Numerical insights into sticking and other complex properties of compacted powders

David Freiberg

doi:10.17918/00001924

The compaction of powders allows for the rapid creation of solid powder compacts through mechanical densification. Many industries, including the powder metallurgy, ceramic, and pharmaceutical industries, rely heavily on the creation of solid materials by means of powder compaction. However, the process of systematically creating compacts with acceptable properties from new powders remains difficult and material-intensive, and the problem of 'sticking' - the propensity for powder to adhere to the punches and dies used to compact the powder - is one of the major problems affecting the development of new powders and powder compaction processes. In this work, we simulate sticking at high relative densities with DEM (the Discrete Element Method) for the first time. The simulations demonstrate the role of inhomogeneity of the deformation at the contacts (both inhomogeneity in force distribution and inhomogeneity in powder properties, in the case of mixtures) and the interplay of wall adhesion and interparticle cohesion on the development of sticking. We provide evidence that the punch-tablet separation stress during the separation of the punch from the tablet by itself is not a sufficient proxy for sticking, but normalization with strength is possible with single material compacts. We show that DEM predicts that this normalization cannot be generalized to two materials differing only in interparticle cohesion; compositions of such mixtures may produce separation stress and yet have broadly varying sticking behaviors. Also, the results provide insights in the evolution of contact of a tablet as it separates from the punch and demonstrates that a fracture-like process is characteristic of the separation with the opening progressing from the outer diameter of the punch-tablet contact to the its middle. Differences in the punch/tablet properties as well as die wall friction cause the behavior. Further, we use DEM to simulate and explore the evolution of the compact's mechanical properties (such as Young's modulus) throughout the unloading and reloading of external stress, and compare with experimental results showing irreversible changes up to and including severe compact damage during the unloading process. We confirm that DEM can replicate these irreversible changes, providing confidence that the underlying model is capable of predicting nontrivial emergent properties, and show that the breaking of internal particle-particle bonds in tension, even in early unloading while the powder compact is under a net compressive stress, is the root cause of the changes of the compact's elastic properties.

Numerical insights into sticking and other complex properties of compacted powders

Files and links (1)

Abstract

Metrics

Details

Numerical insights into sticking and other complex properties of compacted powders

Files and links (1)

Abstract

Metrics

Details

Drexel University Social media