Extended rheological software
An extruder is not a very accurate instrument to measure rheological data of the melt that is being processed. Yet, various suppliers of micro compounders claim to be able to give reliable values for the melt viscosity and the corresponding shear rate and shear stress by rheological measurements in the backflow channel and analyzing these with proprietary software.
However, rheological experts know that these estimated values can be dozens % off the true values, as these estimates usually do not take into account the Bagley correction (for entrance and exit effects in a capillary) and/or the Rabinowitsch correction (for non-Newtonian fluids). In addition the motor torque is assumed to be equal to the screw torque, thereby neglecting the very significant frictional losses in the gear box. Finally, many of these measurements are performed outside the main mixing volume under different shear conditions, not relevant for the extrusion rheology.
In close cooperation with prof. dr. ir. H.E.H. Meijer, a world expert in extruders, extrusion, upscaling and rheology at Eindhoven University of Technology, TUE (The Netherlands), Xplore recently derived approximate, but very reliable rheological relationships based on modeling the rheological process in the conical, twin-screw extruder. Thus our approach is based on the relevant extrusion conditions which is essential for correct upscaling. This analysis relates the measured vertical force in our micro compounder/extruder with the actual melt torque, the estimated shear viscosity and the corresponding estimated average shear rate and shear stress.
Possible errors, as mentioned above, are circumvented by this new and innovative approach. These sophisticated, proprietary and very useful relationships are now programmed in a new, dedicated software package that is available for Xplore customers of the MC 5, MC 15 and PME 5.