Melting and densification of thermoplastic powders

The present work focuses on the transformation of a loosely packed, low density powder compact, to a fully densified polymer part, when processed at temperatures above the melting (or glass transition) point of the polymer. The purpose of this study is to elucidate the mechanisms involved in the process and to examine the applicability of models available in the materials science literature for the description of the overall densification of molten polymer particles. The evolution of density as a function of time during sinter-melting was measured experimentally using a heating oven. The results revealed that the overall process consists of two stages. The first stage Involves particle coalescence, which depends on viscosity, surface tension and powder properties. During this stage air pockets, which eventually become bubbles, are entrapped inside the melt. The second stage involves the diffusion controlled shrinkage and eventual disappearance of the bubbles. The experimental results were compared to models commonly used for the densification of particulate compacts in the ceramics, glass and metals processing literature, Application of models based solely on viscosity and surface tension phenomena, can describe satisfactorily the process until the point where closed pores (bubbles) form. A bubble dissolution model has been successfully applied to provide predictions of density as a function of time for the late stages of densification.