I was asked this great question the other day...
eMailer
I heard something about machine response (reaction time during switching from fill to hold).
How does this response influence my process?
My Response
There are many aspects which must be considered when reviewing the transfer from 1st to 2nd stage. Most of these can be seen on your machine's pressure curve.
Under-Damping:
- As the injection pressure graph transitions from first to second stage, the curve should not dip far below the specified packing pressure. If this occurs, the molding machine is ‘under damped’ and is not capable of transferring from velocity control to pressure control without losing significant pressure. Under damping often results in sinks and short shots due to insufficient packing pressure. This only occurs when molding with hydraulic machines, and indicates a faulty hydraulic valve.
Over Damping:
- If the pressure curve transfers between stages gradually, the molding machine is ‘over damped’. This indicates that additional material is being forced into the mold at a pressure higher than the set point. An ‘over damped’ molding machine will often result in flash or overpacking due to the fact that the hydraulic valve can not transition into pressure control adequately. The controller may need to reprogrammed or a hydraulic valve may need replacement to resolve ‘over damping’.
Overshoot:
- Similar to over-damping, many machines have heavy injection units which are difficult to slow down when the machine transfers from 1st to 2nd stage. This can often cause inconsistent fill. Although most electric molding machines have accurate servo motor controls which prevent this, many hydraulic molding machines are very susceptible to this situation. On these machines, it is a great idea to use one or two transitional speeds to prevent this condition.
Fluttering:
- Fluttering or erratic changes within the injection pressure curve during transfer indicate faulty hydraulic valves or electric servo motors. These fluctuations, as with the other conditions above, affect shot-to-shot consistency and should be improved to the best of the machine's capabilities.
Additional Thoughts
You should always evaluate the capabilities of your molding machines with rigorous testing. This is another reason why machine-independent process documentation is so important... process outputs tend to ignore the idiosyncratic behavior of any individual machine.
-Andy
Andy,
Which of your courses is best if I want evaluate my injection machine?
MJ
MJ,
Our Intelligent Molder series covers nine machine, mold, and process tests:
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We have a special package designed for high level processors which includes the Intelligent Molder Series for only $50 more. This can be found at:
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-Andy
What would cause what looks like an under-damped curve, but on an all electric machine? My injection pressure is 8900ppsi and pack pressure is 8750ppsi. At transfer the pressure dips to 7700 and then comes back up to the set point of 8750. It takes 0.26 seconds to reach the set point. Is that acceptable for such a small pressure change? The max pack velocity is set very high and not limiting the screw speed... and the part is 95% full on fill stage so I'm not transferring too soon. I suspected it had something to do with the hot runner system, but tried a cold runner and had a similar dip. The odd thing is the larger difference I have between peak injection pressure and pack pressure, the smalled the dip.
Good question. I will address this in a different blog post later this week.
-Andy
Hi tnk1311.
Which type of decoupled molding process are you running? DII or DIII?
Milan
Hello Milan,
I exchanged a few emails with this person and DII-style process is being rung. This person tried and checked a few things and it turned out his machine limits the speed at which it packs. There is a setting for this, but making an upward change has no effect on the process. The next step this person will make s to check with the machine manufacturer on how to correct this obvious flaw in the machine performance.
-Andy
Hi tnk1311.
I have experienced this before and found some manufacturers use first pack as a ramp timer to transition from 1st stage fill to second stage pack creating a desired response ramp in coordination with the pressure and velocity at which packing occurs. This is generally with machines that have multiple pack and hold pressure settings.If you have a .25sec pack on your timer this could cause the change.
-Dave