Note: Descriptions are shown in the official language in which they were submitted.
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SINTERING PROCESS AND TOOLS FOR LTSE IN
METAL INJECTION MOLDING OF LARGE PARTS
BACKGROUND OF THE INVENTION
The present invention relates to the art of sintering metal inj ection molded
preforms
or flowbodies, and more particularly to a two-step sintering process and
related tools for
controlling flowbody deformation which typically occurs during the sintering
process.
Metal injection molding (MIM) is a well lcnown technique for the cost
effective
production of complex multidimensional parts. Typically such parts are of
comparatively
small size with a weight within a range of about 25 to about 250 grams and are
often made in .
high production volumes. Metal injection molding is most commonly used in the
automotive, firearms, and medical industries.
In general, the MIM process involves mixing a powder metal, water and a
binder.
The binder is typically composed of an organic aqueous based gel. The mixed
powder metal
and binder composition produces a generally flowable mixture at relatively low
temperature
and pressure. The proportion of binder to powder metal is typically about 40-
60% binder by
volume. The goal is to produce a flowable mixture with a viscosity such that
the mixture will
fill all of the crevices and small dimensional features of a mold. The
flowable mixture is
typically transferred to the mold, via an injection molding machine.
2 0 Injection molding machines are l~nown in the art and are typically capable
of applying
several hundred tons of pressure to a mold. The mold is typically constructed
with internal
cooling passages to solidify the flowable material prior to removal. The mold
cavity
'typically is larger than that of the desired finished part to account for the
shrinkage that
occurs after binder removal. The mold structure may be formed from either a
rigid or a
2 5 flexible material, such as metal, plastic, or rubber. Preferably, the mold
is equipped with
vents or bleeder lines to allow air to escape from the mold during the molding
process.
Alternatively, the mold may be equipped with a porous metal or ceramic insert
to allow air to
escape from the mold. After the mold has been filled with the flowable
mixture, pressure is
applied to the moldlmixture to form the molded part, otherwise known as the
preform.
3 0 Typical injection mold pressures for a preform are in the range of about
10-12 lcsi. The as
molded prefonns may be referred to as "green" parts. The green preform may be
dried by
oven heating to a temperature sufficient to vaporize most of the remaining
water. Then, the
preform is placed in a furnace to vaporize the binder. To achieve a part with
high density and
thus a sufficient working strength, the preform is subsequently sintered.
3 5 Sintering is an elevated temperature process whereby a powder metal
preform may be
caused to coalesce into an essentially solid form having the same or nearly
the same
mechanical properties as the material in casted or wrought form. Generally,
sintering refers
to raising the temperature of the powder metal preform to a temperature close
to, but not
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exceeding, the melting point of the material, and holding it there for a
defined period of time.
Under these conditions, interparticulate melting occurs and the material
densifies to become
solid.
In general, complete solidification does not occur, but sintered density can
approach
99% with some materials. As the densification process occurs, the interstitial
voids in the
preform shrinlc in size and decrease in number. As a result, the bully volume
of the sintered
preform is considerably less than that of the pre-sintered preform. As the
preform shrinks,
geometric deformation of the preform may occur. This deformation is relatively
minor in
small parts and can be easily remedied by secondary machining operations.
However, in
large parts, those with net weights over 250 grams, undesired deformation is
more
problematic.
In general, during the period of densification, while the preform is subjected
to high
temperature, preforms of certain configurations, such as tubular or other
shapes, have less
strength to resist deforming influences and it is a recognized challenge in
sintering such metal
parts to achieve final geometries congruent to the preform. See, e.g., U.S.
Patent No.
5,710,969. This problem is particularly apparent when sintering preforms with
large
cylindrical sections and irregular high mass protrusions. For example, a large
cylindrical
preform section will deform under the influence of gravity to a densified
section in the form
of an oval. For this reason, the use of MIM and sintering technology has not
expanded to the
2 0 production of comparatively large parts weighing in excess of about 250
grams, or to parts
having cylindrical sections with diameters in excess of about 3.8 cm. What is
needed
therefore is a sintering method and tools which will allow for comparatively
larger parts to be
sintered while maintaining the geometric stability of the parts.
2 5 SUMMARY OF THE INVENTION
The invention provides a process and/or tools that can be used to make
dimensionally
accurate MIM parts of a size and/or complexity heretofore unachievable and
includes
improved drying, binder removal, and sintering processes which may be used in
conjunction
with specialized sintering tools to provide for the geometrically stable
sintering of large,
3 0 complex, MIM parts.
By way of example only, the improved processes include a four-stage drying
process,
a single stage binder removal process, and a two-stage sintering process.
Drying of wet green
preforms is particularly important as cracks often form during the drying
process, resulting in
a large number of scrap parts. This problem is particularly prevalent with
large MIM parts.
3 5 The novel two stage sintering process includes a first fixing stage where
the MIM
molded preform may be densified to about 60% to 80% of its maximum density at
a first
sintering temperature, and then allowed to cool. Generally, the sintering
temperature used in
the first sintering stage is sufficiently below the melting point of the
powder metal material
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used in the molding process to prevent the preform from taping an improper set
due to the
force of gravity acting over any large unsupported surfaces. It may prove
desirable to keep
the first sintering temperature below the solidus temperature of the alloy
(i.e., the temperature
at which the alloy begins to melt). This first stage serves to fix the overall
shape of the
preform.
In the second stage, the prefonn is heated to a second sintering temperature
near the
melting point of the powdered metal material at which a denser part density is
developed.
Typically, in a preform part containing both large and small cylindrical
features, heat
resistant sintering tools such as inserts of predetermined sizes may be used
in both the first
and second sintering stages. Heat resistant materials, such as aluminum oxide
ceramic may
be used for the inserts. In the first sintering stage, the inserts are used to
support the preform
and control the diameter of any small cylindrical features. In the second
sintering stage, the
larger cylindrical features may be fitted with a second set of inserts to
prevent undue
deformation of these features due to the force of gravity that otherwise would
cause the
features to take an oval or other undesired shape during the sintering.
These and other features of the invention will become more apparent from the
following detailed description of the invention, when taken in conjunction
with the
accompanying exemplary drawings.
2 o BRIEF DESCR.TPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a valve flowbody prepared for first stage
sintering with
sintering tools in accordance with the present invention.
FIG. 2 is another perspective view of the flowbody of FIG.1 prepared for
second stage
sintering with additional sintering tools in accordance with the present
invention.
2 5 FIG. 3 is a perspective view showing the sintering tools of FIG. 1 in more
detail.
FIG. 4 is a perspective view showing the sintering tools of FIG. 2 in more
detail.
FIG. 5 is a flow chart illustrating the steps of the present invention drying,
binder
evaporation, and sintering processes.
3 0 DETAILED DESCRIPTION OF THE INVENTION
In this specification the term "preform" is meant to include conventional
powder
metal preforms where the powder metal is compacted without the use of a
binder. The term
"preform" is also meant to include MIM flowbodies where the flowbody is
produced from a
mixture of a powder metal, water and a binder. A flowbody is a structure or
part with a flow
3 5 passage formed therein, such as the portion of a valve assembly having the
fluid flow passage
formed therein.
Throughout this specification the process and tools of the present invention
will be
referred to in reference to a particular flowbody produced from a commercially
available
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Inconel 718 powder metal composition mth a nominal chemistry composition of
5Z.5Ni-
18.SFe-18.SCr-S.lNb-3Mo-0.9Ti-O.SAl-0.4C (% by weight) mixed with a binder
comprising
an aqueous agar solution.
In general, the various temperatures and heating times are applicable to any
Inconel
alloy composition. Those skilled in the art will understand that the sintering
process of the
present invention may be applied to virtually any metal alloy, including but
not limited to
iron, nickel, and titanium based alloys. Sintering temperatures and times for
alloys other than
Inconel 718 will of course vary from those described. Further, the processes
of the present
invention may be used with virtually any preform or MIM flowbody configuration
and the
tools of the invention may be used with any preform or flowbody having laxge
and small
cylindrical features.
With reference to FIG. 1, there is shown an exemplary flowbody 26 prepared for
first
stage sintering. The flowbody is a butterfly valve housing having a large
cylindrical bore 30
with an inside diameter of about 8.8 cm and a pair of smaller cylindrical
bores 28 having an
inside diameter of about 3.0 cm. The typical wall thickness of the flowbody's
features is
about 3 mm. The flowbody has a weight of .about 1000 grams or substantially in
excess of
parts typically made by MIM processes. The flowbody includes a diaphragm 20
which is
formed during the molding process and which helps provide support for
roundness of the
flowbody. The diaphragm, however, is not required for all applications and is
removed
2 0 before or after sintering, as desired.
The flowbody is produced using the processes and tools of the present
invention and
is dimensionally and geometrically representative of the type of large
flowbodies wluch may
be successfully produced using the present invention processes. The processes
and tools can
also be used to malce other large complex MIM parts. It is believed that the
present invention
2 5 processes are suitable for sintering flowbodies with weights of up to at
least 1500 grams and
with cylindrical features having diameters in excess of 8 cm.
As shown in FIG. 1, supporting the flowbody are specialized sintering tools.
In
particular, within each small bore is placed a ceramic insert, e.g., a
cylinder 34 (see also FIG.
3). Each cylinder functions to maintain the geometry of the respective bore in
which it is
3 0 placed, and to support, via a ceramic rod 32, the flowbody during first
stage sintering. Each
of the cylinders includes a throughbore 35 (FIG. 3) which slidably receives
the ceramic rod
32. The ceramic rod, which may be solid or tubular, rests in a ceramic support
structure 40,
such as a firebriclc support structure. The support structure may include a
base 42 and a pair
of V-notch blocks 41 (FIG. 3) for receipt of the ceramic rod. The
configuration of the first
3 5 stage sintering tools 32, 34, 41, and 42 are shown with more particularity
in FIG. 3. The
flowbody 26 is supported by the ceramic rod 32, through the cylinders 34 such
that the
flowbody is spaced from the base 42.
It will be appreciated that for smaller parts having smaller bores, the
cylinders 34 may
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be removed and the part supported by the ceramic rod 32 only. In this case,
the ceramic rod
may or may not be used to insure rounchiess of the bore. For example, the rod
may be used to
support the part, but is not needed to maintain roundness of a relatively
small bore. In
addition, the orientation of the flowbody relative to the support structure
may be varied as
desired. For example, FIG. 1 depicts the large cylindrical bore 30 having a
horizontal axis.
The part may be rotated on the ceramic rod, however, such that the bore 30 has
a vertical
axis.
Refernng now to FIG. 2, the flowbody 26 is shown prepared for second stage
sintering. Placed within the large bore 30 are two large diameter ceramic
inserts, e.g.,
cylinders 38 (see also FIG. 4). Lilce the smaller ceramic cylinders used
during the first stage
sintering, these cylinders serve to maintain the geometry of the bore and to
support the
flowbody during sintering, via a ceramic rod 36. The ceramic rod can be the
same rod as
used in the first stage sintering. Referring now to FIG. 4, the second stage
sintering tools are
shown in more detail. The cylinders 38 each have a throughbore 37 for slidable
receipt of the
rod 36. The rod 36 supports the cylinders and consequently the flowbody in the
firebrick
support structure 40. The same support structure can be used for the first and
second
sintering stages. In the second sintering stage, the flowbody is also
supported by the ceramic
rod through the cylinders such that the flowb.ody is spaced from the base.
The sintering tools are preferably produced from commercially available
aluminum
2 0 oxide ceramic. Aluminum oxide is a durable material that will neither
deform nor stick to the
W conel 718 metallic flowbody during sintering. The sintering tools may be
made by
machining aluminum oxide bar stock or by an injection molding process known in
the art.
Preferably, the outside diameter of the cylinders 34 and 38 is machined to the
desired inside
diameter of the final dimensions of the bores in which they are placed. In
this manner, the
2 5 desired final dimensions of the flowbody cylindrical features may be more
easily controlled
as the flowbody shrinks around the cylinders during sintering. In many
instances it will be
desirable to machine the diameter of the cylinders 34 and 38 to a diameter
smaller than the
final inside diameter of the flowbody's cylindrical features to provide a
small amount of
excess material for secondary machining operations. It should be appreciated
that the inserts
3 0 could instead be of any shape needed to form the bore during the sintering
process, as may be
required by the geometry of the desired end part.
With reference to FIG. 5, the present invention sintering process will be
described in
detail. Steps 12-18 comprise the wet green MIM part drying process. Prior art
drying
processes call for quickly drying M1M parts at an elevated temperature. This
procedure is
3 5 effective with small parts. However, large MIM parts with comparatively
large cylindrical
features tend to crack during a quick drying process leading to an
unacceptably high number
of scrap parts. It is believed that this is due to the rapid vaporization of
water from the
flowbody binder causing differential shrinkage between thick and thin flowbody
sections and
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between drier outer (external) portions and wetter internal portions. Thus, an
important step
in successfully producing large MIM parts is removing the water from the parts
without
producing cracks.
In step 12, one or more of the freshly-molded green flowbodies are sealed in
containers or bags, which may be made of plastic or any other suitable
material. The sealed
containers are stored for a 2-3 day period at room temperature and atmospheric
pressure.
During this time water vapor evaporates from each flowbody and condenses on
the container
or bag walls. In step 14, the sealed container or bag is vented to the
atmosphere to initiate a
slow drying rate. The flowbody is then stored in this state for a period of
three to five days.
During this period, water evaporates from the formerly sealed container or bag
and water
vapor continues to evaporate from the flowbody. '
In step 16, each flowbody is removed from the vented container and is allowed
to dry
on a shelf or other support for an additional two to three days. In general,
testing has
revealed that it is important to slowly dry the green flowbody to prevent
crack formation.
However, the duration of time the flowbody is dried in the sealed and vented
container and
on the shelf may vary considerably depending upon factors such as the size and
wall
thickness of the particular flowbody. Therefore, the drying times mentioned
are meant to be
examples only.
The time periods stated above were used to produce crack free flowbodies of
the type
2 0 shown in FIG. 1. In step 18, the flowbody is baked at 60° ~
5°C in an oven at atmospheric
pressure for about 24 hours. The low temperature oven baling vaporizes any
remaining
water in the flowbody. At the completion of the drying process, a dry green
flowbody
typically loses about 7% of its "as molded" weight. In step 20, the flowbody
is heated in a
furnace to about 275°C ~ 5°C for about two hours. This step
vaporizes the non-aqueous
2 5 portion of binder from the flowbody. At this point, the dry green flowbody
is ready for
sintering.
Further testing has indicated that the addition of one or more additives to
the binder
may permit a quicker drying process, which does not require placing the green
flowbody in a
container or bag, and which, for some applications, may result in a product
that is ready for
3 0 sintering after drying the green flowbody at room temperature for 2-3 days
or less. This
quicker drying method, however, appears to adversely affect surface finish,
e.g., pitting.
Testing is not complete and it has not been determined whether this addition
of additives to
the binder to reduce drying time is preferred for any particular application.
While the drying
method depicted in FIG. 5 is believed to be an acceptable method, it should be
appreciated
3 5 that other drying methods are contemplated and that the sintering method
to be described may
be used with any suitably dried green MIM part.
For first stage sintering, the flowbody is setup with the ceramic tools 32,
34, 41 and
42 as described above. In step 22, the flowbody is placed in a high-vacuum
furnace and is
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1 heated preferably to about 1235°C for a period of about thirty
minutes. The goal of first
stage sintering is to substantially fix the overall shape of the part. Thus,
at 1235°C for a
duration of thirty minutes, some inter-particulate melting will occur in the
flowbody.
Generally, this melting occurs on the exterior surfaces of the flowbody. The
typical density
of an Inconel 718 flowbody after first stage sintering is about 60% to 80% of
the maximum
obtainable density. During the first stage sintering, the flowbody is not
heated close enough
to the melting point of the metal alloy to become sufficiently plastic such
that gravity acting
on the flowbody can cause significant deformation of the flowbody.
Although temperature control during the sintering process is important, some
variation in temperature is permissible. For example, for first stage
sintering 1100°C to
1240°C is an acceptable working range for the flowbody. A temperature
range of 1230°C to
1240°C may also be used. The duration for W hick the flowbody is heated
may also vary
depending upon the geometry of the flowbody. Flowbodies with thin walls may
require less
sintering time, and correspondingly, flowbodies with thick walled sections may
require
longer sintering times.
Generally, after first stage sintering, the flowbody is removed from the high-
vacuum
ftirnace and allowed to cool for a period of several hours between first and
second stage
sintering. This cooling period is not critical to the process and primarily
allows the first stage
sintering tools to be removed from the flowbody and the second stage sintering
tools to be
2 0 installed in the flowbody. One or more flowbodies may be processed
simultaneously using
the process and tools described herein.
In step 24, the second stage sintering tools 36, 38, 41, and 42 are installed
in the
flowbody which is again placed in the high-vacuum furnace. The flowbody is now
heated to
a temperature of about 1280°C ~ 5°C for a period of about thirty
minutes. A temperature
2 5 above about 1270°C may also be used. The goal of second stage
sintering is to achieve
increased or even maximum densification of the flowbody. Temperature control
is more
critical in second stage sintering as the flowbody is heated to a temperature
near the melting
point of the alloy composition. In this regard, the sintering temperature
should not exceed the
melting point of the alloy. Test results reveal that using the 1280°C ~
5°C second stage
3 0 sintering, the densification approaches 99% of the density of the alloy in
its wrought form.
Conducting the second stage sintering at temperatures below 1275°C is
entirely possible. At
lower second stage sintering temperatures, less flowbody densification is
achieved in a given
time and correspondingly the finished part has a higher porosity and somewhat
reduced
working strength. This is entirely acceptable for parts where maximum strength
is not
3 5 required. After the second stage sintering, the flowbody may be machined
and/or heat treated
as desired. For example, the flowbody is solution heat treated and further
treated by
precipitation hardening to reach the desired mechanical property. This
procedure is known in
the art.
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A cast flowbody and an MIM flowbody typically have different surface
characteristics. A cast flowbody has a surface roughness of about 250 micro
inches, while an
MIM flowbody has a surface roughness of less than about 30 micro inches. Less
material is
wasted in the MIM process and less machining is required as compared to
casting, and
therefore it is less expensive to malce parts with the MIM process.
It will be appreciated that a new multi-stage MINI part drying and sintering
process
has been presented. These new processes allow for comparatively large MIM
parts to be
sintered while maintaining good dimensional control of the part's geometry. In
addition,
specialized aluminum oxide ceramic sintering tools which assist in maintaining
precise
dimensions of large cylindrical features have also been presented. While only
the presently
preferred embodiments have been described in detail, as will be apparent to
those slcilled in
the art, modifications and improvements may be made to the system and method
disclosed
herein without departing from the scope of the invention. Accordingly, it is
not intended that
the invention be limited except by the appended claims.
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30
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