Vacuum forming a laminate charge

A method of forming a laminate charge, and a forming tool and apparatus for forming a laminate charge is disclosed. In a forming step, a suction force is generated between a forming tool and an impermeable sheet which causes the impermeable sheet to press the laminate charge against a male corner of the forming tool and into a female corner of the forming tool. The male corner of the forming tool is positioned between first and second parts of the laminate charge, and the female corner of the forming tool is positioned between second and third parts of the laminate charge. During the forming step, gas is injected between the forming tool and part of the laminate charge to create a gas cushion between the forming tool and the third part of the laminate charge. This gas cushion inhibits the third part of the laminate charge from becoming clamped against the forming tool as the laminate charge is pressed into the female corner of the forming tool.

CROSS RELATED APPLICATIONS

This application is the U.S. national phase of International Application No. PCT/GB2017/052591 filed on Sep. 6, 2017, which designated priority to United Kingdom (GB) patent application 1615213.4, filed Sep. 7, 2016, the entire contents of each of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a method of forming a laminate charge. The invention also relates to a forming tool and apparatus for forming a laminate charge. The charge is typically, although not exclusively, either a composite material or a preform which is subsequently infused to form a composite material.

BACKGROUND OF THE INVENTION

A known method of vacuum moulding a composite material is described in U.S. Pat. No. 4,562,033. Prepregs are placed on a heated former and covered with a sheet of a micro-porous film material, a breather material and an impermeable membrane.

A problem with some known vacuum moulding processes is that it can be difficult for the charge to be easily sucked into the female corner of the forming tool without becoming clamped against the forming tool.

Another known method for the shaping of a sheet of preform material is described in U.S. Pat. No. 4,946,640.

SUMMARY OF THE INVENTION

A first aspect of the invention provides a method of forming a laminate charge as set out in claim1. During a vacuum forming step, gas is injected between the forming tool and a third part of the laminate charge to create a gas cushion between the forming tool and the third part of the laminate charge. The gas cushion inhibits the third part of the laminate charge from becoming clamped against the forming tool as the laminate charge is pressed into the female corner of the forming tool.

After the forming step, the charge may be removed from the tool without applying a suction force to the third part of the charge. However, more typically the method further comprises stopping the injection of gas between the forming tool and the third part of the laminate charge, and generating a suction force between the forming tool and the impermeable sheet, which causes the impermeable sheet to press the third part of the laminate charge against the forming tool.

Optionally the third part of the laminar charge is a flange with an edge, and at least some of the gas injected between the forming tool and the third part of the laminate charge exits the gas cushion by flowing past the edge of the flange. Alternatively the gas injected between the forming tool and the third part of the laminate charge may exit the gas cushion by flowing through the thickness of the laminar charge, or by flowing into vacuum openings in the second face of the forming tool.

The third part of the laminar charge may be a flange with an edge, or it may extend to form a further corner—for instance to form a stepped shape.

The suction force which causes the impermeable sheet to press the laminate charge against the male corner of the forming tool and into the female corner of the forming tool, and the suction force which causes the impermeable sheet to press the third part of the laminate charge against the forming tool, may be generated by the same vacuum generator or by different vacuum generators. The corners of the forming tool may be of any angle or shape. The surfaces of the forming tool may be of any angle or shape.

Typically the gas is a compressed gas—for instance from a compressed gas canister.

The gas may be air, or any other suitable gas.

The method may be used, at a minimum, to form the charge into a “Z” shape, but it may also be used to form more complex shapes, such as a top-hat shape, an omega shape, or a stepped shape. In a preferred example the laminate charge has fourth and fifth parts; the forming tool has a second male corner and a second female corner; and the method further comprises: in the forming step, generating a suction force between the forming tool and the impermeable sheet which causes the impermeable sheet to press the laminate charge against the second male corner of the forming tool and into the second female corner of the forming tool, wherein the second male corner of the forming tool is positioned between the first and fourth parts of the laminate charge, and the second female corner of the forming tool is positioned between the fourth and fifth parts of the laminate charge; and during the forming step, injecting gas between the forming tool and the fifth part of the laminate charge in order to create a second gas cushion between the forming tool and the fifth part of the laminate charge.

The second gas cushion performs the same function as the first gas cushion—it inhibits the fifth part of the laminate charge from becoming clamped against the forming tool as the laminate charge is pressed into the female corner of the forming tool. This enables the charge to be formed into a top-hat or omega shape.

The male corner is positioned between a first face of the forming tool and a second face of the forming tool; and the female corner is positioned between the second face of the forming tool and a third face of the forming tool. The forming tool may have vacuum openings in the first, second and fourth faces of the forming tool, and/or in the male corner of the vacuum tool, and/or in the female corner of the forming tool. Where such vacuum openings are provided, then the suction force is generated by sucking gas through the vacuum openings. Alternatively the vacuum may be applied without the use of such vacuum openings.

Gas injection openings may be provided in the third and fifth faces of the forming tool. Where such gas injection openings are provided, then the gas is injected between the forming tool and the laminate charge through the gas injection openings. Alternatively the gas may be may injected without the use of such gas injection openings.

A second aspect of the invention provides a forming tool for forming a laminate charge. The forming tool has vacuum openings in first and second faces of the forming tool. The vacuum openings are arranged to create a suction force adjacent to the first and second faces of the forming tool by sucking gas away from the first and second faces of the forming tool through the vacuum openings. A vacuum port is provided in fluid communication with the vacuum openings, optionally via a vacuum manifold.

Gas injection openings are also provided in a third face of the forming tool. The gas injection openings are arranged to create a gas cushion adjacent to the third face of the forming tool by injecting gas out of the gas injection openings. These gas injection openings may comprise an array of holes, slots or grooves in the forming tool. Alternatively the forming tool may be formed by a porous material with micro-pores which provide the vacuum openings. A gas port is provided in fluid communication with the gas injection openings, optionally via a gas manifold.

Optionally a vacuum manifold couples the vacuum port to the vacuum openings, wherein the vacuum port is in fluid communication with the vacuum openings via the vacuum manifold.

Optionally a gas manifold couples the gas port to the gas injection openings, wherein the gas port is in fluid communication with the gas injection openings via the gas manifold.

A vacuum generator, such as a motorised pump, may be coupled to the vacuum port. The vacuum generator is arranged to generate the suction force by sucking the gas away from the first and second faces of the forming tool through the vacuum openings and the vacuum port.

A gas source, such as a compressed air cylinder or motorised pump, may be coupled to the gas injection port. The gas source is arranged to create the gas cushion by injecting the gas through the gas port and out of the injection openings.

The forming tool may be used, at a minimum, to form a charge into a “Z” shape, but it may also be used to form more complex shapes, such as a top-hat shape, an omega shape, or a stepped shape. Optionally the forming tool has fourth and fifth faces; a second male corner positioned between the first and fourth faces of the forming tool; a second female corner positioned between the fourth and fifth faces of the forming tool; a second set of vacuum openings in the fourth face of the forming tool, wherein the second set of vacuum openings are arranged to create a suction force adjacent to the fourth face of the forming tool by sucking gas away from the fourth face of the forming tool through the vacuum openings, and the vacuum port is in fluid communication with the second set of vacuum openings; a second set of gas injection openings in the fifth face of the forming tool, wherein the second set of gas injection openings are arranged to create a second gas cushion adjacent to the fifth face of the forming tool by injecting gas out of the second set of gas injection openings; and a second gas port in fluid communication with the second set of gas injection openings. This enables the forming tool to form a charge into a top-hat or omega shape.

The vacuum openings may only be in the first and second faces, but more preferably male corner vacuum openings are provided in the, or each, male corner of the forming tool, wherein the male corner vacuum openings are arranged to create a suction force adjacent to the male corner(s) of the forming tool by sucking gas away from the male corner(s) of the forming tool through the male corner vacuum openings, and the vacuum port is in fluid communication with the male corner vacuum openings. Similarly female corner vacuum openings may be provided in the, or each, female corner of the forming tool, wherein the female corner vacuum openings are arranged to create a suction force adjacent to the female corner(s) of the forming tool by sucking gas away from the female corner(s) of the forming tool through the female corner vacuum openings, and the vacuum port is in fluid communication with the female corner vacuum openings.

Optionally the forming tool comprises a pair of peripheral grooves, wherein each groove runs along a respective peripheral edge of the first face, round a respective peripheral edge of the male corner, along a respective peripheral edge of the second face and terminates at the female corner.

A third aspect of the invention provides apparatus for forming a laminate charge. A vacuum system is arranged to create a suction force adjacent to first and second faces of the forming tool by sucking gas away from the first and second faces of the forming tool; and a gas injection system is arranged to create a gas cushion adjacent to the third face of the forming tool by injecting gas adjacent to the third face of the forming tool. The vacuum system may create the suction force using vacuum openings in the first and second faces of the forming tool, or it may create the suction force in some other way for instance by sucking air out through a bagging film. Typically the vacuum system comprises a vacuum generator, such as a motorised pump. Similarly the gas injection system may be arranged to create the gas cushion by injecting gas out of the third face of the forming tool, for instance via gas injection openings in the third face of the forming tool; or it may create the gas cushion in some other way, for instance by injecting air under the edge of the charge from the side. Typically the gas injection system comprises a gas source, such as a compressed air cylinder or motorised pump.

A fourth aspect of the invention provides a method of forming a laminate charge. Gas (typically air) is sucked through openings in a forming surface in a sequence of stages, each stage of the sequence causing a different part of the laminate charge to be pressed against the forming surface. Generating vacuum forces in a sequence of stages, rather than simultaneously across the full extent of the forming tool, enables the charge to be pressed against the forming surface with a wave-like or progressive motion which can result in better consolidation and less wrinkling in the final product.

A fifth aspect of the invention provides a forming tool for forming a laminate charge. First and second control valves or vacuum generators can be operated individually, to independently control the suction forces generated by first and second sets of vacuum openings respectively.

The following comments apply to the method claims, and where appropriate also to the forming tools.

Optionally after the second stage of the sequence, the first and second stages of the sequence are repeated in order, so that gas is alternately sucked and then not sucked through a second set of the vacuum openings.

During the second stage of the sequence, gas may be sucked through both the first and second sets of the vacuum openings, or only through the second set of the vacuum openings.

Optionally during the first stage of the sequence the second part of the laminate charge is not in contact with the forming surface, and during the second stage of the sequence the second part of the laminate charge comes into contact with the forming surface.

The forming surface may have any shape, including a relatively flat shape or a continuously curved shape with no corners. More typically the forming surface comprises a pair of faces; and a male or female corner positioned between the pair of faces. The pair of faces may be substantially planar, or they may be formed with ramps or other non-planar features. Optionally the first set of the vacuum openings are in one of the faces, and the second set of vacuum opening are in the other of the faces. Alternatively the first set of vacuum openings are in one of the faces and the second set of vacuum openings are in the male or female corner. In either case, the sequential application of vacuum can reduce the risk of wrinkles forming at the corner and/or increase consolidation of the laminate charge at the corner. Optionally a third set of the vacuum opening are in the other of the faces, and during a third stage of the sequence gas is sucked through the third set of the vacuum openings so that a third part of the laminate charge is pressed against the other of the faces. Gas is typically not sucked through the third set of vacuum openings during the first or second stage of the sequence.

The forming surface may have a single corner only, but in a preferred embodiment the forming surface comprises first, second and third faces; a male corner positioned between the first and second faces; and a female corner positioned between the second and third faces. Optionally the first set of the vacuum openings are in the first face and/or the male corner and/or the second face, and the second set of the vacuum openings are in the female corner and/or the third face. In one embodiment the second set of the vacuum openings are in the female corner, and a third set of the vacuum openings are in the third face; and during a third stage of the sequence gas is sucked through the third set of the vacuum openings so that a third part of the laminate charge is pressed against the forming surface. Gas is typically not sucked through the third set of vacuum openings during the first or second stage of the sequence. Alternatively the first set of the vacuum openings are in the first face, the second set of the vacuum openings are in the second face, and a third set of the vacuum openings are in the third face. During a third stage of the sequence gas is sucked through the third set of the vacuum openings so that a third part of the laminate charge is pressed against the forming surface. Gas is typically not sucked through the third set of vacuum openings during the first or second stage of the sequence.

Optionally, during the first and/or second stage of the sequence, gas is injected through the third set of the vacuum openings to create a gas cushion between the forming surface and the third part of the laminate charge.

In a preferred embodiment the sequence comprises five stages in which gas is sucked through vacuum openings in the first face, the male corner, the second face, the female corner and then the third face, in that order.

The (or each) vacuum generator may be a pump such as a motorised pump, or any other suitable source of vacuum.

In the case of the forming tool, the first and second control valves can be operated individually, to independently control the suction forces generated by the first and second set of vacuum openings respectively. In other words the first and second control valves can be operated independently of each other. Optionally the control valves are ball valves, or any other valve with an “on” setting and an “off” setting.

In the case of the forming tool, the first and second vacuum generators can be operated individually, to independently control the suction forces generated by the first and second set of vacuum openings respectively. In other words the first and second vacuum generators can be operated independently of each other. For instance the first and second vacuum generators may be turned on and off to independently control the suction forces, or the first and second vacuum generators may each have a respective control valve, such as a ball valve, which can be turned on and off to independently control the suction forces.

Optionally the second vacuum generator can be switched between a vacuum generation mode in which it sucks gas away from the forming surface through the second set of vacuum openings, and a gas injection mode in which it injects gas (typically air) through the second set of vacuum openings so as to generate a gas cushion adjacent to the forming surface.

The following comments apply to all aspects of the invention.

The faces of the forming tool may be substantially planar, or they may be formed with ramps or other non-planar features.

The openings may comprise an array of holes, slots or grooves in the forming tool. Alternatively the forming tool may be formed by a porous material with micro-pores which provide the openings.

Optionally an impermeable sheet, such as a bagging film, is provided which can be fitted over the forming tool.

The impermeable sheet may be a bagging film, a membrane, or any other impermeable and flexible sheet material. The impermeable sheet may be tensioned over the forming tool. The impermeable sheet may typically be tensioned in a direction away from a top edge of the forming tool. The tension applied to the impermeable sheet may typically be an axial tension. The tension may be applied manually. The tension may be applied mechanically. One method in which the impermeable sheet may be tensioned mechanically is via a cam arrangement. The impermeable sheet may stretch when tension is applied. The tension may be released during the forming operation, typically when the charge has been formed around the male corner of the forming tool. The tension may be released by enabling the cam arrangement to rotate as the sheet forms around the tool. The impermeable sheet may be clamped. The impermeable sheet may typically be clamped against the forming tool. The forming tool may be bigger than the charge. The impermeable sheet may be clamped manually. The impermeable sheet may be clamped mechanically via a clamping arrangement. The impermeable sheet may be releasably clamped. The clamp release mechanism may be automated. One method of mechanically clamping the impermeable sheet is via a cam arrangement wherein the cam clamps the impermeable sheet. Alternatively the impermeable sheet may be clamped by releasable blocks. The clamping arrangement may be retractable. The impermeable sheet may be clamped and tensioned simultaneously. The clamping force is typically applied in a different direction to the tension force. The clamping force may be applied in a direction perpendicular to the third forming tool surface. The impermeable sheet may be clamped on an opposite surface of the forming tool.

The laminate charge comprises a stack of two or more plies, typically ten or more plies.

The laminate charge may comprise a stack of non-fibrous plies, but typically it comprises a stack of fibre plies. The fibres of the fibre plies may be carbon fibres, or fibres made from any other suitable material. Fibre plies are preferred because they can have relatively high porosity, which is beneficial to enable the flow of gas through the charge.

The fibre plies may be prepreg plies, each prepreg ply comprising a layer of fibres impregnated with a matrix material. However more preferably the laminate charge comprising a stack of dry-fibre plies, optionally containing a binder. Such dry-fibre plies have a higher degree of porosity than prepreg plies, which is beneficial to enable the flow of gas through the charge. Optionally the stack of dry-fibre plies includes resin film plies interleaved with the dry-fibre plies, as a precursor for a resin-film-infusion process.

Optionally the method further comprises infusing the dry-fibre plies of the laminate charge with a liquid matrix material after the impermeable sheet has pressed the third part of the laminate charge against the forming tool. This may be achieved by a resin transfer moulding process, a resin film infusion process, or any other suitable method.

The dry-fibre plies may be infused on the forming tool, but more typically they are infused on an infusion tool. The infusion tool may have the same male profile as the forming tool, but more typically the method further comprises removing the laminate charge from the forming tool after the impermeable sheet has pressed the third part of the laminate charge against the forming tool; placing the laminate charge on an infusion tool with a female corner of the infusion tool positioned between the first part of the laminate charge and the second part of the laminate charge, and a male corner of the infusion tool positioned between the second part of the laminate charge and the third part of the laminate charge; and infusing the dry-fibre plies of the laminate charge on the infusion tool with a liquid matrix material.

An infusion tool may be provided, with an inverted complementary shape to the forming tool. Optionally the infusion tool has first, second and third faces, a female corner positioned between the first and second faces of the infusion tool, and a male corner positioned between the second and third faces of the infusion tool.

DETAILED DESCRIPTION OF EMBODIMENT(S)

A forming tool10for forming a laminate charge is shown inFIG. 1. The forming tool comprises a forming part20shown inFIGS. 2-4, fitted onto a base30shown inFIGS. 5-7.

The forming part20has five parts1-5forming an “omega shape”. The five parts1-5are connected by curved corner parts6-9. Each part1-5has a respective outer face1a-5awhich is approximately planar, and each corner part6-9has a curved outer face6a-9aforming either a male corner or a female corner. Thus a first male corner6ais positioned between the first and second faces1a,2aof the forming part; and a first female corner7ais positioned between the second and third faces2a,3aof the forming part. The other half of the forming part20is a mirror image, as indicated by mirror plane X inFIG. 1, with a second male corner8apositioned between the first and fourth faces1a,4aof the forming part; and a second female corner9apositioned between the fourth and fifth faces4a,5aof the forming part10.

As shown inFIG. 2, the forming part20is perforated to form arrays of vacuum holes21in the first, second and fourth faces1a,2a,4aof the forming part10, and similar arrays of gas injection holes22in the third and fifth faces3a,5aof the forming part10. Male corner vacuum holes21aare also provided in the male corners of the forming part, along with female corner vacuum holes21bin the female corners of the forming part. The vacuum holes21,21a,21bare arranged to create a suction force adjacent to the faces1a,2a,4aof the forming tool as will be described in further detail below. Similarly the gas injection holes22are arranged to create gas cushions adjacent to the faces3a,5aof the forming tool as will be described in further detail.

A pair of peripheral grooves23,24run along respective peripheral edges of the faces1a,2a,4a, and respective peripheral edges of the first and second male corners6a,8a. The grooves23,24have vacuum holes25,26and they terminate at the female corners7a,9a.

The base30comprises a support network shown inFIG. 6, and a mounting structure31shown inFIG. 5. The support network comprises five horizontal plates32, ten axial ribs33,33′ running parallel with the corners6a-9a, and seven lateral ribs34running transverse to the corners6a-9a. The plates32and lateral ribs34are perforated with holes32aand34arespectively. Eight of the axial ribs33are perforated with holes33ain a similar fashion, but two of the axial ribs (labelled33′) are non-perforated—that is, they are continuous with no such holes. These continuous ribs33′ will be referred to below as dividing ribs33′.

FIG. 7is a side view of the base30with the two dividing ribs33′ shown. The dividing ribs33′ and the mounting structure31segment the base30into three chambers labelled inFIG. 7: a central vacuum chamber35, and a pair of positive pressure chambers36. The perforated ribs32-34enable gas to flow across the ribs within the chambers35,36; and the dividing ribs33′ prevent gas from flowing between the chambers35,36.

Before the forming part20is fitted onto the base30, vacuum tape37shown inFIGS. 7 and 8is fitted along the top edges of the dividing ribs33′. Similarly a frame of vacuum tape38is fitted around the top edge of the mounting structure31. The vacuum tape37,38seals the interface with the forming part20.

A vacuum port40shown inFIG. 5passes through the mounting structure31and into a central channel41of the vacuum chamber as shown inFIG. 6. The vacuum port40is in fluid communication with the vacuum holes21,21a,21b,25,26via the vacuum chamber35. The central channel41acts as vacuum manifold coupling the vacuum port40to the vacuum holes, i.e. the vacuum port40is in fluid communication with the vacuum holes via the vacuum manifold41.

Gas ports42, also shown inFIG. 5, pass through the mounting structure31and into central channels43of the positive pressure chambers as shown inFIG. 6. The gas ports42are in fluid communication with the gas injection holes22via the positive pressure chamber36. Each central channel43acts as a gas manifold coupling a respective gas port to the gas injection openings, i.e. the gas port42is in fluid communication with the gas injection openings via the gas manifold43.

In order to operate the forming tool, a vacuum pump shown inFIG. 9is coupled to the vacuum port40. The vacuum pump generates the suction force by sucking the gas away from the faces1a,2a,4aof the forming tool through the vacuum holes and the vacuum port40. A suitable vacuum pump is part no. SGBL-FU 335-250-ER available from Schmalz. This is a frequency-regulated vacuum blower with continuously adjustable suction volume.

Similarly, compressed air supplies shown inFIG. 9are coupled to the gas ports42. Each gas source creates a respective gas cushion, as described in further detail below, by injecting compressed air through the gas port42and out of the gas injection openings. The compressed air supplies may be compressed gas canisters, or compressed gas pumps for example.

FIG. 10shows a first step in a method of forming a laminate charge using the apparatus previously described. A laminate charge50is laid up on a horizontal lay-up surface provided by the (upper) first surface1aof the forming tool and a pair of retractable lay-up tables58. The laminate charge comprises a stack of dry-fibre plies56shown inFIG. 11which are laid up one-by-one on the horizontal lay-up surface. The number of plies56typically varies between about 15 plies (for a 3 mm charge) up to about 120 plies (for a 40 mm charge). Each ply comprises a knitted or woven fabric of carbon-fibres or other fibres, typically stitched by tricot stitching although other stitching methods are possible such as pillar stitching. Each ply contains binder material and/or a toughener material, typically in the form of a powder or a veil, but it is “dry” in the sense that it is not pre-impregnated with resin so has a relatively open porous structure. Optionally the stack includes a peel ply, or any other porous membrane such as a breather layer, at the top and/or bottom of the stack.

The retractable lay-up tables58are then lowered, following the paths indicated by arrows inFIG. 10, so that the charge50drapes down under its own weight over the male corners6a,8aas shown inFIG. 12.FIG. 13shows the charge50after the retractable lay-up tables58have been removed, and the edges of the charge50are in contact with the outer faces3a,5aof the forming tool. Next a bagging film60is fitted over the forming tool as shown inFIG. 14. Note that the edges of the bagging film60are not sealed.

In a forming step shown inFIG. 15, the vacuum pump is turned on to generate a suction force between the forming tool and the bagging film60. Tension is applied to the bagging film60in a direction down and away from face1aso that the bagging film is tensioned over the plies51at the face1a. The bagging film is then clamped around the edge of the outer faces3aand5a, ensuring that the flanges53and55are not clamped between the clamp and the tool. The bagging film can be clamped manually or via a clamping arrangement62at one or more positions along the length of the edge of the outer faces3aand5a, as illustrated inFIG. 15. The suction force between the tool and the bagging film60causes the bagging film60to press the laminate charge around and against the male corners of the forming tool. The clamping arrangement62increases the tension acting at the male corner region, enabling air present between the plies51adjacent surface1ato be washed out preferentially to air present between the plies adjacent other surfaces of the charge being washed out during the forming process, which achieves a bulk reduction in the part and also prevents wrinkling at the male corners. As the suction force causes the bagging film to press the laminate charge parts52and54against faces2aand4a, the clamping arrangement is allowed to rotate which releases the bagging film, allowing the bagging film to press the laminate charge around and into the female corners of the forming tool. This vacuum force is indicated by inwardly directed arrows70inFIG. 15which also indicate the negative flow of air into the vacuum chamber35via the vacuum holes. The step of clamping the bagging film may be omitted where desired, e.g. for thinner plies stacks which have less bulk.

The charge50has first, second, third, fourth and fifth parts numbered51to55respectively. The male corners6a,8aof the forming tool are positioned between the first and second parts51,52of the charge, and between the first and fourth parts51,54of the charge respectively. The female corners7a,9aof the forming tool are positioned between the second and third parts52,53of the charge, and between the fourth and fifth parts54,55of the charge respectively. The third and fifth parts53,55of the charge will be referred to below as flanges53,55.

During the forming step, the compressed air supplies are also turned on so that compressed air71is injected between the forming tool and the flanges53,55of the charge to create gas cushions72between the forming tool and the flanges53,55. Each gas cushion72inhibits its associated flange53,55from becoming clamped against the forming tool as the charge is pressed into the female corners7a,9aof the forming tool. The gas cushions72effectively lift up the flanges53,55so that there is relatively little friction between the flanges53,55and the forming tool. The flanges53,55may be completely lifted up by the gas cushion so that there is no contact with the forming tool, although more typically the flanges will perform a flapping or rippling motion causing intermittent contact with the forming tool.

The relatively high porosity of the dry-fibre charge50means that some of the air from each gas cushion72exits by passing through the flange53,55of the charge and then out from under the bagging film, as indicated by arrows73inFIG. 15. Note that the bagging film60is not sealed against the forming tool, so the compressed air can escape easily via this route. Some of the compressed air will also exit the gas cushions72by flowing past and underneath the edge of the flange53,55and out from under the bagging film. Some of the compressed air will also flow directly from the gas injection holes and into the vacuum chamber35via the vacuum holes. This will slightly impede the efficiency of the vacuum pump, but such a drop of efficiency has been found to be acceptable.

FIG. 16shows the flow of air in and out of the network of ribs during the forming step described above. Optionally the flow through the ribs can be controlled by fitting permeable diffusing cloth over selected ones of the holes in the ribs, for instance to achieve a more even flow distribution or to slow down the flow.

The vacuum holes25,26pull the bagging film down into the peripheral grooves23,24. This helps the bagging film60to be held securely against the top of the forming tool and seals this particular interface between the bagging film and the forming tool.

FIG. 17shows the charge at the end of the forming step. The low friction caused by the gas cushions72has enabled the flanges53,55to slide or float easily into the female corners of the forming tool without wrinkling. There will also be a certain degree of relative sliding motion between the dry-fibre plies during the forming step, and this may be assisted by the action of the gas cushions which help to loosen the stack and reduce inter-ply shear forces.

At the end of the forming step, the compressed air supplies are disconnected or turned off to stop the injection of compressed air between the forming tool and the charge, although the vacuum pump is left running. The clamping arrangement62is also retracted. The edges of the bagging film are then sealed against the forming tool by vacuum tape80shown inFIG. 18. Next, as shown inFIG. 19, additional vacuum pumps are coupled to the gas ports42, reversing the flow of air though the gas ports42so that the gas ports42become vacuum ports. This generates a suction force between the forming tool and the bagging film sheet which causes the bagging film sheet to press the flanges53,55of the laminate charge against the forming tool. The suction force into the positive pressure chambers36via the gas injection holes is indicated by arrows85inFIG. 20.

In an alternative process, rather than connecting additional vacuum pumps to the gas ports42, these ports may remain inactive and the vacuum forces pulling the flanges into contact with the forming tool are instead generated by the vacuum pump coupled to the vacuum port40, with the air flowing into the vacuum chamber via the vacuum holes.

The suction forces air out of the pores in the charge50, reducing its bulk ready for the next steps. Once the charge has been fully formed, it is heated on the forming tool, with the vacuum pump(s) on, to activate a binder in the dry-fibre plies. Next, the shaped charge is removed from the male forming tool, turned upside down, and fitted into a female infusion tool90shown inFIG. 21. The infusion tool90has an inverted complementary shape to the forming tool with faces91-95corresponding with the faces1-5of the forming tool; female corners96,98corresponding with the male corners6,8of the forming tool; and male corners97,99corresponding with the female corners7,9of the forming tool.

A peel ply100is laid onto the charge50, followed by an infusion mesh101, and a bagging film102which is sealed to the infusion tool by sealant tape103. Alternatively, the infusion tool may be sealed by an integral vacuum system. A vacuum is applied via vacuum valves104in the bagging film. A liquid resin source105is coupled to infusion ports106in the bagging film. The dry-fibre plies of the laminate charge are infused with liquid resin from the source105which flows through the infusion mesh101and into the charge. The charge could be infused on a male infusion tool, but it has been found that a female tool, as shown inFIG. 21, is preferred. In this preferred arrangement, a female corner96of the infusion tool is positioned between the first part1of the laminate charge and the second part of the laminate charge, and a male corner97of the infusion tool is positioned between the second part2of the laminate charge and the third (flange) part3of the laminate charge.

The infused charge is then cured and is ready for use in its particular application.

An alternative forming tool will now be described with reference toFIGS. 22 to 31.

Similar to the forming tool10shown inFIG. 1, the forming tool200has five parts201-205, and four curved corner parts206-209located between each of the five parts201-205. A first male corner part206is located between the first part201and the second part202, and a first female corner part207is located between the second part202and the third part203. Similar to the tool ofFIG. 1, the forming tool200has a second male corner part208located between the first part201and the fourth part204, and a second female corner part209located between the fourth part204and the fifth part205forming an “omega” shape.

Referring now toFIG. 23, the forming tool200is assembled from a base210, a first sidewall212, a second sidewall214and an upper section216. The base210has holes210aand210bwhich receive pins (not shown) of the sidewalls212,214to secure the sidewalls212,214to the base. The sidewalls212,214also have holes212a,214awhich receive pins (not shown) of the upper section216.

Each of the parts201-209has a respective outer surface201a-209a, the outer surfaces201a-209atogether providing a forming surface of the forming tool. The surfaces201a-205aare planar faces, and the surfaces206a-209aare curved male and female corners. Note that the faces2a,4ainFIG. 1are parallel, but the faces202a,204ainFIG. 22taper inwardly towards the top of the forming tool200.

As shown inFIG. 24, a pair of Aluminium or steel guide rails218a,bare fitted on each sides of the first (upper) face201a, to retain the laminate charge (not shown) in place during the forming operation.

As with the forming tool10shown inFIG. 1, nine sets of holes221-229are provided in the nine outer surfaces201a-209aof the forming tool200. These holes will be described in greater detail with reference toFIG. 24.

Whereas the forming tool10shown inFIG. 1has a manifold arrangement, each of the nine parts201-209shown inFIG. 22has a respective chamber231-239arranged in fluid communication with the holes221-229for that part only, with no fluid communication between the chambers231-239. The chambers231-235,237,239are closed by respective plates231a-235a,237a,239aand the chambers236,238in the male corner parts206,208are closed by the upper faces of the sidewalls212,214.

Referring toFIG. 24, each surface201a-209aof the forming tool200has a respective set of holes221-229. Each set of holes221-229is in fluid communication with a respective chamber231-239, and each chamber231-239is in fluid communication with a respective port241-249.

Referring toFIG. 25, the ports241,242,244,246-249are connected in parallel to a first pump260via respective ball valves251,252,254,256-259which are also shown inFIG. 27. The ports243and245are connected in parallel to a second pump262via respective ball valves253,255, also shown inFIG. 27.

The first pump260can be turned on and off via a switch (not shown). When turned on with the ball valves open, the pump260provides a suction force at the first, second and fourth faces201a,202a,204a, and at the corners206a-209a. This vacuum force is applied to the laminate charge via the ports241,242,244,246-249, the chambers231,232,234,236-239and the vacuum holes221,222,224,226-229.

The second pump262can be turned on and off via an on/off switch (not shown), and also switched between a vacuum generation mode and an air injection mode by a second switch (not shown). When in the vacuum generation mode with the ball valves open, the second pump262sucks air away from the third and fifth faces203a,205avia the ports243,245, the chambers233,235and the holes223,225, thereby applying a suction force to the flanges of laminate charge. When in the air injection mode with the ball valves open, the second pump262injects compressed air via the same route to generate an air cushion adjacent to the third and fifth faces203a,205a.

The flow of air through the various sets of holes221-229can be controlled independently by operation of the switches and the ball valves251-259as described in further detail below.

An alternative arrangement is shown inFIG. 26in which the ports247,249are connected to the second pump262rather than the first pump260.

It will be appreciated that in other embodiments, the forming tool200and apparatus may comprising additional pumps, additional vacuum chambers, additional ball valves and/or additional ports arranged to independently control the suction forces row by row, or hole by hole.

A method of forming a laminate charge50using the forming tool200will now be described with reference toFIGS. 28-31. The laminate charge50is the same as the laminate charge50described above with reference to theFIGS. 10-21, so it will not be described again

Similar to the method described inFIG. 10, in a first step the laminate charge50is laid-up onto a horizontal lay-up surface provided by the first face201aof the forming tool200, between the guide rails218a,b. The laminate charge50is then draped over the forming tool200and a bagging film60is applied as shown inFIG. 29.

The right-hand side of the laminate charge is formed first in a series of sequential forming stages set out in Table 1 below. In Table 1, the symbol 0 indicates that a ball valve is closed, the symbol − indicates that air is being sucked away to apply a vacuum force, and the symbol + indicates that air is being injected to generate an air cushion.

Initially, before stage 1, all of the ball valves are closed. In stage 1 the first pump260is switched on, and ball valve251is opened so that air271shown inFIG. 28is sucked through the set of holes221in the first (upper) face201ato generate a first suction force271abetween the holes221and the bagging film60as shown inFIG. 29. The suction force271acauses the bagging film60to press the first part51of the laminate charge against the first face201aof the forming surface. During stage 1, the remaining ball valves252-259remain closed so no suction forces are generated between any of the other holes222-229and the bagging film60.

In stage 2, the ball valve253is opened with the second pump262switched on and in air injection mode, so that compressed air273is injected into the port243to create an air cushion273abetween the third surface203aof the forming tool200and the flange53as shown inFIG. 29.

In stages 3, 4 and 5 the ball valves256,252and257are opened one after the other so that air272,276,277is sucked through the vacuum ports246,242and247as shown inFIG. 28. This generates further suction forces276a,272a,277ashown inFIG. 29via the sets of holes226,222,227along with the vacuum force271awhich continues to be applied via the set of holes221. The suction forces271a,276a,272aintroduced in stages 1, 3 and 4 cause the bagging film60to press the parts51,52of the laminate charge50against the upper part of the forming tool and then progressively around the male corner while the flange53remains floating on the air cushion. Introducing the suction force277ain stage 5 causes the charge to be sucked into the female corner.

During stages 1 and 2 of the sequence, the second part52of the laminate charge is not in contact with the second face202aof the forming surface, and during stage 4 the second part52of the laminate charge comes into contact with the second face202adue to the suction force272a.

In stage 6 the ball valve257is closed so that the suction force277ain the female corner is removed, then in stage 7 the ball valve257is re-opened to re-apply the suction force277a. Stages 6 and 7 are then repeated in stages 8 and 9 respectively. Alternately turning on and off the suction force277ain this way assists with progressively sucking the charge into the female corner.

Finally, in stage 10 the second pump262is switched to vacuum generation mode to suck air273through the port243which generates a suction force273bshown inFIG. 31which causes the bagging film60to press the flange53against the forming tool200.

The same process is then repeated to form the left-hand side of the laminate charge as set out in Table 2 below.

In this example, the left and right-hand sides of the laminate charge are formed one after the other. Alternatively, the processes of Tables 1 and 2 may be run at the same time, so that the left and right-hand sides of the laminate charge are formed simultaneously.

After the forming process has been completed, the laminate charge is then cured in the same fashion as described above with reference toFIG. 21.

One preferred application for the charge formed by any of the tools described above is as a component part of a torsion-box of an aircraft wing (known as a wing-box). In such a wing-box, the second and fourth parts of the charge provide the fore and aft spars of the wing-box, the first part provides a lower wing skin, and the flanges act as attachment points for an upper wing skin. Other applications for the charge include automotive (car floor panel), mass transit, wind turbine, boat hulls, marine turbine or engine parts (jet engines).