I received this question from a frequent blog contributor, and believe it makes a good discussion on decoupled processes…
On some of our machines, we can adjust the hold velocity. How should this feature be used when molding a decoupled II process.
Before I respond, I want to relate the general attributes of this process:
- A process that uses one injection speed to fill – whenever possible
- The mold fills 95 to 98 percent full during first stage
- All cavities are short shot during first stage
- First stage fill is velocity-controlled and not pressure limited
- Second stage pack is pressure-controller and not velocity limited
- Process uses only 20 to 80 percent of the machine’s available shot size
- The final cushion is approximately 10 percent of the overall shot size
In essence, the purpose of a DII process is to ‘decouple’, or separate, velocity controlled fill from pressure controlled pack. To do this, you should either turn the velocity control off so the pressure control takes prominence.
Always make sure you understand the specific controls on your machines. In some cases, the 2nd stage velocity is actually a limit rather than an actual setting. In this case, you should set the speed near the maximum so that 2nd stage pack does not become velocity-limited.
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And when I should adjust hold pressure velocity?
For what is good this feature? For DIII molding?
I have not encountered a situation where adjusting the hold pressure was necessary… nor can I think of a situation where it may be helpful.
What in case, when I can’t turn off hold pressure velocity. How big value I should adjust?
If you set the value at the maximum… then it will be pressure limited. This is typically the best way.
As Always… You are Welcome.