GB2381492A

GB2381492A - Forming composite structures

Info

Publication number: GB2381492A
Application number: GB0126011A
Authority: GB
Inventors: Robert Charles Backhouse
Original assignee: Trysome Ltd
Current assignee: Trysome Ltd
Priority date: 2001-10-30
Filing date: 2001-10-30
Publication date: 2003-05-07
Anticipated expiration: 2021-10-30
Also published as: GB0126011D0; GB2381492B

Abstract

A method for forming a composite article, comprising: placing a body of heat-expanding material in a mould, with a layer of a pliable material between the heat expanding material and an interior wall of the mould; heating the heat-expanding material to cause it to expand to force the pliable material against the interior wall of the mould; and reinforcing the pliable material.

Description

FORMING COMPOSITE STRUCTURES This invention relates to a method for forming composite structures.

Composite materials such as resin-reinforced carbon fibre offer advantages in the fabrication of structural elements such as beams. One preferred form for such beams is a box structure, with the composite material at or near the beam's exterior.

This form is typically stronger than beams that use the same amount of composite material, but at the beam's centre.

Resin reinforced carbon fibre composites are typically formed by impregnating carbon fibres with resin and then curing the resin to form a rigid structure. The carbon fibres are typically in the form of a pliable fabric. In order to ensure that the finished structure has the desired shape, with the fibres in the desired configuration, the fabric must be held in place relative to a mould. Then the resin is introduced into the mould to impregnate the fibres and then cure. Reinforced composites using other materials systems can be formed by an analogous route.

Forming flat panels in this way is relatively straightforward. The fabric can be placed in a mould and the resin poured over it and then cured. Altematively, the carbon fibre can be pre-impregnated with resin, to form a pre-preg; and the pre-preg can be placed in the mould and cured. However, forming box structures is difficult since in order to arrange that the fabric is at the exterior of the finished component it must be held against the interior of a hollow mould.

Figure 1 shows one method for forming box structures. A composite precursor, for example a carbon fibre fabric 1 is arranged carefully around the interior of a mould 2, with an inflatable bag 3 located within the precursor. Then the bag 3 is inflated so as to squeeze the precursor against the interior of the mould. At that stage resin can be introduced if necessary, and the resin cured to form a rigid structure. Finally the bag is deflated and removed from the interior of the cured structure, and the structure is removed from the mould. The finished structure is hollow and its exterior adopts the

interior shape of the mould. This process has the disadvantage that because of the precursor's pliability it is very awkward to arrange the precursor and the bag so that the precursor will be properly pressed against the mould. Arranging the precursor and the bag must be done by hand, making this process unsuitable for mass manufacturing.

In an alternative process the composite precursor is wrapped around a block of elastically compressible material, normally of foam rubber, of a slightly larger size than the interior of the mould. Then the precursor and the foam block are loaded into a split mould, and the mould is closed with the precursor wrapped around the block inside it. In closing the mould the block is compressed somewhat, so the precursor is held by the block against the interior of the mould. Then, if necessary, resin is introduced ; and the resin cured to form a rigid structure. After curing, the foam block can be removed to leave a the reinforced composite as a rigid hollow element. This method has the disadvantage that it is very awkward to arrange the precursor around the foam. Also, because the foam is slightly larger than the mould space and pushes the precursor outward it is difficult to close the mould without trapping the precursor between the mould halves.

Further problems arise if the element that is to be formed is of a complex shape.

First, because of the unitary nature of the bag or alternatively the foam block it is difficult to make them force the precursor accurately against a detailed mould.

Second, it is difficult to arrange the bag or the foam to allow a branching structure to be formed. For instance, figure 2 illustrates a roll cage for a car. The roll cage is formed by a network of mutually intersecting beams, which could usefully be formed as a unitary box section composite article. Because of the interior spaces within the network a complex mould must be used if it is to be fitted around a bag or a foam block. And arranging for the bag or the foam block to squeeze the precursor adequately against the interior of the mould at the intersections between the beams is very difficult.

Similar difficulties apply to the formation of other pliable materials into wall structures.

There is therefore a need for an improved means of forming such structures.

According to one aspect of the present invention there is provided a method for forming a composite article, comprising: placing a body of heat-expanding material in a mould, with a layer of a pliable material between the heat expanding material and an interior wall of the mould; heating the heat-expanding material to cause it to expand to force the pliable material against the interior wall of the mould ; and reinforcing the pliable material.

According to a second aspect of the present invention there is provided a composite article comprising an outer layer of a reinforced fibre material and an inner core including a zone of a heat-expanded material. Preferably the inner core also includes a zone of a non-heat-expanded material.

The pliable material is a fibrous cloth, for example a cloth knitted an or woven from fibres. The fibres are suitably high-strength fibres such as carbon fibre, Kevlar or glass fibre.

The step of reinforcing the pliable material suitably comprises applying a resin to the pliable material and curing the resin. The resin is preferably a heat curable resin, most preferably an epoxy resin.

The resin could is preferably applied after the step of heating the heat-expanding material to cause it to expand. Altematively, it could be applied before that step.

The step of curing the resin preferably comprises heating the resin, most preferably to accelerate cross-linking of the resin.

The reinforcement of the pliable material preferably renders the pliable material rigid.

The step of heating the heat-expanding material is preferably performed by heating the mould. The step of heating the resin is preferably performed by heating the mould.

Preferably the mould is an enclosed and/or hollow mould.

The step of reinforcing the pliable material is performed whilst the pliable material and the heat-expanding material remain in the mould.

The method preferably comprises the step of removing the pliable material and the heat-expanding material from the mould between the steps of heating the heat- expanding material and reinforcing the pliable material.

Preferably the heat-expanding material is capable of expanding in volume by at least 100% on heating, most preferably on heating from 50DC to 250 C.

Preferably, on expansion the heat-expanding material becomes a foam, most preferably a dry foam.

Preferably on expansion the heat-expanding material becomes rigid.

The method may comprise the step prior to the said placing step of setting the pliable material in a semi-rigid state in which it generally conforms to the interior of the mould.

Preferably the said placing step also comprises placing a core in the mould, with the layer of a pliable material between the core and the interior wall of the mould. The core occupies most of the interior volume of the mould. The core is preferably rigid and/or incompressible. Preferably the core is not heat-expandable. The core may be formed of a foam material.

The composite article may be a beam or a network or lattice structure. The composite article preferably a vehicle component and most preferably a vehicle roll cage.

The present invention will be described by way of example, with reference to the accompanying drawings, in which: figure 1 illustrates a method for forming a composite article ; figure 2 shows a vehicle roll cage; figure 3a is a cross-section of a composite article including a heat-expanding core before curing; figure 3b is a cross-section of a composite article including a heat-expanding core after curing; figure 4a is a cross-section of an alternative form of composite article including a heat-expanding core before curing; and figure 4b is a cross-section of an alternative form of composite article including a heat-expanding core after curing.

Figure 3 shows cross-sections of composite articles before and after curing. In its initial form, before curing, the article is shown in figure 3a. In initial form the article includes a carbon fibre cloth 10 which is pre-impregnated with curable resin. The cloth is wrapped loosely around a core of heat-expanding material 11. The heat- expanding material is a material that expands grossly in response to heating. The article in its initial form is placed in a mould 12. At this stage the article is substantially smaller than the intemal mould volume, so it is easy to locate the article in the mould without trapping the cloth 10 between the mould halves.

Once the undersized article has been placed in the mould the mould is closed as shown in figure 3b, and the mould is heated to a temperature at which the heatexpanding material is activated and expands grossly. The temperature to which the mould is raised depends on the properties of the materials that are used for the article, but typically the mould is raised to around 1200C or more. When the heatexpanding material expands it forces the cloth 10 against the interior of the mould 12, causing the cloth to adopt the shape of the mould interior. The mould is then maintained at elevated temperature to accelerate the curing of the resin. When the resin is cured, the mould is cooled and opened and the finished article can be removed. The finished article has a core of heat-expanded material surrounded by an exterior layer of resin-reinforced carbon fibre.

The expansion action of the heat-expanding material also acts to consolidate the fabric and the resin, to help density the outer layer of the article.

Instead of a pre-impregnated (pre-preg) cloth, resin transfer moulding can be used.

A non-impregnated cloth could be wrapped around the heat-expanding core, and resin injected once the article has been placed in the mould, either before or after the heat-expanding material has been activated.

It should be noted that the heat-expanding material is a material that on heating is capable of expansion other than by simple thermal expansion. The heat-expanding material is conveniently a material in which a chemical reaction is activated at a temperature above at least 50 C, and which is capable of causing the material to expand in volume by at least 100%. One example of a suitable material is Synspand available from Dexter Hisol, which is available initially in the form of a pliable sheet and expands at around 1200C to form a rigid foam. It is preferred that the heat- expanding material expands to form a foam since that can results in a light-weight core to the final composite article. Most preferably the foam is a dry foam. Most preferably the foam is a closed cell foam. To achieve a foam, the chemical reaction preferably results in the evolution of gas inside the bulk of the heat-expanding material, so as to blow the material into a foam. The heat-expanding material should be triggered to expand by a temperature that is sufficiently far above room temperature to avoid accidental activation, but lower than that at which the materials of which the article is comprised degrade. If a pre-impregnated or semi-impregnated cloth is used then the temperature at which the heat-expanding material expands should be less than that at which rapid curing of the resin occurs.

The mould could be heated to a single temperature at which the heat-expanding material expands and at which the curing of the resin is accelerated. Alternatively, the mould could be heated to a first temperature to cause the heat-expanding material to expand, and then heated further or cooled to a second temperature at which the curing of the resin is optimised.

To increase the likelihood that the carbon fibre cloth adopts the correct configuration when it is pressed against the mould, it can be pre-formed using a binder such as starch to a semi-rigid shape that generally matches the interior of the mould. In its semi-rigid form the cloth is suitably self-supporting.

The resin could be an epoxy resin.

Figure 4 shows a cross-section of an alternative form of article. In figure 4 like parts are numbered as for figure 3. The article of figure 4 additionally includes a rigid core 13. in the unexpanded form of the article, as shown in figure 4b, The heat-expanding material 11 is wrapped around the core 13, and the carbon fibre cloth 10 is wrapped around the heat-expanding material. Figure 4b shows the structure of the final article after having been placed in a mould and headed as for the article of figure 3 The article of figure 4 has the advantage that less of the heat-expanding material is needed, which can save cost if the material used for the rigid core 13 is less expensive than the heat-expanding material.

The rigid core 13 could be formed of a foam, for example a polyurethane foam, a phenolic or an expanded foam. The rigid core could be formed simply from cut lengths of rigid foam block.

In the embodiment of figure 4, the initial volumes of the rigid core 13 and the heatexpanding material 11 can be selected to minimise cost and maximise the performance of the final article. Typically the rigid core is best around 2mm smaller all round than the mould void.

In some situations it may be helpful to pull a vacuum in the mould at the same time as heating it to cause the heat-expanding material to expand and/or accelerating the curing of the resin.

The process could be performed in two separate stages. The initial article as shown in figures 3a and 4a could be placed in a mould and the heat-expandable material

activated in order to form the article to the shape of the interior of the mould. Then the article could be removed from the mould before the resin has cured or even before it has been applied. Subsequently the article could be cured outside the mould.

A unitary piece of reinforcing fibre cloth could encircle the core of the article, or a number of pieces of cloth could be arranged around the core. In the latter case it is preferred that the joints between the pieces of cloth overlap. If it is desired that the cloth does not completely envelope the core then the cloth could only partially enclose the core.

The principles described above are not limited to use for resin-reinforced carbon fibre. Instead of carbon fibre other fibres, preferably high-strength fibres such as glass fibre or Kevlar, could be used. Other materials could also be used for the matrix. There may be a flow medium to improve flow of the resin into and/or around the fibres. The fibres could be knitted or woven into a cloth.

The present invention may include any feature or combination of features disclosed herein either implicitly or explicitly or any generalisation thereof, irrespective of whether it relates to the presently claimed invention. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims

1. A method for forming a composite article, comprising: placing a body of heat-expanding material in a mould, with a layer of a pliable material between the heat expanding material and an interior wall of the mould ; heating the heat-expanding material to cause it to expand to force the pliable material against the interior wall of the mould ; and reinforcing the pliable material.

2. A method as claimed in claim 1, wherein the pliable material is a fibrous cloth.

3. A method as claimed in claim 1 or claim 2, wherein the step of reinforcing the pliable material comprises applying a resin to the pliable material and curing the resin.

4. A method as claimed in claim 3, wherein the resin is applied after the step of heating the heat-expanding material to cause it to expand.

5. A method as claimed in claim 3 or 4, wherein the step of curing the resin comprises heating the resin.

6. A method as claimed in claim 5, wherein the steps of heating the heat-expanding material and heating the resin are performed by heating the mould.

7. A method as claimed in any preceding claim, wherein the step of reinforcing the pliable material is performed whilst the pliable material and the heat-expanding material remain in the mould.

8. A method as claimed in any of claims 1 to 6, comprising the step of removing the pliable material and the heat-expanding material from the mould between the steps of heating the heat-expanding material and reinforcing the pliable material.

9. A method as claimed in any preceding claim, wherein the heat-expanding material is capable of expanding in volume by at least 100% on heating.

10. A method as claimed in claim 9, wherein the heat-expanding material is capable of expanding in volume by at least 100% on heating from 500C to 250 C.

11. A method as claimed in any preceding claim, wherein on expansion the heat- expanding material becomes a foam.

12. A method as claimed in claim 11, wherein the foam is a dry foam.

13. A method as claimed in any preceding claim, wherein on expansion the heat- expanding material becomes rigid.

14. A method as claimed in any preceding claim, comprising the step prior to the said placing step of setting the pliable material in a semi-rigid state in which it generally conforms to the interior of the mould.

15. A method as claimed in any preceding claim, wherein the said placing step also comprises placing a rigid core in the mould, with the layer of a pliable material between the core and the interior wall of the mould.

16. A method as claimed in claim 15, wherein the rigid core occupies most of the interior volume of the mould.

17. A method as claimed in claim 15 or 16, wherein the core is not heat-expandable.

18. A method as claimed in any of claims 15 to 17, wherein the core is formed of a foam material.

19. A method substantially as herein described with reference to figures 3 and 4 of the accompanying drawings.

20. A composite article comprising an outer layer of a reinforced fibre material and an inner core including a zone of a heat-expanded material.

21. A composite article as claimed in claim 20, wherein the inner core also includes a zone of a non-heat-expanded material.

22. A composite article substantially as herein described with reference to figures 3 and 4 of the accompanying drawings.