The pressure gradient in the molten polymer is the pressure drop per unit of length along the flow path. The pressure drop from one location to another is the force that pushes the molten polymer to flow during filling. Polymer always moves from higher pressure to lower pressure, akin to water flowing from higher elevations to lower elevations.

Filling with a more uniform pressure gradient is desirable. Non-uniform pressure gradients often indicate problems such as flow hesitation, over-packing (resulting in flash) and under-packing (resulting in excessive shrinkage).