made the surface of Talc particles mechanically coated with hydrophobic silica

Made the surface of talc particles mechanically

This preview shows page 39 - 41 out of 172 pages.

made the surface of Talc particles mechanically coated with hydrophobic silica by dry coating to create new functionalitie. Blümel et al. found that flowability and bulk density of PE-HD particles were enhanced by dry particle coating, thus improving laser beam melting (LBM) processes and the resulting product quality. These researches inspire us a thought for improving the flow property of GP. However, some method about enhancing flow property of gypsum powder is relatively complex and expensive. Regardless of the fabrication technique, a critical prerequisite before creating solid parts is to spread powders on top of a deposition bed (building platform). The efficacy of the manufacturing process is a strong function of the uniformity with which the metal powder is spread on the deposition surface, which in turn, depends on the gradation of the metal powder. The correlation between particle size distribution (PSD), bed quality, and powder
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- 40 - flowability is of great interest to metallurgists and engineers because choosing proper powder gradation and processing parameters can significantly enhance the quality of the AM product and reduce the cost. However, the ambiguity surrounding this issue primarily owes to a general inability to characterize the grain scale interactions. Recent studies have shown that the uniformity with which the powder can be spread on the depositional surface is a strong function of the grading of the input powder. Specifically, experiments with stainless steel powder of three different PSDs suggests that a proper amount of fine particles can improve the properties of additive manufactured part in terms of the surface roughness, density and mechanical strength. That substantial fines content improves the smoothness and increases the density of finished parts is not surprising. From previous work, it is known that materials with a wider gradation are capable of achieving a denser packing. At the microscale, this is attributable to the fact that smaller particles will migrate to fill voids between larger particles. However, an excessive amount of fine particles can significantly hinder the particle flow behaviors, leading to unexpected voids and defects. Therefore, understanding the effect of fine fractions on the flowability of metal powder and determining the optimum amount of fine particles is of great interest to engineers. Indeed, prior experimental work on flow behaviour of fine powders has been reported by researchers in different fields but the interpretations of the results are not consistent because of the complex physics and large number of governing factors. In general, the correlation between powder flowability and particle size depends on inter-particle adhesions caused by inter- molecular forces including van der Waals (vdW) force, electrostatic charges, and capillary attractions. Of the many microscale forces, the van der Waals force is the most common because it occurs in all situations and plays a dominant role in determining the quality of the deposition bed in AM. Unfortunately, experimental study of the spreading process under
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  • Fall '15
  • Selective laser sintering, Particle size distribution, sintering, Additive manufacturing

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