AU2002328694A1 - Method of manufacturing composite sandwich structures - Google Patents

Description

METHOD OF MANUFACTURING COMPOSITE SANDWICH STRUCTURES

Field of the Invention

This invention relates to a method of manufacturing composite sandwich structures and refers particularly, thought not exclusively, to a method of manufacturing such structures having integral multi-ply facings. The invention also relates to such structures, particularly to such structures manufactured by the method.

Definitions

Throughout this specification reference to a vacuum source is to be taken to include a source of suction, and includes a suction or vacuum source or sources, or any other source of suction or vacuum.

Background of the Invention

Three dimensional textiles for sandwich structures are known. Such textiles have two sheets connected by a plurality of extensible threads. During manufacturing, the extensible threads and the inner surfaces of the sheets are coated with a resin. The sheets are then moved apart by the required distance so that the extensible threads extend generally perpendicularly to both sheets. The resin is then allowed to cure, thereby creating a sandwich structure of relatively high stiffness yet being relatively light weight. Additional load carrying capacity, hence usefulness, may be added to this sandwich structure by adhering face sheets to the outer face of each sheet in a secondary process, the adhesion generally being achieved by the use of film adhesives, normally a thermoset adhesive.

Difficulty has been experienced in achieving reasonable perpendicularity of the extensible threads and ensuring flat face sheets. The known processes for achieving this are slow and relatively complex, thus making the composite sandwich structure so produced relatively expensive. It also means the process is quite slow, making it difficult for mass production.

Furthermore, the known manufacturing techniques are not suitable for large or complex shapes, thereby restricting their use.

It is therefore an object of the present invention to provide a method of manufacturing composite sandwich structures which at least in part addresses the problems of the known methods.

A further object is to produce a composite sandwich structure from the new method.

Summary of the Invention

With the above and other objects in mind, the present invention provides a method of manufacturing a composite sandwich structure using a basic preform as its basis, this preform having at least two sheets connected by a plurality of extensible threads. The method consists of two stages.

The first stage, known as the preform assembly method, is a technique to create the preform stacking sequence, and incorporate a bond between individual layers in this stacking sequence sufficient to withstand the loads and chemicals applied during moulding, and during the design life of the created structure. A permanent bond is applied between layers incorporated in the final structure. A non-permanent bond is employed between outer face sheets of the final structure and caul plates (if used) or diaphragm, and between the caul plates (if used) and diaphragm.

The preform assembly method involves affixing to the outer face of a first sheet and/or second sheet a face sheet, the adhesion being by a film adhesive placed over at least substantially all of each outer surface, and subjecting the preform to pressure and/or heat to set the film adhesive. Preferably, the film adhesive is one or more of a thermoplastic or thermoset material, a non-porous film, a net- or grid-like open structure, and/or a random filament. The film adhesive may create a permanent or non-permanent bond and will not react with the resin used to impregnate the preform.

The face sheet may be of any suitable material, and the material of the first face sheet may be different to that of the second face sheet. These variations of these suitable materials may include unimpregnated fabric(s) to be impregnated in-situ with the three dimensional fabric, or hardened skin(s) such as metals of previously cured composite materials, etc, or caul plate(s) used to give dimensional stability and quality finish to the final moulding. Also, no additional face sheets may be incorporated.

The present invention also includes a composite sandwich structure preform produced by the above method.

The present invention also provides a second stage in the method of manufacturing a composite sandwich structure, which is a moulding method. During the description of this second stage, all sheets assembled against the first sheet during the preform assembly method will be collectively referred to as the first sheet. Likewise, all sheets assembled against the second sheet during the preform assembly method will be collectively referred to as the second sheet.

The moulding method includes the steps of:

(a) placing a preform (which may or may not be in accordance with the earlier described method) in a mould, the preform having a first sheet and a second sheet, there being a plurality of extensible threads extending between the first and second sheets, the mould having a first portion and a second portion, the first sheet being in the first portion, and the second sheet being in the second portion; (b) placing a diaphragm between the first sheet and the first portion of the mould, the diaphragm extending to at least an outer periphery of the first portion, there being a first chamber between the diaphragm and the first portion, and a second chamber between the diaphragm and the second portion;

(c) non-permanently adhering to the first sheet of the preform to the diaphragm and non-permanently adhering the second sheet to the second mould portion;

(d) creating a pressure differential between the second chamber and the first chamber by reducing the pressure in the second chamber;

(e) using the pressure differential to feed a resinous compound into the second chamber to coat the extensible threads and surfaces of the first and second sheets capable of being infused;

(f) ceasing the feeding of the resinous compound into the second chamber;

(g) ceasing the reduction of the pressure in the second chamber to substantially equalise the pressures in the first and second chambers;

(h) creating a second pressure differential between the second chamber and the first chamber by having the pressure in the first chamber below that in the second chamber to thus cause the diaphragm and hence, the first sheet to move away from the second sheet thereby causing the extensible threads to extend; (i) maintaining the second pressure differential; and

(j) allowing the resinous compound to set and cure.

Preferably, the second pressure differential at step (h) is created by applying sufficient vacuum pressure to the first chamber. Alternatively, or additionally, it may be created by applying a positive pressure in the second chamber. More preferably, the vacuum pressure is created by a first vacuum source.

Preferably, the first chamber is vented during step (d), the venting ceasing upon the commencement of step (h)

More preferably, the second chamber is vented during step (h). This venting may be slow, or rapid.

Advantageously, the reduction of the pressure in step (d) is by means of the application of the vacuum source, preferably to a second vacuum port in the second portion of the mould. Alternatively, a second vacuum source may be used. More preferably the feeding of the resinous compound is by at least one inlet port in the second portion. Alternatively, the at least one inlet port may be between the first and second portions.

More advantageously, the vacuum port is located at or adjacent to the centre of the second portion; the at least one inlet port being located outside the periphery of the preform.

Preferably, the vacuum source is connected to the vacuum port as well as the first chamber by separate tubes, the separate tubes being separately openable and closable by appropriate devices such as, for example, clamps, valves, or the like. Alternatively, the inlet port may be at a first end of the second portion and the vacuum port at a second end of the second portion, the preform assembly being located between the first end and the second end. More preferably, the connection to the first chamber is by means of a first vacuum port in the first portion. Advantageously, the first vacuum port is also substantially centrally located.

Advantageously, heat is applied for a predetermined time during step (j). Preferably, the heat is applied by heating the first portion and/or the second portion. Alternatively, the heat is applied by introducing heated air into the second chamber. The heated air may be introduced through the at least one inlet port and/or the second vacuum port and/or a further inlet part. Further alternatively, the heat may be applied by placing the mould in a heated oven.

The first sheet, second sheet and extensible threads form a three dimensional fabric, which may be of any suitable material such as, for example, glass fibre, carbon fibre, or any other suitable textile material.

Preferably, a caul plate is releasably adhered to the diaphragm above the first sheet, and to the preform.

The resinous compound may be fed using a positive pressure. A carrier mesh may be used. If a carrier mesh is used it is preferably attached to the inlet ports and carries the resinous compound to the preform.

The resinous compound may be of any suitable type such as, for example, a vinylester, phenolin, epoxy, or the like, resin. A carrier may be used, if desired.

The diaphragm may also extend around the preform, in the form of a bag.

In that instance, the vacuum inlet and/or outlet ports may pass through the bag.

The mould may be any mould which is capable of incorporating a diaphragm and of holding vacuum in a cavity under the diaphragm sufficient to conduct a successful infusion of the preform assembly. This includes a sealed, fully closed mould, a base plate-diaphragm combination, an envelope bag, or any other suitable means. The mould preferably incorporates the means for the infusion of the preform assembly with resin.

The mould advantageously includes means for creating a pressure differential on either side of the diaphragm and is capable of maintaining the pressure differential until the sandwich structure is cured.

The mould preferably includes means for supporting the predetermined, final component dimensions. This may include:

• caul plate(s) in the internal cavity (usually limited in separation distances by an external system);

• shaped mould(s) external to the diaphragm; and/or

• in the case of hardened face sheet(s), an external system to limit the extension between said face sheet(s).

Alternatively, the cavity dimensions of the mould (between the first and second portion) may be mechanically controlled and varied by the use of mechanical devices such as actuators. In this way the first and/or second portions may be relatively moveable to cause the separation of the first and second sheets and thus the extension of the extensible threads. This feature may also be used to control cavity dimensions at the resin injection stage (e). This may result in the non-use of the diaphragm.

Brief Description of Drawings

In order that the invention may be fully understood, there shall now be described, by way of non-limitative example only, preferred embodiments of the present invention, with reference to the accompanying illustrative drawings in which: Figure 1 is a schematic side view of a typical preform;

Figure 2 is a schematic plan view of the preform assembly of Figure 1 during the preform assembly method;

Figure 3 is a schematic side view of the preform under vacuum prior to infusion;

Figure 4 is a schematic side view of the preform at full height within the mould;

Figure 5 is a schematic side view of the mould prior to insertion of preform;

Figure 6 is a schematic side view corresponding to Figure 5 after insertion of the preform;

Figure 7 is a top view corresponding to Figure 6;

Figure 8 is a schematic view of components of the external moulding assembly used with the embodiment of Figure 11 ;

Figure 9 is a schematic side view showing an alternative mould design; Figure 10 is a schematic side view prior to infusion;

Figure 11 is a schematic side view of a second embodiment; and

Figure 12 is a schematic side view of an alternative preform

Description of Preferred Embodiments

To refer to the drawings, Figure 1 shows a typical preform assembly 8 created by the Preform Assembly Method (PAM) having a distance fabric 10 which has a first sheet 12 and second sheet 14 having extensible threads 16 extending therebetween. A first face sheet 18 is attached to first sheet 12 by a first thermoadhesive film 20; and a second face sheet 22 is attached to a second sheet 14 by a second thermoadhesive film 24.

The attachment of the face sheets 18 and 22 to the distance fabric 10 occurs during PAM. The PAM process depends strictly on the requirements of the adhesive film used to attach the adjacent sheets. The process to be described is for the adhering of the face sheets 18 and 22 to the distance fabric 10 using two thermoplastic adhesive films 20 and 24 requiring heat and pressure for correct application.

Figure 2 shows the preform 8 in place on a flat base plate 1. Over the plate is placed a vacuum diaphragm 2. This diaphragm 2 extends beyond the perimeter of the flat base plate 1. Under the diaphragm 2 is placed a vacuum port 36, breather material 35, and thermocouple schematically shown as 3, the thermocouple being to measure the temperature of the adhesive films 20 and 24. The diaphragm 2 is sealed against the base plate 1 using a sealing bead 4 of mastic. The assembly is then placed in an oven. A vacuum source is applied to the vacuum port 36 to give an appropriate consolidation pressure for the adhesive films 20 and 24. The oven heats the adhesive films to the appropriate bonding temperature. Alternatively, heated pressure rollers and/or a heated press (with or without pressure) may be used. When the appropriate temperature is shown by the thermocouple 3, the temperature is maintained for the recommended bond time for the adhesive film. The base plate 1 and preform assembly 8 are then removed from the oven and allowed to cool. Upon cooling, the preform is removed from the base plate 1 and checked for good adhesive film bonding. If the bond is adequate, the preform 8 is ready for infusion.

During the following description, all sheets assembled against the first sheet 12 during the preform assembly method will collectively be called the first sheet 12, and all sheets assembled against the second sheet 14 during the preform assembly method will collectively be called the second sheet 14.

The preform 8 is located in a second portion 26 of a mould generally designated as 30, which also has a first portion 28 (Figure 4). The second portion 26 has a peripheral frame 32, to which is applied a sealing bead 34. Between first face sheet 18 and first portion 28 there is located a diaphragm 38, which extends to and beyond plate 32 and over the sealing bead 34. The first face sheet 18 is temporarily bonded to the diaphragm 38. As is shown in Figures 5 and 6, rather than peripheral frame 32 there may be provided a tapered frame 48 around the periphery of second portion 26 to aid the deformation of the diaphragm 38. Both frame 32 and frame 48 assist to create a first chamber 40 between diaphragm 38 and first portion 28, and a second chamber 42 between diaphragm 38 and second potion 26. Seals 44 may be provided in addition to, or in place of, sealing bead 34.

Both first portion 28 and second portion 26 have a vacuum ports 36 which, as shown in Figure 5, may be centrally located in the relevant portions 26, 28. Resin inlet ports 46 are provided in second portion 26, although they may be in frame 48, if desired. The resin inlet ports 46 are preferably located between the periphery 50 of preform 8, and the inner edge 52 of frame 32, or inner edge 54 of tapered frame 48. As will be realised from the above description, the frame 48 is used when the frame 32 is not used, and vice versa.

Therefore, upon first portion 28 engaging with second portion 26, diaphragm 38 seals on sealing bead 34 and is secured between the peripheries of the first and second portions 26, 28. Suction (vacuum) is then applied to vacuum port 36 in second portion 26, so that the preform 8 will be securely drawn into second portion 26, as will be the attached diaphragm 38 (Figure 3). With an appropriate resinous compound (not shown) being fed into second chamber 42, the reduced pressure in second chamber 42 will draw the resinous compound into and through the chamber 42 and the preform 8. The resinous compound may have a positive pressure applied to it, if desired. If a positive pressure is used a higher positive pressure must be applied in the first chamber 40 to restrict the diaphragm 38 from rising. As diaphragm 38 presses on first face sheet 12, and as second face sheet 14 presses on second portion 26, the resinous compound is drawn through the preform 8 and thereby coats the infusible surfaces of those sheets, as well as the extensible threads 16 extending therebetween. Any surplus resinous compound will exit through vacuum port 36, where it can be recovered in an appropriate trap.

The feed of the resinous compound continues until all threads 16 and the infusible surfaces of first sheet 12 and second sheet 14 are coated. This time will depend on the nature of the resinous compound, the sheets 12, 14 the number and size of threads 16, and the size of the preform 8.

When the resinous compound has fully infused the preform 8, the resin inlet ports 46 are closed and the vacuum port 36 in second portion 26 is closed. The suction (vacuum) is then applied to vacuum port 36 in first portion 28. The second chamber 42 is simultaneously or earlier vented to the atmosphere by the vent port 37, and/or by resin inlet ports 46, so that second chamber 42 returns to atmospheric pressure. The vent port 37 may, if desired, be through frame 48. Alternatively, a positive pressure can be applied to second chamber 42 so that the first chamber 40 is at a relatively lower pressure. By virtue of the vacuum applied to vacuum port 36 in first portion 28, first chamber 40 is of reduced pressure. This therefore creates a pressure differential between first chamber 40 (low pressure) and second chamber 42 (higher pressure), causing diaphragm 38 to be drawn towards first portion 28, thus drawing first sheet 12 upwardly and away from second sheet 14 which is non-permanently adhered to the second mould portion 26 so that it will not move relative thereto during the moulding process. Therefore, the extensible threads 16 are extended. The distance between the first and second face sheets 12, 14 is that which is desired, as set by the mould cavity height (the sum of the heights first chamber 40 and second chamber 42). This height is usually predetermined by the height of frame 32 or frame 48. Hence, due to the pressure differential the diaphragm 38 and first sheet 12 are drawn up to the first mould portion. The suction (vacuum) is maintained in the vacuum port 36 in first portion 38, and second chamber 42 is sealed to allow the resinous compound to set and cure.

Alternatively, self-foaming resins can be used to infuse the preform. In this case, the pressure differential between the second chamber 42 and the first chamber 40 is created by the foaming of the resin. In this case, after the resinous compound has fully infused the preform 8, the resin inlet ports 46 are closed and the vacuum port 36 in second portion 26 is closed. The second chamber 42 is then vented to the atmosphere by the vent port 37 and/or by resin inlet ports 46, so that the second chamber 42 returns to atmospheric pressure. The first chamber 40 remains at atmospheric pressure. The foaming reaction of the resin, initiated chemically or by the application of heat, creates the pressure differential between the first chamber 40 (low pressure) and the second chamber 42 (high pressure), causing diaphragm 38 to be drawn towards the first portion 28. Thus, the first sheet 12 is drawn upwardly and away from the second sheet 14. The distance between the first and second face sheets 12, 14 is that which is desired, as set by the mould cavity height. The penetration of the first sheet 12 by the foaming resin is resisted by the bonding of the impermeable diaphragm 38 directly on the first sheet 12 during preform assembly. The penetration of the second sheet 14 is similarly resisted by the bonding of the impermeable second portion 26 directly onto the second sheet 14. When an appropriately engineered foaming resinous compound is used, an excellent quality infusion and finish in the skin in combination with a quality foam core will result. When using this type of foaming resin to create a sandwich structure, the distance fabric 10 may or may not be left out of the preform assembly 8 and, hence, the resulting sandwich structure. In the case of the distance fabric 10 being left out of the preform 8, the first sheet 12 is bonded to the diaphragm 38 and the second sheet 14 is bonded to the second portion 26, but they are not bonded to each other. This allows for their separation during the foaming of the resin. Maximum separation distance is again set by the mould cavity.

If desired, the mould 30 may be heated to assist the setting and curing of the resinous compound. Heating may be by heating elements being placed in mould 30, or by placing mould 30 in an oven. Alternatively, hot air could be introduced to second chamber 42 through resin inlet ports 46 and/or venting port

37 in second portion 26.

Upon setting, and preferably curing, of the resinous compound, the mould 30 is separated, diaphragm 38 removed, and the expanded composite structure removed.

If desired, external caul plates 56 and 60 (Figure 8) may be used, particularly for the embodiment of Figure 11. For the embodiment of Figure 11 clamping bars 62, spaced apart by spacer elements 64, are used to support the separation of the external caul plates 56 and 60, which in turn give the desired final dimensions of the preform 8 during the later stages of the infusion process and during curing processes. The vacuum ports 36 and/or inlet ports 46 and/or vent port 37 may be beyond the periphery of the preform 8, if desired. However, the vacuum port 36 and inlet ports 46 should not be adjacent.

The diaphragm 38 is preferably a non-porous film capable of holding a vacuum of the order of 100 kPa. It may be elastic or semi-elastic. Examples of suitable materials include silicon rubber sheet, latex rubber sheet, and a nylon bagging film, etc. As the resinous compound may contaminate the diaphragm 38 during infusion and/or expansion of the preform 8, the diaphragm 38 and the resinous compound should be such that there is no chemical interaction between them.

If desired, a caul plate 59 may be used (Figure 6). The caul plate 59 is placed between the first sheet 12 and the diaphragm 38 to aid control, and/or to improve the surface quality of first sheet 12. In addition, the use of caul plate 59 may assist in reducing peeling-off effects as the diaphragm 38 may initially stretch over the entire area of the caul plate 59. Furthermore, the caul plate 59 may slightly enhance the infusion of the resinous compound as improved flow paths may result. The caul plate 59 is preferably at least as large as the preform 8 and may, if desired, be releasably or securely attached to the diaphragm 38 or first sheet 12 by, for example, double sided tape. The attachment to the caul plate 59 may also occur during preform assembly method.

Furthermore, a carrier mesh 58 may also be used to assist the resinous compound to pass from the inlet ports 46 to the preform 8. The carrier mesh 58 is attached to the interior of second portion 26 at or adjacent the inlet ports 46 and extends to and along the side edges of preform 8. The carrier mesh 58 may extend totally or partially around the periphery of preform 8. EXAMPLE

One example of the production of a composite sandwich structure in accordance with an embodiment of the present invention is described below.

Preform Assembly Method

The surface of plate 1 is cleaned with acetone, with the surface being flat and free of debris and lumps.

A single layer of non-perforated adhesive film is cut to dimensions identical to the preform 8. This layer 20 is to provide a bond between the outer preform surface 18 and the bagging film 38. Layers of perforated adhesive film 20 are cut to provide a bonding/interleaf layer between all preform-to-preform surfaces.

The lower preform surface 22 is placed on plate 1. Alternate layers of perforated adhesive film 20 and preform fabrics are then placed above lower sheet 22 as required. A layer of the perforated adhesive 20 is placed on all preform-to-preform surfaces to bond the preform surfaces and also allow resin to move between the surfaces. A layer of non-perforated adhesive film is placed on top of preform 8.

Mastic tape or other similar sealant is applied to the tooling plate, outside of the perimeter of the preform 8. A thermocouple 3 is placed on the edge of the preform 8, such that it is contacting the adhesive film 20.

The vacuum source fittings are connected to the vacuum port 36 in plate 1.

A full vacuum (-100 kPa) is applied to enable checks of vacuum leaks.

The oven is preheated to the required temperature for adhesive bonding (or just above bonding temperature), and the vacuum is set to the bonding pressure.

The plate 1 with preform 8 is placed in the oven and heated until the thermocouple 3 shows that the thermoadhesive film 20, 24 has reached its/their bonding temperature. The temperature is held for the appropriate time.

The plate 1 and preform 8 are removed from the oven and allowed to cool. The vacuum fittings and thermocouple 3 are removed. The bagging/preform is removed from the plate 1 without peeling the bag from the preform 8. The excess bag/preform is removed to give a net-shape preform with bagging on the skins.

Liquid Moulding preparation

All mould surfaces (inside of mould, caul plate) are cleaned with acetone, with there being no remains of resin flash on mould surfaces and resin inlet channels.

All required hoses are fastened to the appropriate ports in the mould by using compression fittings.

Assembly

Strips of double sided tape are applied to the edges of both sides of the preform 8 and the caul plate 59. The preform 8 is fixed to the caul plate 59 using this tape. The preform 8 is placed in the centre of the second portion 26 of the mould and pressed to achieve a proper bond. The carrier mesh 58 is attached to both inlet ports 46. Mastic tape is applied to the outside of the mould, using extra strips to seal the corners. The diaphragm 38 is fixed to the mould plate, hence, sealing the mould.

Process Set-up

The vacuum and the first venting hose are connected to a resin trap to catch any excess resin. The vacuum source is then attached to the resin trap lid and the resin inlet and the venting hoses are clamped. The lid is closed and securing bars placed in position, and fastening bolts gently tightened. Full vacuum is applied for infusion. Liquid Moulding Procedure

An appropriate amount of resin is mixed for a predetermined gel time. The two resin inlet hose(s) are inserted into the cup. The infusion time is recorded. At the first sign of resin in the exit line, the vacuum pressure is reduced to 40 kPa for 3 minutes. The resin inlet lines and exit line are then closed. A full vacuum is then applied to the resin trap, and, after connecting in the vacuum line of the lid directly to the vacuum, full vacuum is applied. The venting hoses are opened and the vacuum in the resin trap reduced to zero at a rate of 2kPa/sec. The resin lines are removed, and the resin cup and excess hoses also removed. After approximately 2 hours the first venting hose is closed and 5 kPa vacuum applied to the first venting hose. This ventilates the mould and accelerates the curing by replacing the styrene. Curing for several hours is allowed. The clamps are then removed and excess lines cut. The clamping bars are removed from the tool and the lid opened. The diaphragm is removed and the tooling plate released from the expanded structure. The expanded structure is removed from the mould.

It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.

Claims (38)

CLAIMS:

1. A method of manufacturing a composite sandwich structure, the method including:

providing a preform structure, the preform having a first and second sheet, each of the first and second sheets having an inner face and an outer face, the inner faces being connected by a plurality of extensible threads;

affixing to the outer face of the first sheet and/or second sheet a face sheet;

adhering the face sheet to the respective first and/or second sheet by locating a film adhesive over substantially all of the respective outer face of the first and/or second sheet and subjecting the preform to pressure and/or heat to set the film adhesive.

2. A method according to claim 1 , wherein the film adhesive is one or more of a thermoplastic or thermoset material, a non-porous film, a net- or gridlike open structure, and/or a random filament.

3. A method according to claim 1 or 2, wherein the film adhesive creates a permanent or non-permanent bond between the face sheet and the first and/or second sheet.

4. A method according to any preceding claim, wherein the face sheet is one of an unimpregnated fabric(s) or a hardened skin(s) such as metals of previously cured composite materials, or a caul plate(s).

5. A method according to any preceding claim, wherein the first face sheet and the second face sheet are made of different materials.

6. A method according to any one of claims 1 to 4, wherein the first face sheet and the second face sheet are made of the same materials.

7. A composite sandwich structure preform produced by the method of any preceding claim.

8. A method of moulding a composite sandwich structure, the method including:

(a) placing a preform in a mould, the preform having a first sheet and a second sheet, there being a plurality of extensible threads extending between the first and second sheets, the mould having a first portion and a second portion, the first sheet being in the first portion, and the second sheet being in the second portion;

(b) placing a diaphragm between the first sheet and the first portion of the mould, the diaphragm extending to at least an outer periphery of the first portion, there being a first chamber between the diaphragm and the first portion, and a second chamber between the diaphragm and the second portion;

(c) non-permanently adhering to the first sheet of the preform to the diaphragm and non-permanently adhering the second sheet to the second mould portion;

(d) creating a pressure differential between the second chamber and the first chamber by reducing the pressure in the second chamber;

(e) using the pressure differential to feed a resinous compound into the second chamber to coat the extensible threads and surfaces of the first and second sheets capable of being infused;

(f) ceasing the feeding of the resinous compound into the second chamber;

(g) ceasing the reduction of the pressure in the second chamber to substantially equalise the pressures in the first and second chambers; (h) creating a second pressure differential between the second chamber and the first chamber by having the pressure in the first chamber below that in the second chamber to thus cause the diaphragm and hence, the first sheet to move away from the second sheet thereby causing the extensible threads to extend;

(i) maintaining the second pressure differential; and

(j) allowing the resinous compound to set and cure.

9. A method according to claim 8, wherein the second pressure differential at step (h) is created by applying vacuum pressure to the first chamber.

10. A method according to claim 8 or 9, wherein the second pressure differential is created by applying a positive pressure in the second chamber.

11. A method according to claim 9, wherein the vacuum pressure is created by a first vacuum source.

12. A method according to any one of claims 8 to 11 , wherein the first chamber is vented during step (d), the venting ceasing upon the commencement of step (h).

13. A method according to claim 12, wherein the second chamber is vented during step (h).

14. A method according to claim 12, wherein the reduction of the pressure in step (d) is by means of the application of the first vacuum source.

15. A method according to claim 14, wherein the first vacuum source is applied to a second vacuum port in the second portion of the mould.

16. A method according to claim 14, wherein a second vacuum source is applied to a second vacuum port in the second portion of the mould.

17. A method according to any one of claims 8 to 16, wherein the feeding of the resinous compound is by at least one inlet port in the second portion.

18. A method according to any one of claims 8 to 16, wherein the feeding of the resinous compound is by at least one inlet port located between the first and second portions.

19. A method according to claim 15 or 16, wherein the second vacuum port is located at or adjacent to the centre of the second portion; the at least one inlet port being located outside the periphery of the preform.

20. A method according to claim 19, wherein the first or second vacuum source is connected to the second vacuum port as well as the first chamber by separate tubes, the separate tubes being separately openable and closable.

21. A method according to claim 15 or 16, wherein the inlet port is at a first end of the second portion and the second vacuum port at a second end of the second portion, the preform assembly being located between the first end and the second end.

22. A method according to claim 9, wherein vacuum pressure is applied to the first chamber by means of a first vacuum port in the first portion.

23. A method according to claim 22, wherein the first vacuum port is substantially centrally located in the first portion.

24. A method according to any one of claims 8 to 23, wherein heat is applied for a predetermined time during step (j).

25. A method according to claim 23, wherein the heat is applied by heating the first portion and/or the second portion.

26. A method according to claim 24, wherein the heat is applied by introducing heated air into the second chamber.

27. A method according to claim 26, wherein the heated air is introduced through the at least one inlet port and/or the second vacuum port and/or a further inlet port.

28. A method according to claim 24, wherein the heat is applied by placing the mould in a heated oven.

29. A method according to any one of claims 8 to 28, wherein a caul plate is releasably adhered to the diaphragm above the first sheet, and to the preform.

30. A method according to claim 8, wherein, in step (e), the resinous compound may be fed using a positive pressure.

31. A method according to claim 8, wherein, in step (e), a carrier mesh is used to feed the resinous compound, the carrier mesh being attached to the inlet ports.

32. A method according to any one of claims 8 to 31 , wherein the resinous compound is selected from one of a vinylester, phenolin, epoxy, or the like, resin.

33. A method according to any one of claims 8 to 32, wherein the mould is capable of incorporating a diaphragm and of holding vacuum in a cavity under the diaphragm sufficient to conduct a successful infusion of the preform assembly.

34. A method according to claim 33, wherein the mould is one of a sealed, fully closed mould, a base plate-diaphragm combination or an envelope bag.

35. A method according to claim 33 or 34, wherein the mould incorporates means for the infusion of the preform assembly with resin.

36. A method according to any one of claims 33 to 35, wherein the mould includes means for creating a pressure differential on either side of the diaphragm and is capable of maintaining the pressure differential until the sandwich structure is cured.

37. A method according to any one of claims 33 to 36, wherein the mould includes means for supporting the predetermined, final component dimensions.

38. A composite sandwich structure produced by the method of any one of claims 8 to 37.