[learn_more caption=”Dr. Pingjun Tang”] Dr. Pingjun Tang is a Senior Scientist in pharmaceutical development at Vertex Pharmaceuticals Inc. in Boston, MA., where he is responsible for powder mechanical property characterization and its application in formulation and process development. He received his Ph.D. at the Pennsylvania State University in 2004. His Ph.D. research was focused on particulate material mechanics including powder flow, segregation, and compaction. He was the founder of the Vertex Compaction Simulator Lab. [/learn_more]

Abstract: Here we present a novel method for the advanced study of the residual radial stresses observed at the powder‐die wall interface during confined, uniaxial compression of pharmaceutical powders. In this approach, the powder compact is formed on a hydraulic compaction simulator while material is in contact with pressure transducers embedded in the interior walls of a custom‐made instrumented die. The advanced punch control capabilities of the compaction simulator are then utilized to manipulate the compact within the die, moving it out of and then back into contact with the transducers over a fixed cycle time. The results show a significant reduction in the measured stresses upon compact relocation, indicating that initial radial stress values measured immediately after tablet decompression may not sufficiently describe the stress state of a powder compact prior to ejection from the die. For instance, the initial residual stress measured for a compact of anhydrous dicalcium phosphate compressed to 250 MPa was approximately 30 MPa, whereas the residual stress measured after the in‐die manipulation was observed to be 15 MPa. This analysis of in‐die stress data was further coupled with careful measurements of out‐of‐die changes in compact geometry, and a correlation between residual radial stresses and out of die expansion (or contraction) was established. One notable material treated by this technique was HPMC, which exhibited in‐die residual stresses that approached zero. The diameter of compacts ejected from the tooling, however, were measured be smaller than that bore diameter of the die itself. This observation suggests that the presented approach to analyzing in‐die residual radial stresses can help in explaining out‐of‐die tablet relaxation phenomena.