In italiano, per favore
Original Article
Published on 20/10/97
Metal Injection Molding in Orthodontics
Gabriele Floria DDS, Lorenzo Franchi DDS PhD Cand.
Keywords: orthodontic materials, metal brackets, high technology,
MIM, CIM,
Introduction
How do advancements in technological innovations affect our way of
work in orthodontics?
There is no doubt that metallurgy plays a large role in clinical orthodontics; however,
are we sufficiently prepared to fully understand the quality of the new industrial
proposals in our field?
Should we or shouldn't we evaluate and try out the ever increasing
number of high tech "gadgets"?
In the health sciences, it is our professional responsibility to our
patients to constantly update our knowledge. In orthodontics, we deal mostly with
appliances made of metal, ceramic, plastic, etc. It is important for orthodontists to have
an understanding of the way these materials are produced, in order to prevent allergies
and undesirable effects on our patients.
The new European laws on materials require orthodontists to keep the security document for
each product purchased in the office, to assure patients the best care. Of course we are
not expected to be experts in every field but it is certainly wise to learn about a wide
variety of pertinent topics. At times, I personally feel somewhat unprepared when a new
acronym reaches my ears. For example many orthodontic components today are produced by
M.I.M. technology. M.I.M. stands for Metal Injection Molding.
M.I.M.
The M.I.M (Metal Injection Molding) and the C.I.M. (Ceramic
Injection Molding) processes were conceived and developed in USA in the early 1980s. Even
though the MIM process is one of the most promising powder metallurgy processes, its
application was limited for a long time due to its high complexity and high cost. Ten
years later industrial needs changed, and there was demand for high precision small parts
by the aerospace, medical, optical, and computer fields. Today the demand for MIM products
doubles each year (1).
The MIM process is especially suitable for small parts production in
medium-high amounts. In fact traditional methodology is expensive because the housings are
complicated, the materials are difficult to machine, and the starting feedstock is costly.
In addition to the initial material cost, 50% to 75% of the material typically becomes
scrap during machining. (2) As well, in many cases, MIM can facilitate the design of parts
not previously achievable in metal. At the very least, it can streamline the manufacturing
process and result in substantial benefits over traditional manufacturing methods. (3)
In the MIM process fine metal powders are combined with organic
binders, lubricants, and dispersants to construct a feedstock for forming a shape in a
mold. After the shape or part is molded, the organic components are removed from the
structure by a solvent or thermal process and the remaining metal powder is sintered in a
furnace at a high temperature.
The final part is of high density and replicates the shape of
the original part after molding, but it is typically 18% to 20% smaller because of
shrinkage during sintering. As a result of that shrinkage, the use of metal injection
molding in industry has been limited to manufacturing electronic packages that are 2
square inches. (2) In contrast to traditional sintering, MIM uses a large amount of
binder, very refined metal powder (particle diameters are usually less than 15 microns),
and a binder extraction technique which is very accurate.
Before starting a production with MIM, it is essential to determine
whether the part is economically and physically suited to the process. This is often
accomplished at the field sales level. Certain design changes may be appropriate in order
to derive the maximum MIM cost and quality benefits. As in plastic injection molding, wall
thickness should be uniform to avoid uneven shrinkage during sintering.
Raw material preparation (feedstock)
This is the first step. The metal powder or "metal dust"
is mixed with the binder to obtain a homogeneous mix. For orthodontics components the
breakdown is usually 55-65% metal dust and 45-35% binder. The binder is wax-based organic
material and special machines assure a good mixed composition. This phase is essential to
obtain quality in the final product. Orthodontic parts are usually created from stainless
steel powder (316L, 430L) obtained by an atomization process and selected by grain size.
The smaller particle in this case is 15 microns. The Ceramic Injection Molding (CIM)
process instead uses Al2O3, ZrO2, Si3N4, SiC, and Y2O3.
Injection molding
The next step is the molding process. There is no difference in
technique from plastic injection molding.
Certain devices are specifically created; however, it is critical that all parameters such
as pressure temperature, injection speed, and so on, are extremely constant and
controllable.
After molding, the part is called "green body" and it is relatively fragile. It
is also 20% larger than the final product.
Debinding or debunking
The "green" parts are exposed to heat, solvent or a
combination of the two in order to remove most (at least 90%) of the binder material. At
the end the "brown" parts are approximately the same size as the green parts but
are quite porous. This step is crucial and the residual 10% will be removed at the
sintering phase. Several debunking processes are required for different binding materials.
Sintering
"Brown" parts are sintered in vacuum type furnaces or with
minor success controlled atmosphere furnaces. This furnace reaches 1400 C° for MIM and
2000 C°for CIM process and they have sophisticated regulation systems to optimize all
parameters and if necessary treat the final product thermally.
Here, they shrink 17 to 22% to nearly full density and are then complete. The vacuum
furnaces are preferable to the others because the final product does not contain gas
inclusions and it is more compact because the heat diffuses uniformly.
Conclusion
This process is quite long and delicate but allows for production of metals
and alloys that cannot be used in traditional way as their melting temperatures are too
high. Converting those stronger metals into a powder helps solve that problem. Powder
metal parts can be made of some of the strongest elemental metals, as well as superalloys.
And because they are formed to net shape, or near net, there is no money wasted on
machined-away scrap metal. This cost-saving feature is one reason for the increasing use
of powder metal for making a variety of parts. The shape of a powder particle, which
depends on the material and the way it was made, influences the density, surface area,
permeability, and flow characteristics of the powder composition. By controlling these
characteristics, the properties of the end-product can be altered.
Orthodontists use many appliances produced by MIM. These include
brackets and NiTi palatal expanders. This technology allows the production of new alloys
with different characteristics for different purposes. Metallic properties are essential
in almost all orthodontic treatment and this MIM process could aid us tremendously in
developing the quality of our materials.
References:
ACKNOWLEDGMENTS: The Authors would
like to thank Leone S.p.A. for showing the MIM production details.
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