The present invention relates to a method of making a wax pattern using a mold, for example for use in resin transfer molding or investment casting, by introducing wax in a molten, solid or flowable form into a mold cavity having a wall, the mold cavity having an internal shape corresponding to an outer shape of the wax pat tern. The invention also relates to a wax pattern per se.

The investment casting or lost wax casting process has been in use for a very long time and has been used to make a highly diverse range of articles. Basically it in volves making a wax pattern of a desired article such as a piece of jewelry or a turbine blade, coating the wax pattern with a plurality of layers of a refractory coat ing, drying the coating to form a refractory shell and melting out the wax pattern to leave a hollow shell into which molten metal is then cast to form the desired article.

In recent times a related technique has been developed for the making of fibre reinforced hollow plastic parts. In this technique a wax pattern is made which forms a core around which at least one layer consisting of a mat or strands of rein forcing fibers is laid which is either in the form of a prepreg, meaning a layer of reinforcing fibers pre-impregnated with a resin, or into which a resin is subsequent ly permeated to form a fiber reinforced resin shell. The fiber reinforced resin shell is cured chemically or thermally or by way of UV-light and the wax core is subse quently removed to leave a generally rigid shell. Sometimes the fibre covered wax pattern is placed in a mold which defines the outer shape of the fiber reinforced shell and impregnated with a resin in that mold known as resin transfer molding (RTM).

In both techniques, i.e. lost wax casting and resin transfer molding problems arise due to the inherent shrinkage of the wax which occurs as it makes the transition from a liquid state to a solid state. This shrinkage results in the wax pattern no longer having the desired shape and thus to problems with the article which is made from the wax pattern. Various proposals have been made to deal with this problem of wax shrinkage. For example in the field of manufacture of turbine blades it is known to make a mold for a wax pattern with the mold cavity having a shape to compensate for the shrinkage that occurs. That is very complicated be cause the degree of shrinkage is related to the local thickness of the wax pattern- Another proposal uses very high pressures to make the wax flow at lower temper atures to reduce shrinkage. This does not however completely reduce shrinkage effects but is also complicated as multiple steps have to be conducted and has limitations relating to venting of the molds and the filling of intricate spaces.

The present invention has the principle object of providing a novel and generally applicable technique for at least substantially mitigating and preferably completely overcoming the problems associated with wax shrinkage when making wax pat terns.

In order to satisfy this object there is provided, in accordance with the present in vention, a method of the above named kind including the steps of:

- filling the mold cavity with flowable wax,

- rotating the mold cavity about at least one axis to line the wall of the mold cavity with wax,

- cooling the wax lining the wall of the mold cavity to solidify the wax into a wax pattern with a hollow interior thereby maintaining the wax in the shape of the wax pattern during the solidification process; and

- removing the wax pattern from the mold cavity.

This method has the advantage that the pressurized fluid in the hollow interior of the wax pattern pushes the closed wall of the ax pattern outwardly during solidifi cation of the wax so that it fully conforms to the shape of the wall of the mold cavi ty, thereby maintaining the solidified removable wax in the shape of the pattern and at least substantially avoiding shrinkage effects at the outer surface of the wax pattern. The shrinkage of the wax that occurs during cooling is thus compensated by an increase in the volume of the hollow interior of the wax pattern and this re- suits in the wax pattern having the desired external shape as determined by the mold.

In this connection it should be noted that said step of rotating said mold may be carried out, in particular continuously, during said step of cooling. By way of ex ample, said step of rotating said mold may start before or after said step of cooling is initiated and may be conducted either during the complete cooling step or during the at least 50 to 90 % of the duration of said cooling step.

In a preferred embodiment of the invention the volume of wax introduced into the mold cavity comprises 25 to 99%, in particular 40 to 95%, especially 50 to 90% of the volume of the mold cavity and the volume of wax in the mold cavity is prefera bly selected such that an average wall thickness of the wax pattern lies in the range from 1 5mm to 20mm, excluding any thickening which may occur at locally tapered regions of the mold cavity. Average wall thicknesses of these dimensions ensure that the wax can be adequately dilated by the internal pressure during shrinkage to compensate for the shrinkage.

In this connection it should be noted that the gas present in the mold cavity may be compressed during the introduction of the wax into the mold cavity. While the wax is cooled and rotated this increase in pressure may aid in urging the wax such that it covers the complete surface of the mold cavity, thereby improving the re sultant wax pattern.

The method may further comprise the step of filling the mold cavity with a pressur- izable fluid, preferably such that during said step of cooling, the hollow interior of the solidified pattern is subjected to the pressurized fluid during the solidification process. In this way one can further ensure that the complete surface of the cavity is covered with wax during said step of cooling such that the outer surface of the wax pattern corresponds to the inner surface of said cavity.

The pressurizable fluid preferably comprises a gas such as air, CO2 or Nitrogen. The use of a gas, particularly air or a constituent of air such as oxygen or nitrogen, means there is no contamination of the environment, it also allows bottled gas to be used instead of a compressor which can be advantageous as the quantity of gas required is not necessarily large for small to medium scale production of smaller articles.

Alternatively the pressurizable fluid could comprises a liquid having a vapor phase with an adequate partial pressure at the solidification temperature of the wax.

Such a technique could be used to ensure the pressure regulation sets in automat ically avoiding the need for pressure regulators.

In a particularly preferred embodiment the mold cavity is defined in at least one part of a mold having at least first and second parts and the wax is added to the mold when this is opened into at least one of the first and second parts, optionally only partially filling a cavity in at least one of the first and second parts and sub sequently closing the first and second parts by moving the cavities of the first and second parts towards one another and subsequently pressurizing a space of the mold not filled with wax, prior to rotating the mold to line the wall of the mold cavity with liquid wax.

In this connection it should be noted that a seal, e.g. an elastic seal such as an O- ring or a seal made from an elastomer, may be present between the first and sec ond parts. The use of such seals enables the gas present in the mold cavity to re main in the mold cavity as the wax is filled into the mold cavity.

A technique such as this makes it relatively easy to fill the open mold part with wax as it can be simply poured into the open mold part which is subsequently closed by at least one further mold part to form the mold cavity. The open mold part can be selected in the design of the mold to have a volume greater than any other mold parts forming the total volume of the mold cavity in order to ensure that the desired total volume of wax can poured into one part of the mold. Also it is not es sential for the wax to be poured into the open mold part, it could be added in the form of wax granulate or pieces of solid wax and the mold can be heated to melt the wax prior to or during the rotation step. In a method of the above named kinds the wax is preferably selected to be a wax molten at a higher temperature in range from 60 to 140°C, in particular to 70 to 120 ^° C and to be solid at a lower temperature in a range from 30 to 100 ^° C, in par ticular 60 to 90°C. It is particularly preferred if the higher temperature is only slight ly higher than the lower temperature. This reduces the amount of time and energy needed to cycle the mold over a range of temperatures and speeds up the rate of production of the individual wax patterns produced from the mold.

For this purpose the temperature difference between the higher temperature and the lower temperature is selected to be less than 40°C, in particular less than 30 ^° C, preferably less than 20 ^° C and especially less than 10 ^° C.

The pressure in the hollow interior of the wax pattern is preferably selected to lie in the range from 1.02 to 5bar, i.e. pressure difference between the interior of the wax pattern and an exterior of the mold or wax pattern is 0.02 to 4 bar, in the mol ten state of the wax. It has been found that pressures of this order of magnitude are sufficient to press the wax outwardly into contact with the wall of the wax cavity during solidification with the higher pressures up to about 4bar gage pressure be ing more appropriate for waxes having a higher temperature difference between the lower and higher temperatures as defined above or having more shrinkage.

In this connection it should be noted that the pressure in the hollow interior of the wax pattern may be less after the wax has solidified than before the wax has solid ified. As the liquid wax is introduced into the mold, the gas, such as air, present in the mold is compressed due to the presence of the liquid wax, as the liquid wax is cooled such that it can solidify, the volume of the wax reduces enabling the gas, such as air, in the hollow cavity to expand and the pressure in the cavity to reduce.

The mold cavity may be heated to a temperature below a solidification tempera ture of the wax, in particular to a temperature selected in the range of 1 to 40°C, in particular 5 to 25°C, below the solidification temperature of the wax. In this way the wax can be solidified in a more controlled manner and the shrinkage effects at the surface of the wax pattern can be reduced as the wax does not automatically so lidify on contact with the surface of the mold cavity.

In this connection it should be noted that particularly complex structures it may be beneficial to heat the mold to a temperature above the solidification temperature of the wax, e.g. in a range of 0.1 to 5°C above the solidification temperature of the wax, to ensure that the liquid wax can flow into undercuts etc. possibly present in the mold.

When the mold cavity has a generally longitudinal direction it is preferably, but not necessarily, rotated about the longitudinal direction as axis. Alternatively the mold can be rotated about an axis orthogonal to the longitudinal direction. This has been found to facilitate the manufacture of the hollow wax pattern. It is also possi ble to rotate the mold about at least one further arbitrarily selected axis, particular ly if the mold cavity has a complicated shape which makes it difficult to ensure wax flows into all areas of the mold cavity.

Rather than rotating the mold about a second axis of rotation which may be desir able for wax patterns having a complex shape, one can also achieve this effect by adding a higher percentage of wax to the mold and increasing the pressure differ ence of the residual gas present in the mold such that the wax can cover the com plete inner surface of the cavity due to the increased wax volume and gas pres sure in the mold.

Generally it is considered sufficient for the mold to be rotated about one axis but it can also be rotated about at least two preferably orthogonal axes. This may be convenient if molds for a variety of diverse articles are to be produced on the same piece of production machinery as this can then be laid out for rotation about the said two axes and be generally applicable to a wide range of diverse articles.

In the method of the invention the step of cooling the wax pattern is preferably ef fected by cooling the mold, e.g. by means of fluid cooling, such as liquid cooling, said liquid optionally comprising at least one of water and a coolant or additive such as a wetting agent which ensures good heat transfer to and from the mold.

The methods of the invention preferably include the further step of forming the mold cavity to define at least one feature of shape by which the finished wax pat tern can be located in a further mold in a resin transfer molding process or inte grated into a wax tree in an investment casting process.

Such features of shape can for example include wax stubs which allow the wax pattern to be precisely located in a further mold during a resin transfer molding process. At least one of the stubs can be a hollow stub which facilitates pressuriz ing and/or venting the hollow interior of the wax pattern during its manufacture.

This the mold preferably has at least one passage for applying pressure to a liquid or gas present in the hollow interior of the wax pattern and/or for draining a liquid or gas from the hollow interior following solidification of the wax pattern.

The flowable wax that is used should be present at a temperature above the melt ing point of the wax and below a boiling point of said wax at standard tempera tures and pressures, i.e. wax may be introduced into the mold cavity in the liquid state.

The mold cavity may be evacuated to a pressure selected in the range of 0.02 to 0.95 bar, in particular to a pressure selected in the range of 0.05 to 0.5 bar prior to the introduction of the wax. Reducing the pressure in the chamber may aid select ing the pressure within the hollow cavity in a more controlled manner in depend ence on the amount of wax introduced into the mold cavity.

The wax may have a viscosity of greater than 2000 mPas for a temperature of less than 85°C and a viscosity of less than 800 mPas for a temperature greater than 105°C. Such waxes have found to be particularly stable in shape up to their melt ing point and the transition between the liquid state and the solid state takes place over comparatively small temperature ranges making said waxes more cost effec tive in their use.

One or more inserts may be positioned at one or more predefined positions at said mold and said wax is solidified around said inserts at said one or more predefined positions. The inserts may be functional elements such as elements having an outer thread, e.g. bolts, elements having inner threads, e.g. nuts, or supports for functional elements. By embedding these inserts in the outer surface of the wax pattern, these can be integrated into devices that are formed using the wax pat tern. For example if the wax pattern is used in a resin transfer molding process, the inserts can be integrated into the device by being adhesively bonded to the wall formed during the RTM process.

The present invention is also directed to a wax pattern obtainable by the method of any one of the preceding claims, the wax pattern having a hollow interior and an external surface substantially free of shrinkage effects. Such a wax pattern can be used either in an investment casting process or in a resin transfer molding pro cess.

In a wax pattern of this kind the wax pattern preferably has an average wall thick ness of the wax pattern between the outer surface of the wax pattern and the hol low interior thereof lies in the range from 1 5mm to 20mm excluding any thickening which may be present at locally tapered regions of the wax pattern.

The wax pattern may comprise one or more inserts present at an outer surface of said wax pattern at pre-definable positions. The inserts may, for example be func tional elements which can be embedded in a device formed using said wax pat tern. Devices formed using such wax patterns may for example be parts of hous ings of robots with integrated functional elements at pre-defined positions in order to mount components within the housings of said devices. The invention will now be described in more detail by way of example only and with reference to embodiments as illustrated in the accompanying drawings in which are shown:

Fig. 1 a longitudinally sectioned schematic view of a opened mold having first and second parts defining a mold cavity for the formation of a wax pattern,

Fig. 2 a schematic view of the closed mold of Fig. 1 illustrating preferred rotations thereof,

Fig. 3 a perspective view of the finished wax pattern removed from mold cavity of mold of Figs 1 and 2,

Fig. 4 a schematic sectional view of a further mold similar to Fig. 2,

Fig. 5 a longitudinally sectioned view of the hollow wax pattern of Fig. 3 showing its hollow interior and illustrating the average wall thickness thereof.

Fig. 6 an alternative design of a mold for a wax pattern in which the wax is intro duced via a port into the closed mold.

Turning now to Fig. 1 there can be seen a mold 10 for forming a wax pattern. The mold comprises upper and lower mold parts 12, 14 each of which defines in this embodiment a respective part 16, 18 of the mold cavity. The lower mold part 14 defines a larger part 18 of the volume of the mold cavity and the upper part 12 a smaller part 16 of the volume of the mold cavity. This is not essential, each mold part 12, 14 can define one half of the volume of the mold cavity or one part could define the entire volume of the mold cavity, the other part them being essentially flat at the side confronting the mold cavity. The mold can be made of a metal such as aluminum or an aluminum alloy or can be made of a fiber reinforced plastic. If necessary a release agent can be sprayed onto the wall of the mold cavity to stop the wax adhering to the wall of the mold cavity. It can be seen from Fig. 1 that the part 18 of mold cavity is partially filled with mol ten wax 20, although it could be filled with wax granulate or pieces of wax. An air space 22 is also present in the lower mold part 14 above the wax 20.

At its left hand side the mold cavity in the lower part 14 has a half cylindrical re cess 24 to form, together with a complementary half cylindrical recess 26 in the upper mold part 12, a first locating stub 28 (only shown in Figs 3 and 4) on the fin ished wax pattern. The locating stub need not necessarily be cylindrical, it could have a conically tapering shape to facilitate the removal of the wax pattern from the mold or to form a tapering vent or in-gate in a lost wax casting process. Alter natively it could have a different shape such as a rectangular shape in cross sec tion to facilitate orientation of the wax pattern in a further mold in a resin transfer molding process.

At its right hand side the mold cavity in the lower part 14 has a half cylindrical re cess 30 to form, together with a complementary half cylindrical recess 32 in the upper mold part 12, a second locating stub 34 (only shown in Figs 3 and 4) on the finished wax pattern 36. The locating stub 34 need not necessarily be cylindrical, it could have a conically tapering shape to facilitate the removal of the wax pattern from the mold or to form a tapering vent or in-gate in a lost wax casting process. Alternatively it could have a different shape such as a rectangular shape in cross section to facilitate orientation of the wax pattern in a further mold in a resin trans fer molding process. In this connection it should be noted that the wax pattern 36 does not require stubs to be provided for the subsequent handling of the wax pat tern 36.

In the design of Fig. 1 there is shown a hollow needle 38 extending from a disc 40. The disc 40 which fits in semicircular recesses 42, 44 formed in the mold parts 12 and 14 serves to close the half recesses 30 and 32 when the mold is closed and the needle 38 serves to pressurize the interior cavity 16, 18 of the mold. The disc 40 prevents the hollow needle being blown out of the closed mold when pressure is applied to it to pressurize the cavity 16, 18 of the mold. For this purpose the disc 40 is connected to a rigid tubular extension 46 to the right of the disc 40 in Fig. 1. The tubular extension 46 includes a manually operated shut-off valve 48 and a quick release coupling 50, similar to an air-line connection, which allows the tubu lar extension to be connected via a flexible line 52 and a pressure regulating valve 54 to a gas bottle 56. The flexible line 52 could also be connected to a receiver of an air compressor or the like (not shown).

Fig. 2 now shows the situation when the mold is closed by placing the upper and lower mold parts 12, 14 together at the parting face 58 and clamping them togeth er. Clamping can be effected in any desired manner, for example using cuffs (not shown) which fit around and apply pressure to the two mold parts 12, 14 or by the use of bolts (not shown) which pass through one mold part 12, 14 and engage in screw threads (also not shown) in the other mold part 14, 12 to hold the first and second mold parts 12, 14 together. Clamping of some form is necessary in order to prevent gas pressure applied vial the line 52, the quick release coupling 50, the shut-off valve 48, the tubular extension 46 and the hollow needle 38 to the mold cavity 16, 18 from forcing the mold parts 12 and 14 apart.

Once the mold has been closed the aligned cylindrical recesses 24, 26 and 30, 32 define a longitudinal axis of the mold cavity 16, 18.

In the method of the invention for making a wax pattern 36 using the mold 10 the wax 20 is present in a molten or flowable form in the mold cavity 16, 18 which has an internal shape corresponding to an outer shape of the wax pattern. The method includes basically the steps of:

- filling the mold cavity 16,18 with flowable wax and optionally with a pres- surizable fluid (see Figs. 2 and 6),

- rotating (arrow 62 in Fig. 2) the mold cavity 16, 18 about at least one axis 60 to line the wall with wax 20,

- cooling the wax 20 lining the wall of the mold cavity 16,18 to solidify the wax into said wax pattern 36 with a hollow interior 64 (only shown in Fig. 5) thereby maintaining the wax 20 in the shape of the wax pattern 36 during the solidification process and hereby at least substantially avoiding shrink age effects at the outer surface of the wax pattern 36; and - removing the solidified wax pattern 36 from the mold cavity 16, 18.

If a pressurizable fluid is added to the mold cavity then this increases the pressure within the hollow interior 64 of the wax 20 used to form the solidified wax pattern 36 aiding in urging the wax 20 towards the surface of the mold cavity 16, 18, in order to avoid shrinkage effects at the surface of the solidified wax pattern 36. The pressurizable fluid may be added if the outer surface of the wax pattern 36 has complex shapes present therein such as undercuts, recesses etc. with the pres surizable fluid then aiding in the flow of wax 20 into the regions of the wax pattern 36 having the complex shape.

There are several ways of carrying out this basic method. A first approach will now be described in detail. First of all the wax 20 can be added in liquid form into a part 18 of the mold cavity before the mold 10 is closed. In this case the mold is prefer ably preheated to a temperature corresponding to the melting point of the wax or to a temperature slightly above or below the melting point. Thereafter the mold is closed, the mold parts are clamped together and pressure is applied to the mold cavity from the gas bottle 50 via the pressure regulating valve 54, the flexible line 52, the quick release coupling 50, the opened shut-off valve 48, the tubular exten sion 46 and the hollow needle 38 so that the pressure determined by the pressure regulating valve 54 prevails in the mold cavity 16, 18. The shut-off valve 48 is then closed and the quick release coupling 50 disconnected from the flexible line 52 so that the closed mold can be rotated in the direction of the arrow 62 or in the oppo site direction to allow the molten wax to run around the wall of the mold cavity 16, 18 and wet this wall as well as into the space defined by the recesses 24, 26 and into the space defined by the recesses 30, 32 around the hollow needle 38.

During this rotation the mold 10 is cooled so that the wax lining the wall of the mold cavity 16, 18 gradually solidifies. The cooling can be done via a coolant flow ing through coolant passages in the mold or by full or partial immersion of the closed mold 10 in a liquid such as water. The fact that the closed mold 10 is dis connected from the supply of pressurized gas means that the flexible line does not hinder the rotation. The pressure prevailing in the hollow interior of the wax pattern presses the solidifying wax against the wall of the mold cavity 16, 18 and into the recesses 24, 26 and 3032 so that shrinkage effects do not occur at the surface of the wax pattern 36 but are compensated for by the pressure in the hollow interior 64 of the wax pattern 36.

The rotation of the closed mold about the longitudinal axis 60 should be carried out such that the molten wax 20 runs around the wall of the mold cavity until it is fully solidified but preferably not so fast that molten wax drips from one side of the mold cavity 16, 18 to the other. The rotation can be a continuous rotation in one direc tion or can be alternately effected in opposite directions. Important is that the mol ten wax wets the wall of the mold cavity and that excess molten wax not yet adher ing to the wall of the mold cavity runs around the mold cavity and progressively solidifies on the wax already solidifying at the cooled wall of the mold cavity. This ensures a relatively even thickness of the wax coating at the time of solidification. The rotation can be effected manually and can be supplemented by rotation or oscillation or rocking about another axis, for example an axis orthogonal to the axis 60. The rotation or movement of the closed mold to effect wetting of the wall of the mold cavity 16, 18 and a generally uniform thickness of the wall of the hol low wax pattern can also be effected by a machine or robot if desired (both not shown).

The finished wax pattern 36 is shown in a perspective view in Fig. 3 and in a longi tudinal section in Fig. 5. Fig 5 shows that the wall thickness of the finished wax pattern 36 measured generally radially from the hollow interior 64 is relatively con stant in the middle section but greater at the tapered ends.

It should be noted that the method of the present invention is generally applicable to a wide variety of wax patterns, essentially without limitation. For very intricate patterns with many undercuts or internal structures it may be necessary to use more complicated motions of the closed mold to ensure all desired spaces are filled with wax and it may be necessary to use multipart molds or sliders to ensure the finished wax pattern can be released from the mold. Another way of carrying out the basic method is to partially fill the mold cavity 16,18 with wax 20 in a solid form, for example with pieces of wax or in the form of a granulate. In this case after closing the mold it is necessary to heat the mold 10 to a higher temperature than the melting point of the wax to melt the wax and then to cool it during rotation of the mold 10. Otherwise the process is as described above.

In another embodiment the wax can be added to the mold cavity 16, 18 after the mold 10 has been closed. One way of doing this is shown in Fig. 6. A further way of doing this is shown in Fig. 4.

Fig. 4 shows a schematic sectional view of a further mold 10 similar to the mold 10 shown in Fig. 2. In Fig. 4 a hollow tube 66 is inserted from above the mold part 12 (or from any other direction) via a valve 76 and a shutter 78. The hollow tube 66 is connected to a cylinder 70 containing, in this case molten wax 20. Once the mold is closed pressure can be applied to the wax 20 via the piston 72 movable in the cylinder and the piston movement is used to control the volume of wax added to the mold cavity 16, 18. In order to bring about the rotation of the mold 10, the tube 66 is disconnected between the valve 76 and the shutter 78.

In this connection it should be noted that the supply of the wax 70 does not have to be designed in the manner described herein, but also other apparatus for the dosed provision of wax 70 can be used. For example, a gear pump can also be used.

Prior to filing the mold with the wax 20, the mold 10 can be evacuated using a vacuum pump 80, for example to pressures in the range of 0.02 to 0.95 bar. The vacuum pump is connected to the mold cavity 16, 18 via a passage 82 a valve 84 and a shutter 86. In order to bring about the rotation of the mold 10, the passage 82 is disconnected between the valve 84 and the shutter 86 in a manner similar to the cylinder 70 for the supply of wax 20. The seal 74 is provided between the mold parts 12, 14 if the mold 10 is to be evacuated, so that one can efficiently and expediently obtain the desired pres sures in comparison to if no seal were provided.

Also indicated at the surface of the mold part 14 is an insert 88 that is to be inte grated into the wax pattern 36 on solidification of the wax 20. The insert 88 is held at the mold part 14 in the cavity 18 via a support 90. The support can be a mag netic support, if the insert is magnetic, the support can be a holding apparatus or the support could be provided by a vacuum applied at the mold 10.

On use of the mold 10 shown in Fig. 4 a vacuum may be applied using the vacu um pump 80. Once the desired vacuum of e.g. 0.3 bar is achieved in the mold 10, the valve 84 and the shutter 86 are each closed. Thereafter, the mold is filled with wax 20, e.g. 80% of the volume of the wax cavity is filled with wax 20. Thereby the residual air in the mold 10 is compressed through the addition of the wax 20 in such a way that an overpressure is now achieved within the mold 10. The pres sure in the mold may now be in the range of 1.02 to 4 bar depending on the initial vacuum pressure and the amount of wax added.

Thereafter the valve 76 and the shutter 78 are each closed, the mold 10 is then rotated about the axis of rotation 62 while the mold 10 is cooled, due to the pres surized gas in the mold 10 and the rotation of the mold 10, the initially liquid wax covers the complete surface of the cavities 16, 18 in contact with the mold parts 12, 14 such that a wax pattern 36 is formed having the outer shape resembling that of the inner shape of the mold 10, with the wax pattern 36 having the hollow interior 64.

In a different variant of use of the mold 10, the vacuum pump 80 may be discon nected and the mold 10 is partially filled with wax 20, once e.g. 60 to 70% of the desired volume of wax has been filled into the mold 10, the valve 84 is closed and the remaining wax is filled, also in this case the residual air present in the cavity is compressed to a pressure e.g. in the range of 1.4 to 2.5 bar in dependence on the volume of the wax filled and the amount of air that is permitted to leave the cavity 16, 18 prior to closing the valve 84.

A further variant can now be explained with reference to Fig. 6 where the pressur- izable fluid is added to the cavity before or after the filling of the mold 10 with a wax.

In Fig 6 the hollow tube 66 is inserted into the left hand side of the recesses 24, 26 of the mold parts 12, 14 and is retained therein via a ring bead 68 engaging into a corresponding ring recess. The hollow tube 66 is connected to the cylinder 70 con taining, in this case molten wax 20. Once the mold is closed pressure can be ap plied to the wax 20 via the piston 72 movable in the cylinder and the piston move ment is used to control the volume of wax added to the mold cavity. Pressurization of the mold cavity can then take place, for example as previously described. Alt hough not shown in Fig 5 the valve 76 (see Fig. 4) can be provided in the tube 66 or at the base of the cylinder 70 and the cylinder 70 and piston 72 can be discon nected from the valve as necessary to permit the required rotation of the mold.

In another variant of this embodiment the wax 20 in the cylinder 70 can be in a solid or softened but not molten state and a higher pressure can be applied in the cylinder 70 via the piston or ram 72 to cause the wax to flow into the mold cavity 16, 18 in a solid or softened state. This is what is meant when the appended claims refer to flowable wax, i.e. the wax is solid or softened but not melted and can still be caused to flow as a result of an applied and sufficiently high pressure. Naturally the wax must be able to run around the wall of the mold cavity and the mold must have an appropriate temperature for this to happen and to be cooled during the rotation.

Another possibility could be for the wax to be aerated rather like an aerated choco late so that the cavity 16, 18 is filled with the aerated wax and the air bubbles coa lesce during melting and separate from the wax. The temperature increase during melting could then be sufficient to obtain the desired internal pressure and main tain it during solidification of the wax without the need for an external source of pressure. In theory at least the wax could be added to the cavity of the mold to gether with a liquid which has a suitable vapor pressure at the temperature of the molten and solidified wax to ensure sufficient pressurization of the mold cavity and that shrinkage effects are avoided.

Yet another possibility is to include a piston movable in a chamber forming an ex tension of the mold cavity. The piston can then be moved to either pressurize or depressurize the hollow interior of the wax pattern.

It is also entirely conceivable that the connection between the mold and the source of pressurization includes a rotatable joint which permits rotation of the closed mold without the need for disconnection from the gas supply. Many other variants of the invention are also conceivable. For example the process can be highly au tomated and the resultant wax pattern 36 may be removed from the mold 10 using a robot, which could forward the wax pattern 36 to a different process stage where it is e.g. used in an RTM process to form a device having an interior space having the shape of the wax pattern 36.

Reference numeral list:

10 mold

12 upper part of mold

14 lower part of mold

16 mold cavity in upper part of mold

18 mold cavity in lower part of mold

20 wax

22 air space

24 semi-cylindrical recess

26 semi-cylindrical recess

28 stub of wax pattern formed by 24 and 26

30 semi-cylindrical recess

32 semi-cylindrical recess

34 stub of wax pattern formed by 30 and 32

36 wax pattern

38 hollow needle

40 disc

42 semi-circular recess 44 semicircular recess

46 tubular extension

48 shut-off valve

50 quick release coupling 52 flexible line

54 pressure regulator

56 gas bottle

58 parting face

60 longitudinal axis 62 rotation about longitudinal axis

64 hollow interior of finished wax pattern 36

66 hollow tube

68 ring bead

70 cylinder 72 piston or ram

74 seal

76 valve

78 shutter

80 vacuum pump 82 passage

84 valve

86 shutter

88 insert

90 support for insert