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Injection Molding Cushion Variation Up and Down

I have seen the problem of an incremental change in cushion each cycle for about 20 shots and then begin to decrease and return to nominal once before on a different make of injection molding machine. The shot size was minimal, 20% of the max. It was an amorphous resin, (I believe PC/ABS Blend). As the cushion would change we would go from making good parts to parts that were over packed and showed ejector marks. The pressure at cutoff changed with the cushion, as you would expect.
We plotted all of the TC outputs by cycle, barrel zones, feed throat, thermolator, and thermocouples we attached to the tool. All of the zones appeared to be working normally. We spent a lot of time watching the feedback from the transducer tracking the screw position. We even hooked it up to a PC to monitor.

What it turned out to be was harmonics...
Raising the feed throat temperature would change the cycle, (cushion variation up and the down), but the cycle was still there. We went through each zone on the barrel, one by one, each would have a similar effect. Finally we put a reverse profile on the barrel and the cycling went away.

One month later, when the injection molding machine manufacturer told us we were crazy, we put the same mold back into the injection machine, different lot of material, and we encountered the same cycling, the cushion would creep up, then the cushion would creep down. Nothing on the injection molding machine controls would change it. He went back to the engineers that designed the screw and barrel assembly, the answer that came back was harmonics. They had seen it before, but rarely.

This made sense to me as everything from the heaters turning on and off to the cycle itself presented a waveform. Cycle everything just right and you get a spike.

Since then, I have noted cycling on occasion, but never again anything like that where it affected quality. Now I have heard of another.

Don't forget that even virgin material from the same batch of material has a viscosity standard deviation of +/- 10%. Now this should not create an abrupt change all at once, but over time you will find that process adjustment will need to be made due to this variation.

Process monitoring (especially pressure sensors or thermocouples in the mold cavity) can verify what is happening with the material and the injection molding machine. By comparing hydraulic pressure, cavity pressure (pack and hold), relative to injection screw position, temperature and other factors allow you to create a relative viscosity curve. With all of this information, you can determine what the issues, could be:

-Viscosity shifts (but probably not due to the abrupt nature of the problem)
-worn screw/barrel
-blocked screw tip or non-return valve (non-melted pellets, poor tip design, etc.)
-plugged hot runner
-etc.

The more information that you have to look at, the better your troubleshooting will be. RJG is a great way to take a look into the mold and see what is truly happening relative to injection, pack, hold and recovery (your injection process). Without the look into the mold, it really becomes more of a game of trial and error.
Cushion Up & Down in injection molding , I am unable to solve ,
- - - -> by: VIJAY
What would cause high cushion and what can be done as a solution?
- - - -> by: Antonio
What causes high cushion and what are some solutions?
- - - -> by: Antonio
Cushion Up & Down in injection molding , I am unable to solve ,
- - - -> by: Arnel pagsanjan

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