Plastics Design Principles: Avoid Underflow

By Jay Shoemaker

Related links: Moldflow Plastics Design Guide | Moldflow Communicator™ | Moldflow Design Guide: A Resource for Plastics Engineers

Underflow occurs when a flow front changes direction during filling. In the example in Figure 10: Underflow, underflow occurs because the flow front is not balanced due to the gate location. The contour lines and the velocity arrows should be perpendicular as they are in the upper left corner of the figure. In the lower right side of the figure they are parallel, indicating a significant shift in the flow direction.

The problem with underflow is its effect on orientation. The initial filling direction for an area on the part is represented by the fill time contours. The flow direction is perpendicular to the contour line. The molecules are initially oriented in the direction of that flow. If, later on during the filling phase, the flow direction changes, the molecules closer to the center of the flow channel are oriented in the new flow direction. Molecules want to generally shrink more in the direction of orientation, and so if there is underflow there is significant internal stress in the location of the underflow.

Figure 10: Underflow

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