Inhaltsverzeichnis
Drag-pressure flow
Visualisation of the drag pressure flow
Interpretation of the curves
- Blue curve: The blue curve represents the dimensionless mass flow rate $\pi_\dot m$ and, together with the dimensionless pressure gradient, is a measure of the flow profile and the local conveying effect in the extruder.
- Red curve: The red curve represents the dimensionless pressure gradient $\pi_p$. If the value is 0, there is a pure drag flow. If the value is greater than 0, there is an increase in pressure and the pressurised flow flows against the drag flow (the dimensionless mass flow rate decreases); if the value is less than 0, there is a drop in pressure and the pressurised flow supports the drag flow (the dimensionless mass flow rate increases).
The parameters cannot be calculated in shear and mixing parts.
Analysing the drag pressure flow
A dimensionless pressure gradient greater than 0 means that the pressurised flow flows against the drag flow. Although this reduces the mass throughput, at the same time the flow in the opposite direction ensures good mixing of the melt. Continuous ‘overrunning’ of the screw with a negative dimensionless pressure gradient and thus a high dimensionless throughput (as is usual with grooved barrel extruders) is therefore bad for the mixing effect in the extruder.
The influence of the different screw zones can be analysed based on the progression of the parameters.