thickness (in 0.5mm increments), the stroke can be adjusted by 1-5mm to accommodate multiple product transitions.
The blade design of the push plate cutting mechanism must be differentiated according to the gate type to ensure cutting efficiency and part quality. For point gates (1-2mm diameter), the push plate blade can be designed as a ring with an inner diameter 0.2mm larger than the gate to ensure complete gate coverage. This design enabled a cosmetic bottle cap mold to achieve a point gate cutoff residual length of less than 0.1mm. For side gates (5-10mm width), a rectangular blade is used, 1mm wider than the gate. After cutting the side gate of a home appliance panel mold, the cutout achieved a flatness of Ra 1.6μm, eliminating the need for subsequent polishing. The blade should be regularly ground (every 50,000 mold cycles), and burrs should be repaired promptly if they are discovered. In one case, blade wear (corner radius 0.1mm) resulted in 2% of the sprue material not being fully cut. Re-grinding to a sharp edge (corner radius 0.02mm) restored the cutout to normal.
The power source for the push plate mechanism should be selected based on the cutting force. Small plastic parts (cutting force <5kN) can be driven by a spring, medium-sized parts (5-15kN) are suitable for cylinder-driven operation, and large parts (>15kN) require a hydraulic motor or servo motor. For a 500g PP plastic part, the calculated cutting force is 8kN. Using a 50mm diameter cylinder (working pressure 5MPa), the output force reaches 10kN, meeting the cutting requirement while providing a safety margin. The response speed of the power source must match the production cycle. The response time of a cylinder-driven mechanism is approximately 0.1 seconds, making it more suitable for high-speed production than a spring-driven mechanism (0.3 seconds). Some high-speed injection molding machines (with a cycle time of 10 seconds) use a cylinder-driven push plate, ensuring that the total cycle time is not affected by the cutting action. In addition, a safety protection device needs to be installed to automatically shut down the machine when the cutting force exceeds the set value (such as 120% of the rated force). In the case of a mold where the cutting force suddenly increased due to the entry of foreign matter, the safety device acted in time to avoid damage to the cutting edge, reducing maintenance costs by more than 5,000 yuan.
