Compared with ordinary molds, the pouring system in hot runner molds is always in a high temperature state during use, and plastic melt is prone to leakage at the connection of parts in the hot runner system under high temperature and pressure. Melt leakage not only affects the quality of plastic parts, but also seriously damages the mold, leading to inability to produce. There are two main areas where leakage occurs in the hot runner mold: the end face of the runner on the hot runner plate, and the junction surface between the hot runner plate and the nozzle (including the gate nozzle and the main runner nozzle). There are various reasons for melt leakage, which can be summarized into three aspects: improper operation, improper assembly process, and unreasonable sealing design. This article introduces the causes and prevention of melt leakage one by one.

 Melt leakage caused by process factors

Improper operation process is one of the main reasons for melt leakage during mold production. In order to compensate for the thermal expansion of components in the hot runner system, there is often a certain cold gap between the parts during the design and assembly of the mold. Only at the specified operating temperature can the thermal expansion of the parts completely eliminate cold gaps and achieve sealing and leak prevention. Melt leakage caused by improper operation mainly occurs in the following situations:

(1) Inject in advance when the system does not reach the specified operating temperature. As shown, after the system is heated, the support ring 6, hot runner plate 5, and gate nozzle 4 undergo thermal expansion in the axial direction. The support ring is pressed against the fixed mold installation plate 3 and generates a certain amount of thermal pressure between the hot runner plate and the nozzle. If injection is carried out without reaching the specified temperature, the thermal pressure generated by thermal expansion will not be sufficient to offset the melt pressure, causing separation between nozzle 4 and hot runner plate 5 and resulting in melt leakage.

(2) Melt leakage caused by system heating temperature exceeding operating temperature. In this case, excessive thermal expansion generates significant thermal pressure, which can cause deformation of system components and result in melt leakage. On the other hand, when the temperature of the pouring system drops to the operating temperature, melt leakage may also occur due to the poor adaptability of the hot nozzle with rigid edges to thermal expansion.

(3) Melt leakage caused by improper system heating process or uneven temperature control. During the heating process, if the nozzle heating rate is higher than the heating rate of the hot runner plate, the axial thermal expansion of the system components will limit the lateral thermal expansion of the hot runner plate, causing deformation of the hot runner plate and resulting in melt leakage. Uneven temperature distribution of parts in the pouring system can cause uneven expansion of the parts, as well as distortion and deformation of the parts, resulting in melt leakage.

In summary, following the correct steps and process conditions is a prerequisite for avoiding melt leakage. The general hot runner mold can be operated according to the following steps:

(1) Heat the mold to the set temperature. Especially for large molds, they are heated before injection and then cooled during injection.

(2) Heat the injection machine barrel to the set temperature.

(3) Heat the hot runner system to the set temperature. It is generally divided into two steps:

Firstly, it is a soft start to eliminate moisture in the heater.

Secondly, heat the system to the set temperature at full load. First, heat the nozzle temperature to 2/3 of the temperature of the hot runner plate. Wait until the temperature of the hot runner plate reaches the design temperature before heating the nozzle temperature to the set temperature.