I. Primary Cause of Failure
The 316 stainless steel manifold expands by 0.33mm for every 100mm of surface heated to 200℃ after the hot runner has heated up. Template and mounting hole were rigidly locked together as a result of inadequate expansion clearance that was not anticipated.
The hot runner and the upper template were not separated by an axial floating space of ≥0.5mm during the installation. The displacement allowance is nonexistent following thermal expansion, resulting in the direct jamming of the component into the mounting cavity.
The configuration of the positioning ring tolerance is unreasonable, as it exceeds 0.02mm in coaxiality deviation. Ultimately, local blockage results in an overall lock-up during thermal expansion.
II. Emergency Procedures for On-Site Situations
Immediately cease the heating of the equipment, activate the cooling system, and reduce the temperature of the hot runner to room temperature, thereby enabling the metal to contract naturally. Attempt to manually loosen the locked components.
To prevent damage to the component, use a copper hammer with specialised tools to slowly tap along the opposite direction of thermal expansion if it still cannot be removed after cooling. Avoid direct impact on the heated nozzle.
Before attempting to loosen the part, apply a high-temperature-resistant molybdenum-based grease to the locally blocked areas and allow it to penetrate for 2 hours. Avoid employing brute force to pry, as this may result in the mould core being cracked.
If the blockage is severe, heat the mould plate locally to 120°C. Before removing the part, establish a small gap by utilising the expansion difference between the mould plate and the heated runner.
III. Optimisation and Prevention Strategies for the Long Term
Calculate the reserved margin in strict accordance with the thermal expansion formula ΔL = L × α × ΔT, allowing for 1.5-2 times the expansion amount. Allow a gap of 0.5-0.7mm for a 100mm manifold.
Allow for an axial floating space of ≥0.5mm by using floating positioning rings to counteract thermal expansion displacement; the hot runner plate should not be rigidly locked.
In order to prevent jamming caused by localised off-center loading, the positioning ring tolerance must rigorously adhere to the H7/js6 standard, with coaxiality controlled within ≤0.02mm.
The thermal expansion allowance for each mating surface should be clearly indicated on the machining drawings, such as the "0.7mm gap between hot runner plate and upper mould plate," to prevent insufficient clearance issues during the machining stage. This solution can promptly address on-site locking failures and prevent the recurrence of similar issues from the design stage, thereby preventing severe losses such as mould core cracking and repair costs exceeding 50,000 yuan as a result of mounting hole locking.
