Modeling the Air Pressure Increase Within a Powder Bed During Compression-A Step Toward Understanding Tablet Defects.

The cause of tablet defects, such as cracking, bubbling, and capping, during compression is currently not fully understood. Prior experimental work suggests that an increase in internal air pressure on powder compression can directly contribute to the formation of cracks within a tablet. The present study examines the air pressure increase on compression in a fully two-dimensional axisymmetric tablet geometry while being coupled to a plasticity model describing the evolution of tablet relative density on consolidation. It is shown numerically that increasing compression speed results in a large air pressure increase on the order of 1-1.5 MPa which approaches the diametrical tensile strength of tablets. In addition, it is shown experimentally through X-ray microcomputed tomography scans of tablets made at various dwell times that increasing dwell times equivalent to that on a tablet press has no effect on the degree of cracking within the tablet. Only when dwell times reach a time scale of 10 to 100 s does the air pressure diminish to a point at which cracking is eliminated. The reduction in air pressure during these extended dwells is captured by the current model. The experimental and numerical work presented here couples for the first time an air pressure model and plasticity model on compression. In addition, it provides a foundation for understanding how realistic tableting aspects such as precompression and tablet size impact the air pressure increase on consolidation.