Molding apparatus having elements made of composite material

A molding apparatus for the hot pressing of parts made of refractory material includes at least one element, such as a mold or an intermediate element, which is formed by a material capable of withstanding high temperatures, having adequate mechanical resistance and possessing particular anisotropic properties of thermal conductivity, in particular as a result of a structure including thermally conductive fibers oriented so that a preferential heat transfer occurs towards the areas of the part which are furthest away from its edges of the part, ensuring homogeneous heating or cooling throughout the part at all times.

BACKGROUND OF THE INVENTION 
1. FIELD OF THE INVENTION 
The invention relates to molding apparatus for the hot pressing of parts 
made of a refractory material. 
Known processes for shaping parts made of refractory materials, especially 
composite materials, and in particular parts made of ceramic materials 
intended for use in aircraft engines, generally involve a hot pressing 
operation which can be carried out in a press furnace at high temperature. 
Various heating means may be used for these furnaces, whether for radiant 
heating, or for induction heating, possibly with a susceptor, the heat 
being propagated from the outside towards the center. 
2. Description of the Prior Art 
Traditionally, the pressing assembly proper is realized in accordance with 
a so-called monobloc concept, in which the blank of the part to be 
obtained is placed between two dies which, for applications requiring 
temperatures ranging, for example, between 1200.degree. C. and 
2200.degree. C. and the enclosure to be placed in a vacuum or a neutral 
atmosphere, are made of graphite. However, during high temperature heating 
tests (at 1400.degree. C. for example) on refractory materials, it is 
possible to observe differences of temperature in excess of 200.degree. C. 
between the outer edge and the center of the part. This radial thermal 
gradient is generally unacceptable for the quality of the parts to be 
obtained. Attempting to obtain temperature uniformity by insulation leads 
to excessively prolonging the duration of the cycle and, anyway, makes it 
impossible to achieve the necessary cooling rates after pressing, which in 
certain cases may reach some 1200.degree. C. per hour in order to obtain 
conditions equivalent to oil hardening. 
Complying with these cooling rates leads to providing a separate mold which 
receives the blank and which permits pressing between the dies of the 
press-furnace as described above. In this case, the mold containing the 
pressed material may be removed from the furnace and rapidly cooled. 
The techniques now used, for example, for shaping parts made of composite 
ceramic materials of the type comprising SiC fibres in a glass matrix and 
for the associated temperature ranges and conditions of operation 
(enclosure in a vacuum or neutral atmosphere), comprise the use of dies 
and a separate mold of graphite. 
However, tests have shown that the thermal gradients, particularly in a 
radial direction, between an outer edge and the center, remain too high 
and incompatible with satisfactory implementation corresponding to the 
quality criteria of the results to be obtained. 
French Specification No. 2 532 585 discloses an example of controlling the 
temperatures of a mold used for hot pressing with a view to obtaining 
uniformity of the mold temperatures. However, this solution, which uses a 
mold of electrically-conductive material associated with heating means 
involving a travelling wave field generator, is limited to low-temperature 
applications, particularly in the field of rubber. This field is far 
removed from the aims of the present invention, particularly from the 
point of view of the characteristics of the material worked and the 
thermal conditions employed. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide molding apparatus for the hot 
pressing of parts made of a refractory material which enables the 
above-mentioned conditions of use to be met without suffering the 
drawbacks of the known solutions. 
To this end, according to the invention, the molding apparatus includes at 
least one element formed of a composite material which is resistant to 
high temperatures, said composite material comprising long, thermally 
conductive refractory fibers disposed in a refractory matrix which is less 
thermally conductive than said fibers, said fibers being oriented so as to 
ensure heat transfer, in one direction or the other, towards the areas of 
said part furthest from its outer edges, thereby enabling homogeneous 
heating or cooling to be achieved throughout said part at all times, the 
arrangement of said fibers also being such that said element possesses 
adequate mechanical resistance to compression stresses. 
As will be appreciated, the construction of the element is such that it 
will improve both the heating and the cooling of the regions of the part 
furthest from its outer edges. 
The fiber of the element are preferably carbon based, and preferably the 
element is constituted either by the mold itself or by intermediate 
elements cooperating with a conventional mold of isotropic material. 
Other features and advantages of the invention will become apparent from 
the following description of preferred embodiments of the invention with 
reference to the attached drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A hot pressing assembly 1 is shown in FIG. 1 suitable for use in carrying 
out shaping operations on parts, particularly parts made of composite 
ceramic material, such as parts comprising SiC fibers in a glass matrix 
intended for use in aircraft engines. The assembly 1 is associated with 
heating means indicated at 2 and comprises jack operated pressing means 3 
associated with thermal insulation elements 4 and acting on dies 5. 
Between the dies 5 a pressing mold 6 is placed which contains a blank 7 
for the part to be obtained. 
In accordance with a first embodiment of the invention, the pressing mold 6 
is formed of a material having anisotropic thermal conductivity 
properties. This material, particularly in the case referred to above 
involving the shaping of parts made of composite material comprising SiC 
fibers in a glass matrix, may be a composite material of the carbon/carbon 
type having carbon fibers in a carbon matrix. The thermal conductivity 
parallel to the strata of such a material may be one hundred times higher 
than in the perpendicular direction. 
FIGS. 2 and 3 show examples of pressing molds for use in the shaping of 
flat axisymmetrical plates. In FIG. 2 the fibers 8 of the pressing mold 6a 
are oriented parallel to the median Plane of the plate, the structure of 
the material of the mold 6a in this case being termed bi-dimensional. FIG. 
3 illustrates the use of a material of more complex structure, termed 
n-dimensional, for the pressing mold 6b. 
In all cases, the orientations of the thermally conducting fibers 8 may be 
adapted to the particular shape of the part to be obtained, such as shown 
in FIG. 4 by the pressing mold 6c for the shaping of an axisymmetrical 
part. 
In all cases, the thermal anisotropy of the material constituting the 
pressing molds 6 permits a preferential heat transfer from the heating 
means 2 towards the areas of the part to be shaped furthest from the 
heating means 2. The same phenomenon also occurs when the part cools down, 
in this case favoring the removal of heat from the areas of the mold 
furthest away from the outer edges of the part. 
In a second embodiment of the invention, which may be preferred in cases 
where, depending on the application, it is desired to retain a mold of 
conventional form made from a material having isotropic properties, 
intermediate elements of the molding apparatus, such as the dies 105 of 
the molding apparatus diagrammatically shown in FIG. 5, have a structure 
similar to that just described for the mold 6 of the first embodiment 
illustrated in FIGS. 1 to 4. In this case it is the intermediate elements 
105 which have controlled conductivity properties and which are made of a 
composite material comprising heat conducting refractory fibers disposed 
in a refractory matrix of lower thermal conductivity than the fibers, said 
fibers being orientated so as to ensure preferential heat propagation, in 
one direction or the other, towards or away from the areas of the parts 
furthest from its outer edges. As before, this arrangement permits 
homogeneous heating or cooling of the entire part 7 to be achieved at all 
times during the working of the part. 
In all cases, and irrespective of the particular embodiment selected, the 
choice of the arrangement and orientation of the fibers 8, as illustrated 
in particular in FIGS. 2, 3 and 4, takes into account the criterion that 
the element 6a, 6b, 6c or 105 should possess adequate mechanical 
resistance to compression stresses during the shaping operation. A 
compromise is thus sought between this strength requirement and the 
thermal results to be obtained. 
By reducing the thermal heterogeneity of the molded part, both during 
heating and hot pressing and during cooling, the use of a mold or 
intermediate elements in molding apparatus of the invention as described 
above enables a substantially uniform microstructure to be obtained 
throughout the material of the part finally obtained. In addition, by 
limiting the thermal gradients in the part during processing, the residual 
stresses in the material of the finished part are also reduced. 
The molds or intermediate elements of molding apparatus in accordance with 
the invention as just described may be made using known techniques, 
particularly wire reeling, filament winding or fabric stacking. 
Apart from composite materials of the carbon/carbon type, possibly 
employing graphite or pyrolitic graphite, which have been used for making 
molds for carrying out hot pressing operations in a neutral atmosphere or 
under vacuum, it is also possible to envisage using composite materials of 
the carbon/carbon type for applications involving operation in an 
oxidizing or reducing atmosphere when the materials have been subjected to 
an anti-oxidizing impregnation and/or to a vapor phase deposition of a 
protective refractory material, for example a SiC base. Alternatively, a 
composite material may be used comprising carbon fibers in a refractory 
matrix of the SiC type. More generally, depending upon the applications 
and the conditions of use envisaged, the choice of material may involve 
any composite material having long, heat-conducting, refractory fibers and 
a non-oxidizing matrix, the orientation of the said fibers being arranged 
in the particular manner described above in accordance with the invention.