Detecting a hot runner leak can often be difficult, as it is not always readily visible from the outside of the mold. In the worst case, plastic can be seen coming out through the electrical connectors and other openings in the system.

By that time the hot runner is most likely completely filled with plastic. However, prior to such drastic failure there are signs that an attentive operator can watch out for.

A first indication that there is something wrong with the system is when plastic is injected, but no plastic reaches the cavities. During start-up of a brand new hot runner the operator should know how many shots of material are contained within the hot runner system. For example, if the hot runner contains three shots (volume of the melt channels/cavity volume) there should be material appearing in the cavities after three shots, otherwise the resin most likely disappeared into the manifold pocket through a leak. Always be sure to verify that the shot volume set on the machine is correct when starting up a system.

During operation another indication that a leak exists is a sudden loss of random cavities or parts that are not packed properly. Since a portion of the shot supplied by the machine leaks into the manifold pocket, the parts cannot be properly filled. On the machine controls, this will show up as a loss of process cushion.

With a massive leak the heaters will draw more heat since heat loss in the system is increased due to a lack of thermal insulation. Because an insulating air gap between plates and hot runner components is filled with plastic, more heat is required – increasing the power consumption of the system.

If the operator suspects a leak, the hot runner should be shut down and inspected after the system has cooled. The first step is to remove the cavity plate to inspect the nozzle seal-offs. Some systems allow you to do this in the machine minimizing downtime.

In addition to the manifold and nozzle interface, the system also may have leaked at the nozzle-to-gate interface. However, most leaks occur between nozzle and manifold if the system is not preloaded. After the front end of the nozzles have been inspected to ensure that no leakage has occurred (i.e. no plastic can be seen), the hot runner should be taken out of the machine and the backing plate removed. If there is only a small leak around one drop, the components of that drop should be removed and thoroughly inspected. In addition to the nozzle inspection, the bore depths need to be measured.

For systems in which there has been a significant amount of leakage, it may be advantageous to put the backing plate back on and run the system again with a different color for a few shots (20 to 50); this will determine the source of the leak. Fewer shots will be required to identify the source for bigger leaks. The location of the last color will clearly indicate where leakage initially occurred in the system.

Depending on how much leakage there has been, the system can either be cleaned by hand or sent to a polymer cleaning process. With the polymer cleaning process the complete hot runner – after removal of the nozzle heater and thermocouple – is put into a fluidized bed and heated up to 850xF, vaporizing all of the remaining resin in the hot runner.

Once a system has been cleaned and the cause of the leakage detected, it is recommended that you check the dimensions of all of the components. Damage could have occurred due to the overheat situation or during the cleaning process. In either case the components will need to be replaced.

Hopefully, you will never experience a hot runner leak and have read this only as a preventive measure. The advantages of quality hot runner molds far outweigh any concerns you should have about their operation and any possible leakage problems. The best advice is to take time to examine several hot runner designs for performance before making your purchasing decision. Be sure to follow all of the critical dimension requirements in manufacturing and to provide proper training for everyone who might be involved with the system.