Metal Injection Molding atomization, gas atomization and carbonyl-decompo-
sition are shown in Fig. 2. The metal powder and
Metal injection molding (MIM) is a technology for binder are hot mixed above the softening point of the
manufacturing complex, precision, net-shape com- binder constituents to provide for a uniform coating of
ponents for use in medical, automotive, industrial, the binder on the powder surface. A shearing action
firearms, and consumer industries. The potential of using a continuous extruder or batch mixer aids in
MIM lies in its ability to capture the shaping ad- mixture homogeneity. The mixture is pelletized to an
vantage offered by injection molding by using a appropriate shape for feeding into the molding mach-
starting mixture of fine metal powder and organic ine. Typical binder volume fraction in feedstock range
binder. The MIM technology provides advantages of from 0.3 to 0.45.
cost reduction at volumes ranging from a few thousand
to over millions of parts per year due to its ability to 1.2 Molding
hold close tolerances. Typical tolerances achievable
range from p0.3% to p0.5%, making the process The injection molding process is principally identical
highly cost competitive as compared to machining and to conventional plastic injection molding. However,
investment casting. Further, MIM overcomes the some machine hardware changes are usually required
dimensional and productivity limits of isostatic press- to process specific feedstocks based on compressibility
ing and slip casting, the defects and tolerance limita- and viscosity. The relatively high viscosity of feed-
tions of investment casting, the mechanical strength of stocks (from 10# to 10$ Pa s at shear rates of 10 to
die-cast parts, and the shape limitation of traditional 10$ sV"), coupled with the high thermal conductivity of
powder compacts. a metal filled binder, and a tendency for the powder–
The evolution of MIM technology has spanned over binder mixture to separate at high shear rates defines
two decades and is an offshoot of ceramic injection the constraints for the molding process. Control of the
molding (German and Bose 1997, Mutsuddy and Ford molding process is vital for maintaining tight toler-
1995). Along its evolution path are rapidly changing ances in subsequent steps. Most design advantages of
variants, reflecting different combinations of powders, MIM technology are captured during molding by
binders, molding techniques, debinding routes, and relying on the flexibility of incorporating complexities
sintering hardware. It is important to understand the in the tool. A molded part is called a ‘‘green’’ part
processing steps, design guidelines, and limitations to and is oversized to allow for part shrinkage during
fully appreciate the potential of this technology. sintering.
1.3 Debinding
1. Processing Steps
The debinding step involves the removal of the organic
There are four primary steps to producing metal binder from a molded part using one of three different
injection molded parts as shown in Fig. 1. methods: thermal, solvent, or catalytic, depending on
the composition and constituent of the binder. Ther-
mal debinding involves application of heat to remove
1.1 Feedstock Formulation
the binder by degradation, evaporation, or liquid
The starting material for metal injection molding is a extraction (wicking) (Angermann and Van Der Biest
homogeneous pelletized mixture of metal powder and 1995). Temperatures for thermal debinding can range
organic binder termed feedstock. The binder is simply from 60 mC to 600 mC with the incorporation of dwell
a carrier medium for the powder and once a part is times at selected temperatures specific to the thermal
molded, the binder is removed in a subsequent step decomposition of its binder constituent. The relatively
(termed debinding). A binder formulation is based on long debind time associated with thermal debinding is
its ability to provide for a rigid molded part, ease of greatly reduced using a solvent such as heptane or
removal from a molded part, recyclability, and non- trichloroethane which can dissolve the soluble binder,
toxicity. Process considerations usually dictate three e.g., wax in a wax-polyethylene-polypropylene-based
components to a binder system, a backbone that binder. Some glycol- and polyvinyl alcohol-based
provides strength, a filler phase that is easily extracted water-soluble feedstocks use water as a solvent, while
during initial stages of debinding, and surfactant to gelation-type binders, such as those based on poly-
tailor feedstock rheology. Commonly used binder saccharides plus water, rely on air drying to remove
systems, as listed in Table 1, include thermoplastics, the water. With acetal-polyolefin-based feedstocks, a
thermosets, and gelation systems. hybrid of thermal and solvent processes, termed
Powder selection for the MIM process involves a catalytic debinding, uses nitric or oxalic acid vapor at
combination of tailored particle size distribution to temperatures between 110 mC and 150 mC to depoly-
maximize packing densities, with a mean particle size merize polyacetal into formaldehyde gas (Krueger et
5–15 µm to achieve a high sintered density. Examples al. 1993). The solid (acetal) to vapor (formaldehyde)
of MIM-grade powder morphology using water catalytic degradation results in faster debinding (as
1
Metal Injection Molding
fine particle size used in the MIM process results in
high sintered density, which is concomitant with
shrinkage (ranging from 14% to 20%). Therefore,
molded parts are oversized to compensate for the
sintering shrinkage. Sintered density ranges from 95%
to 99% of theoretical, thus providing superior mech-
anical and corrosion properties as compared to press
and sinter technology. Both batch and continuous
sintering is employed for production. The underlying
principles of sintering are discussed in Sintering and
Liquid Phase Sintering: Metals. The sintering time and
temperature uniformity are of paramount importance
in achieving a combination of desired mechanical
properties and dimensional tolerance. Common sint-
ering atmospheres employed for MIM alloys are
reducing (H -based), inert (N - and Ar-based) or
vacuum. # #
2. MIM Alloys
Common MIM alloys include austenitic stainless
steels (304L and 316L), precipitation-hardened mar-
tensitic stainless steel (17-4PH), martensitic steels (420,
440C), alloy steels based on Fe–2%Ni and Fe–8%Ni
(containing up to 0.5% carbon), 4140, 8620, tool steels
including M2, soft magnetic alloys including Fe–
50%Ni, Fe–3%Si, 430L, and alloys for glass-to-metal
sealing applications such as kovar. The mechanical
properties of MIM alloys show a broader range as
Figure 1
compared to the wrought values. These variations
Process schematic of metal injection molding.
result primarily from the differences in commercial
compared to thermal debinding) while offering su- production practices using different binders, debind-
perior handling strength as compared to thermal or ing methods, sintering technology, and powders.
solvent-based feedstocks. However, optimal processing results in mechanical
In all debinding methods, a skeleton of backbone and corrosion properties that are comparable (in some
binder remains to impart adequate strength and shape case superior) to its wrought counterpart.
retention up to the onset of sintering. This remaining
backbone is thermally removed between 200 mC and
600 mC in a presintering step. 3. Design Considerations and Limitations
Parts for metal injection molding are designed based
on shape, size, and geometry, for optimum perform-
1.4 Sintering
ance. A summary of desirable design features is listed
Depending on the alloy, debound parts are sintered at in Table 2. From a tooling perspective, the designed
temperatures ranging from 1200 mC to 1350 mC. The part must incorporate a parting line, gate, and ejector
Table 1
Commonly used binder systems for MIM.
Thermoplastic binders
Wiech-type binders: polyolefin (PE\PP\PS)jwaxes, oiljdispersant (fatty acids\
esters)
Water soluble binders: PEG\PVAjpolyacrylates\polyamides\PVB
Polyacetal binder: POMjpolyolefinjadditives
Thermoset binders
Cross-linking of polymers: epoxy, phenoxy-resins
Gelcasting: methacrylamidejcross-linkers
Gelling\freezing binders
Hot gelling: methylcellulose\boric acidjwater
Cold gelling: polysaccharidesjwater
2
Metal Injection Molding
Figure 2
Scanning electron micrographs showing the morphology of MIM-grade powder manufactured using (a) water atomization,
(b) gas atomization, and (c) carbonyl decomposition.
system. The avoidance of sharp corners, thick-to-thin debinding also has potential for manufacturing com-
transitions, and the use of draft angles, and corner ponents with thick cross-sections.
radii provide for optimum moldability. Additional The relatively high raw-material cost as dictated by
design restrictions such as the maximum part thickness unique powder specification also constraints the MIM
are determined by the debinding limitations imposed technology to parts weighing 100 g. However, many
by a specific feedstock. Generally, most MIM parts heavier components whose complexity overrides the
have a maximum cross-section thickness 7 mm. In cost constraints have been produced using MIM. The
this respect, acetal-based feedstocks (using catalytic best examples for this technology are parts that have
debinding) have proven processing and handling been designed specifically to take advantage of the
advantages, and are capable of manufacturing parts ability of the MIM process to capture design com-
with thicker cross-sections with virtually no debind plexity as offered during the molding step.
constraints. The polysaccharide–water (gelation-type) Generally, the MIM process is not competitive as
system with its air-drying step replacing conventional compared to stamping or screw machining. It com-
Table 2
Typical design guidelines for MIM.
Specifics Reason
Restrictions No undercuts on internal bores Difficult to tool
No inside close cavities Impossible to tool
Corner radius 0.1 mm Avoid sharp edges\reduce stress
0.5 to 2m draft Facilitate ejection
Smallest hole diameter l 0.1 mm Poor steel condition
Minimum wall thickness l 0.5 mm Difficult to fill thinner sections
Desirable features Gradual section thickness change Ease of fill\reduce stress
Largest dimension 100 mm Processing difficulty
Weight 100 g Material cost\debinding constraints
Wall thickness 6–10 mm Facilitate debinding
Incorporation of flat surfaces Provide sintering support
Allowed design features Holes at angles to one another All these features can be easily incorporated
Hexagonal, square, blind, D-shaped, into the part design and can be molded
keyed, and flat-bottom holes
Stiffening ribs and knurled features
Protrusions and studs
External threads
Part number, identification logo in die
3
Metal Injection Molding
petes with investment casting by providing better materials to be available in the market. The MIM
surface finish, fine-scale homogeneous microstructure, technology continues to penetrate new markets with
thinner and more complex wall sections, and better the introduction of alloys, including cobalt-based
dimensional tolerances. With respect to machining, alloys for wear resistant applications, and titanium-
the MIM process results in cost reduction at medium and nickel-based alloys for medical and aerospace
to high volumes especially if the parts require multiple applications.
machining operations. In cases where superior mech-
anical performance is required, MIM also competes
with press and sinter by providing parts with a higher
density, but at a higher cost. Bibliography
Angermann H H, Van Der Biest O 1995 In: Bose A, German
R M, Lawley A (eds.) Reiews in Particulate Materials. Metal
4. Concluding Remarks Powder Industries Federation, Princeton, NJ
The MIM technology has seen rapid advances in the German R M 1999 Wear applications offer further growth for
1990s and is a strong competitor to machining and powder injection molding. Met. Powd. Rep. 54 (6), 24–8
investment casting given the right combination of German R M, Bose A 1997 Injection Molding of Metals and
Ceramics. Metal Powder Industries Federation, Princeton, NJ
complexity and production quantity. Annual world-
Krueger D, Blomacher M, Weinand D 1993 Rapid catalytic
wide growth rates for MIM is projected to be between debinding MIM feedstock: a new technology grows into a
20% and 30% (German 1999). The ability of the manufacturing process. In: Gaspervich T, German R M (eds.)
process to maintain tight tolerance is strongly de- Adances in Powder Metallurgy and Particulate Materials.
pendent on several factors ranging from raw material PMTech93, Tennessee, pp. 165–80
selection to process control. Commercially available Mutsuddy B C, Ford R G 1995 Ceramic Injection Molding.
feedstocks (with rapid debinding) have resulted in Chapman and Hall, London
distinct processing advantages such as fabrication of
thicker components, while allowing for standard raw R. Tandon
Copyright ' 2001 Elsevier Science Ltd.
All rights reserved. No part of this publication may be reproduced, stored in any retrieval system or transmitted
in any form or by any means : electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or
otherwise, without permission in writing from the publishers.
Encyclopedia of Materials : Science and Technology
ISBN: 0-08-0431526
pp. 5439–5442
