Causes for Screw Bounceback…

I received this interesting question yesterday…

Normally, I end filling with a transfer position and end holding pressure with time. In a specific process, using PP and a consistent filling time of 2.80 sec. Using the gate seat study, it was determined that the hold time should be 8.0 seconds.

Even though the transfer position is set to 6mm, the final cushion is 8mm. When I raise the holding pressure to 20kgf, the cushion drops to 4mm which confirms that holding pressure is working.

My questions are:
Does the low pressure cause the screw retractions during holding pressure?
Why dues the screw not pass beyond the transfer position during holding?
Is there melt pressure present in front of the screw, pushing the screw backward?

My Response
Although there are a lot of questions, I think it can be handled from a couple different angles.
1) Pressure is created by resistance to flow – During injection, a significant amount of pressure is being created in front of the screw. Since polymers are compressible, there is a counteracting pressure pushing the screw backward. If your hold pressure is less than the polymer pressure in front of the screw, then the screw will move backward.
2) Excessive mold filling during first stage – Ideally, your mold should not be completely full during first stage injection. If you completely fill the mold during first stage, then there is a spike in injection pressure as the polymer begins to pack out the mold cavity. When this spike in pressure is followed by a lower packing pressure, there is often a backflow of material out of the mold cavity… resulting in screw bounceback. Overall, this is not a good approach to processing because changes such as material viscosity or melt temperature will cause significant variation in your process. 
Additional Thoughts
Your best approach is to:
1) Increase your transfer position to obtain a short shot during first stage.
2) Increase your hold pressure until you obtain appropriate part quality.
3) Perform a gate seal study to ensure the 8 sec. hold time is appropriate for the adjusted hold pressure.

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