Effects of granule size and distribution on the cold isostatic pressed alumina

Green alumina parts large in dimension are usually fabricated by cold isostatic pressing (CIP), whose properties, including green strength, green density gradient, and compression ratio, are usually affected by the mean size and size distribution of the spray-dried alumina granules. Such effects on the green parts subsequently dictate the sintered structure of alumina, microscopically and macroscopically, if the pressing profile during pressing is not optimized. In this study, alumina granules were spray-dried using an emulsified acrylic polymer as the binder. Granules of various sizes and size distributions were sieved, including sizes ranging between 200 mesh and 180 mesh, ranging between 180 mesh and 120 mesh, ranging between 120 mesh and 80 mesh, ranging between 80 mesh and 40 mesh, larger than 40 mesh and nature granules form without sieved. These six kinds of granules along with the as-spray-dried granules were then cold-isostatic-pressed into green parts. The pressure profile during pressing was varied, in order to optimize the properties of the green parts and the sintered parts. The granules were examined using scanning electron microscope while the green parts were analyzed by green density measurement. The green parts were subsequently sintered at 1600 °C, and density measurement, and scanning electron microscopic examination were carried out to compare the differences caused by the variation in granule size distribution. The results indicated that small granules experienced a very large pressing shrinkage ratio, which could cause severe shape distortion in under-cut region. The drawbacks of green parts arising from the granule size distribution could be compensated by optimal pressure profile design. With such a compensated action during cold isostatic pressing, uniform sintered structures could be attained.