Machine Variation vs. Process Variation

I received the following question in an email yesterday.Since it was lengthy, I have done my best to reduce it to a more concise inquiry…

We have an LCP process where the tolerances for cushion, charge time, fill time, and pressure are very narrow. We are maintaining a consistent cushion with only 0.2mm deviation, yet we are noticing short shots consistently. The detection capabilities of the machine do not seem to be adequate for detecting short shots, is there anything you suggest?
My Response
The biggest problem I see is that you have set you tolerances for both the inputs and outputs which relate directly to each other. Although LCPs are highly crystalline and have a very sharp melting point, their ability to flow into the mold is highly dependent on the rate of injection. As a result, any shift in molecular weight, molecular weight distribution, or additives during lot changes will result in a non-compliance. Eventually, you need to re-evaluate your tolerances.
Ultimately, it appears your issues are not related to variations in your machine, but a lack of flexibility in your process resulting in variation.
Additional Thoughts
Such a process often requires a short shot with a high packing pressure to complete mold filling.  This will ensure that there is always enough pressure available to fill the part completely.
I have a few questions to address this week, but I intend to post a blog about high-pressure molding next week. This should help you better understand how to establish such a process.

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