Understanding Back-Flow Within the Barrel…

I received an interesting follow-up question regarding one of my earlier posts…

One of our techs stated that there is back-flow in the barrel. Is this true? How does this happen?

My Response
Yes, back flow does occur within the barrel in three ways:
1) Material will flow back over the check ring – During injection, all check rings will leak a little, causing some back-flow along the screw.
2) Material will flow back down the screw – During injection the material that leaks around the check ring will push polymer back down the screw channel.
3) Material will flow over the screw flights – During screw recovery, polymer will pass over the flights of the screw. This mixing will increase with an increase of back pressure.
Additional Thoughts
All this back-flow exists in virtually all reciprocating-screw molding machines. If you are processing a heat sensitive material… you need to do your best to minimize these affects by maximizing the efficiency of your screw recovery, and monitor the performance of your check ring.

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