Powder Mechanics

Dietmar Schulze Shear Cell. This device is capable of testing as little as 12 g of powders. The main part of Schulze RTS-XS ® Shear Tester is an annular cell, where the powder specimen is placed. A series of vanes on the bottom of the cell and on the lid prevent powder slipping against the cell boundaries during shearing. The lid is subjected to a vertically acting normal force in order to attain the desired compression level. The normal load is applied by a loading system, which also contains a displacement transducer to measure the lid displacement during shearing in order to capture volumetric changes occurring in the assembly. The shear stress acting on the assembly is continuously monitored by two additional load beams connected to the lid of the annular cell.

Ball indentation is a method that has been explored previously to assess powder flowability of cohesive beds, both experimentally and via computational modelling. Ball indentation determines the cohesive bulk powder failure based on indentation hardness measurements. This can then be expressed as the constraint factor, which is the ratio of indentation hardness to yield stress. Tests have previously been conducted using the Instron 5566 mechanical testing machine, where samples are indented using high precision spherical ball indenters.

The FT4 powder rheometer (Freeman Technology) consists of a cylindrical column into which powder is placed and a range of attachments for measuring various aspects of powder flow.  The standard test drives a rotating impeller downward into a powder bed and measures the force acting on the cell base and the torque acting on the impeller.  A value of ‘flow energy’ is determined by summing the rotational and translational work required to drive the impeller a given distance into the bed.  The bed can be pre-conditioned by gently slicing into the bed prior to the test with the impeller rotating in the opposite direction.  A porous plate can be placed at the base and air driven up through the bed at a range of velocities to see the influence on the flow energy.  The permeability of the bed can be assessed by measuring the pressure drop across the bed under varying consolidation stresses and air flow rates.  In addition, a shear cell attachment with radial vanes can be attached instead of the blade to measure the shear stress required to initiate steady-state flow under a given normal stress.

The Granudrum (GranuTools) is a method of assessing powder flowability properties by using a rotating drum. The drum is rotated at varying angular velocity and images are recorded. These images are then analysed to provide a flowing angle (dynamic angle of repose), where a lower flowing angle indicates better flowability properties. The measurements are highly reproducible and are suitable for a wide range of sample materials, such as those involved in additive manufacturing or pharmaceuticals.