Occurs when flow fronts from two directions meet and then pause momentarily, long enough for a frozen layer to develop. The polymer in one of the flows then reverses direction and flows back between the outer frozen layers.
When the flow reverses, shear heating causes the frozen layer to partly re-melt. This flow reversal gives poor quality surface appearance and structure.
To avoid underflow, balance all flows so that their flow fronts meet at the end of fill.
