I have seen more and more new tooling for Injection Molding designed and built with hot runner manifold systems. Over the years this has proven to be a very effective way to reduce your material consumption and waste during the molding process. However, the hot runner manifold is yet another opportunity for contamination and high scrap rates. Color and material layers will develop leading to carbon build-up if you are not maintaining or shutting down these tools properly. Likewise, I have experienced an increased demand for hot runner system purging due to these issues and have listened to countless stories of failed purging attempts.
When purging your hot runner manifold tool, we can do this by either an open mold purging process or by molding parts with our purging compound. Typically, when molding parts (if possible) we can achieve faster results. After many recent hot runner purging trials there are several things to know but here are two very important factors to having a successful and effective purging experience on your hot runner manifold tools.
The gate size on the tool is designed for your specific material and part geometry which allows the material to flow and the part to be molded according to spec. From the purge compound point of view, we need to flow through your tool without clogging your gates to clean. Some of our purging compounds have minimal gate clearances. It is very important to know your gate sizes on your tool and match one of our purging compounds accordingly.
Once we have selected the purging compound options that will work for your gate sizes, the next important factor is to further narrow down which of these options closely matches your production material melt flow index (MFI or MFR). Once again, we want our purging compound to flow easily through your tool, clean, and then be removed without causing issues. Having a purge compound melt flow that is much higher than your production material will not clean effectively as needed and potentially could cause flashing issues if trying to mold parts during the purging process. An MFI that is much lower than your production material would be very stiff and could be difficult to pass through your manifold potentially causing severe short, residue issues or pressure limiting your machine if trying to mold parts during the purging process.
Having an in-depth conversation on your purging goals and our purging options will allow us to move forward by choosing the best-recommended purge compound grade that is safe for your tool, and meets your needs and expectations. Starting off on the right track allows for an easy and smooth testing period and gets you on to reducing your scrap rate as quickly as possible.
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Hank Moeller is an Injection Molding and Purging Expert with over 20 years of experience in a processing environment.