Injection mold for wax model of a turbine blade having
an isostatic core holder
The field of the present invention is that of the
manufacture of parts for turbine engines and, in
particular, that of the manufacture of the turbine
blades thereof by a so-called lost wax casting
technique.
The lost wax casting technique firstly consists in
producing a model, made of wax or any other material
which is able to be easily removed afterwards, of the
part to be produced; said model comprises an internal
part forming a ceramic core which represents the
desired cavities to appear inside the blade assembly.
The wax model is then immersed several times in
slurries consisting of a suspension of ceramic
particles to produce, by so-called stucco-coating and
drying operations, a mold shell.
Then the wax is removed from the mold shell which is an
operation by which the wax or the material forming the
original model is removed from the shell. After this
removal process a ceramic mold is obtained, the cavity
thereof reproducing all the shapes of the blade and
still enclosing the ceramic core designed to generate
the internal cavities thereof. The mold is then
subjected to a thermal treatment at high temperature or
"firing" which provides it with the required mechanical
properties.
The mold shell is now ready for the manufacture of the
metal part by casting. After checking the internal and
external integrity of the mold shell, the following
step consists in casting a molten metal which fills the
spaces between the internal wall of the mold shell and
the core, and solidifying said metal. In the field of
lost wax casting, currently a plurality of
solidification techniques are differentiated, i.e. a
plurality of casting techniques, depending on the
nature of the alloy and the expected properties of the
part resulting from the casting. This may be columnar
structure directional solidification (DS), singlecrystal
structure directional solidification (SX) or
equiaxis solidification (EX).
After the casting of the alloy, the shell is broken by
a knocking-out operation. During a further step, the
ceramic core which remains enclosed in the resulting
blade is chemically removed. The resulting metal blade
is then subjected to finishing operations which permit
the finished part to be obtained.
Examples of producing turbine blades by the lost wax
casting technique are provided by the applicant in the
patent applications FR2875425 and FR2874186.
To form the wax model of the blade, a tool or wax
injection mold is used, in which the core is placed,
and then the liquid wax is injected via a channel
provided to this end.
It is necessary to place this core extremely accurately
in the injection-mold as any shifting of the
positioning thereof is manifested by non-uniformities
in the thickness of the walls of the blade. As the
metal of the blade is subjected to very high
temperatures, these faults will be manifested as a
considerably reduced service life of the blade. It is
thus necessary to ensure the position occupied by the
core in the mold with a high degree of accuracy. To
achieve this, the molds currently used comprise small
vertical rods which support the core. Bearing points on
one face of the core (suction face or pressure face)
correspond to said rods, which constitute a reference
for the positioning of the core relative to the lower
mold cavity. A second series of rods, which correspond
b spatially to the rods of the aforementioned bearing
points, extends from the upper face of the mold to form
I clamping points on the second face of the core. By
I being clamped between the rods of the bearing points
5 and those of the clamping points, the core is then
correctly and securely positioned in the i'njection
mold.
In current practice, the mold comprises rods on its
10 suction face cavity and pressure face cavity as
follows: four rods corresponding to bearing points on
the core serve to support the core via the suction face
of its aerofoil; they are oriented substantially
vertically. The two last bearing points serve as a
15 lateral abutment for the core in the mold; four
clamping points on the pressure face of the core
correspond to these four bearing points of the suction
face opposite the four bearing points of the suction
face. One example of the positioning of the bearing
20 points and the clamping points in the prior art is
provided in the patent applications EP 1775054, EP
0099215 or GB 2073094; it is also shown in figures 2
and 3.
25 Whilst the length of the supporting rods and clamping
rods is the thickness which is desired for the walls of
the blade and will be denoted as long rods in the
remainder of the present document, the two faces of the
mold also comprise complementary shorter rods, located
30 opposite points on the blade denoted as false points
and, as a result, denoted short rods. Said short rods
also extend vertically but have a slightly shorter
length than that of the rods of the bearing points and
clamping points, such that they do not normally come
35 into contact with the (suction face and pressure face)
surfaces of the core. The purpose thereof is only to
come into contact with the core in case of the
deformation thereof and thus create a tolerance
& envelope around the core. To achieve this, the length
thereof extends up to, for example, 5/100th of a
millimeter of the maximum tolerance of the core. It
follows that if the core is badly positioned and
5 exceeds said tolerance envelope of 5/100th of a
millimeter, the short rods come into contact with the
false points on the surface of the core and either make
the closure of the mold impossible or cause the
breakage of the core when the injection mold is closed,
10 thus making the injection process impossible. Thus,
only cores with dimensions within the tolerance
envelope are able to be used for the injection-molding
of a wax model. It is preferable that the core breaks
at this stage rather than the wax being injected around
the core with an incorrect wall thickness of the wax.
Maintaining the correct wall thicknesses on the metal
part is thus ensured.
The problem encountered with a device for positioning
the core as described above is that the core may still
be deformed or adopt an incorrect position when
positioned ,and clamped in the wax mold. The core is in
fact held and positioned in the region of the four
bearing points and the four clamping points, which are
25 preferably positioned in the region of the upper and
lower ends of the leading edge and the trailing edge of
the aerofoil of the core. However, when positioned and
clamped in this manner, the core is braced and may be
subjected to positioning stresses and clamping stresses
30 which are too high.
The core, being naturally slightly twisted before its
positioning and clamping in the wax injection mold, may
be subjected to too much straightening at the bearing
35 and clamping points. It may then break or even lose its
given position which is released when the mold is
opened. This release of stress may be even greater when
removing wax from the wax model in the shell. The
.& variation of the position of the core when removing the
wax may thus lead to faults in the thickness of the
walls on the metal part after casting.
5 The object of the present invention is to remedy said
drawbacks by proposing a system for positioning a core
in an injection mold for a wax model which does not
have some of the drawbacks of the prior art and, in
particular, which avoids the appearance of stresses in
10 the aerofoil of the core when installed in the mold and
when injecting the wax.
To this end, the subject of the invention is an
injection mold for producing a model of a turbine
15 engine blade to be produced by the lost wax casting
technique, comprising an upper face which folds onto a
lower face, the two faces having a footprint
reproducing the external shape of the blade to be
produced and carrying long rods for supporting a core
20 designed to generate cooling cavities inside the blade,
said core having the shape of an aerofoil between a
root and a tip, said long rods of the lower face being
designed to come into contact with a first face of the
aerofoil in the region of bearing points and said long
25 rods of the upper face being designed to come into
contact with a second face of. said aerofoil in the
region of clamping points, each clamping point of the
second face being positioned opposite a bearing point
of the first face, characterized in that it further
30 comprises at least one short rod extending from one of
the faces of the mold and pointing toward a false point
positioned on one of the faces of the aerofoil, the
length of said short rod being less than the distance
between the face of the mold and that of the aerofoil
35 in the region of said false point and the difference
between said length and said distance being equal to
the tolerance at the position of said false point in
the case of deformation of the core.
"Tolerance at the position of a point" is understood as
the maximum acceptable deformation at that point. With
the specified length for the short rod, it is ensured
that if the core .comes into contact with the short rod,
it would have exceeded fixed tolerances for the
deformation of the core and thus interference with the
short rod would either result in not being able to
close the mold or breakage of the core.
Advantageously, the number of pairs consisting of a rod
associated with a bearing point on the first face and a
rod associated with a clamping point on the second face
of the aerofoil of the core is at most equal to 3. The
limitation to three of the number of corresponding
bearing points and clamping points on the blade permits
the aerofoil of the core to be held without torsional
stress or flexural stress on said aerofoil and thus the
absence of undesirable movement of the core before or
during the injection of wax. In the case of three pairs
of rods, an isostatic support of the aerofoil of the
core is obtained i.e. without harmful stress.
In a particular embodiment, the pairs of long rods are
associated with bearing points on said first face and
with clamping points on said second face, positioned
for a first pair in the region of the center of the
aerofoil, on the trailing edge side, and for the two
other pairs in the region of the upper and lower ends
of the aerofoil on the leading edge side.
In a further particular embodiment, the pairs of long
rods are associated with bearing points on said first
face and with clamping points on said second face,
positioned for a first pair in the region of the center
of the aerofoil on the leading edge side and for the
two other pairs in the region of the upper and lower
ends of the aerofoil on the trailing edge side.
B
Preferably, said short rod is positioned longitudinally
in the region of one of the bearing points or clamping
points of one of the faces of the aerofoil, and
5 laterally on the leading edge side or trailing edge
side opposing that on which said bearing-. point or
clamping point is positioned. 1
More preferably, short rods point toward three false
10 points of the same face, said false points being
positioned in a staggered manner relative to the
bearing points or clamping points of said face.
Even more preferably, short rods point toward three
15 false points on each of the faces of the aerofoil of
the core, said false points being positioned on each
face in a staggered manner relative to the bearing
points or clamping points.
20 The staggered arrangement relative to the bearing
points and clamping points achieves a more uniform
distribution and the ability to control the thickness
of the walls of the blade at the points furthest away
from one another.
25
Advantageously, the rods are of cylindrical shape,
having ends following the local shape of the core. Thus
markings on the surface of the core and local
deformations thereof are avoided.
30
Preferably, the short rods have a greater diameter than
that of the long rods.
In a particular embodiment, the short rods have a
35 length which is shorter by 5/100th of a millimeter than
that of the long rods..
The invention will be understood more clearly and
further objects, details, features and advantages
thereof will appear more clearly during the following
detailed explanatory description of an embodiment of
the invention provided purely by way of illustrative
and non-limiting example, with reference to the
accompanying schematic drawings, in which:
- figure 1 is a view from above of a core in position
on the lower face of a mold, in position before closing
the mold and injecting the wax;
- figure 2 is a front view of the suction face of a
core designed to be placed in an injection mold
according to the prior art;
- figure 3 is a front view of the pressure face of a
core designed to be placed in an injection mold,
according to the prior art;
- figure 4 is an oblique view of the suction face
cavity of an injection mold according to an embodiment
of the invention;
- figure 5 is an oblique view of the pressure face
cavity of an injection mold according to an embodiment
of the invention;
- figure 6 is a front view of the suction face of a
core designed to be placed in an injection mold
according to an embodiment of the invention;
- figure 7 is a front view of the. pressure face of a
core designed to be placed in an injection mold
according to an embodiment, and
* - figure 8 is a sectional view of the core of figure 1
and of the injection mold, taken in the region of one
of the bearing points and one of the false points.
5 The lower surface of an injection-mold 1 forming a
footprint for a suction face of a turbine blade is seen
with reference to figure 1. Arranged on said lower
surface, via its suction face, is a ceramic core 2
having substantially the shape of a blade with a root
10 2a, an aerofoil 2b extending between a leading edge 3
and a trailing edge 4 and a tip 2c. In the remainder of
the text, the terms "longitudinal" or "transverse"
respectively relate to the direction connecting the
root 2a to the tip 2c of the blade and to the direction
15 connecting the leading edge 3 to the trailing edge 4.
The suction face of the aerofoil 2b is in contact with
the long rods (not visible in the figure) carrie.d by
the surface of the suction face footprint of the mold.
20 The mold 1 is produced in three parts carried by a
base: a root block la, a blade block lb and a tip block
lc on which respectively the root 2a, the aerofoil 2b
and the tip 2c of the core 2 rest. In the version
shown, the tip block and root block are mobile relative
25 to the base of the mold 1 to serve as ejectors of the
wax model once the injection process has been carried
out; the aerofoil block 2b is, in turn, fixed and
rigidly connected to the base of the mold 1.
30 With reference now to figures 2 and 3, it is possible
to see respectively the suction face 12 (with the
suction face of its root 12a, its aerofoil 12b and its
tip 12c) and the pressure face 22 (with the pressure
face of its root 22a, its aerofoil 22b and its tip 22c)
35 of a core 2 designed to be placed in the mold 1
according to the prior art for the injection. of wax.
& On the suction face 12 are shown, in the form of
blacked-out circles, bearing points A1 to A6 against
which the long rods extending from the lower surface of
the mold 1 bear and, in the form of transparent
5 circles, the position of false points F1 to F4 located
opposite short rods which delimit the tolerance
envelope for the suction face of the blade. Similarly
on the pressure face 11 are shown, in the form of
blacked-out circles, clamping points S1 to S4, on which
10 the pressure exerted by the upper surface of the mold
is applied when closing said mold and the position of
false points F5 to F7 located opposite short rods for
delimiting the tolerance envelope for the pressure face
of the blade.
15
In the prior art of figures 2 and 3, the six bearing
points are distributed in the following manner: four
points A1 to A4 are positioned in the region of the
upper and lower ends of the suction face of the
20 aerofoil 12b, on the leading edge side 3 for two
thereof and on the trailing edge side 4 for the two
others; the two last bearing points A5 and A6 are
positioned firstly on the root and secondly on the tip
of the core. Said two last bearing points are oriented
25 so as to maintain the core in abutment against the mold
in the two directions of its median plane.
The false points are, in turn, positioned for two
thereof F1 and F2, in the region of the center of the
30 suction face 12b of the aerofoil respectively on the
side of the leading edge 3 and the trailing edge 4 as
are the bearing points of the leading edge and the
trailing edge which enclose them, and for the two
following points F3 and F4 on the suction face of the
35 root 12a of the core.
On the pressure face side, the four clamping points S1
to 54 are positioned opposite the four first bearing
b points A1 to A4, i.e. in the region of the upper and
lower ends of the pressure face 22b of the aerofoil, on
the trailing edge side 4, for the two first ones
thereof and on the leading edge side 3 for the two
others. Three false points F5 to F7 are positioned on
the pressure face 22 of the core, two points F5 and F6
in the center of the pressure face 22b of the aerofoil
transversely.opposite the false points F1 and F2 of the
suction face, and the third F7 on the pressure face 22a
of the root of the core.
Now described with reference to figures 4 to 7 is the
positioning of a core 2 in an injection mold 1 having
long rods and short rods according to the invention.
15
Figure 4 shows the footprint located on the lower face
of an injection mold, known as the suction face
footprint, which is designed to receive the suction
face of a core 2. This suction face footprint bears six
20 rods, three long rods referenced TA51 to TA53, which
are designed to come into contact with the bearing
points A51 to A53, on the suction face of the core 2
and three short rods, referenced TF51 to TF53, which
are designed to monitor the thickness of the wall of
25 the blade in the region of the three false points F51
to. F53 on the core 2.
Similarly, figure 5 shows the footpr'int located on the
upper face of the injection mold. Said pressure face
30 footprint in turn carries six rods, three long rods
TS51. to TS53, and three short rods TF55 to TF57,
corresponding to three clamping points S51 to ,953, and
three false points F55 to F57, on the pressure face of
the core 2.
35
The long rods have a relatively reduced diameter as
their ends are in contact with- the surface of the core
and the presence of said long rods is manifested as a
& hollow tube in the wax model of the blade. It is
necessary, therefore, to minimize this interference
which is present between the core and the long rods, by
the greatest possible reduction of the diameter
5 thereof. In contrast, the purpose of the short rods is
to oppose the resistance when closing the mold, if the
core is badly positioned. The diameter thereof is thus
greater than that of the long rods.
10 With reference to figures 6 and 7, the positioning of
the long rods of the mold 1 is defined so that the
aerofoil- 2b has only three bearing points A51 to A53,
and three clamping points S51 to S53, the three other
bearing points A54 to A56 being transferred to the root
15 2a and the tip 2c of the core.
The long rods of the mold are positioned such that
three bearing points on the suction face side are
located on the trailing edge side 4, for the first A51
20 positioned in the center of the suction face 12b of the
aerofoil, and on the leading edge side 3, for the
following A52 and A53 in the region of the upper and
lower ends of the aerofoil 12b of the core. The
clamping points S51 to S53 are positioned on the
25 pressure face 22b of the core opposite said three
bearing points. Alternatively, a symmetrical
positioning could also be implemented with the point
A51 located on the leading edge side, and the points
A52 and A53 located on the trailing edge side.
30
The three last bearing points A54 to A56 are positioned
on the faces of the core which are transverse to the
median plane thereof, so as to come into abutment with
the mold 1 and correctly position the core by
35 translatory movements in said median plane.
False points F51 to F53 for the suction face 12b and
F55 to F57 for the pressure face 22b are also
a positioned on the aerofoil 2b of the core 2. On the ! suction face 12b and on the pressure face 22b, they are
positioned longitudinally in the same region as the
bearing points or clamping points on the aerofoil i.e.
5 in the center thereof and at its two ends, but on the
trailing edge side or leading edge side which does not
already have a bearing point or clamping point. These
false points are positioned in a so-called staggered
manner relative to the bearing points for the suction
10 face side, and relative to the clamping points for the
pressure face side.
With reference now to figure 8, a core 2 is seen in
section in position in an injection mold 1. The core is
15 held in place in the mold, amongst other things, by a
pair of long bearing rods TA53 and TS53 and it has two
short rods TF53 and TF57 which do not come into contact
with the surface of the core, said core being correctly
positioned in the mold. It is observed that the ends of
20 the long rods are profiled so as to be adapted to the
shape of the core in the region of its corresponding
bearing points. Similarly, the ends of the short rods
are profiled so as to reproduce the shape of the core
in the region of its false points which the short rods
25 are liable to touch. Thus the core is prevented from
being marked when the mold is closed if the core is not
correctly positioned.
The relevance of the invention for the strength of the
30 core 2 when positioned in the mold 1 or during the
injection of wax may be explained as follows:
The leading edge 3 of the core 2 is positioned and
clamped at its upper and lower ends and the trailing
35 edge 4 is only positioned and clamped by a single point
which is located in the center of said trailing edge.
This permits the core to be centered in the most
appropriate manner in the wax. The core 2 is thus less
b subjected to stress in the injection mold, as the
trailing edge 4 at its ends has greater freedom to be
I freely positioned.
5 The aerofoil 2b of the core 2 is positioned,by only
three points, clamped by the long rods TA51 to TA53 and
TS51 to TS53 between, on the one hand, the bearing
points A51 to A53 for the suction face and, on the
other hand, the clamping points S51 to S53 for the
10 pressure face, which provides it with an isostatic
positioning. This results in an absence of flexural
tension or twisting which would generate stresses in
the core and, as a result, the risk of breakage or poor
positioning in the mold 1; this prevents the appearance
15 of faults in the thickness of the walls defining the
internal cavities of the blade produced after casting.
By modifying the arrangement, on the one hand, of the
bearing points on the suction face 12 and, on the other
20 hand, of the clamping points on the pressure face 22 of
the core 2, in addition to the position of the bearing
I points at the root 2a and the tip 2c of the core, the
I distribution of stresses which are applied to the core
I 2 is modified when the core is positioned and clamped
25 in the injection mold 1. By being subjected to less
*
stress, the core deforms less when the mold 1 is opened
I after the injection process and also deforms less when
wax is removed from the shell. he wall thicknesses on
> the cast metal part are thus more carefully maintained.
30
The -invention has been described by positioning a
bearing point on the trailing edge of the suction face
and two bearing points on its leading edge; it is
obvious that the positioning may be carried out in a
35 symmetrical manner with a bearing point on the leading
edge side and two bearing points on the trailing edge.
The positioning of the clamping points on the pressure
face is thus adapted, as a result. Similarly, the
-a description has been made with the suction face side
arranged on the lower face of the mold and having the
bearing points; the invention could also be implemented
if the pressure face were placed against the lower face
5 of the mold and its long support rods.

WO 2012/052665
Claims
1. An injection mold for producing a model of a
turbine engine blade to be produced by the lost wax
casting technique, comprising an upper face which folds
onto a lower face, the two faces having a footprint
reproducing the external shape of the blade to be
produced and carrying long rods (TA51 to TA53, TS51 to
TS53) for supporting a core (2) designed to generate
cooling cavities inside the blade, said core having the
shape of an aerofoil (2b) between a root (2a) and a tip
(2b), said long rods of the lower face (TA51 to TA53)
being designed to come into contact with a first face
(12) of the aerofoil (2b) in the region of bearing
points (A51 to A53) and said long rods of the upper
face (TS51 to TS53) being designed to come into contact
with a second face (22) of said aerofoil in the region
of clamping points (S51 to S53), each clamping point of
the second face being positioned opposite a bearing
point of the first face,
characterized in that it further comprises at least one
short rod (TF51 to TF53, TF55 to TF57) extending from
one of the faces of the mold and pointing toward a
false point (F51 to F53, F55 to F57) positioned on one
of the faces (12b, 22b) of the aerofoil (2b), the
length of said short rod being less than the distance
between the face of the mold and that of the aerofoil
in the region of said false point and the difference
between said length and said distance being equal to
the tolerance at the position of said false point in
the case of deformation of the core.
2. The injection mold as claimed in claim 1, in which
the number of pairs (TA51 and TS51, TA52 and TS52, TA53
and TS53) consisting of a rod associated with a bearing
point on the first face and a rod associated with a
clamping point on the second face of the aerofoil (2b)
of the core (2) is at most equal to 3.
4
3. The injection mold as claimed in claim 2, in which
said pairs of long rods are associated with bearing
points on said first face (12) and with clamping points
5 on said second face (22), positioned for a first pair
(TA51 and TS51) in the region of the center of the
aerofoil (2b), .on the trailing edge side (4), and for
the two other pairs (TA52 and TS52, TA53 and TS53) in
the region of the upper and lower ends of the aerofoil
10 (2b) on the leading edge side (3) .
4. The injection mold as claimed !in claim 2, in which
said pairs of long rods are associated with bearing
points on said first face (12) and with clamping points
15 on said second face (22), positioned for a first pair
in the region of the center of the aerofoil (2b) on the
leading edge side ( 3 ) , and for the two other pairs in
the region of the upper and lower ends of the aerofoil
(2b) on the trailing edge side (4).
20
5. The injection mold as claimed in one of claims 1
to 4, in which said short rod is positioned
longitudinally in the region of one of the bearing
points or clamping points of one of the faces (12b,
25 22b) of the aerofoil (2b), and transversely on the
leading edge side or the trailing edge side opposing
that on which said bearing point or clamping point is
positioned.
30 6. The injection mold as claimed in claim 5, in which
short rods (TF51 to TF53, TF55 to TF57) point toward
three false points of the same face (12b, 22b), said
false points (F51 to F53, F55 to F57) being positioned
in a staggered manner relative to the bearing points
35 (A51 to A53, S51 to S53) or clamping points of said
face.
h 7. The injection mold as claimed in claim 6, in which
the short rods (TF51 to TF53, TF55 to TF57) point
toward three false points on each of the faces of the
aerofoil (2b) of the core (2), said false points (F51
5 to F53, F55 to F57) being positioned on each face in 'a
staggered manner relative to the bearing points or
clamping points (A51 to A53, S51 to S53) .
8. The injection mold as claimed in one of claims 1
10 to 7, in which the rods are of cylindrical shape,
having ends following the local shape of the core.
9. The injection mold as claimed in one of claims 6
to 8, in which the short rods have a greater diameter
15 than that of the long rods.
10. The injection mold as claimed in one of' claims 6
to 9, in which the short rods have a length which is
shorter by 5/100th of a millimeter than that of the
20 long rods.