Second plate formwork automatic pouring cut-off mechanism
The two-platen automatic gate-off mechanism is a key feature in achieving runner-less or low-runner injection molding. By automatically severing the connection between the gate and the part during mold opening, it eliminates the need for manual gate trimming and significantly improves production efficiency and part quality. This mechanism is particularly suitable for mass-produced small and medium-sized parts, such as electronic connectors and toy parts. Highly integrated within the two-platen mold structure, it takes up no additional space, making it a core component of automated injection molding lines.
The automatic shutoff mechanism consists of a sprue bushing, a shearing blade, a guide assembly, a reset mechanism, and a drive mechanism. The sprue bushing is connected to the main runner and features a special necking structure at its end, reducing the melt thickness at the gate to 0.5-1mm, facilitating shearing. The shearing blade, typically ring-shaped or blade-shaped, is attached to the movable or fixed mold. During mold opening, the drive mechanism moves perpendicular to the gate, severing the gate. The guide assembly, consisting of a guide post and a guide bushing, ensures the straightness of the shearing blade’s movement, achieving a shearing accuracy of ±0.02mm. The reset mechanism, typically a spring or pneumatic cylinder, pulls the shearing blade back to its initial position during mold closing. The drive mechanism utilizes either the mold opening force or an independent hydraulic cylinder to provide the shearing force. The mold opening force-driven mechanism is simple and suitable for small molds, while the hydraulic cylinder-driven mechanism offers adjustable shearing force and is suitable for large molds. For example, the automatic shutoff mechanism for a two-plate mold for an electronic connector utilizes a ring-shaped shearing blade coupled with a hydraulic cylinder to shear a 2mm diameter gate in just 0.3 seconds.
The automatic pouring shutoff process is precisely coordinated with the opening of the second mold. During the mold closing and injection stage, the melt fills the mold cavity through the sprue bushing. The constriction at the gate creates a weak point for the melt. Initially, the movable and fixed molds separate. When the mold opening distance reaches a set value (typically 5-10 times the gate diameter), a drive mechanism drives the shear blade to cut the gate along the constriction. The plastic part now moves with the movable mold, while the sprue aggregate remains in the sprue bushing on the fixed mold side. As the mold opening continues, the demolding mechanism releases the plastic part from the movable mold core, and the sprue aggregate is ejected from the sprue bushing by the ejector mechanism. During mold closing, the shear blade returns to its original position, and the sprue bushing reconnects to the main runner, ready for the next injection. This synchronized action ensures a smooth pouring shutoff process and a smooth cut. For example, the cutout roughness of the automatic pouring shutoff on a certain PP mold can reach Ra1.6μm, meeting appearance requirements without the need for further processing.
The design of key parameters for the gate shutoff mechanism must balance cutting effectiveness and part quality. The diameter and length of the gate neck are key parameters. The diameter is typically 1/3-1/2 the part wall thickness, and the length is controlled between 1-3 mm. A shorter diameter can result in incomplete shutoff, while a longer length increases the cutting force. The cutting blade angle should be designed based on the material properties. For tough materials (such as PE and PP), a 30°-45° cutting edge is recommended; a sharp edge reduces strain. For brittle materials (such as PS and PMMA), a 60°-90° cutting edge is recommended to avoid chipping during cutting. The calculation of the cutting force takes into account the gate diameter, material strength, and cutting speed. The formula is F=πdLσ, where d is the gate diameter, L is the neck length, and σ is the material’s shear strength (approximately 20 MPa for PP). For example, a PP gate with a diameter of 2 mm and a length of 1 mm has a shear force of approximately 125 N. In actual design, the calculated value needs to be multiplied by a safety factor of 1.5-2 to prevent cutting failure due to melt pressure fluctuations.
The debugging and optimization of the automatic shut-off mechanism should focus on the cutting quality and the life of the mechanism. During the mold trial, first check whether the incision is smooth. If wire drawing occurs, increase the pressure of the cutting knife or reduce the diameter of the gate neck. If the incision is concave, it may be that the gap between the cutting knife and the gate sleeve is too large (should be controlled within 0.01-0.03mm), and the mating surface needs to be re-grinded. For crystalline plastics, the mold temperature can be appropriately increased by 5-10°C to maintain a certain toughness of the melt at the gate and reduce brittle cracking during cutting. The smooth movement of the mechanism is crucial. The friction coefficient can be reduced and the service life can be extended by applying solid lubricants (such as molybdenum disulfide) to the guide column. During the production process, the plastic residue on the cutting knife needs to be cleaned regularly, and the blade wear needs to be checked daily. When the blade radius exceeds 0.1mm, it needs to be re-sharpened to ensure stable cutting effect.
The development direction of the two-plate mold automatic pouring shutoff mechanism is modularization and intelligence. The modular design enables the mechanism to adapt to different sizes of gates and plastic parts. By replacing the gate sleeve and cutting knife assembly, automatic pouring shutoff can be achieved for multiple plastic parts on the same mold, reducing the switching time to less than 30 minutes. In terms of intelligence, some high-end mechanisms integrate pressure sensors and visual inspection systems to monitor the cutting force and incision quality in real time. When anomalies are detected, the cutting speed or pressure is automatically adjusted. For example, if a system detects incision stringing, it automatically increases the cutting force by 10% and extends the cutting time by 0.1 second, effectively solving the problem of quality fluctuations. With the advancement of Industry 4.0, the automatic pouring shutoff mechanism can be connected to the MES system to record the pouring shutoff parameters and quality data of each mold, realize the quality traceability throughout the entire life cycle, and provide strong support for the intelligent upgrade of injection molding production.
