Melting and densification of thermoplastic powders

The present work focuses on the transformation of a loosely packed, low den sity powder compact, to a fully densified polymer part, when processed at t emperatures above the melting (or glass transition) point of the polymer. T he purpose of this study is to elucidate the mechanisms involved in the pro cess and to examine the applicability of models available in the materials science literature for the description of the overall densification of molt en polymer particles. The evolution of density as a function of time during sinter-melting was measured experimentally using a heating oven. The resul ts revealed that the overall process consists of two stages. The first stag e Involves particle coalescence, which depends on viscosity, surface tensio n and powder properties. During this stage air pockets, which eventually be come bubbles, are entrapped inside the melt. The second stage involves the diffusion controlled shrinkage and eventual disappearance of the bubbles. T he experimental results were compared to models commonly used for the densi fication of particulate compacts in the ceramics, glass and metals processi ng 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 succ essfully applied to provide predictions of density as a function of time fo r the late stages of densification.