Advantages and Limitations

Process Attributes. As emphasized already, PIM can produce a range of components from powders. A main attraction is the economical production of complex parts from high-performance materials. Because of the high final density, the PIM products are often superior to those produced by other powder fabrication routes. Most materials are available, including all common ceramics and alloys: steel, stainless steel, tool steel, silicon nitride, cemented carbide, silicon carbide, copper, tungsten heavy alloys, nickel-base alloys, alumina, cobalt-base alloys, and composites that include tungsten-copper and molybdenum-copper.

Besides the primary advantages of shape complexity, low cost, and high performance, several other attributes are worthy of notice. Producing both internal and external threads in the molded component is possible, thereby avoiding post-sintering machining. Also, waffle patterns and insignias can be molded directly into the component. Furthermore, the surface finish is typically good.

PIM vs. Die Compaction. Some confusion exists about the difference between PIM and traditional die compaction. The latter process is widely employed in forming powders into squat (low-height) shapes that can be ejected from compression tooling. Die compaction usually employs high forming pressures but gives less shape complexity. Most important, uniaxial die compaction results in density gradients in the compact, unlike PIM, which is hydrostatic. Density gradients result in distortion during sintering. Consequently, components fabricated from pressed powder are either sintered at lower temperatures, where sintering shrinkage is avoided, or machined after sintering. Otherwise, dimensional scatter becomes unacceptably large. Because of forming pressure gradients, sintering temperature differences, and differing performance levels, PIM and traditional powder compaction are usually not applied to the same structures. The PIM approach is suited for complex shapes sintered to near-full density, while die compaction is suited to simple shapes, sintered at lower temperatures to lower performance levels. The low porosity in PIM materials gives a high strength, toughness, ductility, conductivity, magnetic response, and so on.

Advantages. Besides the traditional materials, PIM can also produce specialty materials such as nickel superalloys, intermetallics, precious metals, refractory metals, and ceramic-fiber reinforced ceramic composites. Co-molding is another possibility, where two materials are combined to make a laminated structure. Such components can be joined in the green condition. This option has merit for creating corrosion barriers, wear surfaces, electrical interconnections, or high-toughness structures.

For the producer, injection molding is a desirable option because of manufacturing ease, including process control, flexibility, and automation. Inherently, injection molding is associated with large production volumes. Various components are produced at rates approaching 100,000 per day. On the other hand, small production runs are possible, with as few as 5000 parts per year being economical. This flexibility fits well with the current desires for quick response in manufacturing.

As with all technologies, the essence centers on economics. Powder injection molding is cost advantageous for the more complex shapes. The largest advantage comes from the elimination of the secondary operations, such as grinding, machining, drilling, or boring, that typically are required for precision components. Also, since the feedstock material (runners, sprues, and damaged moldings) can be recycled, material use is nearly 100%. This is especially important for costly raw materials such as refractory metals, specialty ceramics, and precious metals.

Process Limitations. Generally, PIM is viable for all shapes that can be formed by plastic injection molding. Still, for shapes with simple or axial-symmetric geometries, it is not competitive with traditional screw machining or die compaction and sintering.

In some cases another limitation is the component size. Large components require more powder (a large expense in some compositions), and large molding and sintering devices, which are more expensive and difficult to control. Small components with simple geometries can be more economically produced by standard machining, die compaction, or casting techniques.

Debinding is a key problem with PIM because the time for binder removal depends on the section thickness. Consequently, various manufacturers have set upper limits on section thicknesses, ranging from 10 to 50 mm. On the other hand, PIM has been used to form section thicknesses less than 0.5 mm. In practice, dimensional tolerances are typically within 0.3% of a target, although holding tighter tolerances on critical dimensions is possible. For better dimensional control, machining or coining is required after sintering. Density gradients can result from uneven filling, thickness variations, or direction changes during molding, and they often cause component warpage in sintering. Thus, manufacturers minimize changes in section thickness and if possible hold the variation in thickness within a factor of two. However, 10- to 100-fold section thickness changes are possible. Maximum sizes depend on several factors, including tool costs, powder costs, and equipment capacities. Typically, the largest dimension is below 100 mm, with the total component volume being below 100 cm3. However, much larger components are in production.

The small particles used in PIM are more expensive than larger powders or wrought materials. This becomes a barrier to large component fabrication, because powder cost becomes a larger fraction of the production cost. On the other hand, as consumption increases, the powder cost continues to decline.

Substantial problems facing the technology are the lack of knowledge on the part of end users, missing property data, and the shortage of personnel trained in the basic process. These problems are acute with the ceramic materials because of the high sensitivity of ceramics to microstructural flaws induced during molding. Further, a lack of design guidelines has inhibited PIM from being adapted as a replacement for other production routes. These problems are being addressed via seminars and educational literature. However, the many process variants create initial confusion.

Powder Injection Molding

Randall M. German, The Pennsylvania State University

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