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Title:
MOULDED PLASTIC ARTICLES AND A METHOD AND APPARATUS OF MOULDING PLASTICS OF PARTICULAR APPLICATION IN THE MOULDING OF THERMOSETTING PLASTICS
Document Type and Number:
WIPO Patent Application WO/2011/083331
Kind Code:
A2
Abstract:
A method and apparatus for moulding thermosetting plastic articles using pressurised fluid to force the thermosetting plastic to conform to the mould for curing is described. The pressure for curing the thermosetting plastic is provided by the pressurised fluid instead of a hydraulic press or ram. A mould tool is described having a movable section to relieve unwanted pressure in the mould after the work piece has been cured. A process is described in which pressurised heated liquid is used to transfer heat to the plastics to cure them and in which the pressure of the liquid may be controlled by the application of pressurised gas.

Inventors:
SEDDON MARK (GB)
Application Number:
PCT/GB2011/050013
Publication Date:
July 14, 2011
Filing Date:
January 06, 2011
Export Citation:
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Assignee:
SEDDON MARK (GB)
International Classes:
B29C33/20; B29C33/24; B29C43/36; B29C70/44
Domestic Patent References:
WO2010130879A12010-11-18
Foreign References:
GB1000138A1965-08-04
Attorney, Agent or Firm:
KAZI, Ilya et al. (120 Holborn, London EC1N 2SQ, GB)
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Claims:
Claims

1 . A method of manufacturing a work piece from thermosetting plastics the method comprising: introducing thermosetting materials into a mould, heating the mould and providing pressurised fluid into a flexible membrane within the mould to force the thermosetting plastic to conform to the shape of the mould for curing.

2. The method of claim 1 in which the mould comprises at least two parts and in which the method comprises bringing the at least two parts together to close the mould and applying a restraint to hold the mould closed before providing the pressurised fluid.

3. The method of claim 2 wherein the work piece comprises at least one cavity and wherein providing pressurised fluid into the mould comprises providing pressurised fluid into the cavity.

4. The method of claim 2 or 3 in which at least one of the two mould parts comprises a movable section movable to relieve strain in the restraint, the method comprising selectively releasing the movable section to facilitate opening of the mould.

5. The method of any of claims 2 to 4 in which at least one of the two mould parts comprises a movable section movable to relieve strain in the restraint, the method comprising controlling a pressure applied to the movable section to facilitate opening of the mould.

6. A method of manufacturing a work piece according to any of claims 1 to 5, wherein providing pressurised fluid into the mould comprises providing heated pressurised liquid into the mould to force the thermosetting materials to conform to the shape of the mould for curing.

7. A method of manufacturing a work piece according to claim 6 the method further comprising providing pressurised gas into the mould to force the liquid out of the mould after curing.

8. A method of manufacturing a work piece according to claim 6 or 7, wherein the pressurised liquid is provided into the mould at a first pressure condition, the method comprising further pressurising the liquid in the mould to a higher pressure condition by applying pressurised gas to the liquid.

9. A method of manufacturing a work piece from plastics the method comprising: introducing plastic moulding materials into a mould comprising at least two mould parts; bringing the at least two mould parts together to close the mould;

applying a restraint to hold the mould closed before providing pressurised fluid into the mould to force the materials to conform to the shape of the mould, wherein at least one of the two mould parts comprises a movable section movable to relieve strain in the restraint caused by moulding the work piece.

10. The method of claim 9 in which the plastic moulding materials comprise thermosetting plastics and the fluid comprises a gas.

1 1 . The method of claim 9 or 10 in which the plastic moulding materials comprise thermosetting plastics and the fluid comprises a heated liquid.

12. The method of claim 6 as it depends from any of claims 2 to 5, or any of the claims dependent thereon, or the method of claim 9, comprising controlling the temperature of a first one of the two mould parts to be within a first temperature range and controlling the temperature of the heated pressurised liquid to be a within a second temperature range.

13. The method of claim 12 in which at least a part of the first temperature range is greater than at least a part of the second temperature range.

14. The method of claim 12 or 13 in which the first temperature range is selected such that the curing time of the thermosetting materials at the first temperature range is long in comparison with the dwell time of the thermosetting materials in the mould.

15. The method of claim 12, 13 or 14 in which the second temperature range is selected such that the curing time of the thermosetting materials at the second temperature range is less than or substantially equal to the time for which the thermosetting materials are pressed in the mould.

16. The method of any of claims 12 to 15 further comprising controlling the temperature of the respective other one of the at least two mould parts to be within a third temperature range, preferably wherein at least a part of the third temperature range is greater than the first temperature range.

17. The method of any of claims 12 to 16 in which providing thermosetting materials into the mould comprises laying up the thermosetting materials in the first one of the two mould parts.

18. The method of claim 16 or 17 in which the third temperature is selected such that the curing time of the thermosetting materials at the third temperature is less than or substantially equal to the dwell time in the mould.

19. The method of claim 4 or any of claims 5 to 8 as dependent on claim 4, or claim 9 or any of claims 10 to 18 as dependent on claim 9, in which the movable section is movable by selectively pressurising an actuation cavity within the at least one of the mould parts, the method comprising controlling the pressure in the actuation cavity to allow the movable section to move to relieve strain in the restraint caused by moulding the work piece.

20. The method of claim 19 as dependent on claim 8 or any of the claims dependent thereon, comprising pressurising the actuation cavity to a pressure at least equal to the first pressure condition and then pressurising the actuation cavity to a pressure at least equal to the higher pressure condition.

21 . The method of claim 20 further comprising reducing the pressure in the actuation cavity to enable the movable section to move to facilitate opening of the mould.

22. The method of claim 19 or any of the claims dependent thereon, in which the movable section comprises a first surface arranged to provide at least part of a wall of the mould cavity and a second surface exposed to the actuation cavity, wherein the second surface has an area that is at least equal to the area of the first surface.

23. A moulding apparatus according to claim 22 comprising providing fluid communication between the mould cavity and the actuation cavity to provide substantially equal fluid pressure condition in the mould cavity and the actuation cavity.

24. The method of any preceding claim wherein the work piece comprises at least one work piece cavity and wherein providing pressurised fluid into the mould comprises providing pressurised fluid into the work piece cavity.

25. The method of any of claims 9 to 24 in which providing pressurised fluid into the mould comprises providing pressurised fluid into a flexible membrane within the mould.

26. The method of claim 1 , or any of the claims dependent thereon, or the method of claim 25 in which the membrane comprises blown film tubing.

27. The method of claim 26 comprising selecting a length of blown film tubing based on a size of the mould cavity, and securing fluid couplings to the tubing to enable fluid to be introduced to the tubing.

28. The method of claim 27 in which the fluid couplings comprise two parts and securing fluid couplings to the tubing comprises providing a first of the two parts inside the tubing and securing the first part onto a second part outside the tubing, through the tubing wall to provide a fluid coupling into the tubing.

29. The method of claim 28 in which the fluid coupling parts are threaded and securing the parts comprises screwing them together.

30. The method of claim 27, 28 or 29 further comprising heat sealing the tubing to form a closed membrane or sac.

31 . The method of any of claims 26 to 30 in which the membrane comprises at least two fluid couplings and providing pressurised liquid into the mould comprises providing pressurised liquid into the mould through one of the fluid couplings and exhausting the pressurised fluid from the membrane through the respective other one of the two fluid couplings.

32. A moulding apparatus comprising: a mould having at least two mould parts, the mould parts being adapted to be held together by a restraint to provide a mould cavity between the parts; means for introducing a pressurised fluid into the mould cavity to mould a work piece by forcing it to conform to the shape of the mould cavity; wherein at least one of the mould parts comprises a movable section that is movable to relieve strain in the mould restraint caused by moulding the workpiece.

33. A moulding apparatus according to claim 32 adapted for moulding materials comprising thermosetting plastics and in which the means for introducing a pressurised fluid comprises means for introducing a pressurised liquid into the mould cavity and comprising a pressure control means adapted for introducing a heated pressurised liquid into the mould at a first pressure condition and further adapted to apply pressurised gas to liquid in the mould to further pressurise said liquid to a higher pressure condition.

34. A moulding apparatus according to claim 32 or 33 in which the movable section is movable by selectively pressurising an actuation cavity within the at least one of the mould parts.

35. A moulding apparatus according to claim 34 in which the movable section comprises a first surface arranged to provide at least part of a wall of the mould cavity and a second surface exposed to the actuation cavity, wherein the second surface has an area that is at least equal to the area of the first surface.

36. A moulding apparatus according to claim 35 comprising means for providing fluid communication between the mould cavity and the actuation cavity to provide substantially equal fluid pressure in the mould cavity and the actuation cavity.

37. A moulding apparatus according to any of claims 32 to 36 adapted to perform the method of any of claims 1 to 31 .

38. A moulding apparatus according to any of claims 32 to 37 further comprising a pressure vessel coupled to the means for introducing pressurised fluid wherein the pressure vessel is resistant to corrosion by superheated steam.

39. A moulding apparatus according to claim 38 comprising a source of pressurised liquid at the first fluid pressure condition.

40. A moulding apparatus according to any of claims 32 to 39 further comprising a source of pressurised gas, preferably wherein the gas is an oxygen free or combustion suppressive gas such as nitrogen.

41 . A moulding apparatus according to any of claims 32 to 40 in which the means for introducing pressurised fluid comprises at least two fluid couplings to enable pressurised fluid to be introduced into, and exhausted from the moulding cavity.

42. An article comprising a hollow body moulded from a single piece of thermosetting plastic and having a membrane at least partially bonded to an interior surface of the hollow body.

43. A canoe moulded from a single piece of thermosetting plastic.

44. A structural support for use in the construction of buildings moulded from a single piece of thermosetting plastic.

45. A car body shell or a part thereof, such as a door moulded from a single piece of thermosetting plastic.

46. A manhole cover moulded from a single piece of thermosetting plastic.

47. A windmill blade for a wind turbine moulded from a single piece of thermosetting plastic.

Description:
Moulded plastic articles and a method and apparatus of moulding plastics of particular application in the moulding of thermosetting plastics

This invention relates to the moulding of plastics and more particularly to a method of moulding thermosetting plastic and to a method of making large thermosetting plastic mouldings without the need for high locking force presses and to articles made by these methods.

This application claims priority from UK Patent Application Number 1000138.6 which is hereby incorporated by reference in its entirety.

It is known to use injection moulding and compression moulding to mould thermoplastics into a variety of intricate shapes. Sheet thermosetting materials (ie those containing a mesh reinforcement) are not suitable for use in injection moulding and the range of shapes which can be formed from thermosetting materials are constrained by the properties of the materials and the requirements of the manufacturing process.

A mould for thermosetting plastics comprises a pressing tool having a void between two or more parts of the tool. The parts can be brought together to apply a high load and heated. In typical manufacture of thermosetting plastic products, thermosetting material is placed into a mould and cured by heating the tool under high pressure. The applied load forces the material to take the form of the void in the tool and heat from the tool causes the material to cross link into a rigid compound. Typically a mould for forming thermosetting plastic comprises two tool halves - a female mould and a male mould plunger which are forced together, so that a thermosetting work piece in the mould is forced to take the form of the void between the two tool halves.

Presses used for forming robust thermosetting plastics such as those used in load bearing structures need to be capable of applying high pressures, of the order of 80 to 100 bar (8 to 10 MPa). Applying pressure over a large area requires a large force. Typically a hydraulic ram is used for this process and this can cause difficulties for the manufacture of large and/or hollow structures from thermosetting plastics because a very large press is required.

Large and/or hollow structures can be made from thermosetting plastics by pressing component parts and subsequently securing the parts together to form a larger whole and/or a hollow body. It is desirable to be able to produce large and/or hollow structures from thermosetting plastics without the need to use correspondingly larger hydraulic rams and for that structure to be a single part, not an assembly of smaller parts.

In overview, to address these and other problems the inventors in the present case have proposed a method and apparatus in which a moulding tool is heated sufficiently to enable it to cure a thermosetting material within a time period of between a few minutes and a few hours. The tool may then be lined with a thermosetting work piece with a sac or membrane inside it or adjacent to the work piece. Pressurised fluid can then be pumped into the sac or membrane to press the work piece whilst it is cured in place of the use of a hydraulic press or ram. Once the thermosetting material has had sufficient time to cure, the fluid pressure may be reduced so that the mould can be opened, the moulded thermoset removed, and the process repeated.

The inventors have also provided certain improvements to this process, namely; (1 ) reduction of gas leakage from the membrane; (2) addressing the problem that, if parts of the moulding formed between the tool and its lid are solid (rather than hollow) considerable pressure may remain in the tool after the gas pressure has been removed and this pressure makes it difficult to open the lid of the mould; (3) addressing the problem that the thermosetting material may be partially cured whilst it is being laid up in the tool prior to moulding.

Although this method and apparatus are of particular advantage in the manufacture of articles from thermosetting materials they may also be applied to thermoplastics

Aspects and examples of the invention are set out in the claims.

In a first aspect there is provided a method of manufacturing a work piece from thermosetting plastics the method comprising: introducing thermosetting materials into a mould, heating the mould and providing pressurised fluid into the mould to force the thermosetting plastic to conform to the shape of the mould for curing. Preferably providing pressurised fluid into the mould comprises providing pressurised fluid into a flexible membrane within the mould. In one possibility the method comprises bringing the at least two parts together to close the mould and applying a restraint to hold the mould closed before providing the pressurised fluid. In one possibility providing pressurised fluid into the mould comprises providing pressurised fluid into the cavity. These and other examples of the invention have the advantage of enabling articles to be manufactured from thermosetting materials which are of a size and/or shape that, according to conventional methods would require a hydraulic press of very large size. Advantageously, methods according to the invention are performed without the use of a hydraulic press. The use of a membrane to receive the pressurised fluid in the mould is of particular advantage because it transfers the pressure (and heat) from the fluid to the work piece whilst controlling the manner in which pressure is applied to the work piece.

In some possibilities at least one of the two mould parts comprises a movable section movable to relieve strain in the restraint, the method comprising selectively releasing the movable section to facilitate opening of the mould. In one possibility at least one of the two mould parts comprises a movable section movable to relieve strain in the restraint, the method comprising controlling a fluid pressure applied to the movable section to facilitate opening of the mould. The inventors in the present case have appreciated that, particularly where a moulded article comprises some solid (as opposed to hollow) parts, substantial outward pressure may be applied to the mould by the work piece even after fluid pressure in the mould has been released and it may be difficult to open the mould. This and other examples of the invention have the advantage that these unwanted pressures/strains can be relieved to enable the mould to be opened.

In one possibility providing pressurised fluid into the mould comprises providing heated pressurised liquid into the mould to force the thermosetting materials to conform to the shape of the mould for curing. This and other examples of the invention have the advantage that the heat transferred from the fluid can cure the thermosetting material whilst also transferring the force which presses the work piece. Preferably the liquid comprises superheated water or heated oil. In some examples pressurised gas may be provided into the mould to force the liquid out of the mould after curing. This has the advantage that the heated fluid can be safely flushed from the mould.

Preferably the pressurised liquid is provided into the mould at a first pressure condition, and the method comprises further pressurising the liquid in the mould to a higher pressure condition by applying pressurised gas to the liquid. This and other examples of the invention have the advantage that a source of pressurised gas is easy to obtain so the pressure of the gas can be used to press the work piece whilst heat from the liquid may assist in curing the thermosetting material. In one possibility the first pressure condition comprises a pressure of at least 4 bar, preferably at least 6 bar, preferably at least 7 bar. In some possibilities the first pressure condition comprises a pressure of less than 15 bar, preferably less than 12 bar, preferably less than 10 bar. The first pressure condition may be selected according to the boiling point of water at that pressure and the required curing temperature. Preferably the higher pressure condition comprises a pressure of between 80 and 400 bar. Advantageously the higher pressure condition may be selected according to the pressure required for curing the work piece.

In an aspect there is provided a method of manufacturing a work piece from plastics the method comprising: introducing plastic moulding materials into a mould comprising at least two mould parts; bringing the at least two mould parts together to close the mould; applying a restraint to hold the mould closed before providing pressurised fluid into the mould to force the materials to conform to the shape of the mould, wherein at least one of the two mould parts comprises a movable section movable to relieve strain in the restraint caused by moulding the work piece. Preferably the plastic moulding materials comprise thermosetting plastics and the fluid comprises a gas, in some examples the plastic moulding materials comprise thermosetting plastics and the fluid comprises a heated liquid.

In some possibilities the method comprises controlling the temperature of a first one of the two mould parts to be within a first temperature range and controlling the temperature of the heated pressurised liquid to be a within a second temperature range, preferably in which at least a part of the first temperature range is greater than at least a part of the second temperature range. In some possibilities the first temperature range is selected such that the curing time of the thermosetting materials at the first temperature range is long in comparison with the dwell time of the thermosetting materials in the mould. In some possibilities the second temperature range is selected such that the curing time of the thermosetting materials at the second temperature range is less than or substantially equal to the time for which the thermosetting materials are pressed in the mould. These and other examples of the invention have the advantage that the thermosetting materials do not begin to cure prematurely but, once the materials have been laid up in the mould they can be cured rapidly without the need to wait for the mould to heat up. This is of particular advantage in the manufacture of structurally complex work pieces.

Some examples comprise controlling the temperature of the respective other one of the at least two mould parts to be within a third temperature range, preferably wherein at least a part of the third temperature range is greater than the first temperature range, preferably in which providing thermosetting materials into the mould comprises laying up the thermosetting materials in the first one of the two mould parts. This and other examples of the invention have the advantage of increasing the speed of the process because the lid (or other part) of a mould can be heated to a curing temperature to speed curing without risking prematurely curing the work piece. Preferably the curing time of the thermosetting materials at the third temperature is less than or substantially equal to the dwell time in the mould. As will be appreciated the dependency of the curing time on temperature varies between different thermosetting materials and this information is available form manufacturers of the materials.

In some possibilities the movable section is movable by selectively pressurising an actuation cavity within the at least one of the mould parts, the method comprising controlling the pressure in the actuation cavity to allow the movable section to move to relieve strain in the restraint caused by moulding the work piece. In some possibilities the method comprises pressurising the actuation cavity to a pressure at least equal to the first pressure condition and then pressurising the actuation cavity to a pressure at least equal to the higher pressure condition. In some possibilities the method comprises reducing the pressure in the actuation cavity to enable the movable section to move to facilitate opening of the mould. This and other examples of the invention have the advantage that a source of pressurised fluid can be used to actuate the movable section to facilitate opening of the mould.

In some possibilities the movable section comprises a first surface arranged to provide at least part of a wall of the mould cavity and a second surface exposed to the actuation cavity, wherein the second surface has an area that is at least equal to the area of the first surface. In some possibilities the apparatus comprises means for providing fluid communication between the mould cavity and the actuation cavity to provide substantially equal fluid pressure condition in the mould cavity and the actuation cavity. These and other examples of the invention have the advantage that the actuation cavity can be maintained at substantially the same fluid pressure as the moulding chamber which enables the same pressurised fluid to be used to actuate the lid and perform the moulding process.

In some possibilities the work piece comprises at least one work piece cavity and providing pressurised fluid into the mould comprises providing pressurised fluid into the work piece cavity, preferably pressurised fluid into the mould comprises providing pressurised fluid into a flexible membrane within the mould. The membrane may comprise blown film tubing and the method may comprise selecting a length of blown film tubing based on a size of the mould cavity, and securing fluid couplings to the tubing to enable fluid to be introduced to the tubing. In some examples the fluid couplings comprise two parts and securing fluid couplings to the tubing comprises providing a first of the two parts inside the tubing and securing the first part onto a second part outside the tubing, through the tubing wall to provide a fluid coupling into the tubing. In some possibilities the fluid coupling parts are threaded and securing the parts comprises screwing them together and the method may further comprise heat sealing the tubing to form a closed membrane or sac. These and other examples of the invention have the advantage that a cheaply available product such as lay-flat tubing may be used to provide a membrane of arbitrary size for mouldings of any size or shape.

Preferably the membrane comprises at least two fluid couplings and providing pressurised liquid into the mould comprises providing pressurised liquid into the mould through one of the fluid couplings and exhausting the pressurised fluid from the membrane through the respective other one of the two fluid couplings. This has the advantage that fluid, such as superheated liquid can be efficiently flushed from the mould and recycled/reused.

In an aspect there is provided a moulding apparatus comprising: a mould having at least two mould parts, the mould parts being adapted to be held together by a restraint to provide a mould cavity between the parts; means for introducing a pressurised fluid into the mould cavity to mould a work piece by forcing it to conform to the shape of the mould cavity; wherein at least one of the mould parts comprises a movable section that is movable to relieve strain in the mould restraint caused by moulding the work piece. In some possibilities the apparatus is adapted for moulding materials comprising thermosetting plastics and the means for introducing a pressurised fluid comprises means for introducing a pressurised liquid into the mould cavity and comprising a pressure control means adapted for introducing a heated pressurised liquid into the mould at a first pressure condition and further adapted to apply pressurised gas to liquid in the mould to further pressurise said liquid to a higher pressure condition.

Preferably the comprises a pressure vessel coupled to the means for introducing pressurised fluid wherein the pressure vessel is resistant to corrosion by superheated steam. Preferably the apparatus comprises a source of pressurised liquid at the first fluid pressure condition. In some possibilities the apparatus comprises a source of pressurised gas, preferably wherein the gas is an oxygen free or combustion suppressive gas such as nitrogen.

In one aspect the invention provides a method of manufacturing a work piece from thermosetting plastics, the method comprising: introducing thermosetting materials into a heated mould and pumping pressurised gas into the mould to force the thermosetting plastic to conform to the shape of the mould as it is cured such that the moulding process can be performed without the use of a mechanical press to press the mould together. In this way the gas acts directly or via a membrane which may be disposable or single use directly on the compound to be moulded and a large even pressure can be applied over a large area.

Examples of the invention have the advantage that the size of the work piece is not limited by the size of the machinery/hydraulic ram available (as with conventional thermosetting manufacture) because the heated mould need only be restrained by sufficiently robust clamps to enable the use of gas to provide the compressive moulding force. Preferably the work piece is hollow and optionally the work piece is between 20 cm and 1 metre long, between 1 and 2 metres long, between 2 and 10 metres long or greater than 10 metres long. Advantageously the work piece can be of a length that would normally require a press of impractically large tonnage or bed size

Preferably the thermosetting materials comprise sheet moulding compound (SMC). Typically SMC comprises resin with filler and elongate glass 'threads' or fibres in the resin. Optionally the fibres are arranged in mutual alignment and/or matted. In one possibility SMC comprises carbon fibre. Typically SMC is 3mm thick. SMC may be more than 1 mm thick. Preferably SMC is less than 10mm thick. In one possibility the thermosetting materials comprise a plurality of layers of SMC with intermediate layers of additional glass and/or carbon fibres. In one possibility the thermosetting plastics comprise dough moulding compound short strands of glass (or carbon) fibres arranged in random alignment.

Examples of the invention have the advantage of providing integral hollow mouldings of SMC reinforced with elongate glass fibre which enables the production of strong and cost- efficient parts. Where large volumes of products need to be transported the low weight and high strength of articles produced according to the invention (e.g. a density of between 1.6 and 2.1 kg/m 3 ) offer weight saving effects and therefore an environmental benefit. In one possibility a dry glass mat is used between two sheets of SMC and/or SMC with a higher percentage of glass fibre is used. This and other examples of the invention have the advantage that it is not necessary to apply pressure to the mould itself using a hydraulic ram and the mould can simply be held together using sufficiently robust restraints while pressure is applied using a gas pumped into the interior of the mould.

In one possibility the method comprises introducing a flexible membrane into the mould to receive the pressurised gas and transfer the pressure of the gas to the thermosetting materials. In an example the gas is an inert gas. In one example the membrane is re-usable. Preferably the membrane is a single use membrane so that a moulded work piece can be removed efficiently from the mould without the requirement to retrieve the membrane. In one possibility the membrane has a melting/softening point similar to the curing temperature of the thermosetting plastic. In another possibility the membrane has a melting/softening point higher than the curing temperature of the thermosetting plastic. In one possibility the membrane is nylon or PET. An aspect of the invention provides a kit comprising a mould and a flexible membrane disposable in the mould for containing a pressurised gas.

In one possibility the membrane is reusable to enable it to be used several times. In one possibility two or more membranes can be used where the outer membranes are single use and the inner membranes are multiple use, for example wherein the outer membrane has a softening/melting point that is similar to the curing temperature of the thermosetting materials.

In one possibility the fluid pumped into the interior of the mould comprises a liquid of a sufficiently high temperature to cause the plastic to cure thus enabling the tool temperature to be lower than the curing temperature. This has the advantage of giving more time for the laying up process without the plastic curing prematurely.

In one possibility parts of the tool (for example movable sections of the tool) are not fixed rigidly to the rest of the tool during the moulding process. These parts may be held in the required position by fluid pressure. In some possibilities, this pressure will be the same or close to the pressure applied within the moulding. This has the advantage that the net force on these parts can be much lower and therefore their strength does not need to be as high). Providing tool parts which are not rigidly/fixedly held together has the advantage of reducing the difficulty of opening the tool at the end of the moulding process because the 'floating tool parts', e.g. parts which are not rigidly/fixedly held together, such as a movable section of the tool) can relieve strain in the restraints/clamps used to hold the tool shut.

In one possibility the mould has at least two parts and is arranged such that a thermosetting plastic material can be laid in the mould adjacent or around the membrane before the two parts of the mould are assembled together to close the mould. In one aspect an apparatus for moulding a thermosetting plastic has a closable mould and a gas supply means coupled to supply pressurised gas to a membrane within the mould and a locking means for locking the mould closed around the membrane and an interlock system for preventing the gas supply means from supplying gas to the membrane before the mould has been closed.

In some possibilities the force required to close the mould is less than the force required to compress a thermosetting plastic to force the thermosetting plastic to conform to the interior of the mould.

Examples of the invention have the advantage of providing a method of manufacturing hollow components made from thermosetting materials in one moulding process without the need to fabricate (assemble together) several parts after moulding.

The invention also has the advantage of removing the need for high clamping force moulding machines in making products from thermoset materials because the pressurised gas will provide the internal pressures needed in the process. Preferably the gas is an oxygen free or combustion suppressive gas such as nitrogen. Nitrogen is plentiful and relatively cheap to obtain and relatively safe to work with. Still more preferably the gas is an inert gas. This and other examples of the invention have the advantage of reducing the risk of damaging the workpiece by burning during the curing process.

Preferably the use of a fluid inside the mould forces the work piece against the tool walls and so promotes efficient uptake of heat by the work piece from the heated mould. The use of a gas (or a low heat transfer/capacity fluid) inside the mould has the advantage of thermally insulating the interior of the work piece to promote efficient uptake of heat by the work piece from the heated mould. As an alternative, a fluid may be used to heat (or cool) the interior of the work piece/ mould. Optionally another fluid, such as a liquid can be used in addition to or instead of a gas. In an embodiment reinforcing substrates are laid into the mould according to an intended structure of the work piece and optionally resin binders may be applied to the substrates. This has the advantage that the reinforcing substrate can be aligned and distributed according to the expected stresses that will be applied to the work piece in use.

In one possibility reinforcing substrates are at least partially combined with a resin binder before being fully cured this has the advantage that combined resin and substrate material can be introduced to the mould quickly and efficiently. It is desirable to make hollow mouldings having improved strength and physical characteristics and whereby the time of manufacture is reduced and the amount of material used is decreased in producing a moulding of the same size.

In one possibility the invention is used in the manufacture of thermosetting manhole covers, for example wherein the manhole covers comprise hollow sections to provide improved load bearing properties, such as less deflection using less raw materials and providing structures of reduced weight.

In one possibility the invention is used to manufacture aeroplane wings, hollow windmill blades, road signs with integral poles (and optionally, a base), canoes, boats and car body shells and components thereof by forming them from a single moulding. This has the advantage of providing a product which is lighter and yet stronger than a comparable assembly of parts. Examples of the invention provide window frames having preformed window apertures, preferably wherein the window aperture can be created by having no SMC between the membrane and the interior tool wall over the area of the intended aperture.

In one possibility the invention may be used to manufacture roof sections and structures for use in the construction of buildings from thermosetting plastics wherein the structures comprise at least one cavity (such as a hollow section). This has the advantage of providing construction materials having reduced weight, increased strength and good sound and heat insulation properties which would be easier to transport and erect than conventional materials.

In one possibility the invention may be used to manufacture hollow domestic central heating radiators. Radiators according to this example of the invention have the advantage of having the surfaces which are safe to touch whilst providing equivalent or improved radiating properties as compared with conventional metal radiators, such radiators would also be lighter and would not rust.

An aspect provides a method of moulding a load bearing member from a thermosetting plastic, the mould having a feature corresponding to a form feature of the load bearing member and a gas inlet for providing a pressurised gas to the interior of the mould, the method comprising: introducing a sheet moulding compound into the mould; heating the mould to soften the sheet moulding compound; and supplying pressurised gas to the gas inlet such that the gas pressure forces the sheet moulding compound to conform to the mould.

In an example the mould is heated to above 80 degrees centigrade, preferably above 100 degrees, preferably above 120 degrees centigrade more preferably above 150 degrees centigrade. In an example the mould is heated to less than 250 degrees centigrade, preferably less than 200 degrees centigrade more preferably less than 175 degrees centigrade, still more preferably less than 160 degrees centigrade.

In an example the mould is heated to between 150 degrees centigrade and 160 degrees centigrade. Optionally the gas pressure is at least 40 bar (4 MPa). Optionally the gas pressure is less than 300 bar (30 MPa). Optionally the gas pressure is at least 60 bar. Optionally the gas pressure is less than 120 bar. Optionally the gas pressure is at least 70 bar. In one possibility the gas pressure is less than 1 10 bar. Optionally the gas pressure is at least 80 bar. In one possibility the gas pressure is less than 100 bar.

In one possibility the method comprises coupling a flexible membrane to the gas inlet so that the pressurised gas forces the membrane against the sheet moulding compound to force the sheet moulding compound to conform to the shape of the mould. In one example the load bearing member is hollow. Preferably a load bearing member is a load bearing cover, for example a manhole cover for a maintenance shaft. In an example a load bearing member is a box section to replace an H-beam or I-beam rolled steel joist for use in construction. In one possibility the uncured thermosetting material is placed in the tool, possibly surrounding a sac or flexible membrane bag which is coupled to receive a supply of pressurised gas. In one possibility the sac is a flexible thin film bag for example made from a polymer with a high melting and softening temperature, such as nylon. In an example, once the mould is clamped shut an inert pressurised gas, such as nitrogen, is applied at high pressure. Preferably high pressure comprises pressures of between 10 and 300 bar, or between 80 and 200 bar, preferably between 140 and 150 bar.

In one possibility the mould has at least two parts which may be arranged in an open configuration and a closed configuration. Once sheet moulding compound has been laid into the mould in the open configuration, the mould is arranged in the closed configuration and a force is applied to close the mould wherein the force required to close the mould is less than the force required to press and cure the plastic. Preferably the force required corresponds to a pressure of less than O.I MPa, preferably less than 1 kPa, for example 10Pa. Preferably the gas pressure forces the thermoset against the walls of the tool thus forcing it to take the shape of the tool void and aiding the conduction of heat from the tool into the thermoset material.

Features of any of the aspects or examples may be combined with features of any of the other aspects or examples described herein. Features of apparatus aspects and examples may be applied to method aspects and vice versa.

Summary of Drawings

Embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings, in which

Figure 1 shows a very schematic diagram of a mould with a pressurised fluid system for forming a work piece from thermosetting plastic;

Figure 2 shows a flow diagram to provide a very schematic indication of a process of forming a thermosetting plastic; and

Figures 3A to 3D indicate the process of forming a manhole cover having a reinforcing insert.

Figure 4 shows a very schematic diagram of a more particular example of a mould with a pressurised fluid system;

Figures 5A to 5D shows a set of diagrams indicating a process of forming an article, such as a manhole cover using the apparatus shown in Figure 4. Figure 1 shows a mould comprising two parts 10 and 10' having which define a cavity for receiving a thermosetting plastic workpiece 16. Restraint 18 is arranged to hold the two halves 10, 10' of the mould together.

Gas supply 12 is coupled via connection 14 to provide a supply of gas to the interior of mould 10, 10'. Gas supply 12 and connection 14 is arranged to be fluidly coupled to the mould 10, 10' such that a supply of pressurised gas can be provided to the interior of the mould when it is clamped shut. Connection 14 comprises at least one nozzle arranged in the interior of the mould 10, 10'. Gas supply 12 is operable to provide a supply of pressurized gas such as Nitrogen at a pressure of between about 40 bar to about 300 bar.

Heat source and temperature control unit 19 is connected to both halves of mould 10 and 10' to control the temperature of the mould during the process of curing the work piece 16.

In the example of Figure 1 a single restraint 18 is shown however in operation a number of restraints can be used. An example of a restraint is a clamp such as a G-clamp. In one possibility restraint 18 comprises a plurality of clamps.

Optionally a sac or membrane is provided coupled to the nozzle of gas supply connection 14 so that the gas is maintained within the forming thermoset in a predetermined fashion. The use of a sac, and the particular geometry of the sac can be selected according to the required geometry of the thermoset. The terms sac and membrane are used interchangeably herein and can be considered mutually equivalent.

The operation of an apparatus according to Figure 1 is indicated in very schematic form by the flow diagram of Figure 2. In operation, initially, the material of the work piece 16 is in the form of a resin and a substrate which is introduced to the mould 10, 10' at step 20. The substrate may be laid into the mould according to the intended structure of the workpiece and its intended use. For example more reinforcing substrate can be arranged in areas which, during use of the work piece will experience greater stress and, if the reinforcing substrate comprises elongate elements the alignment of elongate elements of the substrate can be selected according to the stresses to which the work piece is intended to be subjected during use. Typically at this stage the thermosetting workpiece has a volume larger than that of the finished piece. Optionally, at step 20 the resin and substrate may be combined before being introduced to the mould, for example some or all of the resin may be applied before the substrate is laid up in the mould. In one possibility the substrate is laid up into the mould and then the resin is applied.

In an alternative possibility pre-combined sheets of thermosetting material (comprising a thermosetting resin applied to a substrate without curing) are used. In this possibility reinforcing substrates and resin binders are brought together before being fully cured. The reinforcing substrate is bathed in a liquefied resin binder so that the reinforcing substrate becomes saturated or coated depending on the nature of the substrate. Typically glass and carbon fibre cloth substrates tend to become coated at this stage. Optionally the pre-impregnated substrate can be partially dried or baked before being introduced to the mould as a dry semi-cured resin bound with the reinforcing substrate. Once joined and dried in this fashion, the intermediate product in this form can be introduced to the mould 10, 10' for curing.

After the thermosetting workpiece is introduced to the mould the mould is held shut using restraints 18. The two halves of the mould 10, 10' are heated by heat source and temperature controller 19 set to a desired temperature. The temperature is selected according to the properties of the thermosetting resin binder and/or the reinforcing substrate used in the workpiece. At step 24 the thermosetting work piece 16 will be heated until it becomes pliant so that it will take the shape of the mould when subjected to pressure. At step 26 gas supply 12 is controlled to pump a supply of pressurised gas into the interior of the mould such that the work piece 16 is forced to take the form of the mould by the pressure of the gas.

At step 28 the temperature of the mould is controlled to within the required parameters by the heat source and temperature controller 19 while gas supply 12 controls the pressure in the interior of the mould. The duration of the curing process is selected according to the characteristics of the thermosetting material of the workpiece.

Once the temperature has risen sufficiently for the thermosetting material to cure, the gas pressure can be reduced to atmospheric pressure. The tool can then be opened and the hollow moulding removed, with the aid of tool ejectors such as ejector pins mounted in the mould 10, 10' if necessary. At step 30 the mould is undamped and, optionally one or more ejector pins can be used to eject the finished work piece from the mould.

Figure 3A shows a mould for a manhole cover with a thermosetting compound material arranged in the mould to surround two cavities. A nozzle for a gas supply provides a path for a supply of pressurised gas into each of the cavities and a liner, membrane or sac is attached to the nozzle such that, when pressurised gas is supplied to the nozzle the sac will inflate to apply pressure to the dough material while it is cured by heating in the mould

Figure 3B shows the assembly of Diagram 1 during the curing process where pressurised gas is pumped into the sacs through the two nozzles.

Figures 3C and 3D illustrate the same two stages of the curing and pressing process but in this case the structure includes a reinforcing strut in each of the cavities to provide a manhole cover of improved load bearing properties.

Figure 4 shows a moulding tool 108, having a lid 100, 102 comprising an outer part 100 and an inner part 102. A lid cavity 1 10 is defined by a space between the inner 102 and outer 100 parts of the lid. The inner lid part has a first surface 102-1 which provides one side of the lid-cavity, and a second surface 102-2 which provides an interior surface of the mould. In Figure 4 the first surface, 102-1 is larger than the second surface 102-2.

The lid cavity 1 10 is coupled to the exterior of the outer lid 100 by a lid fluid coupling 106. A restraint 1 18, such as a clamp, is provided to enable lid, 100, 102 to be secured in place against the tool body 104.

In the examples shown in Figures 4 and 5 a thermosetting workpiece 1 12 comprises a plastic thermosetting material such as a dough moulding compound, DMC.

The tool body 104 cooperates with the inner lid 102 to define at least one moulding region in which the work piece 1 12 may be disposed for moulding. Heating members 1 14 are arranged in the tool 108 and the inner lid 102 to heat the tool and its lid. Sacs or membranes 1 18 are disposed within the moulding region (adjacent or within the workpiece 1 12 depending on the shape of the finished product required). The sac or membrane 1 18 is coupled to mould fluid couplings 120, 122 by valves 1 16, 1 17 respectively.

Valve 1 16 comprises first valve part 1 16-1 and second valve part 1 16-2 which are threaded to enable them to be screwed together. The first valve part 1 16-1 is disposed inside the membrane 1 18. The second valve part 1 16-2 is disposed outside the membrane 1 18 and secured to the first valve part 1 16-1 by screwing the two parts of the valve together. Valve 1 17 is similarly configured.

Although it is not shown in Figure 4 (which is a single cross section view) a fluid path is provided by membrane 1 18 between the valves 1 16, 1 17.

The membrane or sac 1 18 may be made from a material comprising a tube of heat sealable material such as plastic, for example a blown film lay-flat tubing. These are merely particularly advantageous examples and other materials may be used to provide the membrane/sac 1 18.

The material of the membrane/sac 1 18 may be cut to size to suit moulds of different sizes and shapes. Holes may then be made in the walls of the material in positions that correspond to the position of the fluid couplings 120, 122 of the moulding tool body 104. Once the valves 1 16, 1 17 have been screwed together the membrane/sac 1 18 can be closed to form a sealed pocket which can retain fluid.

A washer may be used to seal the valve parts 1 16-1 , 1 16-2 against the bag. The washer may comprise silicone rubber or another suitable material.

The system used to apply pressurised fluids to the cavities of the tool body and its lid will now be described in more detail.

Fluid coupling 120 is couplable, by a switchable coupling 124, to air at ambient pressure 130 or to a hot-water line 128 or to a closure 126 which closes the fluid coupling 120. When the switchable coupling 124 is in a first arrangement the fluid coupling 120 is open to air at ambient pressure 130, in a second arrangement the fluid coupling 120 is coupled to the hot-water line 128 and in a third arrangement the fluid coupling 120 is closed, to seal that opening of the membrane. Hot-water line 128 is coupled to a hot water tank 136 to receive a supply of hot, preferably superheated, water 136-1 . In the water tank 136 there is a gas head 136-2 of heated, pressurised gas which may comprise steam.

Fluid coupling 122 is couplable by a switchable coupling 127 to a fluid line 134 or to a closure 132 which closes the fluid coupling 122. When the switchable coupling 127 is in a first position the fluid coupling 122 is coupled to the fluid line 134 and in a second position the fluid coupling 122 is closed to seal that opening of the membrane. A tap point 138 on the fluid line 134 is couplable to a source of pressurised gas 140 to couple pressurised gas into the fluid line 134. The pressurised gas in this example is pressurised to between 10 bar and 300 bar.

The lid fluid coupling 106 is couplable by a switchable coupling 144 to one of the fluid line 134 and the gas head 136-2 from the water tank 136. When the switchable coupling 144 is in a first arrangement the lid fluid coupling is coupled to the fluid line 134 and in a second arrangement the lid fluid coupling is coupled to the gas head 136-2.

The switchable couplings may be provided by taps and may comprise valves. The water tank may comprise a pressure vessel and may include a pressure release valve configured to vent fluid from the chamber to regulate the pressure in the tank to a selected value.

The inventor in the present case has appreciated that, if any part of the thermoset moulding is solid (rather than hollow) after moulding and curing then such parts may exert considerable outward pressure on the mould even when the gas/fluid in the mould is returned to the ambient pressure level. This outward pressure may stretch the restraint 180. In examples where clamps have been used to provide restraint 180 the clamps may stretch by between 0.5mm and 1 mm and can exert a force of about 200 tonnes on the mould. This makes it difficult to open the lid of the mould after a thermoset work piece has been pressed.

The function of the arrangement of outer lid 100, inner lid 102 and lid cavity 1 10 is to provide a 'floating lid' to address this problem.

In one example, the lid-cavity 1 10 is approximately 5mm deep. In this example the second surface 102-2 of the inner lid 102 has an area of 2500sq.cm exposed to the pressure in the mould tool 108. In this example the first surface 102-1 (top) of the lid has an area of 51 cmx51 cm=2601 sq.cm.

As described below with reference to the Diagrams of Figure 5, the same pressure is applied to the interior tool 108 as to the cavity above the floating lid 1 10. In the example described a pressure of 100 bar is applied to the tool which results in a net force of (2601 -2500) * 100 = 10.1 tonne to close the lid. So the lid stays in place during the moulding process. This process is described in greater detail below with reference to Figure 5.

Process Overview

Figure 5 includes a series of diagrams, labelled Diagram 1 through to Diagram 6 respectively which illustrate one method of using the apparatus shown in Figure 4. In Figure 4 and Figure 5 like reference numerals are used to indicate like elements.

Before describing the process in detail it is instructive to give a brief overview. The tool body 104 is heated by heating members 1 14 to a temperature of 90°C before being layed up with workpiece 1 12 and the sac or membrane 1 18 and the valves 1 16, 1 17 as described above.

The water tank 136 is pressurised to a pressure of between 7 bar and 10 bar and the water 136- 1 is heated to 150°C. The lid cavity 1 10 is connected to the gas head 136-2 at the top of the water tank 136 whilst the sac/membrane 1 18 is filled with the superheated water. At this stage the lid cavity 1 10 and the interior of the tool are at same pressure. The gas head 136-2 at the top of the tank is then disconnected from the lid cavity and the sac or membrane 1 18 is disconnected from the water 136-1.

Once the interior of the mould has been isolated from the water tank (e.g. switchable couplings 144 and 121 are both connected to 134 coupling 124 is set to closed) the high pressure gas source 140 is coupled to the lid cavity 1 10 and to the sac or membrane 1 18 and the work piece is pressed and heated until it is cured. Once the curing process is complete the high pressure gas source 140 is decoupled and the valve to the water tank is opened to allow residual gas pressure to push the majority of the hot water back into the tank 136. The valve to the tank can then be closed and the sac or membrane 1 18 can be opened to the atmosphere to allow the remaining superheated water to flash off. By following this process the workpiece is cured by heating from the water so the tool body 104 need only be at a temperature of 90°C which substantially reduces the problem of premature curing during the process of laying up the work piece. To speed curing without causing the workpiece to cure prematurely, the inner lid 102 can be heated to 150°C as it only touches the thermoset a couple of seconds before the high pressure is applied. These temperatures are merely examples, as will be appreciated by the skilled reader, the temperatures can be selected based on the curing times of the thermosetting materials being used according to information provided by the manufacturers. Preferably the tool body, in which the work piece is laid up is maintained at a temperature (or within a temperature range) at which the curing time of the thermosetting material is long in comparison with the dwell time of the materials in the mould. For example it may take a few minutes or tens of minutes to lay up a work piece in the mould in which case the tool body can be maintained at a temperature at which the curing time of the thermosetting materials a few hours or more, or at least substantially longer than the time taken to arrange the thermosetting materials in the mould without causing premature curing of the materials. The materials can then be cured by introducing heated pressurised liquid into the mould at a temperature (or within a temperature range) at which the curing time is less than or substantially equal to the desired time for which the work piece is to be pressed in the mould.

Although this process is described as a two stage process involving the use of both gas and liquid it may be performed solely using gas or solely using liquid. In examples which solely use fluid for the moulding process gas may be used to flush fluid from the mould after the workpiece has been cured.

Detailed Description of Process Illustrated in Figure 5

In more detail: Figure 5A shows the system open to ambient pressure to enable the tool 108 to be loaded. The fluid coupling 120 is coupled by switchable coupling 124 to air at ambient pressure and the fluid coupling 122 is coupled by switchable coupling 127 to fluid line 134. Tap point 138 is closed so the high pressure gas is not applied to the fluid line. The lid fluid coupling 106 is coupled by switchable coupling 144 to the fluid line 134. As a result the interior of the mould, between inner lid 102 and tool 108, and the lid cavity 1 10 between the inner and outer lid parts 100, 102 are both at ambient pressure. In this state, the mould can be opened so that a work piece 1 12 of thermosetting material may be arranged in the mould around the sac or membrane 1 18 and valves 1 16, 1 17. Figure 5B shows the system filled with heated water to begin curing the thermosetting material. Fluid coupling 120 is connected by switchable fluid coupling to hot-water line 128 to allow the mould to fill with hot water. The lid fluid coupling 106 is coupled to the gas head 136-2 by switchable coupling 144. Optionally, any gas in the mould is allowed to escape via fluid line 134 as the hot water enters the mould before the fluid coupling 122 is closed off by switchable coupling 127 to seal the hot water in the mould.

Figure 5C shows the high pressure phase of the moulding process. Fluid coupling 124 is closed by switchable coupling 124. The lid fluid coupling 106 is coupled to the fluid line 134 by switchable coupling 144. The fluid coupling 122 is coupled to the fluid line 134 by the switchable coupling 127. The tap point 138 connects fluid line 134 to the source of pressurised gas 140. This pressurises the lid cavity to force the inner lid 102 down onto the workpiece. The hot water in the mould is pressurised by the gas and forces the workpiece 1 12 to conform to the shape of the mould 108 and lid 102.

Figure 5D shows the process of emptying the water from the mould. The tap point 138 is closed to decouple the source of pressurised gas 140 from the fluid line 134. The fluid coupling 120 is coupled to the hot-water line by switchable coupling 124 which enables the residual gas pressure in the lid cavity 1 10 to force the hot water back into the tank 136.

Figure 5E shows the system arranged to return pressures in the mould 108 cavity and lid cavity 1 10 to ambient pressure. The switchable coupling 124 decouples fluid coupling 120 from the hot-water line and is opened to ambient air pressure 130. This enables any strain in the mould due to compression of the workpiece to be relieved by movement of the inner lid part 102 into the lid cavity 1 10. With the fluid system in this configuration the restraint 180 can be removed to open the tool and remove the workpiece 1 12.

The valves 1 16 may then be removed from the thermoset by ripping the membrane/sac 1 18. The holes left in the thermoset can be closed using bungs.

Although the introduction of gas and liquid has been described as if performed at a single pressure the pressure of the gas or liquid may be modulated by ramping the pressure up or down as the gas/liquid is applied to the mould or by applying a selected pressure profile. The invention is further illustrated by the following exemplary aspects:

1 . A method of manufacturing a work piece from thermosetting plastics the method comprising: introducing thermosetting materials into a mould, heating the mould and providing pressurised gas into the mould to force the thermosetting plastic to conform to the shape of the mould for curing.

2. The method of aspect 1 in which the mould comprises at least two parts and in which the method comprises bringing the at least two parts together to close the mould and applying a restraint to hold the mould closed before providing the pressurised gas.

3. The method of aspect 2 wherein the work piece comprises at least one cavity and wherein providing pressurised gas into the mould comprises providing pressurised gas into the cavity.

4. The method of any preceding aspect in which providing pressurised gas into the mould comprises providing pressurised gas into a flexible membrane within the mould.

5. The method of any preceding aspect in which the thermosetting plastics comprise sheet moulding compound (SMC).

6. The method of any preceding aspect in which the thermosetting plastics comprise a thermosetting resin and elongate threads or fibres preferably in which the fibres are arranged in mutual alignment and/or are matted.

7. The method of any preceding aspect in which the thermosetting plastics comprise glass and/or carbon fibres.

8. The method of any preceding aspect in which the gas is an oxygen free or combustion suppressive gas.

9. The method of any preceding aspect in which the gas is nitrogen.

10. The method of any preceding aspect in which reinforcing substrates are laid into the mould according to an intended structure of the work piece.

1 1 . A method of moulding a load bearing member from a thermosetting plastic, the mould having a feature corresponding to a form feature of the load bearing member and a gas inlet for providing a pressurised gas to the interior of the mould, the method comprising:

introducing a sheet moulding compound into the mould;

heating the mould to soften the sheet moulding compound; and

supplying pressurised gas to the gas inlet such that the gas pressure forces the sheet moulding compound to conform to the mould. 12. A method according to aspect 1 1 wherein the mould has at least two parts which may be arranged in an open configuration and a closed configuration and wherein introducing a sheet moulding compound into the mould comprises laying sheet moulding compound into the mould in the open configuration, arranging the mould in the closed configuration and applying force to close the mould wherein the force required to close the mould is less than the force required to press and cure the plastic.

13. A method according to aspect 12 wherein the force required corresponds to a pressure of less than 0.1 MPa, preferably less than 1 kPa for example 10Pa.

14. The method of aspect 13 in which the load bearing member is hollow.

15. The method of aspect 14 in which the load bearing member is an H-beam or I-beam for use as a substitute for a rolled steel joist to be used in building construction.

16. The method of any of aspects 1 1 to 15 comprising providing a reinforcement member with the thermosetting plastic in the mould such that the load bearing member incorporates the reinforcement member, preferably wherein the reinforcement member comprises steel.

17. The method of any preceding aspect in which the gas pressure is between 60 and 120 bar or between 70 and 1 10 bar, still more preferably between 80 and 100 bar or between 20 and 300 bar.

18. The method of any preceding aspect in which heating the mould comprises heating the mould to 150 degrees centigrade.

19. A method of making hollow structures from thermosetting plastic using a mould the method comprising: laying a thermosetting plastic comprising a reinforcement substrate carrying a thermosetting resin binder into the mould; supplying pressurised gas to the interior of the mould and heating the mould such that the pressure of the gas forces the thermosetting plastic to conform to the mould.

20. A method of manufacturing a work piece from thermosetting plastics the method comprising: introducing thermosetting materials into a mould in an open configuration and subsequently closing the mould and locking the mould into a closed configuration, heating the mould to soften the thermosetting plastic and providing pressurised gas into the mould to force the thermosetting plastic to conform to the shape of the mould for curing.

21 . The method of aspect 20 wherein the mould comprises at least one nozzle for providing pressurised gas into the mould.

22. The method of aspect 21 wherein introducing thermosetting materials into the mould comprises arranging thermosetting materials adjacent a flexible membrane coupled to the at least one nozzle for containing a pressurised gas. 23. The method of aspect 22 wherein the work piece is hollow and wherein the membrane is a single use membrane which is left inside the work piece once it is formed.

24. The method of aspect 23 wherein the melting point of the membrane is similar to the curing temperature of the thermosetting plastic such that the membrane is at least partially bonded to the thermosetting plastic during curing.

25. An article comprising a hollow body moulded from a piece of thermosetting plastic and having a membrane at least partially bonded to an interior surface of the hollow body.

26. A method of manufacturing a work piece from thermosetting plastics the method comprising: introducing thermosetting materials into a mould heating the mould to soften the thermosetting plastic and providing pressurised gas into the mould to urge the softened thermosetting plastic to conform to the shape of the mould and varying the pressure of the gas while the work piece is in the mould.

27. The method of aspect 26 in which varying the pressure of the gas comprises varying the pressure of the gas in a series of stages.

28. The method of aspect 26 or 27 in which varying the pressure of the gas comprises providing gas at a first pressure to urge the softened thermosetting plastic to conform to the shape of the mould and providing pressurised gas into the mould at a second pressure during curing of the thermosetting plastic.

29. The method of aspect 28 in which the first pressure is selected based on the pressure required to force the softened thermosetting plastic to conform to the shape of the mould.

30. The method of aspect 28 or 29 in which the second pressure is selected based on the pressure required during curing of the thermosetting plastic.

31 . The method of aspect 27 in which varying the pressure of the gas in a series of stages comprises varying the pressure according to a selected pressure profile.

32. Apparatus for moulding thermosetting plastics the apparatus comprising: a mould holder for receiving a mould moulding thermosetting plastics for, the mould holder having an open configuration and a closed configuration; a heater arranged to heat said mould for softening thermosetting plastic in the mould; and locking means for locking the mould holder in the closed configuration; and a fluid coupling means arranged for providing pressurised gas into the mould such that the pressurised gas can force softened thermosetting plastic in said mould to conform to the shape of said mould.

33. Apparatus according to aspect 32 comprising a flexible membrane coupled to the fluid coupling means to receive and contain the pressurised gas such that the pressure of the pressurised gas acts through the membrane to force softened thermosetting plastic in the mould to conform to the shape of the mould.

34. Apparatus according to aspect 32 or 33 in which the fluid coupling means comprises a nozzle which protrudes into the mould.

35. Apparatus according to aspect 32, 33 or 34 further comprising a mould for moulding thermosetting plastics.

36. Apparatus according to any of aspects 32 to 35 closing means for closing the mould holder by applying a closing force wherein the closing force is less than the force required to force softened thermosetting plastic to conform to the mould.

37. A method of moulding thermosetting plastics substantially as described herein and/or with reference to the accompanying drawings.