The Pressure at end of fill result, which is produced by a Fill analysis, shows the pressure distribution in the cavity at the instant when the cavity is completely filled with polymer.
At the beginning of filling, the pressure is zero, or 1 atm in the absolute pressure scale, throughout the mold. The pressure at a specific location starts to increase only after the melt front reaches that location. The pressure continues to increase as the melt front moves past, due to the increasing flow length between this specific location and the melt front.
The pressure difference from one location to another is the force that pushes the polymer melt to flow during filling. The pressure gradient is the pressure difference divided by the distance between two locations.
Like water flowing from higher elevations to lower elevations, polymer always moves in the direction of the negative pressure gradient, from higher pressure to lower pressure; therefore, the maximum pressure occurs at the polymer injection locations and, the minimum pressure occurs at the melt front during the filling stage, as shown in the following diagram.
The magnitude of the pressure or pressure gradient depends on the resistance of the polymer in the mold. This is because polymer with high viscosity requires more pressure to fill the cavity. Restricted areas in the mold, such as thin sections, small runners, and long flow lengths, also require a larger pressure gradient, and, therefore, a higher pressure to fill.
During the filling stage, large variations in the pressure distribution, which are indicated by closely-spaced contours, should be avoided. Pressure should be zero at the extremities of each flow path at the end of filling.