Injection Unit Inertia…

I was in some online discussions lately and this question came as a follow-up via email…

I have an older {brand name} molding machine and it has a very large & heavy injection unit. What is the best way to compensate for overshoot?
My Response
Once the machine transfers, the ideal result would be for the screw to go directly into packing. Unfortunately, the inertia of the injection unit can cause additional material to be injected during 1st stage. In most cases, you can just transfer from 1st stage to 2nd stage early to compensate. Unfortunately, the inertia creates force applied to the screw resulting in a pressure-controlled situation. Although the slowdown is typically consistent, some high-precision applications need more precise control near the end of fill.
If more control is required, and you cannot change machines, then you can create a slowdown profile at the end of fill. In this situation, the profile should closely match the natural slowdown of the screw using a couple steps. The benefit to this approach is that you have a more consistent slowdown than just allowing the screw to slow down.
Additional Thoughts
This correction should only be used when the natural slowdown of the screw provides inadequate consistency during transfer.

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