Ceramic Powder Compaction

Abstract

Powder pressing, either uniaxially or isostatically, is the most common method used for high-volume production of ceramic components. The object of a pressing process is to form a net-shaped, homogeneously dense powder compact that is nominally free of defects. A typical pressing operation has three basic steps: (1) filling the mold or die with powder, (2) compacting the powder to a specific size and shape, and (3) ejecting the compact from the die. To optimize a pressing operation, experienced press operators generally understand and control parameters such as die-fill density, die-wall friction, packing density, and expansion on ejection.Die filling/uniformity influences compaction density, which ultimately determines the size, shape, microstructure, and properties of the final sintered product. To optimize die filling and packing uniformity, free-flowing granulated powders are generally used. Spherical granules (i.e., agglomerates or clusters of finer particles) range in size from ~44 to 400 μm with the average size being ~100–200 μm. They are typically produced from 0.5 to 10-μm median particle-size powders by spray drying a ceramic powder slurry. To produce processable powders, various organic additives are typically added to the slurry prior to spray drying. These include binder(s) for strength, plasticizers that produce deformable granules, and lubricants that mitigate frictional effects. Consistent batching and dispersion of the granulated feed are critical for reproducible and uniform die filling. Granule densities that are 45–55% of the theoretical density (TD), and bulk-powder and die-fill densities of 25–35% TD are typical for ceramic powders.