Measurement of friction for powder compaction modelling - comparison between laboratories

A ʹround robinʹ assessment of methods for measuring friction for die compaction of powders has been undertaken as part of the European PM Modnet programme Measurements have been made on a Distaloy AE with 0.5% graphite and l.0% wax using five different shear based techniques and six techniques using instrumented dies. The potential accuracy of friction measurement required as input to models of the compaction process has been assessed using results from modelling sensitivity studies where friction was varied, together with specifications from industry of the accuracy of model output required. These studies suggest that an accuracy of measurement of friction coefficient of ± 0.01 is preferred, but that an accuracy of 4 ± 0.02 will still give predictions of acceptable levels of accuracy. Friction data have been compared by separating the measurements into three regimes, respectively relevant to: surfaces perpendicular to punch movement during compaction (Type 1), surfaces parallel to punch movement during compaction (Type 2), surfaces parallel to punch movement during ejection (Type 3). The majority of data relate to the second of these cases. Type 2 friction data from shear techniques fall within a band of ± 0.02 around a linear relation with normal pressure and density. Mean friction falls from 0.14 to 0.07 as density increases from 4 to 7.5 g cm^-3 and from 0.11 to 0.06 as normal pressure rises from 0 to 600 MPa. Type 2 friction data from dies show a wider variation than data from shear techniques. A number of reasons for this has been suggested. The use of a varying ratio of radial to axial pressure at die surfaces during compaction has been demonstrated to bring closer agreement between data from die and shear techniques, especially at pressures above 200 MPa.