Occurs when flow fronts from two directions meet, 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 the frozen layer partly remelts due to shear heating. This flow reversal gives poor part quality, both from a surface appearance and structural viewpoint.
To avoid underflow, balance all flows so that their flow fronts meet at the end of fill.
