Effect of compaction rate during the cold forming of polymeric powders

During many plastics processing methods, both conventional and novel, plastic powders or granules are subjected to pressure. The pressure transmissibility through the powder has an important effect on the success and efficiency of the process, and so it is desirable to be able to predict the pressure at any point from a knowledge of the applied pressure and the geometry of the system. To examine this pressure transmissibility effect, poly(vinylidene chloride) powder has been compacted in a metal die under different rates using either load or displacement control. It has been found that rate effects are particularly important for loading rates greater than 8 MPa/s and displacement rates greater than 7.5 mm/s. The ratio of transmitted pressure ( P 2 ) to applied pressure ( P 1 ) has been measured for a range of compact geometries, and it has been found that the ratio is approximately constant during the latter stages of the compaction process. Various laws for predicting the transmitted pressure have been studied, but it has been found that the simplest equation that agrees with the data is of the form