Background of the invention
The present invention relates to the field of
5 casting, and more particularly to producing ceramic cores
for casting, which cores are to be received in casting
molds for forming internal cavities in castings made in
such molds.
10
Such cores may be used in particular in lost model
or lost wax casting methods. So~called "lost model'' or
"lost wax" casting methods have been known since
antiquity. They are particularly adapted to producing
metal parts of complex shapes. Thus, lost wax casting is
used in particular for producing turbomachine blades. In
15 the present context, the term "turbomachine" is used to
designate any machine in which it is possible to transfer
energy between a flow of fluid and at least one rotor,
such as for example a compressor, a pL®p, a turbine, a
propeller, or a combination of a plurality of such
20 elements. The term "blade" is used to cover any airfoil
element, whether stationary or rotary, that contributes
to this transfer of energy in the turbomachine. Lost wax
casting is particularly useful for production.
In lost wax casting, the first step is normally to
25 make a model out of a meltable material, having a melting
temperature that is comparatively low, such as for
example wax or resin. The model is then integrated with
a casting tree that is to be covered in refractory
material in order to form a mold. After the meltable
30 material of the mold has been removed or eliminated from
the inside of the mold, whence the term "lost" in the
name for such methods, molten metal is cast into the mold
in order to fill the cavity in the mold that is formed by
the model being removed or eliminated from the mold.
35 Once the metal cools and solidifies, the mold can be
opened or destroyed in order to recover a metal part
having the shape of the model. The term "metal" is used
2
in the present context to cover not only pure metals, but
above all metal alloys.
In order of form complex internal cavities within
metal parts that are obtained by such casting methods, it
5 is possible to integrate one or more refractory cores in
each model. These refractory cores remain in position
inside each mold after the meltable material has been
removed, thereby enabling complex shapes to be formed
inside the casting that is made in the mold. They can
10 subsequently be eliminated together with the rest of the
mold when unmolding the part.
Typically, such refractory cores are made of ceramic
material and constitute consumables in the casting
method. To produce them, injection-molding methods have
15 been developed in which a paste comprising a ceramic
granulate and a polymer binder is injected under pressure
into a mold cavity in order to form the core, with the
core then being fired in order to consolidate it. In
order to further increase the ability of the core to
20 withstand the forces to which it is to be subjected
during the casting process, the core may also be
impregnated with a resin after its initial firing, prior
to being fired again a second time.
Nevertheless, the reject rate when producing such
25 cores can be relatively high, particularly when they are
complex in shape. This is due to internal stresses that
are generated during the firing and subsequent cooling of
the core, which internal stresses can give rise to
cracking in certain critical points of the core. In
30 particular, the core may present lateral protuberances,
especially when it is to form a cooling circuit in a
turbomachine blade, which lateral protuberances may then
be intended to form outlet channels leading to an outside
surface of the blade, and in particular outlet slots in
35 the trailing edge of the blade. Under such
circumstances, cracks can form at the root of at least
one of the lateral protuberances.
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Object and summary of the invention
The invention seeks to remedy those drawbacks. ·In
particular, the present disclosure seeks to propose a
method of producing a ceramic core for casting that makes
it possible to avoid forming cracks at the root of at
least one first lateral protuberance of the core.
In at least one embodiment of the invention, this
object is achieved by the fact that the method comprises
at least a step of injecting a p~ste comprising a ceramic
granulate and a polymer binder into a mold cavity in
order to form a part presenting a shape corresponding to
the shape of the core together with at least one
additional lateral protuberance adjacent to said first
15 lateral protuberance, a step of firing the part, and a
step of eliminating said additional lateral protuberance
from the part after said firing step.
By means of these provisions, the internal stresses
tending to generate cracks during firing can be
20 concentrated on the additional lateral protuberance of
the part, which additional later·al protuberance can then
perform a sacrificial role, since it is subsequently
eliminated and is no longer to be found on the finished
ceramic core. The first lateral protuberance of the
25 core, together with any other adjacent lateral
protuberances can then be spared from such cracks being
formed, thereby reducing the reject rate.
In order to better consolidate the part, the method
may also include a step of impregnating the part with a
30 resin after said firing step. This impregnation step may
take place before or after eliminating the additional
lateral protuberance.
In order to better concentrate internal stresses of
the part on the additional lateral protuberance, the
35 additional lateral protuberance may have a cross-section
presenting an area corresponding to 75% to 125% of the
area of a cross-section of the first lateral protuberance
4
in the same section plane. In particular, the additional
lateral protuberance may have a thickness parallel to a
transverse axis of the part corresponding to 90% to 110%
of the thickness of the first lateral protuberance
5 parallel to the same transverse axis, and/or a width
parallel to a longitudinal axis of the part corresponding
to 90% to 110% of the width of the first lateral
protuberance parallel to the same longitudinal axis.
10
15
The shape of the core may present not only a first
lateral protuberance, but also a.plurality of lateral
protuberances spaced apart from one another parallel to a
longitudinal axis of the part, in particular with crosssections
that decrease going away from the first lateral
protuberance. The additional lateral protuberance of the
part may be spaced apart from said first lateral
protuberance along the same axis and in the opposite
direction to the other lateral protuberances of said
plurality of lateral protuberances. Under such
circumstances, and in order to better concentrate
20 internal stresses on the additional lateral protuberance,
a minimum distance between the additional lateral
protuberance of the part and the first lateral
protuberance may be 0.7 times to 2.5 times, and in
particular 1 to 1.5 times, a minimum distance between the
25 first lateral protuberance and the closest of tlte lateral
protuberances of said plurality of lateral protuberances.
In particular, the core may be for being received in
a casting mold for producing a turbomachine blade in
order to form a cooling circuit in the blade, said first
30 lateral protuberance forming an outlet slot in the
trailing edge.
Brief description of the drawing
The invention can be well understood and its
35 advantages appear better on reading the following
detailed description of an embodiment given by way of
I
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non-limiting example. The description refers to the
accompanying drawing, in which:
· Figure 1 is a view of a ceramic core produced by
the method of the invention;
· Figure 2A is a side view of a part that is to form
the core, prior to eliminating an additional lateral
protuberance; and
· Figure 2B is a view of the same part in section on
plane IIB-IIB.
Detailed description of the invention
Figure 1 shows a ceramic core 1 made using a method
constituting a specific embodiment of the invention. The
ceramic core 1 is for use in producing a turbomachine
blade 2 (drawn in dashed lines) by lost wax casting. As
can be seen in figure, the core 1 presents four lateral
protuberances 11, 12, 13, and 14 oriented parallel to a
transverse axis X of the core 1, and offset from one
another parallel to a longitudinal axis Z of the core 1.
20 These four lateral protuberances 11, 12, 13, and 14 are
in the form of teeth and are for forming trailing edge
outlet slots in the blade 2 for a cooling circuit of the
blade 2.
In a first step of producing the core 1, a paste
25 comprising a ceramic granulate, e.g. constituted by
silica grains of micrometer size, and a polymer binder
serving to give the paste a desired degree of viscosity,
is injected at high pressure, e.g. lying in the range
5 megapascals (MPa) to 70 MPa, into a mold cavity
30 presenting a shape corresponding to the shape of the core
1, but together with an additional lateral protuberance.
This produces an intermediate part that is consolidated
by sintering the ceramic granulate during a firing step.
By way of example, the firing step may have a total
35 duration lying in the range 30 hours (h) to 45 h, with
one plateau at 80°C and another plateau at 1200°C so as
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to ensure not only that the ceramic granulate is
sintered, but also that the polymer binder is eliminated.
The intermediate part 10 as consolidated in this way
is shown in Figures 2A and 2B. As can be seen in
5 Figure 2A, the intermediate part 10 that is to form the
core 1 has at least one additional lateral protuberance
15 adapted to be eliminated after fidng the intermediate
part 10 and prior to using the core 1. Thus, in the
embodiment shown, in addition to the four lateral
10 protuberances 11, 12, 13, and 14. of the core 1, the
intermediate part presents an additional lateral
protuberance 15 adjacent to the first lateral
protuberance 11, which in the embodiment shown is that
one of the four lateral protuberances 11, 12, 13, and 14
15 that is the closest to the root of the blade 2. More
specifically, the additional lateral protuberance 15 is
spaced apart from the first lateral protuberance 11 going
away from the other lateral protuberances 12, 13, and 14.
On the part 10, the minimum distance d0 between the
20 additional lateral protuberance 15 and the first lateral
protuberance 11 in a direction parallel to the
longitudinal axis Z lies in the range 0.7 times to 2.5
times the minimum distance d1 between the first lateral
protuberance 11 and the following lateral protuberance
25 12. In particular, the distance d0 may lie in the range 1
to 1. 5 times the distance d1 •
Figure 2B shows the cross-sections of the lateral
protuberances 15, 11, 12, 13, and 14 in the section plane
IIB-IIB of Figure 2A. As shown diagrammatically in this
30 figure, the area s 0 of the cross-section of the additional
lateral protuberance 15 is similar to the area s 1 of the
cross-section of the first lateral protuberance 11. In
particular, the area s 0 may lie in the range 75% ·to 125%
of the area s 1 • Specifically, the two areas s 0 and s 1 may
35 be substantially equal. The areas s 2 , s 3 , and s 4 of the
cross-sections of the other lateral protuberances 12, 13,
and 14 may decrease going from the area s 1 of the cross7
section of the first lateral protuberance 11, in such a
manner that s 1>s2>s3>s4 , or at least all of them are less
than the areas s 0 and s 1 •
In the embodiment shown, each of the lateral
5 protuberances 15, 11, 12, 13, and 14 presents a section
that is substantially rectangular. The width £0 and the
thickness e 0 of the additional lateral protuberance 15 in
directions that are parallel respectively to the
longitudinal axis Z and to a second transverse axis Y
10 perpendicular to the first trans~erse axis X, may each
present a value lying in the range 90% to 110% of the
corresponding dimensions f 1 and e 1 of the first lateral
protuberance 11.
As a result of the part 10 being fired and sintered,
15 it shrinks, thereby generating internal stresses in the
part 10. These internal stresses can produce cracks at
the root of the additional lateral protuberance 15, which
thus plays a sacrificial role in order to avoid cracks
forming in the first lateral protuberance 11 or in the
20 subsequent lateral protuberances 12, 13, and 14.
Nevertheless, in a subsequent step, the additional
lateral protuberance 15 is eliminated, e.g. by machining,
so as to give the part 10 .the final shape for the core 1.
The cracks are thus eliminated together with the
25 additional lateral protuberance 15.
In another step, that may be performed before, or
preferably after, eliminating the additional lateral
protuberance 15, it is possible to impregnate the part 10
with a thermosetting resin in order to give it even
30 better mechanical properties.
35
After these steps, starting from the paste of
ceramic granulate and polymer binder, and going via the
part 10 with an additional lateral protuberance, the
ceramic core 1 is thus.obtained in its final shape.
The resulting core 1 can then be incorporated in a
casting mold in order to form complex internal outlines
in a casting made in the mold.
8
Thus, in a lost wax casting method, the core 1 as
obtained in this way can be incorporated in a model of
the metal part that is to be made, which model is made
using a material that melts at a melting temperature that
5 is comparatively low. Such a material may be a wax or a
resin, for example. The model can then be integrated in
a casting tree designed to be coated in refractory
material in order to form a casting mold. After removing
or eliminating the meltable material of the model from
10 the inside of the casting mold, molten metal is cast into
the mold so as to fill the molding cavity that is formed
by the model in the casting mold after the model has been
removed or eliminated. Once the metal cools and
solidifies, the mold is opened or destroyed in order to
15 recover a metal part having the shape of the model.
Although the present invention is described with
reference to a specific embodiment, it is clear that
various modifications and changes may be applied thereto
without going beyond the general ambit of the invention
20 as defined by the claims. Consequently, the description
and the drawings should be considered in a sense that is
illustrative rather than restrictive.

CLAIMS
1. A method of producing a ceramic core (1} for casting,
said core (1} presenting a shape that includes at least a
first lateral protuberance (11}, and the method
5 comprising at least:
· a step of injecting a paste comprising a ceramic
granulate and a polymer binder into a mold cavity in
order to form a part (10} presenting a shape
corresponding to the shape of the core (1} together with
10 at least one additional lateral ~rotuberance (15}
adjacent to said first lateral protuberance (11};
a step of firing the part (10}; and
a step of eliminating said additional lateral
protuberance (15} from the part (10} after said firing
15 step.
20
2. A method according to claim 1, wherein the method also
includes a step of impregnating the part (10} with a
resin after said firing step.
3. A method according to any preceding claim, wherein the
additional lateral protuberance (15) is eliminated by
machining.
25 4. A method according to any preceding claim, wherein the
additional lateral protuberance (15} has a cross-section
presenting an area (s0 } corresponding to 75% to 125% of
the area (s 1 } of a cross-section of the first lateral
protuberance (11} in the same section plane.
30
5. A method according to any preceding claim, wherein the
additional lateral protuberance (15} has a thickness (e0 }
parallel to a transverse axis (Y} of the part (10}
corresponding to 90% to 110% of the thickness (e 1 } of the
35 first lateral protuberance (11} parallel to the same
transverse axis (Y} .
10
6. A method according to any preceding claim, wherein the
additional lateral protuberance has a width (£ 0 ) parallel
to a longitudinal axis (Z) of the part corresponding to
90% to 110% of the width of the first lateral
5 protuberance parallel to the same longitudinal axis.
7. A method according to any preceding claim, wherein the
shape of the core (1) presents a plurality of literal
protuberances (11, 12, 13, 14) spaced apart from one
10 another parallel to a longitudinal axis (Z) of the part
( 10) .
8. A method according to claim 7, wherein said lateral
protuberances (11, 12, 13, 14) of the core (1) present
15 cross-sections that decrease going away from the ·first
lateral protuberance (11).
9. A method according to claim 7 or claim 8, wherein the
additional lateral protuberance (15) of the part (10) is
20 spaced apart from said first lateral protuberance (11)
along the same axis and in the opposite direction to the
other lateral protuberances (12, 13, 14) of the core (1).
10. A method according to claim 9, wherein a minimum
25 distance. (d0 ) between the additional lateral protuberance
(15) of the part (10) and the first lateral protuberance
(11) is 0.7 times to 2.5 times a minimum distance (d1 )
between the first lateral protuberance (11) and the
closest of the lateral protuberahces (12) of said
30 plurality of lateral protuberances.
35
11. A method according to any preceding claim, wherein
said core (1) is for being received in a casting mold for
producing a turbomachine blade (2) in order to form a
cooling circuit in the blade (2), said first lateral
protuberance (11) forming an outlet slot in the trailing
edge of the blade (2).