You will notice that the flow rate shown in the analysis log does not typically reach the user-specified flow rate. Material compressibility accounts for the difference in flow rate input and output.
The compressibility of the material in the barrel can be significant. The polymer density in the barrel increases as the ram moves forward and the barrel pressure increases. The flow rate based on the ram movement is larger than the flow rate out of the nozzle, and the polymer in the barrel has to be taken into consideration if the predictions are to be used for machine control.
During an analysis, the barrel volume is added to the model in the calculation, with the melt in the barrel assumed to be isothermal at melt temperature. As the ram advances, the melt is transferred from the barrel to the sprue/runner/cavity, and the melt in the barrel is reduced accordingly.