Distance mechanism that controls the opening sequence of the three-plate mold
Three-plate molds enable multi-point feeding and precise gate control for complex plastic parts, making them widely used in precision injection molding. The distance mechanism that controls the mold opening sequence is crucial for ensuring safe mold operation and part quality. This mechanism requires precise control of the distance and sequence between the panel and runner plate, and between the runner plate and cavity plate. In an 8-cavity connector three-plate mold, a failure of the distance mechanism initially led to a misaligned mold opening sequence. The runner and cavity plates opened before the panel, causing gate breakage and a 12% part defect rate. A high-quality distance mechanism should ensure that the panel and runner plate are first opened (a distance equal to the runner length + 10mm) to remove runner condensate; then, the runner and cavity plates are opened (a distance sufficient for part demolding). In one case, the use of a distance mechanism improved the mold opening sequence accuracy from 85% to 100%, reducing the mold failure rate by 90%.
Due to its simple structure and high reliability, tie-rod spacing mechanisms are widely used in small and medium-sized three-plate molds. Their core principle is to limit the mold opening distance through the tie-rod length, combined with a spring to achieve sequential control. A mobile phone case three-plate mold uses a 12mm diameter 45# steel tie-rod. One end of the tie-rod is fixed to the faceplate, while the other passes through a slot in the runner plate. A limit nut is installed at the end. Adjusting the nut position sets the mold opening distance between the faceplate and runner plate (typically 50-100mm). A compression spring (8mm diameter, 50mm free length) is placed between the runner plate and cavity plate. Initially, the spring force holds the runner and cavity plates closed until the faceplate and runner plates reach the set distance, at which point the tie-rod pulls the runner plate open. This mechanism allows the mold opening distance deviation to be controlled to within ±1mm, a significant improvement over the ±5mm tolerance of a mold without a spacing mechanism. This has increased the runner slurry removal success rate from 90% to 99.5%.
The latch mechanism is suitable for large three-plate molds (cavity sizes exceeding 500mm), providing greater locking force and preventing incorrect mold opening sequences. A three-plate mold for a certain automobile bumper utilizes two symmetrically arranged latches, consisting of a fixed latch and a movable latch. During the initial mold opening, the fixed latch locks the runner plate, ensuring that the front panel opens first. When the mold opening distance reaches the set value (120mm), the latch rollers move along an inclined surface, forcing the fixed and movable latches apart, allowing the runner plate to begin opening from the cavity plate. The latch is made of SKD11 (HRC55 hardness) and features a nitriding treatment, which increases wear resistance by three times. The latch showed no noticeable wear during a 100,000-mold test, while maintaining a locking force of over 80kN, making it more suitable for high-load applications than a tie-rod mechanism (40kN). In one case, the latch mechanism reduced the mold opening sequence error rate for a large mold from 8% to 0.1%, avoiding the high repair costs associated with mold collisions.
A swing-hook distance mechanism uses a lever principle to control the mold opening sequence. It offers responsiveness and easy adjustment, making it suitable for precision three-plate molds (such as optical lens molds). Its structure comprises a swing hook, a stopper, and a spring. During mold opening, the swing hook engages the runner plate’s boss, limiting its movement and allowing the first plate to open first. When the plate moves to a set distance (e.g., 60 mm), the stopper rotates the swing hook, releasing the runner plate, which then continues to open with the moving mold. The swing hook’s rotating shaft utilizes a bronze bushing and grease, ensuring a response time of less than 0.1 seconds, faster than the 0.3-second response of a pull-rod mechanism. Using this mechanism, a specific optical lens mold reduced the separation time deviation between runner slurry and the molded part from ±0.5 seconds to ±0.1 seconds, avoiding scratches on the molded part caused by delayed separation. The swing hook’s angle design is determined through dynamic analysis to ensure a shock-free release. In one case, optimizing the swing hook angle from 45° to 60° reduced impact force by 40%.
The auxiliary design and maintenance of the distance mechanism are crucial to ensuring long-term stability, including buffer devices, position detection, and regular maintenance. Polyurethane buffer blocks (80 Shore A hardness) installed at the distance mechanism’s limit contact point absorb mold opening shock, reducing the impact noise of one mold from 85dB to 70dB while also reducing component wear. Proximity switches detect mold opening position, immediately shutting down the production line if the mold opening sequence is incorrect. This device reduced fault detection time on one production line from 30 minutes to 10 seconds, preventing batch defects. Maintenance measures require cleaning of the distance mechanism after every 5,000 molds, inspecting the tie rod threads for looseness and the swing hook for smooth rotation, and applying high-temperature grease (resistant to 150°C). Through standardized maintenance, one company has increased the distance mechanism’s mean time between failures from 30,000 molds to 80,000. In addition, the size of the distance mechanism must match the mold opening stroke of the injection molding machine. In one case, the maximum mold opening distance of the injection molding machine was not taken into consideration, resulting in the distance mechanism being unable to open fully. The problem was ultimately solved by shortening the tie rod length by 10mm to ensure compatibility between the mechanism and the equipment.
