Copperbase

• Stainless steel

• Sterling silver

• Superalloys

• Tungsten heavy alloy

Ceramics and compounds

• Alumina-chromia (Al2O3-Cr2O3)

• Hydroxyapatite (Ca10(PO4MOH)2)

• Nickel aluminide (Ni3Al)

• Silicon nitride (Si3N4)

Cermets and composites

Al2O3-SiC Al2O3-ZrO2 Mo-Cu NbC-Ni

Ni3Al-Al2O3

SiO2-Si

TiC-Ni-Mo

W-Cu

WC-Ni

WC-TaC-Co

ZrO2-MgO

ZrO2-Y2O3

Noticeably absent are magnesium and aluminum; these reactive metals develop powders with surface oxides that are difficult to remove during sintering. Stainless steel is the most widely used material for injection molded parts, as shown by the relative ranking in Fig. 2. Stainless steel 316L is the most commonly used alloy. Ferritic and duplex stainless steels are also used.

Stainless

Fe-Ni steel Alumina Iron Fe-Si WC-Co Silicon nitride Others

Relative use

Fig. 2 Relative material utilization in powder injection molding (PIM) processing on a weight basis, showing the dominant position of steels and stainless steels. Source: Ref 2

Injection molding moved quickly from a laboratory development to a commercial process for two reasons. First, a precision shape could be achieved at a significant cost reduction compared to a conventional manufacturing process. Second, the use of fine powders promoted densification during sintering, and high properties could be achieved. The high potential of the process added a third reason that sustained growth. Availability of fine powders and pellet feedstock from suppliers increased as the process showed early successes. There is still a great deal of development work toward more efficient and economical production of fine powders (<20 /'m) specifically to supply the injection molding industry.

In the as-molded state, the green part is typically —65 vol% powder and —35 vol% binder. Therefore, upon removal of the binder, considerable densification is required to achieve density levels of —95 to 98% of full density. This densification must be accompanied strictly by size change because unwanted shape distortion would render the part useless. Therefore, uniform particle distribution in the binder is essential. Also, to minimize contamination, the binder must either be removed completely before sintering, or it must participate in alloying during sintering.

A binder/plasticizer has also been used to extrude long shapes rather than just mold individual shapes. A heated barrel plasticizes the binder, and a die gives shape to the powder being extruded. With this technique it is possible to form fluted shapes, hollow tubes, tubes with multiple cavities, and other complex shapes. More information is in the article "Powder Injection Molding" in this Volume.

Rapid prototyping technology includes quick methods for making templates, models of parts, prototype molds and dies, and fully functional components. Rapid prototyping methods rely on variants of powder metallurgy capabilities such as selective sintering, powder spraying, ink jet techniques for directed spraying, and deposition. Several methods are discussed in the article "Powder Metallurgy Methods for Rapid Prototyping" in this Volume.

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