How to prevent hot runner leakage

How to prevent hot runner leakage

a common concern of mold manufacturers is that the molten material in the hot runner system will flow into the splitter plate slot after leakage. Generally, there are many reasons for material leakage, but there are ways to avoid each reason. As the externally heated hot runner system represents the main application type of hot runner, this is discussed as an example

what causes the leakage of hot runner

in most cases, the leakage of hot runner system is not caused by poor design, but because it is not operated according to the design parameters. Usually, leakage occurs at the seal between the hot nozzle and the splitter plate. According to the design specifications of general hot runner, there is a rigid edge at the hot nozzle, which can ensure that the height of the hot nozzle assembly is less than the actual groove depth on the hot runner plate. The reason why this dimension difference (usually called cold gap) is designed is to avoid damaging components due to thermal expansion when the system is at operating temperature. For example, for a 60mm thick splitter plate and a 40mm hot nozzle assembly (the total height is 100mm), when the temperature rises to the operating temperature of 230 ℃, it usually expands by 0.26mm. If there is no cold gap, thermal expansion will often cause damage to the edge of the hot nozzle. Therefore, the lack of effective sealing under cooling conditions is the main cause of material leakage in the hot runner (as shown in Figure 1)

Figure 1. The leakage of hot runner is the main cause of mold damage and shutdown.

generally, in order to ensure the sealing of the system (hot nozzle and splitter plate), the system must be heated to the operating temperature so that the force generated by it is enough to offset the injection pressure, so as to prevent the injection pressure from jacking the two parts. Generally, inexperienced operators will not wait for the system to reach the operating temperature, and worse, they may even forget to turn on the heating system. In this way, the pressure generated on the surface of the hot runner with cold gap cannot prevent material leakage before it reaches its operating temperature

in addition, material leakage may also occur in the case of excessive heating. Due to the poor adaptability of the hot nozzle at the edge of the strip steel 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 system damage caused by material leakage, the hot nozzle will also be irreparably damaged due to excessive pressure, so the hot nozzle needs to be replaced

since the preload of the hot nozzle and splitter plate is very important, the size and tolerance requirements provided by the hot runner supplier must be strictly followed in order to effectively prevent material leakage of the system. Therefore, for the mold manufacturer, the height of all hot nozzle components and the size of the groove should be carefully checked according to the drawing requirements of the hot runner supplier. If there is a problem, it should also draw on the strengths of others to communicate with the hot runner supplier and 1. A little perfluoropropyl perfluoroethylene ether and poly (4 fluoroethylene) copolymer

hot nozzle design options to prevent material leakage

Figure 2 shows the sealing of the splitter plate and hot nozzle designed for various hot nozzles. Figure 2a is a design with rigid edges and no leak proof function. This design has no active sealing and overheating protection under cooling conditions, and its operating temperature range is ± 27 ℃. The back of the hot nozzle in Figure 2b is fixed on the shunt plate. If the hot nozzle is fixed abnormally, the high-temperature bolt on the shunt plate can prevent leakage under cooling conditions. Because the rigid edge needs a certain expansion space at room temperature, the system still needs a cold gap. Although this method can prevent the leakage of molten material from the hot nozzle to the splitter plate, it cannot prevent the thermal expansion of components under overheated conditions, and its operating temperature range is ± 27 ℃. Figure 2C is an economical and effective way to prevent material leakage between the hot nozzle and the splitter plate, which can be applied to the small cavity number system. Since the hot nozzle fixed on the splitter plate with bolts can move together with the splitter plate, this design requires the minimum length of the hot nozzle and limits the mold cavity spacing. The hot nozzle edge in Figure 2D adopts elastic rather than rigid design. The elastic edge can provide preload under cooling conditions and prevent the system from being damaged. If it is overheated accidentally, it can also absorb thermal expansion, and its operating temperature range can be expanded to ± 110 ℃

Fig. 2 sealing condition of flow divider and hot nozzle designed for various hot nozzles

how to find material leakage

generally, it is difficult for people to find out whether there is material leakage in the hot runner, because it can't be seen from the outside of the mold. However, if you observe carefully, you can still find the material leakage through some signs. Generally, when setting a new hot runner, the operator is very clear about how many times of injection can be contained in the runner. For example, if the runner can contain three times of injection (each injection is the capacity of the runner/mold cavity), then after three injections, there should be molten material in the mold cavity. If not, it is likely that the molten material leaked into the splitter plate groove. When enabling the hot runner, make sure that the injection volume is set correctly on the machine

during operation, another sign of material leakage is that part of the mold cavity or the product is not fully filled. This is because part of the molten material injected leaked into the distributor slot, resulting in insufficient injection molding of the product. On the control interface of the injection molding machine, this situation is displayed as a sudden change in process parameters

if the operator suspects that there is material leakage, the injection molding machine should be shut down immediately and checked after the system cools down. After cleaning the system and detecting the cause of leakage, all components should also be checked. This is because overheated temperature or cleaning process may cause damage to components. If a component is found to be damaged, it should be replaced immediately

finally, it is worth mentioning that the operation advantage brought by high-quality hot runner mold is far greater than the trouble caused by material leakage that may occur in the operation process. Therefore, when purchasing hot runner, customers should fully understand the design and performance of various hot runner to ensure the machining accuracy of important parameters, and provide necessary technical training for all relevant personnel. (end select sine wave for experiment)