Flow rate result, shown in the analysis log

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.

Material compressibility affects flow rate

For Fill analyses, the effects of material compressibility in the barrel are calculated.

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.

User input and the compressibility calculation

The compressibility calculation does not depend on user-specified barrel dimensions if the following settings are used:

(Fill analysis settings > Filling control > Ram Speed Profile > % shot volume vs % flow rate), or

(Fill analysis settings > Filling control > Ram Speed Profile > % stroke vs % ram speed)

The compressibility calculation does take into account the user-specified shot size and machine screw diameter if any other ram speed profile setting was used.

If the shot size and machine screw diameter were not specified, then an automatic stroke volume of 120% is used. This value is considered sufficient to fill the part and takes the barrel effect into account.

To achieve a 100% flow rate with virtually zero compressibility, set the following pvT Properties in the Thermoplastics material dialog.