Emphasis On Metal Injection Molding
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debind oven where most of the binder is extracted through thermal, solvent, or catalytic processing. The resulting (brown) parts are fragile and
approximately the same size as the green parts
but only 80 percent as dense. Just enough binder
remains to keep the powder particles together.
Finally, the parts are sintered, or solid-state
diffused, in a controlled-atmosphere sintering
furnace at temperatures close to the metal’s
melting point. After the parts are sintered, they
have properties similar to wrought materials.
During the sintering cycle, the remaining
binder is removed – as well as the air space
that was created when the binder was removed
– and the parts shrink approximately 20 per-
cent to their final size. MIM molds need to be
oversized to account for this shrinkage during
the sintering process.
The mass of MIM parts is typically less than
150 grams, and the average part is closer to
10 grams. As part size increases, MIM’s cost
advantages become off-set by the high raw
Ideally, MIM parts should have wall thicknesses not less than 0.1cm (0.04in.) and not
more than 12.7mm (0.5in.). Due to material
flow limitations, care should be taken to minimize the distance from the gate to the part’s
farthest point. MIM part tolerances are nominally ±0.5 percent, although in some cases,
tighter tolerances can be achieved.
Since MIM parts shrink and become soft
Compatible with Other Processes
during sintering, parts should be designed
for proper support throughout the sintering
process to ensure that they maintain their
desired shape. The easiest parts to sinter have a
common co-planar surface that can rest flat on
a ceramic substrate or setter. When unsupport-
ed features are present, support often needs to
be added through special fixturing to minimize
or eliminate distortion of the part.
Many parts are sintered to final dimensions
because the tolerance for the MIM process
falls within ±0.5 percent. If tighter tolerances
are required, secondary metalworking operations can be performed. MIM parts can be
machined, tapped, drilled, broached, sized,
ground, or welded like wrought parts.
MIM offers the same design freedom as plastic
injection molding and is an excellent option
for relatively small, highly complex parts that
otherwise would require extensive finish machining or assembly operations if made by any
other metal-forming process. MDT
For more information, visit www.protolabs.com.
Pellets of MIM feedstock rest in a hopper atop an
injection-molding press. The pellets are volumetrically
metered into a barrel, heated, and injected into a tool to
form a green part.