How to Prevent Hot Runner Leaks
One of the most common concerns expressed by molders about hot runner molds is the threat of a leak of molten plastic causing the manifold pocket to fill.
One of the most common concerns expressed by molders about hot runner molds is the threat of a leak of molten plastic causing the manifold pocket to fill. There are several reasons for leaks and all can be prevented. We will examine only externally heated hot runner systems as they represent the majority of the hot runner systems in use.
What Causes Hot Runner Leaks?
Most systems don't leak because of poor design, but instead because they are operated outside of their designed operating window. The most common location for leakage is at the seal-off between the nozzle and the manifold. The design criteria for a typical hot runner system in which the nozzles have a solid skirt design, requires the height of the component stack to be less than the actual pocket depth in the hot runner plates. The difference between the dimensions - known commonly as the cold clearance - is necessary to protect the components from collapsing due to thermal expansion when the system is at operating temperature.
For example: A 60-mm thick manifold with a components stack height of 40 mm (total 100 mm) will grow by 0.26 mm from room temperature to operating temperature (450xF). If there were no cold clearance the thermal expansion would cause the skirt of the nozzle to collapse. The fact that there is no positive seal in the cold condition is the main reason for hot runner leakage. In order for the system to seal (nozzle to manifold) the operating temperature must be reached in order to create enough force (20,000 pounds, for example) to resist the injection forces that want to push the two components apart. Inexperienced operators might not wait to reach full operating temperature or, in the worst case, may even forget to turn the system's heat on. A hot runner system designed with a cold clearance that has not reached full operating temperature will not have enough surface pressure to prevent leakage.
Leakage also can occur after the hot runner is accidentally overheated. Again, solid skirt design nozzles have little flexibility for thermal expansion. If we use the previous example with the manifold nozzle stack height of 100 mm and the system is accidentally heated to 800°F, the components will grow an additional 0.24 mm. The force then is more than 40,000 pounds, which is above the yield strength of the skirt, ultimately causing it to deform permanently. Once an overheated system returns to its operating temperature (450xF) it will not develop the necessary seal-off pressure to prevent leakage. In addition to the damage caused by leakage of the molten plastic, all of the nozzles will need to be replaced due to the permanent damage caused by over-compression.
Since the correct preload on nozzles and the manifold is so critical it is understandable that the dimensions and tolerances provided by the hot runner supplier must be strictly adhered to in order to avoid leakage of the system. Moldmakers should carefully inspect all stack heights and pocket dimensions against the hot runner manufacturer's print. Any questions should be discussed immediately with the hot runner manufacturer.
Nozzle Design Options to Prevent Leakage
There are various nozzle design options to seal between the manifold and the nozzle - (1) the solid skirt design with no active leakage prevention feature; (2) a nozzle that is bolted from the back to the manifold. High temperature screws are used to bolt the nozzle to the manifold and provide a positive seal in cold conditions. However, the system still requires a cold clearance since the solid skirt of the nozzle requires room to expand at operating temperature. While this approach provides a positive seal from nozzle to manifold it does not protect the components from thermal expansion due to overheating; (3) a nozzle, which is screwed into the manifold, moves with the expanding manifold. There are restrictions for minimum nozzle length and large pitch spacing with this design. This option is a simple and cost-effective way to provide a positive seal between nozzle and manifold for low cavitation systems; and (4) a nozzle with a spring design instead of a solid skirt. The spring provides preload in the cold condition and also prevents the system from damage if accidentally over-heated, by absorbing any thermal expansion. This allows a wide operating window of up to +/-200Fx.
How to Detect a Leak
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.
There are three key areas of concern regarding leakage prevention: design, manufacturing tolerances and training. While there are options on the nozzle design, there are no options on manufacturing tolerances and proper hot runner training for the workforce.
Choose a hot runner system that provides some method of leakage prevention. Hot runner systems having nozzles with a solid skirt and a significant amount of cold clearance are the ones most likely to leak.
The moldmaker must ensure that the critical dimensions and tolerances in the plates are maintained to provide enough seal-off force once the system is at operating temperature.
Training for all shifts on proper assembly, start-ups and maintenance of hot runner systems is necessary.
While both hot runner and cold runner systems have their benefits, for design flexibility as well as reduced cycle times and costs, hot runner systems will probably be the better choice.
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