Note: Descriptions are shown in the official language in which they were submitted.
POWDER INJECTION MOLD ASSEMBLY AND METHOD OF MOLDING
TECHNICAL FIELD
The application relates generally to powder injection mold assemblies, and
more
particularly to metal injection molding mold assemblies and method for molding
a metal
element
BACKGROUND OF THE ART
In powder injection molding (PIM), the solidification of the feedstock can be
achieved by
relying on the heat transferred to the cooler mold, or by heating the mold
during
injection and then cooling it to solidify the part. In either case, there are
many
challenges in creating complex passages and thin walls inside of a molded
part. As the
feedstock cools, its viscosity increases until it finally solidifies. Frequent
changes in
velocity, direction, and/or cross-sectional area of the passages tend to
increase the
heat loss, thus reduce the time before the feedstock solidifies, which can
create
injections defects. Flow lines, trapped air, and/or incomplete filling of the
mold can
occur during cooling/solidification and can cause defects in the shaped
element. For
example, if the shaped element includes adjacent holes, very thin material
portion
between the holes can remains unfilled as, during injection, the feedstock
rapidity
solidifies in the thin cavity portion of the mold assembly due to the high
thermal
capacity and conductivity of the mold material.
Heating the mold to prevent defects in the shaped element greatly increases
the energy
and cost necessary to perform the PIM process, and affects the time required
for
injection because of the difficulties in attaining a steady state. In
addition, careful
control of heat transfer during filling and solidification of the feedstock is
required in
PIM.
SUMMARY
In one aspect, there is provided a mold assembly for powder injection molding
of a
shaped element, the mold assembly defining a mold cavity for receiving a
feedstock
and comprising: a first mold portion having a first surface defining a first
portion of the
mold cavity and releasably engageable to the feedstock, the first mold portion
having a
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first thermal capacity and a first thermal conductivity; and a second mold
portion having
a second surface defining a second portion of the mold cavity and releasably
engageable to the feedstock, the second mold portion having a second thermal
capacity and a second thermal conductivity; wherein at least one of the first
thermal
capacity and the first thermal conductivity is lower than a respective one of
the second
thermal capacity and the second thermal conductivity.
In another aspect, there is provided a mold assembly for powder injection
molding of a
shaped element, the mold assembly comprising: a first mold part and a second
mold
part cooperating to define a mold cavity, the first mold part and the second
mold part
being movable relative to one another to selectively open and close the mold
cavity, the
first and second mold parts being disengageable from the shaped element after
molding; the first mold part comprising an integral mold portion having a
surface
defining the mold cavity, the mold portion made of a material different from
solid metal
and having at least one of a thermal capacity and a thermal conductivity lower
than that
of solid metal.
In a further aspect, there is provided a method of molding a green part, the
method
comprising: injecting a powder injection molding feedstock in a mold cavity
defined in a
mold assembly; extracting heat from the feedstock in the mold cavity through a
first
portion of the mold having a first surface in contact with the feedstock and
through a
second portion of the mold having a second surface in contact with the
feedstock, a
first heat flux through the first surface and first portion being lower than a
second heat
flux through the second surface and second portion to allow the feedstock to
fill the
mold cavity before solidification; solidifying the feedstock to create the
green part; and
disengaging the green part from the first and second portions of the mold by
extracting
the green part from the mold cavity.
DESCRIPTION OF THE DRAWINGS
Reference is now made to the accompanying figures in which:
FIG. la is a schematic cross-sectional view of a mold assembly in accordance
with a
particular embodiment;
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FIG. lb is a schematic cross-sectional view of a mold assembly in accordance
with
another particular embodiment;
FIG. 2a is a schematic cross-sectional view of a mold assembly in accordance
with
another particular embodiment, with hollow metal pins;
FIG. 2b is a schematic cross-sectional view of a hollow metal pin of the mold
assembly
of FIG. 2a;
FIG. 3a is a schematic cross-sectional view of a mold assembly in accordance
with
another particular embodiment, with pins integral to one mold part;
FIG. 3b is a schematic cross-sectional view of the mold assembly of FIG. 3a,
taken
along line B-B;
FIG. 3c is a schematic cross-sectional view of a mold assembly in accordance
with
another particular embodiment, with pins integral to two mold parts;
FIG. 4a is a schematic cross-sectional view of a mold assembly in accordance
with
another particular embodiment; and
FIG. 4b is a schematic cross-sectional view of a mold assembly in accordance
with
another particular embodiment.
DETAILED DESCRIPTION
Referring to Figs. la to 4b, there is provided various mold assemblies 10,
10', 110, 210,
210', 310, 310' (hereinafter, mold assemblies 10-310') used in a powder
injection
molding (PIM) process for producing a shaped element. In a particular
embodiment,
mold assemblies 10-310' can be used in metal injection molding (MIM) to mold a
green
part used to produce a metal element.
In a PIM process a feedstock comprising a material powder and a binder is
injected in a
mold assembly. Examples of possible powder materials include high temperature
resistant powder metal alloys, such as a cobalt alloy or nickel-based
superalloy, or
ceramic, glass, carbide or composite powders or mixtures thereof. In MIM
processes
the powder material is a metal powder. Other high temperature resistant
material
powders which may include one material or a mix of materials could be used as
well.
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The binder can include one or more binding material(s). The binder can include
various
components such as surfactants which are known to assist the injection of the
feedstock into the mold assembly for production of the shaped element. In a
particular
embodiment, the binder includes a mixture of binding materials, for example
including a
lower melting temperature polymer, such as a polymer having a melting
temperature
below 100 C (e.g. paraffin wax, polyethylene glycol, microcrystalline wax) and
a higher
melting temperature polymer or polymers, such as a polymer or polymers having
a
melting temperature above 100 C (e.g. polypropylene, polyethylene,
polystyrene,
polyvinyl chloride). Other suitable materials or mix of materials could be
used as well.
In a particular embodiment, the solid loading of the feedstock (i.e. the
proportion of
binder/powder material) is of 60%, or of more than 60%, where the solid
loading is
determined on a volume basis as VP/(VP + VB) wherein VP is the volume of
powder
material and VB the volume of binder. In a particular embodiment, the solid
loading is
selected to facilitate heat transfer within the feedstock so as to facilitate
cooling and
solidifying of the portions of the part not in direct contact with the
surfaces of the mold
assembly.
Powder injection molding can be performed under low or high pressure
conditions. In a
particular embodiment, the PIM process is performed at a pressure range of
less than
100 psi, preferably in a range of 50 to 100 psi. Lower pressures allow using
mold
portions of lower strength, such as hollow metal portions or plastic portions
coated with
metal.
Mold assemblies 10-310' can be used for molding green parts for obtaining
metal
elements, such as gas turbine engine components for aircraft. In a particular
embodiment, the metal element to be produced, and accordingly the
corresponding
molded green part, has at least one portion that has low thickness or complex
geometry. For example the metal element can be a gas turbine engine component
having a plurality of adjacent holes therein, the holes forming a complex
network of
channels in the mold cavity, where the flow of the feedstock may be difficult.
The metal
element can also comprise thin portions, molded with corresponding thin
portions of the
mold cavity, where the flow of the feedstock may also be difficult. In a
particular
embodiment, the metal element is a heat shield panel including a plate having
a small
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thickness, for example about 0.036 inch. In another embodiment, the metal
element is
a swirler for a fuel nozzle, including a plurality of differently angled
holes, with a
thickness of material between adjacent holes being small, for example 0.010
inch or
less. Other types of elements are also possible.
The mold assemblies 10-310' are used to mold the shaped element as a green
part.
The green part, once separated from the mold assembly 10-310', is then debound
to
produce a brown part, then sintered to produce the final element.
Referring to Fig. la, a mold assembly 10 in accordance with a particular
embodiment is
shown. The mold assembly 10 defines a mold cavity 12 for receiving the
feedstock
therein. It is understood that the configuration shown for the mold assembly
10 is
exemplary only, and that the mold cavity 12 has a shape substantially
corresponding to
the shape of the shaped element to be molded.
In a particular embodiment shown in Fig. la, the mold assembly 10 comprises a
first
mold part 14 and a second mold part 16. The first mold part 14 and the second
mold
part 16 are movable relative to one another to selectively open and close the
mold
cavity 12. In a closed configuration the first mold part 14 and the second
mold part 16
are engaged and secured to receive and retain the feedstock in the mold cavity
12. It is
understood that the mold assembly 10 can comprise more than two mold parts
movable relative to one another and engageable to close the cavity.
The first mold part 14 and the second mold part 16 are disengageable from the
shaped
element after molding. Once the feedstock is molded (i.e. the feedstock is
cooled until
the binder has reached a solid state), the first mold part 14 and the second
mold part
16 are disengaged from one another and from the shaped element. Therefore, the
shaped element is removed from the mold assembly 10 when it is in an open
configuration and none of the first mold part 14 or second mold part the 16
remains
engaged with the shaped element.
The mold assembly 10 includes a first mold portion 18 and a second mold
portion 20
which have different thermal capacities and/or different thermal
conductivities from
each other. Each portion can be defined by a respective one of the mold parts
14, 16,
or both portions can be defined in a same one of the mold parts 14, 16. In
Fig. la, the
first mold portion 18 is an integral portion of the first mold part 14, and
the second
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portion 20 is defined by a remainder of the first mold part 14 and by the
second mold
part 16.
The first mold portion 18 has a first surface 22 defining a first portion 24
of the mold
cavity 12. The second mold portion 20 has a second surface 26 defining a
second
portion 28 of the mold cavity 12. The first and second surfaces 22, 26, are in
contact
with the feedstock when the feedstock is injected and flows in the mold cavity
12 so
that heat can be transferred from the feedstock to the first and second mold
portions
18, 20 through the first and second surface 22, 26, respectively.
The first mold portion 18 and second mold portion 20 are integral elements of
the mold
assembly 10 and releasably engageable to the feedstock. Therefore, the first
surface
22 and the second surface 26 enter in contact with and engage the feedstock
when the
feedstock flows in the mold cavity 12. Heat is then removed through the first
surface 22
and through the second surface 26 and the feedstock solidifies. However, when
the
shaped element is removed from the mold assembly 10, the first portion 18 and
the
second portion 20 are disengaged from the shaped element.
The first mold portion 18 has a first thermal capacity and a first thermal
conductivity.
The second mold portion 20 has a second thermal capacity and a second thermal
conductivity. In a particular embodiment, the first thermal capacity is lower
than the
second thermal capacity and/or the first thermal conductivity is lower than
the second
thermal conductivity. As used herein, "thermal capacity" refers to the ratio
of heat
added/removed from an object to the resulting temperature change, i.e. the
ability of a
material to absorb heat, while "thermal conductivity" refers to the rate at
which heat
flows through a material, i.e. the ability of a material to transfer heat
therethrough;
accordingly, a lower thermal capacity results in a material having a lower
ability to
absorb heat, and a lower thermal conductivity results in a material having a
lower ability
to transfer heat therethrough, both of which resulting in a lower quantity of
heat being
removed from a component (e.g. feedstock) adjacent to that material in a given
period
of time. Therefore, the flux of heat through the first surface 22 and first
portion 18 is
lower than the flux of heat through the second surface 26 and second portion
20 so that
a smaller amount of heat is removed from the feedstock in the first portion 24
of the
mold cavity 12 than from the feedstock in the second portion 28 of the mold
cavity 12.
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In a particular embodiment, the first mold portion 18 having the lower thermal
capacity
and/or thermal conductivity is made of a material different from solid metal.
Therefore,
the first mold portion 18 has a thermal capacity that is lower than the
thermal capacity
of solid metal and/or a thermal conductivity that is lower than the thermal
conductivity of
solid metal. The thermal capacity and thermal conductivity of the first mold
portion 18
can both be lower than that of solid metal, or only one of the thermal
capacity and
conductivity can be lower than that of solid metal. As a consequence, the heat
flux
through the first surface 22 and first portion 18 is lower than through a
similar portion
made of solid metal material.
In the embodiment shown in Fig. la, the first and second mold portions 18, 20
are
made of solid material, with the solid material of the first mold portion 18
having a lower
thermal capacity and thermal conductivity than the solid material of the
second mold
portion 20. For example, the first mold portion 18 can be made of plastic,
ceramic,
glass or a combination thereof while the second mold portion 20 is made of
metal.
Other combinations of materials are also possible. In a particular embodiment
the
thermal conductivity of the first mold portion 18 is in the range of 0.1 to
0.5 W/mK while
the thermal conductivity of the second mold portion 20 is in the range of 10
to 60
VV/mK. Other values are also possible.
Referring to Fig. 1 b, a mold assembly 10' in accordance with another
particular
embodiment is shown, where elements similar to that of the mold assembly 10
are
referred to using the same reference numeral and will not be described further
herein.
The mold assembly 10' differs from the mold assembly 10 of Fig. la in that
first mold
portion 18' includes a core 30 coated with a layer 32 defining the surface 22
of the mold
cavity 12, the core 30 and layer 32 being made of different materials. One of
the core
30 and the layer 32 may be made of the same material as the second mold
portion 20.
Alternately, both the core 30 and layer 32 may be made of a different material
than that
of the second mold portion 20.
In a particular embodiment, the core 30 is made of metal, and the layer 32 is
made of
plastic, ceramic or glass. In that case, the layer 32 has a lower conductivity
than solid
metal and insulates the metal core 30 so that the heat flux through the first
surface 22
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and first portion 18' is reduced with respect to that through the second
surface 26 and
second portion 20.
In another embodiment, the core 30 is made of plastic, ceramic or glass, and
the layer
32 is made of metal, so as to allow a polished finish of the molded surface,
have
increased wear, and/or reduce chemical interactions between the core 30 and
the
feedstock; other advantages could also motivate the use of a layer 32 of
different
material. The presence of the core 30 allows for the first portion 18' to have
a lower
total thermal capacity than a first portion 18' completely made of solid
metal, so that the
heat flux through the first surface 22 and first portion 18' is lower than
through the
second surface 26 and second portion 20.
Alternately, the core 30 may be replaced by an insulating cavity filled with
air or with
any other suitable gas, so that the first mold portion 18' is configured as a
thermal
insulating storage container. As air is a good thermal insulator, the flux of
heat through
the first surface 22 and first portion 18' is reduced compared to a solid mold
portion
made of the same material as that of the layer 32.
Referring to Figs. 2a and 2b, a mold assembly 110 in accordance with another
particular embodiment is shown, which also includes first and second mold
parts 114,
116 movable relative to one another to selectively open and close the mold
cavity 112,
and disengageable from the shaped element after molding. The mold assembly 110
also includes a first mold portion 118 and a second mold portion 120 which
have
different thermal capacities and/or different thermal conductivities from each
other.
In this embodiment, the first mold portion 118 is integral to the first mold
part 114 and
the second mold portion 120 includes the remainder of the first mold part 114
and an
entirety of the second mold part 116. The first mold portion 118 defines a
plurality of
pins, protruding within the mold cavity 112 to allow formation of holes in the
shaped
element. The first mold portion 118 defines a first portion 224 of the mold
cavity 112
that has a cross-section that is smaller than the cross-section of a second
portion 228
of the mold cavity 112 which is defined by the second mold portion 120. In a
particular
embodiment, a smaller cross-section causes change in the direction and/or
velocity of
the flow of feedstock injected and flowing in the mold cavity 112. As shown in
Fig. 3b,
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the pins 118 can define a network 246 within the mold cavity 212, in which the
direction
and/or the velocity of the feedstock flowing in the mold cavity 212 changes.
Referring back to Fig. 2b, each pin includes a layer 138 of material, for
example a
metal layer, defining the first surface 122 in contact with the feedstock, and
having an
inner surface 140 opposed to the first surface 122; the inner surface 140
defines an
insulating cavity 142. The presence of the insulating cavity 142 reduces the
total
thermal capacity of the pins defining the first mold portion 118, which in a
particular
embodiment slows the cooling of the feedstock in the associated portion 224 of
the
mold cavity 112, thus improving the flow in the restricted areas of the mold
cavity 112.
Alternately, the insulating cavity 142 may be replaced by a core made of a
different
material than that of the layer 138, where the core or the layer 138 may be
made of the
same material as the second mold portion 120. As previously mentioned, various
combinations are possible, including, but not limited to, a metal core with a
plastic,
ceramic or glass layer, and a plastic, ceramic or glass core with a metal
layer.
Referring to Fig. 3a, a mold assembly 210 in accordance with another
particular
embodiment is shown, where elements similar to that of the mold assembly 110
are
referred to using the same reference numeral and will not be described further
herein.
Similarly to the mold assembly 110, the first mold portion 218 includes a
plurality of
pins. In this embodiment however, the first mold portion 218 is made of a
solid material
different than that of the second mold portion 120. For example, the first
mold portion
218 can be made of plastic, ceramic, glass or a combination thereof while the
second
mold portion 120 is made of metal. Other combinations of material are also
possible.
Referring to Fig. 3c, a mold assembly 210' in accordance with another
particular
embodiment is shown, where elements similar to that of the mold assembly 210
are
referred to using the same reference numeral and will not be described further
herein.
Similarly to the mold assembly 110, 210, the first mold portion 218' includes
a plurality
of pins. In this embodiment however, the first mold portion 218' is integral
to both the
first mold part 114 and the second mold part 116, i.e. some of the pins are an
integral
part of the first mold part 114 while some of the pins are an integral part of
the second
mold part 116. Although the first mold portion 218' is depicted as being made
of solid
material, it is understood that alternately, it can include insulating
cavities such as in the
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first mold portion 118 of Figs. 2a-2b, or a core surrounded by a layer
defining the
surface of the mold cavity in contact with the feedstock.
Referring to Fig. 4a, a mold assembly 310 in accordance with another
particular
embodiment is shown, which also includes first and second mold parts 314, 316
movable relative to one another to selectively open and close the mold cavity
312, and
disengageable from the shaped element after molding. The mold assembly 310
also
includes a first mold portion 318 and a second mold portion 320 which have
different
thermal capacities and/or different thermal conductivities from each other.
In this embodiment, the first mold portion 318 correspond to the first mold
part 314 and
the second mold portion 320 corresponds to the second mold part 316; the two
mold
parts 314, 316 as a whole thus have different thermal capacities and/or
different
thermal conductivities from each other. Alternate configurations are also
possible.
The first surface 322 of the first mold portion 318 and the second surface 326
of the
second mold portion 320 are in proximity to each other when the mold assembly
310 is
closed, so as to define a mold cavity 312 having a small thickness, defining
for example
a plate or sheet like shaped element, for example a platform of a metal heat-
shield
panel used in gas turbine engine. In this embodiment, the first mold portion
318 is
made of solid material having a lower thermal capacity and thermal
conductivity than
the material of the second mold portion 320 and/or than that of solid metal.
For
example, the first mold portion 318 can be made of plastic, ceramic, glass or
a
combination thereof while the second mold portion 320 is made of metal. Other
combinations of materials are also possible.
Referring to Fig. 4b, a mold assembly 310' in accordance with another
particular
embodiment is shown, where elements similar to that of the mold assembly 310
are
referred to using the same reference numeral and will not be described further
herein.
The mold assembly 310' differs from the mold assembly 310 in that the first
mold
portion 318' includes a core 330 coated by an outer layer 332 defining the
surface 322
in contact with the feedstock, with the core 330 and layer 332 being made of
different
materials, and where the core 330 or the layer 332 may be made of the same
material
as the second mold portion 320. In a particular embodiment, the core 330 is
made of
metal, and the layer 332 is made of plastic, ceramic or glass. In another
embodiment,
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the core 330 is made of plastic, ceramic or glass, and the layer 332 is made
of metal,
so as to allow a polished finish of the molded surface, have increased wear,
and/or
reduce chemical interactions between the core 330 and the feedstock; other
advantages could also motivate the use of a layer 332 of different material.
It is understood that although the first mold portion 18, 18', 118, 218, 318,
318' is
depicted in the Figures as being an element of an upper mold part of the
assembly, it is
understood that the mold assembly 10-310' can have any other suitable
orientation.
In a particular embodiment, mold assemblies 10-310' using mold portions having
a
conductivity and/or a thermal capacity lower than that of solid metal and/or
lower than
that of another mold portion of the mold assembly 10-310' allow complete
filling of the
mold cavity, elimination of weld lines and/or elimination of air entrapment,
by effectively
slowing the cooling and thus slowing the increase in viscosity and the
subsequent
solidification of the feedstock, thus facilitating flow of the feedstock in
restricted regions
of the mold cavity.
In use in a particular embodiment, a shaped element is molded using powder
injection
molding, for example metal injection molding, in accordance with the
following. The
feedstock having a composition and a solid loading as defined above is
injected in the
mold cavity defined in the mold assembly 10-310'. The feedstock is injected as
a
viscous suspension to be solidified by extracting heat through the mold
assembly 10-
310'. Heat is extracted from the feedstock through the first mold portion 18,
18', 118,
218, 218', 318, 318' having a first surface contacting the feedstock, and
through a
second mold portion 20, 120, 320 having a second surface contacting the
feedstock.
The first surface defines a first portion of the mold cavity and the second
surface
defines a second portion of the mold cavity.
In a particular embodiment, the heat flux through the first surface and first
mold portion
18, 18', 118, 218, 218', 318, 318' is lower than the heat flux through the
second surface
and second mold portion 20, 120, 320, so as to slow the increase in viscosity
and the
subsequent solidification of the feedstock in the first portion of the mold
cavity as
compared to what it would be if the two portions were made of material having
the
same thermal properties. The feedstock can therefore fill the entire mold
cavity before
solidifying, thereby preventing formation of flow lines, weld lines, trapping
of air or other
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defects in the shaped element. This can also decrease the pressure required to
fill the
mold cavity and further decrease the number of possible defects.
The first mold portion 18, 18', 118, 218, 218', 318, 318' is made of a
material having a
first thermal capacity and a first thermal conductivity. The second mold
portion 20, 120,
320 is made of a material having a second thermal capacity and a second
thermal
conductivity. In a particular embodiment, at least one of the first thermal
capacity and
the first thermal conductivity is lower than a respective one of the second
thermal
capacity and second thermal conductivity, and/or at least one of the first
thermal
capacity and the first thermal conductivity is lower than that of solid metal.
Once the feedstock is solidified to create the green part, the green part is
disengaged
from the mold portions 18, 18', 118, 218, 218', 318, 318', 20, 120, 320 by
extracting the
green part from the mold cavity. The green part can then be debound and
sintered to
create the metal element.
The above description is meant to be exemplary only, and one skilled in the
art will
recognize that changes may be made to the embodiments described without
departing
from the scope of the invention disclosed. Modifications which fall within the
scope of
the present invention will be apparent to those skilled in the art, in light
of a review of
this disclosure, and such modifications are intended to fall within the
appended claims.
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