Powder Vertical Shear Rig

Anyone who has handled powders has almost certainly come across a clogged hopper, and will know that this is caused by caking, arching or rat-holing. To measure the tendency of a powder to show these properties, the main property to consider is the powder's bulk resistance to the initiation of flow. This helps to imitate the consolidation that occurs under the powder's own weight when the outlet is shut and the feeder is switched off. It is when the outlet is opened and the feeder started again that the blockages can build up.

The likelihood of a blockage forming depends on both the properties of the powder and the conditions surrounding it (humidity, temperature, stress state, hopper geometry). A vertical shear test provides a method to test the commencement of flow of a powder from a packed state, imitating a filled hopper. This test can also help to determine the correct hopper outlet size for reliable flow.

The Powder Vertical Shear Rig is yet another application that can be performed on powder samples using the Texture Analyser. During the test, a known mass of powder is transferred to the main body of the rig and compressed to a known force to create a uniform cake of powder. A trapdoor below the powder cake is released, exposing a circular surface of the cake. A probe slightly smaller than the hole then pushes a plug of the powder cake through the bottom of the rig. The powder is put into an almost perfect shear state (where the force is parallel with the powder movement). The rig offers a unique means of consolidating the powder, removing the applied force and testing the resulting sample without disruption by user handling. Using a special sequence and macro created in Exponent Connect software, which guides the user through the procedure, the important parameter of 'vertical shear strength' is automatically calculated once tests are complete.

Powdered material.