Injection molding built-in small tie rod fixed distance parting mechanism
The built-in tie-rod spacing parting mechanism in injection molding is a sophisticated device that controls the parting sequence and distance in three-plate molds. Its core function is to limit the mold opening stroke using a small tie-rod (typically 8-16mm in diameter) built into the mold plate. This saves more space than external tie-rods. Incorporating this mechanism into a three-plate mold for a mobile phone casing reduced the mold width by 50mm, enabling the use of a smaller injection molding machine (from 120 to 100 tons) and reducing energy consumption by 15%. The tie-rod length must be precisely calculated, typically adding 10-20mm (safety margin) to the main parting line opening. For one mold, the main parting line opening required an 80mm opening, and the tie-rod length was set at 95mm to ensure reliable positioning after parting. The actual opening deviation is controlled within ±1mm. The threaded connections at both ends of the tie-rod must be secure, using fine-pitch threads (such as M10×1) and thread sealant. In one mold, loose threads resulted in inaccurate spacing, with the opening deviation reaching 5mm. Tightening restored the gap to normal.
High precision guidance and fit are required for built-in small tie rods, ensuring smooth movement without binding or deflection during the parting process. The clearance between the tie rod and the mold plate must be controlled within 0.02-0.05mm, using an H7/f6 clearance fit. In one mold, the clearance was excessively large (0.1mm), causing the tie rod to wobble, uneven force on the parting surface, and flash on the part. Adjusting the clearance to 0.03mm eliminated the flash. The tie rod surface requires high-frequency quenching (HRC 50-55 hardness) and polishing (Ra 0.8μm) to reduce movement resistance. In one case, an unquenched tie rod resulted in wear of 0.2mm after 10,000 mold cycles. After replacing the quenched tie rod, the wear was reduced to 0.05mm/10,000 mold cycles. For large molds (template size > 500mm), 2-4 small tie rods need to be arranged symmetrically to ensure uniform force. A certain automobile instrument panel mold uses four tie rods with a diameter of 16mm, distributed at the four corners of the template. The difference in the opening distance at each point during parting is less than 0.5mm, which improves the mold stability by 40% compared to the mold with two tie rods.
The parting force control of the small tie-rod fixed-distance parting mechanism must be coordinated with the mold structure, using springs or hydraulic cylinders to achieve smooth parting and avoid impact. To overcome the clamping force of the parting surface during initial mold opening, a compression spring (12-20mm diameter) can be installed on the small tie-rod. One mold uses a spring with a force of 5kN to assist in the initial opening of the main parting surface, reducing parting force fluctuations from ±30% to ±10%. For large three-plate molds, hydraulic cylinder actuation is required. One mold’s small tie-rod mechanism is coupled with a 63mm diameter hydraulic cylinder, achieving a parting speed of 30-50mm/s, which is smoother than a pure spring actuation and offers adjustable speed. Cushioning during the parting process is crucial. One mold incorporates a 0.5mm thick copper sheet at the end of the small tie-rod stroke to reduce impact acceleration from 10g to 3g, extending the life of the mold plate by two times. Reliable locking after fixed distance is essential. One mold uses a step at the end of the tie-rod to contact the mold plate, with a contact area of ≥20mm², to ensure deformation under parting force.
Maintenance and commissioning of built-in small tie rod mechanisms require meticulous attention to detail to ensure long-term stable operation. Regularly clean impurities from the tie rods and mold platen holes, using compressed air every 5,000 molds. In one mold, iron filings entering the mating clearance caused the tie rods to become stuck. After cleaning, the mating surfaces needed to be reground, increasing costs by 500 yuan. Lubrication should be performed with high-temperature grease (resistant to 150°C) and applied every 10,000 molds, focusing on the contact surfaces between the tie rods and the guide sleeves. Insufficient lubrication in one mold resulted in increased wear on the tie rods; regular lubrication reduced the wear rate by 60%. During commissioning, use a dial indicator to measure the opening distance of each parting surface to ensure consistency with the designed value. In one mold, the actual opening distance was 5mm less than the designed value. Adjusting the tie rod length to meet the required value prevented runner condensate from being removed. For multiple sets of tie rods, ensure consistent length (deviation < 0.1mm). In one 8-cavity mold, a 0.2mm difference in tie rod length resulted in a tilted parting surface and a 2% deviation in part weight. Replacing tie rods of the same length reduced the deviation to 0.5%.
The design of the tie rod mechanism for special scenarios requires flexible adjustment to accommodate varying mold structures and process requirements. The tie rods for three-plate hot runner molds must be positioned away from the hot runner system. In one mold, the tie rods are offset by 30mm, ensuring spacing without affecting the nozzle layout and controlling hot runner temperature fluctuations to ±2°C. For molds requiring multiple partings (such as four-plate molds), multiple sets of tie rods can be used to control the sequence of different parting surfaces. In one four-plate mold, two sets of tie rods (50mm and 100mm in length) were used to first open the runner parting surface (50mm) and then the main parting surface (100mm), achieving 100% accuracy in sequence accuracy. The tie rods for thin mold plates (thickness < 30mm) require reinforcement sleeves. In one 25mm thick mold, a 50mm-long reinforcement sleeve (made of 45# steel) was inserted into the tie rod hole to prevent cracking in the mold plate. The interference fit (H7/r6) between the reinforcement sleeve and the mold plate ensures a secure fit. In addition, the material selection of small tie rods needs to be based on the life of the mold. Ordinary molds use 45# steel (low cost), and long-life molds use 40Cr (tempering treatment, which extends the life by 3 times). A certain automobile mold uses 40Cr tie rods, which still have no obvious wear after 500,000 molds are produced, which is more economical than 45# steel tie rods (200,000 molds).
