An Integrally Heated Mould

Background of the invention

The present invention is concerned with an integrally heated mould, and in particular an integrally heated mould formed substantially from ceramic material and thus suitable for high temperature moulding of polymeric composites, in particular thermoplastic composite components, but not excluding thermosetting composite components. Examples would include a thermoplastic composite wind turbine blade, an aeroplane fuselage or section thereof, the hull of a marine vessel or large automotive and transport panels, enclosures or containers for buses or trucks.

Composite and plastic materials are used to fabricate products using various moulding techniques and devices. The process of fabricating composite and plastic materials usually requires that heat be supplied to the product material which results in that material assuming the form of a mould surface. The heating may also activate chemical curing or polymerisation or some other desired chemical or morphological change in the material.

In thermal processing of composite and plastic materials, the heat may be provided through for example autoclaves and platen presses. These methods often result in lengthy cycle times to achieve the required temperature profile for the part being fabricated and thus the thermal processing cycles are usually defined by the mould and equipment limitations rather than the optimum cycle for the material being processed. As the size of the composite component being processed exceeds certain limits, the cost of autoclaves and presses can become prohibitive.

For composite materials that can be consolidated and processed under vacuum pressures only, it is not necessary to apply extra pressure such as that provided by a press or autoclave. It is well known, in these cases, to use large ovens or purpose-built heating enclosures that have been constructed around the tooling. Forced, re-circulating heated air can be used to heat the tooling, but this method of heating is inefficient and expensive, particularly at large mould sizes.

Heater blankets can also be used to heat mould surfaces and the composite materials being processed, but these blankets are limited in operation to about 200 ⁰ C, and have limited durability and drapeability for complex surfaces. These heater blankets are also limited to only heating the mould and composite material externally.

In general, there can also be problems with press, oven and autoclave moulding in achieving a controlled, sufficiently uniform or profiled thermal distribution.

If metal moulds are used, for example steel and aluminium, there is a fundamental mismatch of the co-efficient of thermal expansion (CTE) between the metal mould and the composite and/or plastic materials being processed therein. This CTE mismatch causes greatest problems with larger moulds, for example having a single dimension, such as length or width, of 2 metres or greater, as the difference in thermal strains is magnified when dealing with moulds of this size. The economics of casting and machining larger sized metal moulds are however generally prohibitive. Metals such as Invar (a high nickel steel) do have lower CTEs but are much more expensive, and more difficult to machine than regular steel moulds.

It is well-known to use carbon or glass fibre epoxy or glass fibre polyester moulds, which have the advantage of having a similar CTE as the composite or plastic material being processed. However, none of these mould materials exhibit any continuous temperature resistance above approximately 18O ⁰ C, and are thus limited to use below this temperature. Epoxy matrix moulds have a particular problem with durability under thermal cycling, and exhibit matrix cracking and degradation after thermal cycling.

It is also known to use carbon fabric as an integral heating element in composite moulds. However, problems are encountered with hotspots in the mould due to the inhomogeneous nature of the carbon fabric, which causes variations in electrical resistance across the fabric. This is particularly problematic when attempting to heat non-planar or curvilinear working surfaces, as the carbon fabric tends to become bunched at particular locations, and stretched at other locations, resulting in non-uniform spacing of the fibres and therefore respectively greater and lesser heat distribution per unit area, which means that this form of heating is not viable for moulds with curvilinear working surfaces. There is also a limit to the amount of power (KW) that can be supplied into such a carbon fabric, and it is

impracticable to employ a three phase power supply with such fabrics. This means that large moulds heated using such a method can only be heated slowly, and are therefore uneconomic. Problems also occur with achieving a robust connection of a power terminal to the edges of the carbon fabric, where it can be difficult to avoid local over-heating of the carbon fibres at their terminations. Difficulties also occur with the clamping of bundles of fibre tows in order to make electrical connections, which means that each individual carbon fibre cannot directly be connected to the power source.

It is known to construct moulds from ceramic matrix composites, for example from materials such as a pre-ceramic alumina matrix. Glass or carbon fibre reinforcements may be used to improve the structural performance of the mould. Ceramics are good thermal insulators, but also have a high density and high specific heat capacity, which can make them slow to heat by external means such as autoclaves or ovens and other forms of heating.

Ceramics are attractive for integral heating provided the heating elements are kept close to the working surface of the mould, so as to provide even heat distribution. The remaining structural layers provide insulation which reduces heat losses from the mould. The use of heating elements in ceramic moulds is made possible by the fact that ceramic is also a good electrical insulator.

It is known to embed carbon fabric in a ceramic mould in order to provide a means of integrally heating the mould. International Patent Application WO00/54949 describes a self-heated mould having a cast ceramic mould base in which heat is generated from a carbon fabric which is either embedded near the surface of the base of the cast mould or supported above the tool base in proximity to the composite material being formed. However, the same problems occur with carbon fabric heating of ceramic moulds as with epoxy or polyester moulds. As detailed above, these problems include non-uniform heat generation due to non uniform distribution of the carbon fibres, limitations of the power input, and the over-heating of the power terminals.

The present invention has therefore been developed with a view to overcoming the above mentioned problems of the prior art, and in particular when manufacturing relatively large ceramic moulds, for example having a single dimension greater than 2 metres in length.

Summary of the invention

The present invention therefore provides, according to a first aspect, an integrally heated mould for moulding polymeric composites, in particular thermoplastic composite components, the mould comprising a mould body formed substantially from ceramic and having a single dimension greater than 2 metres in length and/or a working surface greater than 5 m ² in area; and heating means located at or adjacent the working surface, the heating means comprising a protective covering thereon.

Preferably, the heating means is capable of simultaneously heating different areas of the working surface to different temperatures and/or heating the entire working surface to a single temperature.

Preferably, the working surface is substantially non planar.

Preferably, the mould body comprises fibre reinforcement.

Preferably, the mould body comprises a first layer of ceramic material in which the heating means is integrated; and a second layer of ceramic material adjacent the first layer.

Preferably, the mould body comprises a gel coat layer which defines the working surface.

Preferably, the gel coat layer comprises un-reinforced ceramic.

Preferably, the gel coat layer is of a thickness in the range of 0.1mm to 5mm, preferably in the range of 0.5mm to 3mm, and most preferably in the range of lmm to 2mm.

Preferably, the mould body comprises a support layer adjacent the second layer.

Preferably, the heating means comprises an array of heating elements whose heat output can be individually varied and/or varied in pre-defined groups.

Preferably, the wattage density of the heating means can be locally varied in order to suit local variations of thickness and/or type of material to be processed within the mould.

Preferably, individual and/or groups of heating wires are provided with dedicated electrical terminals.

Preferably, the heating means comprises heating tape.

Preferably, the first layer and/or the second layer comprise fibre reinforcement.

Preferably, the mould comprises a support frame by which the mould body is supported.

Preferably, the mould comprises dislocation means adapted to permit relative movement between the support frame and the mould body.

Preferably, the mould body comprises first and second mould sections locatable in register with one another to define, in combination, the working surface.

Preferably, each mould section comprises a peripheral flange onto which the heating means extends.

Preferably, the mould comprises a sealing layer in spaced relationship to the working surface to enable a vacuum to be applied and maintained at the working surface of the mould.

Preferably, the sealing layer comprises glass and/or carbon reinforced plastic. Alternatively, the sealing layer comprises a metal foil.

Preferably, the mould comprises an insulating layer disposed between the sealing layer and the mould body.

Preferably, the insulating layer comprises micro spheres.

Preferably, the micro spheres are hollow.

Preferably, cooling channels are provided within the second layer and/or the support layer, through which channels a cooling medium may be passed.

According to a second aspect of the present invention there is provided a method of manufacturing a mould according to the first aspect of the invention, the method comprising applying a first ceramic layer to a pattern to form a working surface of the mould; locating heating means having a protective covering within the first ceramic layer; and curing the first ceramic layer such that the heating means are integrated into the first ceramic layer at or adjacent the working surface.

Preferably, the method comprises, in the step of locating the heating means, positioning heating elements individually and/or in groups within the first ceramic layer.

Preferably, the method comprises the further step of applying a second ceramic layer to the first ceramic layer and curing the second ceramic layer.

Preferably, the method comprises, in the step of applying the first ceramic layer and/or the second ceramic layer, locating reinforcing fibres within the first and/or the second ceramic layer prior to curing same.

Brief description of the drawings

The present invention will now be described with reference to the accompanying drawings, in which:

Fig. 1 illustrates a perspective view of a portion of a mould body according to a preferred embodiment of the present invention, comprising first and second mould sections;

Fig. 2 illustrates cross-sectional view of a portion of the mould body illustrated in Fig. 1 ;

Fig. 3 illustrates a section of partially fabricated mould body, illustrating the placement of heating means therein;

Fig. 4 illustrates a perspective view of a mould according to the present invention, which includes a support frame by which the mould body illustrated in Fig. 1 is supported during use; and

Fig. 5 illustrates a schematic perspective view of an alternative embodiment of a mould accordingly to the present invention, incorporating a sealing layer to enable vacuum moulding to be effected with the mould.

Detailed description of the drawings

Referring now to Figures 1 to 4 of the accompanying drawings, there is illustrated an integrally heated mould, generally indicated as 10, for particular use in moulding relatively large composite and/or plastic components such as wind turbine blades (not shown), sections of aeroplane fuselage, or large automotive and transport panels, enclosures or containers for buses or trucks, or any other large scale component which would generally be unsuitable for moulding using a metal mould and autoclave or oven combination. Such components will generally be at least two meters in length, or have a surface area of at least 5m ² . The mould 10, in the embodiment illustrated, is comprised of a mould body 12 which is preferably formed in two sections, hereinafter referred to as a first or upper section 14 and a second or lower section 16, although it will be appreciated that the aforementioned terminology is not intended to limit the invention to use in a particular orientation in which one section is disposed above the other. The sections 14, 16 each include a peripheral flange 18 in order to allow the sections 14, 16 to be securely and accurately located relative to one another, which flanges 18 may also act as a point at which to secure a support frame 36 (omitted from Figure 1 for clarity) in order to support the sections 14, 16 during use, as will be described in greater detail hereinafter.

As mentioned above, the mould 10 is particularly intended for use in moulding large components, for example wind turbine blades, aeroplane fuselages, boat hulls, or large automotive panels or the like. The sections 14, 16 may additionally include an end flange 20 on one or both ends, in order to allow the mould body 12 to be formed from two or more sets (not shown) of sections 14, 16, which may be clamped together in end-to-end

engagement with one another. This would normally be done for practical purposes, in order to simplify the manufacturing process for each of the sections 14, 16, by reducing the overall size, in particular length, of same. However, it will be appreciated that this is merely a preferred feature, and the mould body 12 could be formed from full-length upper and lower sections (not shown). The mould 10 is particularly suited to the processing of materials which can be consolidated under vacuum, for example the processing of thermoplastics and thermosetting composites. In the accompanying drawings, the mould body 12 is illustrated in Figures 1 and 4 as being substantially cylindrical in cross section, as would be the case for moulding, for example, the root of a wind turbine blade or a section of aeroplane fuselage. In Figure 3, the mould body 12 is shown as being substantially elliptical or aerofoil in cross section, as would be the case for moulding the main length of a wind turbine blade. It will of course be understood that these shapes are merely exemplary, and the mould body 12 could be of any desired shape.

Due to the relatively large size of the mould body 12, it is not a practical consideration to heat same in an autoclave or oven or the like, in order to cure the material being formed within the mould 10, as the size and therefore cost and complexity of such an autoclave or oven would be prohibitive. It is therefore necessary to integrally heat the mould body 12, and thus referring in particular to Fig. 2, there is illustrated a cross-sectional view of a portion of the mould body 12, which is indicative of essentially all sections of the mould body 12 surrounding and defining a working surface 22 of the mould body 12, and preferably indicative also of the composition of the peripheral flanges 18 and the end flange 20 if included. Of primary importance when considering the sectional view of the mould body 12 is the provision of integrated heating means in the form of an array of heating elements or wires 24, which are illustrated schematically in Figure 2, but shown in greater detail in Figure 3, which illustrates the upper section 14 in a partially constructed state. The heating wires 24 are each preferably provided as a high temperature flexible metal heating element, and in particular a heating tape, for example as supplied under the trade name AMPTEK AWO heating tape. The heating wires 24 are provided with a protective covering (not shown) thereon, which may for example be glass fibre sheath or any other suitable alternative. This protective covering ensures that the heating wires 24 do not undergo any chemical or physical corrosion during use, which may be an issue when embedded in a ceramic material, and in particular when the heating wires 24 are at an elevated temperature.

The heating wires 24 are located at or adjacent the working surface 22 of the mould body 12, in order to be capable of heating the working surface 22 during use, and therefore the material, for example a thermoplastic composite material, located within the mould 10. The heating wires 24, in heating the mould surface, heats the material, brings said material to a sufficiently molten state in order that the material conforms, preferably under vacuum, to the shape of the working surface 22 in order to create a desired component. It will of course be appreciated that any other suitable alternative or equivalent may be employed to consolidate the material being processed and hold same against the working surface 22 until the component has been formed or suitably cured. For example, an inner core (not shown) or the like could be located between the upper section 14 and lower section 16, in order to maintain the desired shape of the component being moulded. However, if such a component, for example a wind turbine blade, is hollow but closed to the exterior, such an inner core (not shown) could not then be removed from within the component without at least the partial destruction of same.

In order to integrate the heating wires 24 into the mould body 12, the following preferred arrangement is employed. Initially, a suitable blank or pattern 34 (shown only in Figure 3), in the shape of the component to be moulded, is produced by any suitable means. The pattern 34, or at least the surface thereof, must be of a material which is compatible with the materials used to form the mould body 12. Referring in particular to Figures 2 and 3, the pattern 34 is suitably supported, for example on a suitable workbench (not shown) or the like, and the upper exposed side thereof covered first with a mould release agent, and then with a gel coat layer 26, which is preferably formed of un-reinforced ceramic used to provide an improved finish to the working surface 22. The mould release agent applied before the gel coat layer 26 ensures that, once the upper section 14 has suitably cured, it can be separated from the pattern 34 without damage to the upper section 14.

A preferred material for the gel coat layer 26 is pyromeral 23% 000612A liquid and 77% 00612B powder. The gel coat layer 26 is preferably of a thickness in the range of 0.1 to 5mm, more preferably from 0.5 to 3mm, and most preferably from lmm to 2mm in thickness. The gel coat layer 26 is applied to the pattern 34 in the form of a paste or slurry which can be brushed or otherwise applied to the surface of the pattern 34. This process is

described in more detail in the specific example given below. Once the gel coat layer 26 has suitably cured, a first ceramic layer 28 is applied on top of the gel coat layer 26, which first ceramic layer 28 is preferably applied in stages, in order to allow the location and therefore integration of the heating wires 24, as illustrated in Figure 3. The first ceramic layer 28 is again preferably initially prepared as a liquid based slurry, and again suitably applied, for example using a brush, onto the gel coat layer 26. Preferred ceramic materials include reinforced alkali aluminosilicate geopolymeric matrix for composite materials with fibres reinforcement, for example as described in US Patent Nos. 5,798,307 and 5,342,595. A further example of ceramic material is calcium metasilicate. It will be appreciated that there are further ceramic materials that may be used, either individually or in combination.

The first ceramic layer 28 is preferably reinforced, for example using short glass fibres (not shown), which may be applied in sheet form during the application of the first ceramic layer 28. Alternatively carbon fibres may be used instead of glass fibres. Once laid, more ceramic material is applied over the fibre reinforcements, following which the heating wires 24 are applied onto the first ceramic layer 28. Thus once the first ceramic layer 28 is set or cured, the heating wires 24 will be set in the desired position, again as illustrated in Figure 3.

The spacing and orientation of the array of heating wires 24 is particularly important in order to achieve the desired heat distribution across the working surface 22, and in particular to allow different areas of the working surface 22 to be simultaneously heated to different temperatures. In this way the mould 10 is capable of matching the heat output at particular locations on the working surface 22 to the local thickness of the component being produced by the mould 10, as will be described in detail hereinafter.

Once the first ceramic layer 28 has suitably set, a second ceramic layer 30 is preferably applied over the first ceramic layer 28, and preferably in the same manner as that used for the first ceramic layer 28. Once the second ceramic layer 30 has suitably cured, an optional support layer 32 may be applied thereto, in order to provide the upper section 14 with suitable stiffness and/or strength. During the lay-up of the second layer 30 or the support layer 32, cooling channels (not shown) may be formed within either of these

layers. These channels can extend either across the width or along the length of the mould 10.

The combined upper section 14 and pattern 34 are then turned over, such that the other side of the pattern 34 can be accessed, and the entire process is then repeated in order to form the lower section 16. In the embodiment illustrated the mould 10 comprises the support frame 36 which is provided in two parts, one to be fitted to the upper section 14 (omitted from Figure 4 for clarity) and an essentially identical portion fitted to the lower section 16. The frame 36 supports the mould body 12 and gives structural integrity thereto, while allowing the manipulation of the upper section 14 and lower section 16 as required, in particular when demoulding a component. It will of course be appreciated that any other suitable arrangement may be employed in order to provide this support function.

The mould 10 is now ready to be put into use to produce a desired component, for example a wind turbine blade (not shown), which may be in the order of 12 metres or significantly upwards in length. Components of such dimensions are not suitable for manufacture by conventional moulding techniques, which require the mould (not shown) to be heated in an autoclave or oven or the like, in order to facilitate the processing and consolidation of the polymeric composite material from which such wind turbine blades are conventionally manufactured.

The material for manufacturing this type of component is generally commercially supplied in sheet form, which is then laid onto the working surface 22 of each of the sections 14, 16, with the sections 14, 16 then being secured in face-to-face engagement as illustrated in Figures 1 and 4, with suitable steps being taken to retain the material against the working surface 22. These measures may be of any suitable form, for example as previously described the use of an inner core (not shown), or the use of vacuum bagging as is well known in the art. Electrical power is then supplied to the heating wires 24, in order to supply heat to the working surfaces 22. The heating wires 24 are preferably adapted to generate a temperature, at the working surface 22, from 25°c to 800°c and more preferably from 100°c to 500°c. The exact temperature will depend on the nature of the material to be processed in the mould 10. For example, thermoplastic matrix materials such as polypropylene and polyamide 6, polyamide 12, polyamide 1 1 and polybutylene terephthalate may be processed at temperatures below 200 c. Thermoset matrix materials

such as polyesters and epoxies may be processed below 200°c. Other thermoplastic materials such as polyethylene terephthalate may be processed between 250°c and 300°c. Other thermoplastic polymers such as polyphenylene sulphide, polyetherimide, polyetheretherketone and polyetherketoneketone can be processed between 300 c and 400 ⁰ C.

The electrical power rating for the heating means 24 will typically be in the range of 5 kW/m ² to 30 kW/m ² of working surface 22, but can of course be varied to suit the particular production rate required, and also the thickness or local thickness of material to be processed. The wattage density can also be locally varied in order to suit local variations of thickness and type of material to be processed locally within the mould 10. This is one of the major benefits of using the array of heating wires 24, whose heat output can be individually varied, or varied in pre-defined groups, in order to customise the heat output to exactly match the specifications of the component being moulded. To this end, each individual heating wire 24 or alternatively groups of heating wires 24 can be provided with dedicated electrical terminals (not shown) which therefore allow this differential heating of the working surface 22.

This beneficial aspect of the present invention is further enhanced by the use of ceramic materials in forming the mould body 12. Ceramics have a very low thermal conductivity, and thus do not absorb heat or evenly dissipate the heat generated by each of the heating wires 24. Thus by placing the heating wires 24 at or adjacent to the working surface 22, and due to the thermally insulated properties of the ceramic material surrounding the heating wires 24, the heat generated within the heating wires 24 is not dissipated in an even fashion across the working surface 22, thereby allowing local variations in the heat supplied to the working surface 22. Ceramics also have a high thermal mass, with the result that during use of the mould 10, the inner or working surface 22 may be at an operating temperature of, say, 400 ⁰ C, while the outer face of the mould body 12 will remain at a significantly lower temperature, for example room temperature. This has the benefit of minimising the expansion/contraction of the mould body 12 at the points where the frame 36 is connected thereto, thereby eliminating the possibility of cracking at said locations.

A fUrther advantage of the use of ceramic materials for the mould body 12 is the very low co-efficient of thermal expansion (CTE), which closely matches that of composite and plastic materials to be processed by the mould 10. Thus as the combination of the mould body 12 and the component being moulded therein cool following the moulding of the component, negligible stresses are developed within the component, as there is no discernible mismatch in coefficients of thermal expansion between the two. The problems associated with such thermal mismatch are magnified, and thus cause greatest problems with, large sized components, as the difference in thermal strains is greater. It is therefore particularly important to avoid such thermal mismatch when producing large components such as wind turbine blades, boat hulls, aeroplane fuselages, large automotive panels or the like.

The following example sets out the steps in manufacturing the mould 10 of a configuration particularly suited to moulding a thermoplastic composite wind turbine blade under vacuum pressure.

Step One: Preparation of Pattern

A suitable pattern 34 must be used which is compatible with the material used to manufacture the mould. For this particular component, which is a wind turbine blade, a two-part mould was required. The pattern 34 was first machined in two-parts (not shown) from foam block. The two halves were then bonded together and the pattern 34 was sprayed with a polyester gel coat. For a closed mould a one-piece pattern should be used. This ensures accurate closing of the final mould with zero misalignment. A sacrificial flange (not shown) is attached to the pattern 34 at the split line (not shown). This will create a flange for the lay up of the first half 14 of the tool or mould 10. After the first half 14 of the mould 10 is laid up the pattern 34 is rotated 180° and this sacrificial flange removed. The flange 18 of the first half 14 of the mould 10 then becomes a part of the pattern for the second half 16.

For a closed mould 10, standard locators and a seal channel (not shown) may need to be added to the pattern 34. The seal channel can be created on the pattern using an extruded silicon profile. This profile can be bonded to the pattern 34 using any suitable adhesive. Locators can be made from tapered wood sections. They are coated with a polyester gel

coat before being bonded to the pattern 34. Finally the pattern 34 needs to be prepared with a release agent.

Step Two

The next step in the process is to apply a gel coat layer 26, during which the working environment should be maintained at room temperature. The gel coat layer 26 is prepared by mixing 23% liquid and 77% powder with a low speed mechanical mixer.

Apply approximately 2kgs/m gel coat to the pattern 34 with a paint brush or any other suitable means. The gel coat 26 requires 3 to 4 hours curing at room temperature between layers. Apply another 2kgs/m ² to the pattern. The first layer should have cured sufficiently such as not to be displaced while applying the second layer.

Step Three - Reinforcement Layers

The gel coat 26 should have cured enough not to be displaced by applying the reinforcing layers 28, 30.

The first reinforcing layer 28 of resin is prepared by mixing 33.8% liquid with 67.2% powder with a low speed mechanical mixer. Apply the resin over the gel coat 26 with a brush. Ensure good contact is obtained with all the gel coat 26. Disburse approximately 2kg/m ² of resin evenly over the surface and then place glass mats down onto the resin. First use a brush to force the resin through the glass mat. Then use a roller or the like to remove all air. Add more resin to fully impregnate all the glass fibres. Lay up another three further similar layers of glass mat. In the mould layup, carbon fibre mats can also be substituted for glass fibre mats. At this stage the heating elements 24 can be applied.

Step Four - Heating Elements

In the current example the heating elements 24 were supplied by Amptek of the United States. Heating elements 24 should be prepared in advance. Cut AMPTEK AWO standard insulated heating tape 24 to correct resistance. After the tape 24 is cut from the roll the

ends should be prepared and the resistance confirmed. After the tape 24 has been positioned correctly it should be pressed into the resin 28 to ensure it maintains its position. At this stage a detailed sketch of the mould 10 and the lay out of the heating wires 24 should be prepared. The heating wires or elements 24 should now to left to set in the resin 28. Mix an amount of resin and cover the heating elements 24. Use the brush to ensure that all the air is removed from between the heating elements 24. All terminated wire must be brought through the glass mats. Place the glass mat onto the mould. Continue to lay up further reinforcement layers as before. In this case nine layers of glass mat were applied after the heating elements 24 to form the first ceramic layer 28. The size and/or required strength of the tool will determine the number of layers of glass mat required. The use of carbon fibre instead of glass fibre would give higher strength. On completion of the lay up cover the tool with plastic film and seal.

Step Five - Cooling Channels

After the heating elements 24 are embedded within the first ceramic layer 28 (FIG.3) the second ceramic layer 30 is applied. Preferably, before the application of the optional support layer 32, channel-forming inserts (not shown) in the form of rods or tubes are laid on the top of the second ceramic layer 30. After these inserts are secured in position, the reinforcing layers which form the support layer 32 are applied. After the mould 10 is cured the channel-forming inserts may be removed by raising the temperature of the mould 10 above the melt temperature of the inserts. Alternatively, the inserts may take the form of a thin sleeve filled with a fine grained medium such as glass beads or sand. These inserts can easily be removed after the mould 10 is cured. During processing, a suitable cooling medium, preferably air, can be passed through these channels to accelerate cooling.

Step Six - Demoulding and Curing

Before demoulding the mould 10 should be cured at 6O ⁰ C overnight. Smaller tools or moulds can be cured in 4 hours. After demoulding the mould 10 should be cleaned and polished. Post-cure steps are as follows:

Cured 9O ⁰ C for 48 hours (less time for smaller moulds); Ramp to 1 1O ⁰ C at 6 ⁰ C per minute, dwell for 2 hours;

Ramp to 125 ⁰ C at 3 ⁰ C per minute, dwell for 2 hours; Ramp to 135 ⁰ C at 2 ⁰ C per minute, dwell for 2 hours; Ramp to 21O ⁰ C at 6 ⁰ C per minute, dwell for 2 hours; Cool naturally to room temperature.

Step Seven - Terminating and Wiring of Heating Elements

After curing, all the ends of the heating wires 24 should be located and labelled according to the sketch prepared earlier. A ceramic connector block is bonded close to each wire end with a high temperature silicone or some reinforcing resin. After the connector blocks have set, the heater wires 24 are placed into the blocks and tightened. The resistance of each heater wire 24 is checked to ensure that all terminations are correct. The external wiring and controls (not shown) then need to be connected to suitable control circuitry. During use this control circuitry (not shown) can be used to heat particular areas of the working surface 22 to desired temperatures to match the local thickness of the component being moulded. Alternatively the control means can be used to implement uniform heating across the entire working surface 22, if the component being moulded is of a constant thickness. Variations and combinations of these operations may be employed during a single moulding process, depending on the requirements of the moulding process.

As the heating elements 24 are individually located within the first ceramic layer 28, the orientation of and spacing between the heating elements 24 can be exactly specified. Thus in use the heat output at the working surface 22 can be carefully controlled to suit the requirements of the moulding process. In addition, by providing heating elements 24 formed integrally with the flanges 18, 20, during use of the mould 10 thermal strain between the flanges 18, 20 and the main section of the mould body 12 is eliminated. This prevents the occurrence of stress cracks, due to thermal cycling, at the interface between the flanges 18, 20 and the remainder of the mould body 12, thereby prolonging the working life of the mould 10.

Referring now to Figure 5 of the accompanying drawings, there is illustrated a modified embodiment of a mould according to the present invention, generally indicated as 110, in which embodiment like components have been accorded like reference numerals, and unless otherwise stated, perform a like function. The mould 1 10 of this modified

embodiment is similar in configuration and construction to the mould 10 of the first embodiment, and it is not therefore deemed necessary to describe in detail the process involved in manufacturing the mould 1 10.

The mould 1 10 comprises a mould body 1 12 formed substantially from ceramic material, and having a suitably profiled working surface 122, against which a workpiece P is moulded into a desired shape. The mould body 1 12 has an array of integrated heating elements (not shown) located adjacent the working surface 122, as hereinbefore described, in order to heat the workpiece P to enable the moulding thereof. The mould 110 of the present embodiment includes only a lower section 1 16 in which the working surface 122 is formed, and is thus a single sided mould 1 10 adapted for use in a vacuum moulding process as described hereinafter in detail.

While ceramic materials are good thermal and electrical insulators, and thus well suited to the production of an integrally heated mould as hereinbefore described, such ceramic materials, due to their natural porosity, are not capable of sustaining a full vacuum on a working surface of a mould formed therewith. However this has not previously been an issue due to the relatively small size of prior art ceramic moulds, which have not thus been required to be suitable for use with vacuum moulding, which is generally reserved for larger moulds. It was therefore an object of the present invention to render the integrally heated mould of the present invention suitable for use in vacuum moulding.

The mould 1 10 is therefore provided with a sealing layer 50 and an insulating layer 52 disposed between the sealing layer 50 and the mould body 1 12. In the embodiment illustrated, the sealing layer 50 is formed from a glass reinforced plastic, although any other suitable alternative may be employed. The insulating layer 52 is provided to prevent the transfer of heat from the integrally heated mould body 1 12 to the sealing layer 50, which heat might otherwise result in the eventual degradation of the sealing layer 50. Thus the thickness of the insulating layer 52 can be varied depending on the material from which the insulating layer 52 and/or the sealing layer 50 is formed, in addition to the temperature to which the mould body 1 12 is to be heated.

The sealing layer 50 essentially provides an air tight barrier or boundary about all but the working surface 122 of the mould 1 10. In use, the material from which the workpiece P is

to be moulded is laid in contact with the working surface 122, and a conventional vacuum bag 54 is then laid over the workpiece P in the mould body 1 12. The vacuum bag 54 is dimensioned to extend outwardly to overlap a peripheral border 56 formed integrally with the sealing layer 50. The bag 54 is sealed against the border 56 using conventional sealant tape 58, although any other sealing means or mechanism may be used. A vacuum hose 60 extends upwardly from the vacuum bag 54, in order to allow evacuation of the bag 54, in order to apply and maintain pressure against the workpiece P during the moulding thereof. This vacuum pressure draws the workpiece P downwardly against the working surface 122, which is heated by means of the integrated heating elements (not shown), in order to form the workpiece P. Due to the porosity of the ceramic material used to form the mould body 1 12, the pressure across the entire working surface 122 is substantially equal when the vacuum is applied, avoiding any pressure gradients or differences on the working surface 122. This ensures that all parts of the workpiece P are accurately moulded, well consolidated and free of voids.

In order to produce the mould 110, the mould body 112 is manufactured as hereinbefore described with reference to the mould body 12 of the first embodiment, with an array of heating elements (not shown) being embedded therein. It is then necessary to apply the insulating layer 52 to the underside of the mould body 112. Thus a casing (not shown) is first constructed using micro board or the like. A layer of release agent (not shown) is provided on the moulding surface of the micro board, in order to allow de-moulding of the combined insulating layer 52 and mould body 1 12. The casing is required to have a dimension of, in the embodiment illustrated, 50mm greater on all sides of the mould body 1 12. The casing has a height of 60mm for the mould body 1 12 of the embodiment illustrated. The mould body 112 is then placed, working surface 122 face down in the casing, and the insulating layer 52 poured into the casing to surround the mould body 1 12 on all sides apart from the working surface 122, as is illustrated. The insulating layer 52 is produced using a reinforcing resin to which a micro sphere powder is added at a ratio of 30% of the total reinforcing resins weight. The combined insulating layer 52 and mould body 1 12 are then preferably cured overnight, prior to de-moulding. Following de- moulding the mould 1 10 is preferably cleaned and polished, and post cured at 90 ⁰ C for 48 hours, and then at 1 10 ⁰ C for 2 hours, at 125°C for 2 hours, 135°C for 2 hours and 21O ⁰ C for the final 2 hours.

Following curing, the heating element ends (not shown) are located and any ends having multi-strand wire will need to be connected to a solid wire, for example, copper wire, for the vacuum sealing provided by the sealing layer 50 to be effective. Each of the ends are cut so that they protrude approximately 40mm from the underside of the insulating layer 52. A metal ferrule is then crimped to the end of each of the heating element wires, which ferrule should be approximately 9mm in length. The sealing layer 50 can then be applied about the insulating layer 52, as illustrated in figure 5.

The sealing layer 50, in the embodiment illustrated, is formed from glass fibre polyester, although any other suitable alternative may be used. The mould 1 10, including the now cured insulating layer 52, is place on a sheet of micro board, to which a thin layer of release agent has preferably been applied. A border is then marked at a distance, in the embodiment illustrated, of 50mm from each side of the mould 1 10, which will be used to mark the location of the peripheral border 56. To begin applying the sealing layer 50, a resin forming an integral part of the sealing layer 50 is first painted or otherwise applied to the exposed faces of the insulating layer 52, taking particular care to fully wet the areas around the ends of the heating element wires (not shown) projecting from the insulating layer 52. A layer of glass fibre mat (not shown) is then laid onto the wet resin, with a brush (not shown) or the like then being used to fully impregnate the glass with the resin. Three layers of glass fibre mat are applied in this way, with the heating element wires being drawn through each layer as it is applied. The sealing layer 50 is then allowed to cure for a 24 hour period, although this may vary depending on the material forming the sealing layer 50. Glass fibre mats may be replaced by carbon fibre mats to give extra structural strength, if necessary.

In addition to enabling the moulding 1 10 to be used in vacuum forming a workpiece P, the sealing layer 50 provides an additional advantage, whereby the mould 1 10 is sealed to prevent or significantly reduce moisture uptake from the environment during downtime of the mould 1 10, which can be an issue with ceramic moulds, again due to the porosity thereof.

Although not illustrated, as an alternative to the sealing layer 50 and insulating layer 52, it is possible to achieve full vacuum pressure on the mould of the present invention by embedding a metallic foil between the reinforcement layers of the mould body. For

example, a carbon steel foil of 0.05 mm thickness, or a nickel foil of 0.025 mm thickness, can be incorporated into the mould body. Unlike the glass fibre polyester of the sealing layer 50, such metallic foils will not degrade when subjected to heat, and thus the insulating layer 52 of the second embodiment can be omitted when using such foils.