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the bin. In subsequent operations the powder is dried. In space applications it would be necessary to recycle the water or other atomizing fluid.

Finally, mills are now available which can impart enormous rotational torques on powders, on the order of 2.0&times;10 7 rpm. Such forces cause grains to disintegrate into yet finer particles. Operations in free space should permit a variety of related approaches.

4C.4 Powder Pressing

An extensive literature on the various aspects of powder pressing is available and growing rapidly. Although many products such as pills and tablets for medical use are cold-pressed directly from powdered materials, normally the resulting compact is only strong enough to allow subsequent heating and sintering. Release of the compact from its mold is usually accompanied by,small volume increase called "spring-back." In space, compact strength should far exceed that on Earth due to powerful cold-welding effects on pristine grain surfaces.

In some pressing operations (such as hot isostatic pressing) compact formation and sintering occur simultaneously. This procedure, together with explosion-driven compressive techniques, is used extensively in the production of high-temperature and high-strength parts such as turbine blades for jet engines. In most applications of powder metallurgy the compact is hot-pressed, heated to a temperature above which the materials cannot remain work-hardened. Hot pressing lowers the pressures required to reduce porosity and speeds welding and grain deformation processes. Also it permits better dimensional control of the product, lessened sensitivity to physical characteristics of starting materials, and allows powder to be driven to higher densities than with cold pressing, resulting in higher strength. Negative aspects of hot pressing include shorter die life, slower throughput because of powder heating, and the frequent necessity for protective atmospheres during forming and cooling stages.

One recently developed technique for high-speed sintering involves passing high-amperage electrical current through a powder to preferentially heat the asperities. Most of the energy serves to melt that portion of the compact where migration is desirable for densification; comparatively little energy is absorbed by the bulk materials and forming machinery. Naturally, this technique is not applicable to electrically insulating powders (DeCarmo, 1979).

4C.5 Continuous Powder Processing

The phrase "continuous process" should be used only to describe modes of manufacturing which could be extended indefinitely in time. Normally, however, the term refers to Processes whose products are much longer in one physical