WO2019122797A1

WO2019122797A1 - Method of creating a mould from refractory material

Info

Publication number: WO2019122797A1
Application number: PCT/GB2018/000160
Authority: WO
Inventors: James Michael Collins
Original assignee: Castings Technology International Limited
Priority date: 2017-12-23
Filing date: 2018-12-24
Publication date: 2019-06-27
Also published as: GB2569814A

Abstract

A method of forming a shell (205) for investment casting comprises the steps of obtaining a pattern (204) made from a thermoplastic material and creating a shell (205) around the pattern. Layers of slurry are applied around the pattern (204), each of the layers comprising a refractory material, and layers of sand are also applied around the pattern. Each layer of slurry is dried after application, preferably in a microwave. One of the layers of slurry or layers of sand comprises a susceptor material. The final layer of slurry is dried by heating the shell using microwave energy, by radiating the microwave energy towards the shell (205) such that the susceptor material heats up and conducts heat to the refractory material. The shell (205) is heated to a drying temperature which is at or above the melting temperature of the thermoplastic material, such that the pattern (204) melts. The shell (205) is then fired, by using microwave energy to heat it to a temperature that is higher than the drying temperature.

Description

Method of Creating a Mould from Refractory Material

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from United Kingdom Patent Application No. 17 22 229.0.

BACKGROUND OF THE INVENTION

The present invention relates to a method of forming a shell for use as a mould in investment casting.

During the process of investment casting a single-use mould, commonly of ceramic, is formed by applying several layers of slurry and sand to a sacrificial pattern. Due to the necessity to allow the slurry to dry out between coats, this process takes several days. Air-drying a slurry layer takes around three hours. This can be speeded up by heating, but if the sacrificial pattern is made of wax then there is a danger of it melting or deforming.

It is known to use expanded polystyrene foam for the pattern, which is less susceptible to temperature. Using this, a small amount of heat can be applied to dry each layer, reducing the time to about one hour per layer, giving a day’s production time for the shell. However, a disadvantage of using foam is that rather than being melted and reclaimed like wax, it is generally burnt off when the mould is fired, producing environmental emissions.

BRIEF SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided a method of forming a shell for investment casting according to claim 1.

According to a second aspect of the invention, there is provided a method of producing a metal component according to claim 17.

According to a third aspect of the invention, there is provided a method of producing a metal component according to claim 18.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Figure 1 shows steps carried out during the process of investment casting; Figure 2 illustrates steps carried out in Figure 1 ;

Figure 3 illustrates further steps carried out in Figure 1 ;

Figure 4 details steps carried out in Figure 1 to carry out creation of the shell;

Figure 5 details steps carried out in Figure 4 to apply base layers of the shell shown in Figure 2;

Figure 6 details steps carried out in Figure 4 to apply an outer layer to and fire the shell shown in Figure 2;

Figure 7 is a diagrammatic cross-section of the shell shown in Figure 2;

Figure 8 illustrates a first example of apparatus suitable for carrying out the method shown in Figures 4 to 6;

Figure 9 is a diagram of a microwave oven shown in Figure 8; and

Figure 10 illustrates a second example of apparatus suitable for carrying out the method shown in Figures 4 to 6.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1

Figure 1 shows steps carried out during the process of investment casting. At step 101 a master die is obtained. This is a high quality die, generally made of a metal such as aluminium, which can be reused many hundreds of times. The die is the inverse shape of the component to be cast. At step 102 a sacrificial pattern is made by injecting polystyrene into the master die and allowing it to expand to create a foam pattern. Other suitable thermoplastics may be used instead.

After the foam pattern is removed from the master die, a ceramic shell is formed around the pattern at step 103. The shell is dried and the foam is removed.

The ceramic shell has the same internal shape as the master die, and is the mould used for the actual casting process. Thus at step 104 feedstock is poured into the shell and any necessary processing is carried out in order to create the end component. If molten feed stock is used, then it is only necessary to allow it to cool. In a process known as hot isostatic pressing, powered feedstock is used, and the shell and feedstock are subjected to heat and pressure in a containment vessel in order to melt the feedstock. Other casting processes may be used.

At step 105 the component is allowed to cool before the shell is removed at step 106. This is commonly done by hammering the shell, blasting it with water, chemically dissolving it, or any other suitable method. The component is then machined if necessary at step 107 to remove any rough edges or sprues. Figures 2 and 3

The casting process is illustrated diagrammatically in Figures 2 and 3. Figure 2 shows the process of obtaining the ceramic shell, while Figure 3 shows the process of casting the final component. The size of the items shown is dependent upon the size of the metal component to be cast. In general, investment casting is suitable for items that have dimensions anywhere from less than a centimetre to several metres.

A master die 201 is made of two halves 202 and 203. These are placed together and filled, in this example, with expanded polystyrene to form a pattern 204. The two halves 202 and 203 are split apart to leave the pattern. This may be done many times in order to create a pattern for each individual component required. A ceramic shell 205 is then formed around pattern 204 by repeatedly dipping the pattern into slurry and sand and drying it. Circle 206 indicates an area of pattern and shell that will be described further with reference to Figure 7.

Once the layers of slurry are dry, pattern 204 is removed by a suitable method, and shell 205 is fired in order to harden it, leaving it ready for use.

Shell 205 is filled with feedstock 301 which in this example is powdered feedstock suitable for use in a hot isostatic press. This is processed to create a metal component 302. The ceramic shell 205 is removed, and sprue 303 is machined away to finish component 302.

Thus it can be seen that pattern 204 is not exactly the same shape as final component 302. Pattern 204 includes a funnel shape that is then reproduced in shell 205 in order to allow the feed stock to be introduced.

The invention herein described is directed towards a method of producing ceramic shell 205 at step 103, and this is detailed further in Figures 4 to 7. Two examples of apparatus suitable for carrying out the method are then described in Figures 8 to 10.

Figure 4

Steps taken to carry out creation of the shell at step 103 are detailed in Figure 4. At step 401 a number of base layers of slurry and sand are applied to the pattern. For each layer, this involves dipping the pattern, along with any previous layers, into slurry, and coating it with sand before drying it. The slurry is generally a mixture of liquid and refractory powder, and the sand is often also formed from refractory material. Refractory materials are materials that retain their strength at high temperatures. After firing in order to sinter the refractory material, the shell is hard and able to withstand the high temperatures of the molten metal that will be applied to it without cracking or expanding. In addition, many refractory materials have very low reactivity, meaning that they will not react with the metal, and these are commonly chosen for the first few layers of the shell.

In the examples described herein ceramics are used, but any suitable refractory material can be used. Ceramics, although able to withstand high temperatures, are relatively easy to break down physically or chemically in order to remove the shell from the final metal component.

At step 402 a final layer of slurry and sand is applied and the shell is fired. In known methods of investment casting using patterns made from expanded polystyrene, the polystyrene is burnt off during the firing process, which is typically carried out in a gas-fired or electric kiln heated up to around 1000°C. This is an energy intensive method of firing, and the burning of the polystyrene creates carbon emissions and other gases, which must be captured and treated before venting to atmosphere.

The invention herein described instead removes the polystyrene pattern by melting it out during the drying of the last layer. This is made possible by including a susceptor material such as silicon carbide in the final layer of slurry and sand, and drying the layer in a microwave oven. The susceptor material is excited by the microwave energy and conducts heat to the refractory material in the shell. This causes the shell to get hot enough to melt the polystyrene, which can then be reclaimed and either safely disposed of or reused.

Advantageously, the microwave energy can then be increased such that the shell heats up to around 1000°C for firing. This can be done without removing the shell from the microwave oven.

Figure 5

Figure 5 details steps carried out during step 401 to apply the base layers of the ceramic shell 205. At step 501 a suitable slurry and sand are selected, as will be described further with reference to Figure 7. At step 502 the foam pattern, or on later iterations of the step the part-completed shell, is dipped into the selected slurry. At step 503 the slurry is coated with sand, which may for example be achieved using a fluidised bed or a rainfall sander. At step 504 the part-completed shell is placed in a microwave oven and at step 505 the shell is heated to around 50°C until the layer is dry. At step 506 a question is asked as to whether another base layer should be applied, and if this question is answered in the affirmative then control is returned to step 501. Alternatively, all the base layers have been applied and step 401 is concluded.

Thus, in this embodiment each layer of slurry and sand is dried in a microwave oven. The refractory particles and sand are transparent to microwave energy. However, the water in each wet layer is excited by the microwave energy, heats up, and evaporates, thus drying the slurry. As the water is heated, heat is conducted to the ceramic which heats up in turn, and conducts heat to foam pattern 204, which is also transparent to microwave energy.

Therefore, using a microwave oven, the shell could be heated to a theoretical maximum of 100°C, the boiling point of water. When all the water is evaporated no further heating is possible. However, in practice the expanded polystyrene may expand and crack the shell at temperatures above 70°C. Thus the preferred temperature for the drying stage of step 505 is 50°C, although in other embodiments a temperature in the range of 20°C to 60°C could be used.

Figure 6

Figure 6 details steps carried out during step 402 to apply the final slurry and sand layer and fire the shell. Thus at step 601 a suitable slurry and sand are selected. In this embodiment, the sand is silicon carbide, and this provides the susceptor material. In this embodiment the slurry is the same as slurries used in previous layers. However, in other embodiments, the susceptor material might be contained within the slurry; for example it could be made with silicon carbide powder.

Similarly to steps 502 and steps 503, the part-completed shell is dipped into the selected slurry at step 602 and coated with sand at step 603. The shell is then placed in a microwave oven, preferably upside down and above a drip tray into which the melted polystyrene will drain.

At step 605 the atmosphere in the microwave oven is purged with an inert gas such as nitrogen. This is to prevent any accidental combustion of the polystyrene. In other embodiments this step could be emitted, as the polystyrene is expected to melt before combustion temperature is reached, but an oxygen-free atmosphere is preferred to avoid accidents. Other inert gases, for example argon, could be used.

At step 606 the shell 205 is heated using microwave energy to around 300°C. A temperature between 250°C and 380^eC would be suitable. The microwave energy excites the susceptor material which conducts heat to the ceramic shell, which in turn conducts heat to the foam pattern. This heating step dries the final layer of slurry and melts the foam pattern 204; the heating is fast and therefore the point at which the foam pattern might expand and crack the shell is passed relatively quickly. In addition, the completed shell is thicker and less likely to crack than a part-completed shell, and therefore the concern that the foam pattern might expand is less than at previous stages. The melted polystyrene is caught in the drip tray and safely disposed of or reclaimed.

At step 607 the nitrogen is purged so that the atmosphere inside the microwave oven is again standard, and step 608 the microwave energy in the oven is increased so that shell 205 is heated by the susceptor material to around 1000°C until the refractory material is semi-sintered. The exact temperature to which the shell needs to be heated depends on the refractory material used. It is not necessary to heat the shell until it is fully sintered, as it is hard enough for use when semi-sintered. For this reason the temperature that should be reached by the shell does not depend upon the type of refractory material used.

Thus there is described a method of forming a shell for investment casting, comprising steps of obtaining a pattern made from a thermoplastic material, which in this example is expanded polystyrene, and creating a shell by applying layers of slurry and sand around the pattern and drying each layer of slurry. One of the layers of slurry or layers of sand comprises a susceptor material. In this embodiment the drying is carried out using a microwave oven, but in other embodiments the drying of the base layers may use a different method. The outer layer of slurry is dried by heating the shell using microwave energy to or above the melting temperature of the thermoplastic material, such that the layer dries and the pattern melts. In this embodiment, the shell is fired by leaving it in the same microwave oven and increasing the temperature. This is more efficient than heating a furnace to a high temperature.

The shell produced by this method is suitable for use in any investment casting procedure. However, the addition of the susceptor material makes it particularly efficient when used in a hot isostatic pressing procedure. In this procedure, the shell containing powdered metal feed stock is placed in a containment vessel having a heating system, and heat and pressure are applied to the shell so that the feed stock bonds and forms a metal component. Because the mould contains susceptor material, the heating system may comprise a source of electromagnetic energy such as an induction heating system or a source of microwave energy. This can be used to heat the shell using the susceptor material, which may be more efficient than applying radiative heat to the shell.

Figure 7

Figure 7 is a diagrammatic cross-section of the area 206 identified in Figure 2. It shows a corner of foam pattern 204 with shell 205 formed around it. Shell 205 comprises seven layers of slurry and sand. The shell is typically between 5mm and 15mm thick in total, the thickness generally being relative to its overall size. A larger component to be cast will usually require a thicker shell.

In this embodiment, three types of slurry and four types of sand are used. Each slurry is made up of around 30% liquid and 70% refractory powder, and in all three cases the liquid is colloidal silica suspension in water. However, other suitable liquids may be used.

Each of layers 701, 702, 703, 704, 705, 706, and 707 making up shell 205 is made of a layer of slurry and an optional layer of sand. For example, layer 701 comprises layer 708 of slurry and layer 709 of sand. In this embodiment, every layer includes sand, but an embodiment where this is not the case will be described later.

Layer 701 , as the first to be applied to pattern 204, is made of fine materials. The slurry used in layer 708 includes a fine zirconium silicate powder, and the sand of layer 709 is fine zirconium silicate sand. This first layer 701 is used to ensure that pattern 204 is coated to a high degree of accuracy, and is referred to as the prime coat.

Layer 702 is composed similarly to layer 701 but is not as fine. The slurry used also includes zirconium silicate powder, but the slurry has lower viscosity. This ensures that layer 701 is fully covered. The sand used is medium-size zirconium silicate sand, although in other embodiments it could be medium sand of the same composition as the later layers.

After layers 701 and 702, the whole of pattern 204 should be covered. Zirconium silicate is preferred for this prime coat because it has no reactivity with the feed stock 301 and can produce a smooth surface.

Layers 703, 704, 705 and 706 are to provide bulk and strength to the shell and can therefore be made of cheaper materials. In this example, these four layers are identical. The slurry used includes aluminium oxide, and the sand layer is coarse aluminium oxide sand. Other suitable refractory materials are, for example, alumina-silicate, silicon dioxide, zirconium oxide, yttrium oxide, chromium oxide. Other refractory materials not listed are also suitable.

After applying each of layers 701 to 706, the slurry is dried as described with respect to step 505 by heating the part-completed shell using microwave energy. This takes around three minutes per layer. The shell must be removed from the microwave after each drying stage in order to apply further slurry and sand. For this reason, the drying stages cannot be combined with the final drying stage and firing described with respect to Figure 6, and therefore alternative drying methods of these base layers could be used.

The outer layer 707 again comprises a layer of slurry and a layer of sand. In this embodiment, the slurry used is the same as that used in layers 703 to 706. However the sand is silicon carbide sand, which is a susceptor material. In alternative embodiments, the susceptor material could be contained within the slurry of layer 707. As described with respect to Figure 6, once the shell contains susceptor material, it can be heated using microwave energy to a high temperature allowing the pattern 204 to be melted concurrently with the firing of outer layer 707. Advantageously, the shell may be left in the same microwave oven and the energy increased to fire the shell. Firing is done by sintering of the refractory material contained in each of layers 701 to 707.

In this embodiment the susceptor material is contained in the outer layer 707. However, the susceptor material can be included in any of the layers. For example, layer 707 may be duplicated such that there are two layers including silicon carbide sand. In this case, layer 707 would be dried at the lower temperature of 50°C, and the additional layer would become the outer layer and dried at the higher temperature of 300°C. In another embodiment, it might be found that the shell heated more evenly if the susceptor material were placed in a more central layer, such as layer 704. In a still further embodiment, a final layer comprising slurry but not sand is applied, and again it is one of the base layers that includes the susceptor material. Also, as previously described, the susceptor material could be in any of the slurries instead of or in addition to the sand.

Thus whilst it is the final layer that is heated to a high temperature by exciting susceptor material with microwave energy, such that the final layer dries and the pattern 204 melts, it is not necessary that the susceptor particles be themselves included in this final layer. However, if susceptor material is included in a base layer, care must be taken when drying this and subsequent base layers, as the susceptor material will cause the part-completed shell to heat faster than when it is not present. As previously described, it is preferable that the shell be heated to no more than 70°C when drying the base layers, otherwise foam pattern 204 may expand and crack the part-completed shell.

Any suitable slurry liquid, refractory material or sands may be used. In practice, these will be chosen depending on the nature of the casting to be made, for example the size, the method of casting, the metal of the feed stock, and so on.

In addition, any suitable refractory material may be used in addition to or instead of silicon carbide.

Figure 8

Apparatus suitable for mass producing shells using the method described with respect to Figures 4 to 6 is illustrated in Figure 8.

In this example, the microwave ovens are conveyer ovens 801 and 802. The ovens are similar except that oven 801 can be sealed off in order to purge the atmosphere using nitrogen tank 823. It also includes a collection receptacle 824 for the melted polystyrene. Both ovens are provided with ducting 803 and 804 respectively. The ducting provides power to the ovens, and removes water vapour and gases produced during the drying and firing processes. In additional, ducting 803 carries purged gasses via pipe 805. The size of the microwave ovens is chosen to be suitable for the size of ceramic shells to be manufactured.

The production process is controlled by two robot arms 806 and 807. Robot arm 806 controls a tool 808 which is used to pick up a pattern or a part-completed shell, and robot arm 807 controls a similar tool 809.

Foam patterns, such as pattern 810 and 811 , are produced elsewhere and provided in an area close to robot arm 806. Robot arm 806 picks up a foam pattern using tool 808 and dips it into slurry bin 812 and into fluidised bed 814 in order to coat the pattern with a first layer of slurry and a first layer of sand. It then places the pattern on the conveyer belt of microwave oven 801 , which conveys it towards robot arm 807. The conveyer belt moves at such a speed that the pattern with its first layer applied spends around three minutes in microwave oven 801.

Robot arm 807 picks up the part-completed shell from the end of the conveyer belt using tool 809 and dips it in slurry bin 817 and fluidised bed 819 in order to apply the second layers of slurry and sand. It then places the part- completed shell on the conveyer belt of microwave oven 802, in which it spends three minutes being returned to robot arm 806.

Robot arm 806 then applies a layer of slurry from bin 813 and a layer of sand from fluidised bed 815 and places it back on the conveyer belt of oven 801. Robot arm 807 collects the part-completed shell and coats it in slurry from bin 818 and sand from fluidised bed 820 before placing it on the conveyer belt of oven 802. These two steps are repeated again, so that six layers of slurry and sand have been applied and dried. Robot arm 806 collects and stores these part-completed shells in area 821 prior to applying the final layer.

The outer layer is applied by dipping a part-completed shell into slurry bin 813, and fluidised bed 816. Fluidised bed 816 contains silicon carbide sand. Shells that have been coated with this outer layer are then returned to the conveyer belt of microwave oven 801. A number of shells are coated in the final layer at the same time, up to the maximum that oven 801 will contain. Oven 801 is then sealed, and purged using nitrogen from tank 823. Microwave energy is applied so that the shells are heated to around 300°C, and the melted polystyrene is collected in receptacle 824. This takes around 15 minutes, following which the microwave energy is increased so that the shells are heated to around 1000°C to fire the shells, which takes around 30 minutes.

Once this procedure is complete, robot arm 801 removes the completed shells from the conveyer belt of oven 801 and places them in area 822, following which the ceramic shells will be removed by another process.

The example described in Figure 8 is only one apparatus suitable for mass producing ceramic shells using the method described herein. More or fewer microwave ovens may be used, and the application of the layers of slurry and sand may be varied as was described with reference to Figure 7.

Further, although microwave ovens having conveyer belts are particularly suitable for this type of process, any oven in which microwave energy is applied may be used.

Figure 9

Figure 9 is a diagram of microwave oven 801. An oven area 901 may be open at each end or may be sealed, depending on which stage of the process is being performed. The oven should be sealed while the atmosphere is purged of oxygen, but this is not necessary at other times. When the microwave oven is open, then microwave suppressors at each end (not shown) ensure that microwaves do not leave the oven.

Microwave energy is provided by a series of magnetrons 902, 903, 904, 905,

906, 907, 908, and 909 in the roof of the oven. These are provided with power via ducting 803, which also removes water vapour and gases.

Conveyer belt 910 is moved by rollers 911 and 912. Sets of small rollers 913 and 914 at each end move items into and out of the oven.

Belt 910 is made of a fine mesh which allows melted polystyrene to drain through the belt and be caught by drip tray 915, which drains to receptacle 904. Alternatively the belt may be provided with a number of apertures over which the shells should be placed. The inlet and outlet for the nitrogen purge are not shown in Figure 9.

Thus in use, robot arm 806 places a part completed shell such as shell 916 on rollers 914 which move the shell into the microwave oven. If shell 916 has had a base layer applied, then the conveyer belt 910 continues to move while shell 916 passes underneath magnetrons 902 to 909 for approximately three minutes. The microwave energy produced by the magnetrons excites the water in the shell and dries it by the time it exits onto rollers 913 to be picked up by robot arm 807.

However, if part-completed shell 916 has had the final layer applied, then shell 916 is loaded into oven 801 upside down, either on its own or with a number of other part-completed shells. Conveyer belt 910 continues to move until the shells are loaded, following which the oven area 901 is sealed and the atmosphere is purged. Magnetrons 902 to 909 provide microwave energy which excites the silicon carbide or other susceptor material in shell 916. This causes the refractory material to heat up, first drying the layer and then melting the polystyrene, which runs through the mesh of belt 910 to be collected in drip tray 915. The microwave energy is then increased to fire the shell or shells. When this process is complete, the oven area 901 is opened, conveyer belt 910 is restarted and the completed shells are collected from rollers 913 by robot arm 807.

At each stage of the process, the amount of energy produced by magnetrons 902 to 909 must be carefully controlled in order to ensure that the shells contained within are heated to an appropriate temperature. In general, the more shells are present in the oven at one time, the more energy is required. Therefore the process can be calibrated to use the minimum amount of energy necessary. This is in contrast to the drying and firing of shells using direct heat, where the air must be heated to the same temperature irrespective of how many shells are being heated. This can be a particular issue during the firing process, where the heating of a furnace to 1000°C uses the same amount of energy irrespective of how many items are being fired.

Microwave oven 802 is similar to oven 801 and is not shown in detail. However, there is no necessity for doors, a drip tray, or for the conveyer belt to be made of mesh, because oven 802 is used only for the drying of the base layers. Figure 10

Figure 10 illustrates a second example of apparatus suitable for carrying out the method described with respect to Figures 4 to 6. This apparatus can be used for single pieces, such as artisanal works or prototypes, or for mass-producing large pieces that will not fit in a conveyor belt type oven such as oven 801. It could also be used for producing several shells at once, depending on the size of the oven.

The apparatus 1001 comprises an oven area 1002 on a base 1003. Base 1003 includes a funnel 1004 and drip tray 1005 underneath an aperture 1006. In use, the shell 205 around pattern 204 is placed on the base over aperture 1006. If the apparatus were to be used to produce more than one piece, then a plurality of apertures and funnels would be included.

Lid 1007 includes four magnetrons, of which magnetrons 1008 and 1009 are shown. These are controlled by and vented by ducting 1010. Inlet 1011 and outlet 1012 are provided for the purging of the atmosphere.

In use, the pattern 204 is dipped in a layer of slurry and a layer of sand before being placed within microwave 1001. Lid 1007 is put in place, and the magnetrons direct microwave energy into area 1002. As previously described, base layers are dried by heating the shell to around 50°C. The part-completed shell is taken out of oven 1001 to be recoated and replaced for the drying of each layer.

After the final layer of slurry has been applied, the shell is replaced in oven 1001, taking care that it is upside down and located over aperture 1006. Lid 1007 is put in place, and the atmosphere is purged by providing nitrogen through inlet 1011. The magnetrons heat the shell to around 300°C, causing the polystyrene to melt. It flows through aperture 1006 and funnel 1004, to be collected in drip tray 1005. Subsequent to this, the microwave oven is not opened and the energy produced by the magnetrons is increased so that the shell is heated to 1000°C to fire it.

Thus two apparatus suitable for carrying out the method described herein have been described with reference to Figures 8 to 10. Any other suitable apparatus is also envisaged.

Claims

1. A method of forming a shell for investment casting, comprising the steps of:

obtaining a pattern made from a thermoplastic material: and

creating a shell, by:

applying a plurality of layers of slurry around said pattern, each of said layers comprising a refractory material;

applying a final layer of slurry around said pattern;

applying a plurality of layers of sand around said pattern;

drying each said layer of slurry; and

firing said shell;

wherein:

one of said layers of slurry or layers of sand comprises a susceptor material; the step of drying said final layer of slurry comprises heating said shell using microwave energy, by radiating said microwave energy towards said shell such that said susceptor material heats up and conducts heat to said refractory material, to or above a first temperature, which is the melting temperature of said thermoplastic material, such that said pattern melts; and

said step of firing said shell comprises heating it, using microwave energy, to or above a second temperature that is higher than said first temperature.

2. A method according to claim 1 , wherein said step of drying said outer layer is performed in an atmosphere consisting of one or more inert gases.

3. A method according to claim 2, wherein said inert gas is chosen from: argon, nitrogen.

4. A method according to any of claims 1 to 3, wherein said shell is placed in a microwave oven before said step of drying said outer layer, and said step of firing said shell takes place in the same microwave oven without the shell being removed between said steps.

5. A method according to any of claims 1 to 4, wherein the step of drying any of said layers of slurry that is not said final layer comprises:

heating said shell using microwave energy to a third temperature that is lower than said first temperature, such that said layer dries and said pattern is unchanged.

6. A method according to any of claims 1 to 5, wherein said shell comprises more than one refractory material.

7. A method according to any of claims 1 to 6, wherein each said refractory material is chosen from: aluminium oxide, alumino-silicate, silicon dioxide, zirconium silicate, zirconium oxide, yttrium oxide, chromium oxide.

8. A method according to any of claims 1 to 7, wherein said second temperature is around 1000°C.

9. A method according to any of claims 1 to 8, wherein said thermoplastic material is in the form of a foam.

10. A method according to claim 9, wherein said thermoplastic foam is expanded polystyrene.

11. A method according to any of claims 1 to 10, wherein said first temperature is between 250°C and 380°C.

12. A method according to claim 5 or any of claims 6 to 11 when dependent upon claim 5, wherein said third temperature is between 20°C and 60°C.

13. A method according to any of claims 1 to 12, wherein said susceptor material is contained in a layer of sand that is applied before said final layer of slurry.

14. A method according to any of claims 1 to 12, wherein said susceptor material is contained in a layer of sand that is applied after said final layer of slurry.

15. A method according to any of claims 1 to 12, wherein said susceptor material is contained within said final layer of slurry.

16. A method according to any of claims 1 to 15, wherein said susceptor material is powdered silicon carbide.

17. A method of producing a metal component, comprising the steps of: creating a shell using the method of any of claims 1 to 16;

pouring molten metal into said shell; and

removing said shell.

18. A method of producing a metal component, comprising the steps of: creating a shell using the method of any of claims 1 to 16;

pouring powdered metal feedstock into said shell;

placing said shell in a containment vessel having a heating system, and applying heat and pressure to said feedstock such that it melts; and

removing said shell.

19. A method according to claim 18, wherein said heating system comprises a source of electromagnetic energy, and said step of applying heat to said feedstock comprises the step of radiating electromagnetic energy towards said shell such that said susceptor material heats up and conducts heat to said refractory material, which in turn conducts heat to said feedstock.