Abstract:
A process for producing a hollow article made of a composite material of reinforcing fibers embedded in a matrix of hot-polymerized resin includes a step in which at least one core made of a silicone elastomer is draped with at least one layer of resin-impregnated reinforcing fibers. A resultant assembly is moulded to shape the internal and external surfaces of the hollow article by simultaneous inward and outward compression of at least one resin-impregnated fiber layer caused by movement of mould walls towards each other and by thermal expansion of at least one core.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a process for producing a hollow article made of a laminated composite material consisting of reinforcing fibres embedded in a polymerized organic resin matrix, and more particularly to such a process for producing an article having high strength, accuracy and temperature resistance characteristics. 
     Laminated composite materials comprising reinforcing fibres embedded in a matrix of polymerized resin are particularly useful in the aeronautical industry because of their excellent strength-to-weight ratio, and there is an increasing tendency to use such materials instead of metal alloys whenever possible, particularly in the case of thin-walled articles conventionally made by casting or metal fabrication techniques. 
     Endeavours are therefore being made to produce turbomachine parts, especially parts for aircraft turbojet engines, such as casing arms for low pressure compressors or hollow low pressure compressor blades having thin walls defining and extending around cavities which have an opening, i.e. which are not fully closed. 
     These articles must be integral in order to be free from weakening assembly zones. The articles must also be accurate and have a good surface texture so as to avoid the need for subsequent machining. Furthermore, the articles must be able to withstand high temperatures and their cost must be comparable with or below the equivalent metal articles. 
     2. Summary of the Prior Art 
     In the known resin transfer moulding process, known as RTM, reinforcing fibres are placed in a mould having the shape of the finished article, very liquid resin is injected under pressure into the mould and the resin is polymerized while maintaining the pressure. This process enables strong accurate articles to be obtained in a wide variety of shapes. However, the resins used have poor temperature resistance, which limits the use of the process to articles which will remain cool. 
     Resins which can withstand higher temperatures are not sufficiently fluid before polymerization. Consequently, to produce laminated hollow articles with such resins it is necessary to: 
     pre-impregnate layers of fabrics or fibres with the resin; 
     form a core which may or may not be destructible; 
     surround the core by an inflatable elastomeric bladder; 
     drape the pre-impregnated layers of fabric or fibres around the assembly of the core and the bladder; 
     place the assembly of the core, the bladder and the pre-impregnated layers of fabric or fibres in a mould corresponding to the external shape of the finished article; 
     inflate the bladder; 
     polymerize the resin; 
     deflate the bladder and remove the article from the mould; 
     withdraw or destroy the core; and 
     withdraw the bladder. 
     In this process inflating the bladder makes it possible simultaneously to press the resin impregnated layers of fabric which form the composite material against the mould wall, and to compress and cause flow of the material so as to reduce porosities due to air bubbles trapped between the fibre layers, reduce emissions of gas from the resin during polymerization, and expel the excess resin and thereby increase fibre density. A compression corresponding to 20% of wall thickness is usually achieved. 
     Clearly, in such a process only the article surface in contact with the mould wall is accurate, whereas the surface in contact with the bladder is irregular and rough and follows the inevitable heterogeneities of the draping of the reinforcing fibres. It might be conceivable to compress the composite material on the core, but this solution would cause unacceptable creasing of the reinforcing fibres, causing a reduction in the strength of the article. 
     A first problem is therefore to produce, from pre-impregnated fibres or fabrics, hollow articles of a variety of shapes whose internal and external surfaces are accurate and smooth, without creasing the reinforcing fibres. 
     Polymerization of the resin is accompanied by an emission of gaseous components and a reduction in the volume of the resin, both of these phenomena tending to make the resulting composite material porous. This porosity can be reduced, but not eliminated, by the use of gas removal means and by compressing the composite material before the resin hardens in the course of its polymerization, the compression preferably causing a substantial deformation or flow of the composite material. Since the residual porosity reduces the strength of the final article, a second problem is to achieve a general reduction in the porosity of the composite material and, to this end in particular, to increase flow of the composite material during polymerization. 
     When the cavities open to the exterior through openings which are too small, as is often the case, the cores can be eliminated only by destruction of the material of which they are made. Materials are on the market which can be moulded to the required shape, then dissolved by water or a solvent after the article has been moulded. However, such cores are unsuitable in the present case since to produce accurate internal surfaces there would need to be an inward compression of the composite material on the core, with the disadvantages previously described. Consequently, a third problem is to remove the cores after the moulding of the article. 
     The use of composite materials is also hampered by the high production cost of the articles as compared with equivalent metal alloy articles. The high cost is due in particular to the many manipulations required in the production process. The complexity of the manufacturing process should therefore not be increased. 
     French Patent No. 2562834 discloses a process for moulding hollow articles made of a composite fibre and polymerized resin material using an external mould and a core made of a silicone elastomer, this latter material having a very high thermal expansion coefficient. In this process the core compresses the composite material against the mould walls as a result of the thermal expansion of the silicone elastomer during the hot polymerization cycle. The process provides an article having accurate internal and external surfaces but the composite material of the article has appreciable porosity. Therefore, and in order to produce homogeneous and smooth surfaces, French patent No. 2562834 also proposes, at lines 1 to 5 on page 8, to use a paint or gelcoat. 
     SUMMARY OF THE INVENTION 
     With the aim of overcoming the aforementioned problems, the invention provides a process for producing a hollow article made of a laminated composite material consisting of reinforcing fibres embedded in a matrix of hot-polymerized resin, said hollow article having external surfaces and internal surfaces defining at least one cavity, said process comprising the steps of: 
     a) providing a mould having a pair of walls for shaping said external surfaces of the article to be produced, a female part supporting one of said walls, a male part supporting the other of said walls and slidable in said female part to move said walls towards each other, and abutment surfaces on said female part and said male part which are engageable to define accurately the limit positions of said walls when moved towards each other; 
     b) making at least one core corresponding to the shape of said at least one cavity in the article to be produced, said at least one core being made from a thermally expansible silicone elastomer; 
     c) draping said at least one core with at least one layer of reinforcing fibres impregnated with hot polymerizable resin for forming said composite material; 
     d) placing the assembly consisting of said at least one core and said at least one layer of resin-impregnated fibres in said mould; 
     e) sliding said male part of said mould in said female part to move said walls towards each other and thereby compress the resin-impregnated fibre layers between said at least one core and said walls so as to shape said internal and external surfaces of the article simultaneously and to produce sufficient flow of the resin during polymerization; 
     f) raising the temperature of said assembly in said mould to polymerize said resin and to expand said at least one core and thereby stretch said reinforcing fibres during the compression shaping of said at least one layer of resin-impregnated fibres and the polymerization of said resin; 
     g) releasing the moulded article from said mould; and 
     h) withdrawing said at least one core from said moulded article. 
     The external surfaces of the article formed by contact with the mould walls are accurate since the mould walls are rigid and are aprecisely positioned at the end of the their movement towards one another. Typically, an accuracy of 0.05 mm can be obtained. The surfaces are also very smooth, since the porosity of the material is very reduced allowing the mould walls to reproduce their own surface texture on the article. 
     Similar considerations apply to the internal surfaces of the article in contact with the core or cores. However, since the silicone elastomer of which the or each core is made is flexible the accuracy of the internal surfaces is less, but is still satisfactory since the composite material is pressed against the mould walls. 
     The core expansion produced by thermal expansion of the silicone elastomer of which the or each core is made is necessary to restretch the reinforcing fibres which would otherwise be creased by the inward compression produced by the movement of the mould walls towards one another. 
     This double action, i.e. the inward compression caused by the movement of the mould walls towards one another and the outward compression caused by expansion of the core or cores, promotes considerable flow of the composite material so that its porosity is reduced. 
     In the process in accordance with the invention the compression is no longer dependent only on the thermal expansion of the silicone elastomer core or cores as in French patent No. 2562834. Thermal expansion of the core or cores by itself will not have the effect of compressing the composite material against the mould walls, but will simply urge the walls apart because the male part is slidable in the female part. To produce the desired compression, an external force must be applied to the mould. For example by means of a press or by increasing the autoclave pressure, so as to urge the walls towards eachother. 
     Since the polymerization of the resin comprises a gas evolution phase and a hardening phase separated by a phase in which the resin is still liquid and therefore compressible, the compression shaping is preferably effected between the gas evolution phase and the hardening phase. The effect of this is to maintain a relatively large quantity of resin in the mould, with a resultant reduction in the porosity of the composite material end product, the resin remaining in the mould since it is no longer compressed at the start of the polymerization cycle and more particularly during the gas evolution phase. Also, since the compression is effected with a relatively large quantity of resin in the mould, stagnant gases in the mould are more readily expelled. These two factors combine to reduce the porosity of the final composite material. 
     If T1 denotes the transition temperature of the resin from a pasty state to a solid state, it will be advantageous if the core or cores have expanded to the shape and dimensions of the required cavity at or just below this transition temperature T1, with a possible correction for thermal expansion of the polymerized composite, which expansion is very low and is of the order of 1.10 −6 /°C. This enables the cavity to be given the required dimensions at the onset of hardening of the resin. The dimensions of the core when cold are then calculated by applying to the hot dimensions a coefficient corresponding to the thermal expansion of the silicone elastomer between the temperature T1 and the ambient temperature, the elastomer itself usually being prepared by cold polymerization. Typically, an accuracy of 0.1 mm may be obtained for the internal surfaces of the article. 
     The hardness of the elastomer is not critical and one only has to choose an elastomer which is hard enough, for example a hardness of at least 30 Shore A, to ensure that possible deformations of the core remain compatible with the required accuracy of the article. If required the hardness of an elastomer can be increased by a charge of microballs, for example in the form of glass beads. 
     Preferably, the silicone elastomer chosen has a disintegration temperature T2 below the hardening temperature T3 at which the polymerization of the resin is completed, so that the elastomeric core will disintegrate during the moulding of the article. The core thus disintegrates during moulding as a result of the temperature to which the composite material is heated near the end of the polymerization cycle. The material of the core can then readily be withdrawn from the cavity after completion of the moulding process by a simple washing with water or even by scraping or brushing, so that the second problem is solved. 
     In a preferred embodiment, the silicone elastomer selected has a disintegration temperature T2 between the temperature T1 and T3 in order to keep the core or cores in the solid state until the cavity or cavities have been formed to the required dimensions and in order that the core or cores disintegrate when the composite material itself has solidified, so that there is no risk of cracking the composite material. 
     Advantageously, the core or cores may be reinforced by rods of a stronger material, such as a metal alloy, in order to avoid the risk of deformation of the cores during the draping thereof or during the moulding of the composite material. Preferably these reinforcing rods extend beyond the core or cores and are supported in the mould in order to improve the accuracy with which the cores are positioned in the mould and thereby improve the accuracy of the positioning of the cavities in the article. 
     Further preferred features and advantages of the invention will become apparent from the following description of a preferred embodiment with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a transverse cross-sectional view through the mould during moulding of an arm for a low pressure compressor casing for an aircraft engine by a preferred embodiment of the process in accordance with the invention, the thickness of the arm and its constituent parts being increased for the sake of clarity; and 
     FIG. 2 is a longitudinal sectional view through part of the mould at one end of the casing arm, and taken on the line A—A in FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The article  1  which is to be produced is an elongate thin compressor casing arm having two thin lateral parts or side walls  2 ,  3  which meet at a rear edge  4  and a front edge  5 . At the centre of the arm  1  a rib  6  interconnects the side walls  2  and  3  in order to increase their stiffness and thus increase the stiffness of the arm  1 . The side walls  2 ,  3  together with the rib  6  define two elongate cavities  7 . The reference  8  denotes the internal surfaces of the arm  1  which bound the cavities  7 , and the reference  9  denotes the external surfaces of the arm  1 . 
     During moulding of the article  1 , the cavities  7  are formed by cores  10  made of a silicone elastomer. Since the cores  10  are long and thin they are each stiffened by a metal rod  11  which extends through it lengthwise, each rod  11  having a flat rectangular cross-section with rounded edges  12   a . Each silicone core  10  is produced by extrusion with a cavity  13  corresponding to the shape of the rod  11 , followed by cutting to the required length identified by the reference L in FIG.  2 . The rod  11  is then introduced into the cavity  13  of the core  10 , an operation which, even if the core  10  is of reduced resilience, presents no difficulty. 
     The reinforced cores  10  are then each draped with one or more layers  15  of fibres which have been pre-impregnated with resin. The two draped cores are then placed side by side, following which they are together draped with one or more further layers  16  of resin-impregnated fibres so as to form a precursor for the required article. 
     The draped core assembly  10 ,  15 ,  16  is then placed in the female part  21  of a mould  20 , the bottom wall  22  of the female part having a shape corresponding to the external surface of the side wall  2  of the required article  1 . The bottom wall  22  lies adjacent two lateral walls  23   a ,  23   b  which are parallel to one another and are extended by respective flared walls  24   a ,  24   b  each adjacent a respective abutment surface  25   a ,  25   b . The mould  20  also comprises a male part or plunger  26  whose end  27  forms a wall corresponding to the shape of the external surface of the side wall  3  of the article  1 . The wall  27  lies adjacent two lateral walls  28   a ,  28   b  of a shape matching the walls  23   a ,  23   b  of the female part  21  so that the plunger  26  is slidable with a reduced clearance by way of its walls  28   a ,  28   b  between the walls  23   a ,  23   b  which guide the plunger. The lateral walls  28   a ,  28   b  are adjacent respective abutment surfaces  29   a ,  29   b  which contact the surfaces  25   a ,  25   b  of the female part  21  to limit the movement of the plunger into the female part. The mould assembly is surrounded by a felt layer  30  and by a flexible sealed envelope or bag  31  connected by a nozzle  32  to an evacuation source  33 , and the whole assembly is disposed between the two plates  34 ,  35  of an oven press (not shown). 
     As shown in FIG. 2, the rod ends  12   b  extend beyond each end of each core  10  and are engaged between two jaws  40 ,  41  which contact one another at a separation surface  42  and are clamped together by screws  43 . The jaws  40 ,  41  are positioned in a cavity  44  of the mould  20  which opens to the exterior and which has a shape matching the shape of the jaws  40 ,  41  with a clearance e of approximately 0.2 mm. The inside of the cavity  44  has two shoulders  45  for positioning the jaws  40 ,  41  longitudinally, and the opening part of the cavity  44  is filled by a felt pad  46  The clearance e allows the mass of composite material  15 ,  16  to communicate with the felt  46  so that the excess resin can be removed while an adequate internal pressure is maintained. The other end (not shown) of the assembly is symmetrical with the end shown in FIG.  2 . The distance between the pair of jaws  40 ,  41  and the corresponding pair of jaws at the other end is equal to the length L of the cores  10 . 
     In this embodiment the cores  10  are made of a silicone elastomer having a Shore hardness of  70 A, a disintegration temperature T2 of 290° C. and a thermal expansion coefficient of 400.10 −6 C. The article  1  is made from carbon fibres which have a thermal expansion coefficient of substantially zero up to 300° C. and which are formed as fabrics and pre-impregnated with a hot polymerizable resin sold by a company called FIBERITE-USA under the trade name “PMR 15”. This resin has a temperature T1 of transition from a pasty state to a solid state of 280° C., and a resin hardening temperature T3 of 320° C. 
     The process of assembling the components for producing the article  1  comprises the following main operations: 
     a) formation of the cores  10  by extrusion and cutting to the length L; 
     b) introduction of a metal rod  11  into each core  10  so that the rod ends  12   b  project beyond each end of the core  10 ; 
     c) draping at least one layer  15  of resin-impregnated fibres around each of the cores  10 ; 
     d) placement of the assemblies comprising the rods  11 , cores  10  and resin-impregnated layers  15  edge to edge and clamping one end  12   b  of the rods  11  between a first pair of the jaws  40 ,  41  by means of the screws  43 ; 
     e) clamping the other end  12   b  of the rods between a second pair of the jaws  40 ,  41 ; 
     f) draping the resulting assembly between the two pairs of jaws with at least one further layer  16  of resin-impregnated fibres; 
     g) placing the entire assembly in the female part  21  of the open mould  20  and closing the mould by inserting the male part  26  into the female part; and 
     h) placing the felt pads  46  at the entry of the mould cavities  44 , successively placing around the mould  20  the felt layer  30  and the sealed bag  31  with the nozzle  32 , and placing the enclosed mould between the plates  34 ,  35  of the oven press (not shown). 
     A thermal resin-polymerization cycle is then carried out, thus locating the mould walls  22  and  27  accurately relative to one another and to the cores  10  and ensuring the accuracy and quality of the external surfaces of the two sides  2  and  3  of the article  1  and the accuracy of the dimensions and positioning of the cavities  7  relative to the sides  2  and  3 . The sliding is caused by the combined effect of the pressure exerted by the oven press plates  34 ,  35  and of atmospheric pressure acting on the bag  31  which is evacuated to a negative pressure. 
     The pressure causes the excess resin to be extruded into the cavities  44  around the jaws  40 ,  41  via the clearance e left between the jaws  40 ,  41  and the cavity walls, the resin accumulating in the felt pads  46 . If necessary, additional cavities (not shown) can be provided in the mould to receive the excess resin. 
     During polymerisation the liquid resin evolves gases which it is essential to remove if porosities in the composite material are to be avoided. This removal is facilitated by the negative pressure produced in the bag  31  by way of the nozzle  32 , the negative pressure being communicated to the resin through the felt  30  between the bag  31  and the mould  20 , between the walls  23   a ,  23   b  and  28   a ,  28   b , and through the felt pads  46  and the clearance e. 
     Advantageously, the jaws  40 ,  41  also may be pierced by a number of conical holes  47  through which the composite material can communicate with the felt pads  46  to facilitate the removal of excess resin and gases. The conicity of the holes  47  facilitates removal of the hardened resin after completion of the moulding. 
     When the temperature T of the composite material has reached the transition temperature T1 of 280° C., the silicone elastomer cores  10  have expanded to reach the required dimensions of the cavities  7 , thus ensuring accuracy. As the temperature increases above T1 the solidified composite material is held in position by the mould walls  22 ,  27  and opposes further expansion of the cores  10 , the material of which starts to become cross-linked. When the temperature T reaches 290° C., corresponding to the disintegration temperature of the elastomer, the cores  10  start to break up and cease to apply pressure to the composite material. Polymerisation then continues up to the resin hardening temperature T3 of 320° C. 
     The moulded assembly is then removed hot from the mould to ensure that the mould  20 , as it contracts with the reducing temperature, does not crush the composite material which is now solidified and hardened. The excess amounts of resin are then broken and removed, the jaws  40 ,  41  are removed, the rods  11  are withdrawn, and the disintegrated material of the cores  10  is removed mechanically from the cavities  7  by any non-abrasive mechanical means such as scraping, blowing or washing. 
     With such a process a turbomachine casing arm having a length L of 400 mm, a width of 12 mm, and side walls  2 ,  3  having a thickness of 2 mm can be produced directly by moulding, with a geometric accuracy of 0.05 mm in the case of the surfaces formed by the mould walls and a geometric accuracy of 0.1 mm in the case of the surfaces formed by the cores. The external and internal surfaces are completely smooth, and subsequent machining operations are limited to deburring the hardened resin along the lines of removal of the excess resin and to drilling fixing holes in the ends of the article  1 . The porosity of the composite material is reduced to 2%. 
     Because of the resin used the article  1  can be used at a working temperature of 280° C. and can temporarily withstand a temperature of up to 325° C. 
     The invention is not limited to the embodiment which has just been described, but covers all such variants which could be made to it in respect of the required article and the means to be used and which fall within the scope of the claims appearing hereinafter. 
     Of course, the number and shape of the cores can vary according to the required article, and the holes through which the cavities communicate with the exterior can be of reduced size relatively to the cavities shown. It is unnecessary to use the stiffening rods  11  when the cores  10  are short and are therefore unlikely to bend. In such a case a harder elastomer may be used to prevent possible deformations of the cores. 
     If the article  1  is formed with just a single cavity it is possible to use just a single resin-impregnated fibre layer. In the case of flat articles the inward compression is exerted on the article flanks  2 ,  3 . 
     In the embodiment described the walls of the cavities are defined by parallel geometric lines, so that it has been possible to produce the cores  10  directly by extrusion and cutting them to the length L. In the case of cavities of irregular shape, for example a keg shape, the core  10  is made by moulding. The metal rods  11  can be omitted in the case of squat cores  10  and the cores  10  can be stiffened by the choice of an elastomer having a higher Shore A hardness or by inclusion of microballs in the elastomer. 
     The holes through which the cavities communicate with the exterior can be of reduced dimensions since the cores break up during the thermal polymerisation cycle, thus simplifying removal of the core material. 
     Combined use of the negative pressure bag  31  and the oven press make it possible to reach the pressure of 8 bars required for compression of the resin used in this embodiment. An ordinary press and a mould heated, for example, by electrical resistance elements can also be used. Should the resin require a compression of less than 1 bar, the compression can be obtained without using a press and relying solely on the action of atmospheric pressure.