Analysis of hot runner leakage
One of the problems that mold manufacturers are worried about is the leakage of hot runners.
Melt leakage in hot runner systems flows into manifold slots. In most cases, leakage in hot runner systems is not due to poor design, but to failure to operate according to design parameters. Typically, leakage occurs at the seal between the hot nozzle and the manifold. According to the design code of the general hot runner, there is a steel edge at the hot nozzle, which ensures that the height of the hot nozzle assembly is less than the actual groove depth on the hot runner plate. This size-to-size difference (often referred to as cold gap) is designed to avoid damaging components due to thermal expansion when the system is at operating temperature. For example, a 60mm thick manifold and a 40mm nozzle assembly (total height of 100mm) typically expand by 0.26mm when the temperature rises to an operating temperature of 230°C. If there is no cold gap, thermal expansion tends to cause damage to the edges of the hot mouth. Therefore, the lack of effective sealing under cooling conditions is the main cause of hot runner leakage.
Generally, in order to guarantee the sealing of the system (hot nozzle and manifold), the system must be heated to operating temperature so that the force it generates is sufficient to counteract the injection pressure, thus preventing the injection pressure from pushing the two parts apart. In general, inexperienced operators often do not wait for the system to reach operating temperature, and worse, they may even forget to turn on the heating system. In this way, the pressure generated on the surface of a hot runner with a cold gap cannot prevent leakage until its operating temperature has been reached.
In addition, leakage can occur in the case of overheating.Due to the poor
adaptability of the hot nozzle with a steel edge to thermal expansion, when the system is overheated, once it is reduced to the operating temperature, the sealing pressure generated by it cannot prevent leakage due to the influence of steel deformation. In this case, in addition to the damage to the system caused by leakage, it will also cause irreversible damage to the hot nozzle due to excessive pressure, and the hot nozzle needs to be replaced. Since the preload of the hot nozzle and manifold is very important, the dimensions and tolerances provided by the hot runner supplier must be strictly adhered to to effectively prevent leakage in the system.
Therefore, for mold manufacturers, the height of all hot nozzle components and the size of the slot should be carefully checked according to the drawing requirements of the hot runner supplier, and if there is any problem, it should also communicate with the hot runner supplier in time.