When a polymer flows through a large cross-section to a smaller one, it will extend longitudinally. This will result in a pressure drop that will be dependent upon the extensional viscosity of the particular material.

In practical injection moulding situations, this pressure drop due to extensional effects is often seen when a material flows from a large runner into a very small gate. The change in cross-section is sudden and severe, resulting in a large pressure drop across the gate.