Technical Field

This invention relates to a method of producing a composite article having a profile of varying thickness, for example including a tapering section and to such a composite article.

Background Art

One conventional method of manufacturing a composite article is to build up a layer of resin-impregnated fibrous reinforcement, mould it to a desired shape, cure the resin, and thereafter, if a tapered profile is required, to machine the composite article to the desired tapered shape. This procedure is wasteful of the often expensive fibrous reinforcement and resin, which has to be machined away, and involves the extra cost of removing material previously incorporated in the moulded article.

The present invention provides a method of moulding composite articles having a profile of varying thickness, for example a tapered profile, so that, although originally cf even thickness, they come from the mould with the desired shape and do not require to be machined to produce the profile of varying thickness.

Summary of the Invention

According to one aspect of the invention, there is provided a method of producing a composite article having a profile of varying thickness which comprises forming an article of fibre-reinforced matrix material which incor¬ porates a crushable element arranged so that it is susceptible to collapse under the influence of externally- applied forces, and applying external forces to the article

to cause the crushable element to be crushed to impart a profile of varying thickness to the article

The matrix material is conveniently a resin which is or is caused to be deformable during application of the said external forces and which is allowed or caused to set to hold the article in the form of a profile of varying thickness with the crushable element held in the crushed state.

The reinforcing material may comprise fibrous and/cr filamentary material, which may be incorporated in a textile fabric, and the crushable element may comprise similar material and may be in the form of a tube, preferably tubing of braided filaments or yarns. Tubes made of non-woven fibrous material or material woven, knitted or braided from filamentary, textile material can be used.

If tubes are arranged to form at least one layer in the composite article, the crushable element may be constituted by one or more tubes in at least one region of the article which is or are less well supported than one or more other tubes and thus more susceptible to collapse under the influence of externally-applied forces than said one or more other tubes. Thus, the crushable element may be such because of its arrangement and location in relation to other elements.

According to a further aspect of the invention a composite article having a profile of varying thickness is characterised in that the article comprises a fibre- reinforced matrix material incorporating a crushable element which is at least partly crushed to give the article a reduced thickness in the area of the crushed element.

Conveniently a curable resin is used as matrix

material in the composite article, the resin ma/ be a thermoplastic or a thermoset resin.

Conveniently, tubular crushable elements ma, be supported by arranging them parallel to one another side by side so that they provide mutual support. This support is enhanced, without need for additional intermediate elements between the tubular elements, if the elements have complementary shapes so that the peripheries of adjacent parallel tubular elements are contiguous. Simple forms of such shapes are square-section and rectangular-section tubes.

The region of the article in which the tubular crushable elements are less well supported may be an edge region in which the tubular element at the very edge of the article is supported by another tubular element on one side only. On the application of sufficient compressive pressure at right angles to the layer comprising the tubular elements, the edge tubular element will be crushed so that its walls extending in the direction between the surfaces of the layer will collapse leaving little support for the adjacent 'element, which will also collapse, at least partly. In practice, a number of tubular elements in the edge region of the article will be crushed to a progressively smaller degree as one advances further into the article creating an article with a tapered profile.

If tubular crushable elements are left inadequately supported in a central region of the article, the compres¬ sive pressure of the moulding procedure can be arranged to crush these elements and produce a depression in this central region of the article. If a relatively rapid variation in the thickness of an article is required, this can be achieved by introducing a more rigid element than the said crushable elements into the article at the position where the relatively sudden transition in thickness is required. Such a more rigid element may be a

rod of metal or composite material or a tube which is stronger than the other crushable elements, for example because it is made of metal or, if made of filamentary material because, perhaps, it ^' has thicker walls or a more αenεe concentration of filamentary material in its walls than the other tubular elements. On application of compressive pressure, such a more rigid tube, will be crushed less than a weaker tube in the same position, or may be compressed to a virtually imperceptible extent, so that if an adjacent tubular element is crushed considerab¬ ly, a considerable and sudden variation in the thickness of the composite article occurs.

Advantageously, the composite article comprises resin- impregnated fabric outer layers between which is located at least one layer of parallel contiguous tubes arranged side- by-εide.

External compressive forces may be applied by means of a yielding, preferably resilient, plate initially conform¬ ing to and located parallel to a surface of a rigid moulding block having the shape desired for one surface of the composite article. By locating the moulding block and yielding plate in a vacuum press and applying a vacuum, the plate can be urged against the block and will yield and press closer to the blocK where the pressure applied is able to selectively crush the crushable elements of the composite article.

Brief Description of the Drawing

The invention will be further described, by way of example, with reference to the accompanying drawing in which:-

Figure 1 is a side view of a composite article according to the invention, and

Figure 2 is a diagrammatic section through a vacuum press suitable for use in carrying out the method of the invention.

Description of Preferred Embodiment

The composite article 4 shown in Figure 1 comprises a fibre-reinforced matrix material having outer layers 5 and 6 consisting of cured synthetic-resin impregnated faoric sheets. Between the layers 5 and 6 is a layer of parallel side-by-side braided, square cross-section, fabric tubes 7 also impregnated with synthetic resin, which has been cured. The tubes 7 are juxtaposed so that their sides 8 are contiguous and, since they have complementary, in this case straight, sides 8, these sides touch throughout the whole of their extent and provide strong mutual support for one another except in the edge regions 9 and 10 of the composite article where tubes 7 are crushed, or partly crushed, due to the collapse of the tube walls 8 extending in the direction between the surfaces of the layers 5 and 6. The tube 13 or 14 at the very edge of the edge region 9 or 10 is supported on one side only by the adjacent tube 15 or 16. Thus, in the moulding procedure described below, it is more susceptible to wall-collapse and crushing than tubes 7 in the central region 17 of the composite article. The edge tubes 13 and 14 thus collapse virtually completely and the tubes 15 and 16 collapse nearly completely and in fact tubes in the edge regions 9 and 10 are crushed to a progressively smaller degree in the moulding procedure as one advances further into the composite article towards the central region 17. Thus a composite article as shown, with a tapered profile at each edge, is produced.

The composite article 4 of Figure 1 is produced using the vacuum press 20 shown in Figure 2. The press 20 comprises a base 21 incorporating an electrical heater and having an upstanding flange 22 with an aperture 23 to which is connected a vacuum pump 24. A rubber envelope 25 is

6 releasably secured to the base 21 and flange £2 and encloses a block 26 mounted on the base 21. The upper surface of the block 26 is shaped to the configuration required for one face of a composite article of curved sheet-like form.

On top of the block 26 is laid a metal sheet 27 shaped to conform accurately to the shape of the upper surface of the block 26 and a further, resilient metal sheet 28, having the same shape, is spaced from the sheet 27, the twc sheets 27 and 28 enclosing between them a composite article 29 to be moulded. Over the rubber envelope 25 is located a rigid cover 32 releasably secured to the base 21 and incorporating in its upper wall 33 a further electrical heater.

The procedure for moulding a composite article in the vacuum press of Figure 2 is ^' as follows.

The rigid cover 32 and rubber envelope 25 are removed and a release agent is applied to the upper surface of the metal sheet 27 and to the lower surface of the metal sheet 28. A coating of uncured (but curable) synthetic resin is applied to the same surfaces of the metal sheets 27 and 28 and fabric sheets 33 and 34, to constitute surface layers 5 and 6 of the composite article 29, are then applied, one to each of the coated surfaces of metal sheets 27 and 28. Stiff, resin-impregnated tubes 7 made of braided fabric are then juxtaposed with their side walls 8, extending upwardly from the fabric sheet 33, touching one another. The tubes 7 are thus in parallel, side-by-side relationship.

The metal sheet 28 carrying the fabric sheet 34 is then placed on top of the layer of tubes 7, and the rubber envelope 25 and the rigid cover 32 are secured in position. The heaters in the base 21 and the top wall 33 are next switched on and a vacuum is established in the space within the rubber envelope 25 by means of the vacuum pump 24. The

rubber envelope 25 collapses onto the block 26 and the sheet 28 on top of the block 26, and presses the sheet 28 towards the sheet 27, thus applying external, compressive forces to the article 29. These forces act transversely of the article 29 in the direction from one surface layer 6 of the article 29 towards the other surface layer 5.

In the central region (17 in Figure 1) cf the composite article 29, the external compressive forces are resisted by the walls 8 of the tubes 7, the walls 8 being mutually supported in this region 17. However, the tubes at the opposite edges of the composite article 29 are unsupported on their outer sides and they are thus more susceptible to collapse under the influence of transverse compressive forces. Such collapse can occur because of the yielding (and in this case resilient) nature of the metal sheet 28 and after collapse of the edge tubes 7 has commenced, the next outermost tubes (15 and 16 in Figure 1) are liable to start to collapse although they are not in fact crushed to the same degree as the edge tubes. The result is, as shown in Figure 1, the production of ε composite article with tapering edge regions. Curing of the synthetic resin in the composite article is effected by the heat and pressure during the moulding procedure.

To produce an article in which there is a marked and sudden transition in thickness, an element less susceptible to crushing than .the braided tubes 7, that is a more rigid element of the kind mentioned in the introduction above, may be introduced into the composite article in the region where the transition is required.

If a portion of the composite article in the central region 17 is left without tubes, the tubes adjacent this region will tend to collapse on application of transverse compressive forces acting in the direction from one face of the composite article to the other. Thus an article with a central depression can be produced, the extent of the

depression and the degree of taper into the depression depending on a number of factors including the strength of the metal sheet 28, the resistance to lateral compression of the tubes 7, and the forces applied, which depend on the degree of vacuum achieved. The same factors influence the degree of taper achieved in the edge regions 9 and 10 of the composite article of Figure 1.

Because of the curved shape of the sheet 28 it does not deform toward the sheet 27 and block 26 at its ends located above and below the plane of the paper in Figure £, that is the ends of the tubes 7 are not crushed in the composite article of Figure 1. However, by laying the tubes 7 in the direction at right angles to that shown in Figure 1, application of transverse compressive pressure will crush the tube ends, the crushing action being limited by the limitation of the approach of the edges cf the plate 28 towards the plate 27 due to the presence of the material of the crushed tube ends between the plates 27 and 28.

Using flat circular plates 27 and 28 so that the plate 28 can deform towards the plate 27 about the whole of its periphery, a composite article in the form of a disc tapering towards its periphery can be produced.

If braided, or other fabric, tubes are used in making a composite article according to the invention, they can be impregnated with .resin by passing them through a bath of resin, or by placing them in a dish of resin and applying a roller. The fabrics 33 and 34 applied to the metal sheets 27 and 28 may be impregnated with resin after application to the metal sheets by applying resin to them with a roller or brush.

The tubes, constituting crushable elements, used in making a composite article of the kind shown in Figure 1 must have a degree of stiffness so that, some cf them at least, resist crushing in the moulding procedure. This

stiffness or resistance to crushing may be imparted to the tubes in the number of ways. For example, the tubes ma> , after formation as fabric tubes, be lightly impregnated with a resin and cured or partly cured to render them stiff enough to retain their cross-sectional shape whilst they can be bent so that their longitudinal axes become curved. Alternativel , the fabric tubes may have a semi-rigid or collapsible core inserted in them which assists them t retain their cross-sectional shape.