Melting and densification of thermoplastic powders

Abstract

AbstractThe 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 transiton) point of the polymer. The purpose of this study is to dlucidate 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 disapperance 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. Abubble dissolution model has been successfully applied to provide predictions of density as a fuction of time for late stages of densification.