Documenting Open-Loop Processes…

I received this follow-up question from one of our previous blog entries…

I don’t understand why it’s best to be full at the end
of 1st stage on an open loop machine. Our open loop machines have 1st stage and
then total injection forward time which is basically filling and packing time
together. We are still decoupled at the end of 1st stage (filling). If you can
be decoupled at the end of first stage then I think 1 sheet with machine
independent values would work.

note: This question is a response from the following blog: Open-Loop and Closed-Loop Process Sheets…

My Response
Generally speaking, the pressure-limited nature of an open-loop machine will result in an inconsistent fill during 1st stage. There are technologies which can improve the consistency of fill, such as highly accurate proximity sensors and improved flow control valves, but most open-loop machines cannot consistently maintain a short shot around 95% full while using a lower fill time that was established and optimized on a closed-loop machine. For these reasons, most open-loop molders opt for having the mold completely full during 1st stage to help ensure long term consistency. For this reason, it was recommended to document the process differently than a closed-loop process.

If your machine can maintain this consistency while using a fill time comparable to a closed-loop machine, then such process documentation can be very useful for both machine types… but this level of accuracy and consistency in an open-loop machine is typically rare.
Additional Thoughts
In general, the filling behavior of open-loop and closed loop molding machines are such that a different machine-independent process sheet is typically preferred.

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