The fixed-distance parting mechanism is a key device in injection molds, controlling the mold parting sequence and distance. It is widely used in the molding of parts with complex cores, undercuts, or parts that require core extraction before parting. Its core function is to open the mold in stages according to a preset sequence during the mold opening process, precisely controlling the opening distance of each parting surface to ensure smooth demolding without deformation or damage. The rationality of the fixed-distance parting mechanism’s design directly affects mold efficiency and part quality, so it requires targeted design based on the part structure, mold type, and production requirements.
Common types of fixed-distance parting mechanisms include tie rod, swing hook, spring, and hydraulic types. The tie rod type consists of a tie rod, a stop pin, and a parting surface. One end of the tie rod is fixed to the movable mold, while the other end passes through a slot in the fixed mold. When the mold opening force reaches a certain value, the tie rod is stopped by the stop pin, forcing the fixed mold and the intermediate plate to separate first, thus achieving fixed-distance control. This simple and reliable structure is suitable for small and medium-sized molds. The parting distance can be precisely adjusted by the tie rod length, generally within a range of 50-200mm. The swing hook type fixed-distance parting mechanism achieves parting control through the cooperation of a swing hook and a locking latch. Initially, the swing hook locks the fixed mold. When the set distance is reached, the swing hook is released by the pressure block, completing the parting. This type offers high clamping force and is suitable for large molds. The spring type fixed-distance parting mechanism relies on the spring force to open the parting surface first. While compact, the spring is susceptible to fatigue failure and is typically used in applications with low parting forces and short distances. The hydraulic fixed-distance parting mechanism drives the parting action through a hydraulic cylinder, which can achieve stepless speed regulation and precise control. It is suitable for precision molds and automated production lines.
The design of the fixed-distance parting mechanism must focus on determining the parting sequence and distance. The parting sequence should be formulated according to the demoulding requirements of the plastic part. For example, for plastic parts with undercuts on the fixed mold, the fixed mold parting surface must be opened first to complete the core pulling, and then the main parting surface of the movable mold must be opened; for deep-cavity plastic parts, the middle parting surface must be opened first to release internal pressure to prevent the plastic part from sticking to the mold. The determination of the parting distance must meet the demoulding and core pulling requirements. It is generally 1.2-1.5 times the maximum external dimension of the plastic part to ensure that the plastic part can be completely separated from the core or cavity. For example, for a plastic part with a height of 80mm, the parting distance needs to be set to 100-120mm, leaving enough space for operation. In addition, the control accuracy of the parting distance needs to reach ±1mm, which can be achieved through parts such as limit pins and blocks. The hardness of the limit pins needs to reach HRC50 or above to withstand repeated impacts.
Force analysis and strength calculations for the fixed-distance parting mechanism are crucial. During the mold opening process, the mechanism must withstand the clamping force, core-pulling force, and inertia of the plastic part. Therefore, key components such as tie rods, swing hooks, and pins require strength verification. The tie rod diameter can be calculated using the formula d=√(4F/π[σ]), where F is the parting force and [σ] is the material allowable stress (160 MPa for 45 steel). For example, when the parting force is 50 kN, the tie rod diameter must be ≥20 mm. The strength calculation of the swing hook must account for the bending stress at the load point to prevent fracture during the tightening process. 40Cr steel is typically quenched and tempered to a hardness of HRC 35-40. The clearance between the pin and the hole must be controlled within 0.01-0.03 mm to prevent loosening and misalignment. The surface should be nitrided to improve wear resistance.
The installation and debugging of the fixed-distance parting mechanism are crucial to its performance. During installation, it is necessary to ensure that the parallelism error of each parting surface is ≤0.05mm/m, and the coaxiality error of the pull rod and the guide hole is ≤0.1mm to avoid movement jamming. During the debugging stage, it is necessary to gradually adjust the position of the limit pins to ensure that the parting sequence is correct and the parting distance meets the design requirements. During the mold trial, it is necessary to observe the demoulding of the plastic parts. If the parting sequence is disordered, it may be that the pull rod length or the swing hook position is improper and needs to be readjusted; if the parting distance is insufficient and causes deformation of the plastic part, the parting distance needs to be increased by 10-20mm. For molds produced by automated production, it is also necessary to link the fixed-distance parting mechanism with the stroke switch of the injection molding machine to achieve automatic control and improve production efficiency.
Maintenance and optimization of the fixed-distance parting mechanism are key to extending mold life. During daily use, parts such as tie rods and swing hooks should be regularly inspected for wear. When surface wear exceeds 0.1mm, they must be replaced promptly. Spring-type mechanisms require spring replacement every six months to prevent elastic force attenuation. For molds with high production volumes, wear-resistant plates (such as SKD11) can be installed at the contact points of the parting surface to reduce the rate of wear. For optimization, a combined fixed-distance parting mechanism can be used, such as a combination of tie rods and springs, to balance reliability and speed. For complex molds, CAE simulation can be used to analyze stress distribution during the parting process, optimize part structure, and reduce stress concentration. With the development of mold technology, intelligent fixed-distance parting mechanisms have begun to be used. Sensors monitor the parting position in real time, and PLCs automatically adjust the position, further improving the stability and adaptability of the mold.
