Compaction Science and Technology
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provide compact strength. Problems such as die sticking, end-capping defects, and pressing laminations are common to all pressing operations. Similar solutions are employed to optimize all pressing processes (e.g., granulating the powder feed to improve flow and die filling) to achieve high yields. Pressing problems are often related to the characteristics and properties of the powder and the granules thereof, including how the powder flows into and fills the forming die, how the granules deform under the pressing pressure, how uniformly the applied pressure is transmitted through the compact during forming, and the amount of stored elastic-strain energy released upon ejecting the compact from the die. The object of powder pressing is to quickly and reproducibly form a (nominally) defect-free, homogeneously dense powder compact to net shape. The pressing pressure should be high enough to impart sufficient strength for subsequent handling but low enough to avoid excessive wear on the press and tooling. Pharmaceutical tablets must have sufficient strength to survive packaging and transportation but must be weak enough to disintegrate after administration. Ceramic compacts may require higher strength to withstand the high stresses they can be subjected to during green (not fired) compact machining. Unlike pharmaceuticals, some P/M and ceramic parts require thermal processing (i.e., firing, which comprises organic burnout and sintering) after pressing to further densify the compact and make a useful part. Controlled, reproducible firing shrinkage is imperative to maintain specified tolerances and avoid costly postfire machining. Dimensional control is most difficult to achieve with materials that undergo large-volume changes on firing, such as ceramic powder compacts. Likewise pressing problems become more pronounced with materials that undergo large volume changes during the compaction process,
such as high-surface-area and low-bulkdensity powders. Optimum pressing performance occurs with granulated materials that flow freely, and that pack and compact to uniformly high relative densities. As an example, ceramic powders that press well are typically made up of 45-50% dense granules—between 44 and 400 /xm in size—that compact easily to 55-60% relative density. Uniformdensity compacts can be produced by dual-action pressing (i.e., both pressing punches move relative to the die body), by using carbide tooling (i.e., dies and punches), and by pressing simple geometries, all of which tend to minimize die-wall friction and the effects of friction on compaction. Deleterious frictional effects can also be circumvented by isostatic and biaxial pressing. The powder, particle packing, pressing conditions, and discrete particle-particle and particle-die interactions all play important roles in determining compaction behavior. Critical relationships between die geometry, die-wall friction, and the density uniformity of a pressed powder compact are well-recognized. However quantifying these complex relationships and predicting the
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