GB2195933A - Melting and casting non- ferrous and precious metals - Google Patents

Abstract

The method includes producing a ceramic shell from a wax model and including an opening through which molten metal can be poured, joining a crucible (11) containing the metal (12) to be cast to the shell, firing the resulting structure to melt the metal, inverting the structure whilst the metal is molten so that it runs into the shell and, after cooling, obtaining the cast object from the shell. The shell can be made up from a series of coatings of ceramic material and the arrangement is such that the metal is poured directly into the hot shell without contact with the surrounding atmosphere. Where a metal such as silver is to be cast, a lump (14) of charcoal may also be enclosed in the crucible (11) to produce a reducing atmosphere. For cooling, the assembly may be placed in a bed of a fluidised material (16) such as sand or iron filings. <IMAGE>

Description

SPECIFICATION Method of casting non-ferrous and precious metals and apparatus for use in connection therewith This invention relates to a method of casting non-ferrous and precious metals and to apparatus for use in connection therewith.

A conventional method of making small castings in precious metals, such as items of jewellery, uses a lost wax process. With this lost wax process, a wax model of the object to be cast is placed inside a cottel and a mixture of plaster and silica dust is poured around the outside of the wax model and a vacuum is applied to remove any bubbles. The cottel and contents are then placed in a kiln for nine or ten hours, after which time the wax will have evaporated, leaving a bulky hardened mould into which a molten precious metal is poured. Sometimes, the mould is spun whilst the metal is being poured to make the casting. This is a very time-consuming process for less than production runs.

According to one aspect of the present invention, there is provided a method of casting non-ferrous and precious metals using a lost wax process, including the steps of producing a ceramic shell from a wax model, the shell defining within it the shape of the object to be cast and including an opening through which molten metal can be poured, joining in a sealing manner a crucible containing the metal to be cast to the shell so that the crucible is in communication with said opening, firing the resulting structure to melt the metal, inverting the structure whilst the metal is molten so that it runs into the shell, and after cooling obtaining the cast object from the shell.

According to a second aspect of the present invention, there is provided an apparatus for casting non-ferrous and precious metals using a lost wax process essentially as defined in the preceding paragraph, the apparatus including a kiln made of compacted or refractory bound self-supporting ceramic fibre construction forming at least one wall, a base and a removable lid, said at least one wall having an aperture in it through which a flame can be passed to heat the interior of the kiln and the lid of the kiln having a vent in it.

The wall, base and lid of the kiln can each be approximately 5 cm thick. The kiln can be cylindrical. A further kiln can be provided for a dewaxing stage; the wall thickness of this further kiln can be approximately 2 cm.

According to a third aspect of the present invention, there is provided a kit of parts for carrying out a method of casting non-ferrous and precious metals using a lost wax process essentially according to the first aspect of the invention, the kit of parts including a crucible capable of being joined to a said ceramic shell, a kiln according to the second aspect of the invention and at least one blow-torch. -A said further kiln can also be provided. The kit can also include ceramic slurries.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which the Figures diagrammatically illustrate successive stages of the method of casting non-ferrous and precious metals using a lost wax process according to the invention.

The present method is particularly applicable to making items of jewellery and medallions etc.

The original form of the object to be cast is moulded or sculpted from a suitable wax. Figure 1 shows such a wax object 1 from which a replica cast in a precious metal is to be made. The wax object might have been made by pouring or injecting molten wax into a silicon rubber mould which had been formed from an original item. A wax sprue or button cup 2 is also made and the object 1 is provided with a sprue 3 and the object 1 is sprued onto the cup 2 as shown in Figure 2.

Due to the relatively high porosity of the shell, vents are not normally required. The whole assembly is then washed over with a cleaning solution consisting of a solution of a non-ionic surfactant in a solvent such as isopropanol or water and the assembly then is dried briefly in warm air (25-35"C).

As illustrated in Figure 3, a coating 4 of ceramic slurry, preferably zircon slurry, containing up to 50% graphite or another form of carbon by volume is applied to the cleaned wax assembly by painting or dipping and this coating 4 is immediately stuccoed with a fine molochite or fused silica sand (30/80 molochite). Once this has been done, the assembly is dried in warm air (25-35"C) for between half an hour and one hour.

The next stage is illustrated in Figure 4, where second and third coats 5 of slurry, without graphite or carbon, are applied to the assembly, stuccoed and the assembly is again dried but with shorter drying times after each application. A coarser molochite or fused silica grit can be used. Usually three coats are sufficient for small objects to form the desired ceramic shell comprised by the dried coats of slurry.

Figure 5 shows a small box furnace or kiln 6 formed of compacted or refractory bound self-supporting ceramic fibre having at least one opening 7 through which a flame 8 of a blow torch can be projected, the kiln 6 also having a vent 9 in its top. The kiln 6 can comprise four walls and a top integral with one another and open at the bottom, the thickness of each wall and the lid being about 2 cm (3/4 inch), which gives good heat retaining and insulating properties for this part of the process. All sides of the kiln can have substantially equal dimensions or be cylindrical, typical inside volume being one to one and a half litres.

The mould comprising the wax object 1, button cup 2 and ceramic shell is heated quickly to red heat in the kiln 6 in order to expel the wax to leave just the ceramic shell mould. Bulk heat suddenly applied in this fashion ensures total wax extraction or loss without cracking of the shell. The heat can be applied from a domestically-available blowtorch.

Figure 6 shows the ceramic shell (referenced 10) and a ceramic crucible 11 containing a slug 12 of a non-ferrous or precious metal intended to form the casting. The crucible 11 can be formed of the same material as the shell 10.

It will be seen that the ceramic crucible 11 is sealed around its upper rim 11 A to the entire rim of the button cup 2 by applying with the fingers a belt 13 of thickened slurry between the two. The belt 13 is quickly dried by exposing it for a few seconds to a flame.

If the metal to be cast is silicon bronze, for example, then the bronze can be placed in the crucible 11 on its own. Silver, for example, however needs a reducing atmosphere in order to inhibit oxidation to provide a clean surface in the finished article. Accordingly, a reducing agent such as a lump of charcoal 14 is also placed in the crucible to achieve this end.

The assembly shown in Figure 6 is then placed in a box kiln 20 with the crucible 11 containing the metal 12 lowermost. The kiln 20 can comprise four walls 21 and a base 22 integral with one another and a removable lid 23, the thickness of each wall, the base and the lid each being about 5 cm. The walls may have substantially equal dimensions or the kiln may have a cylindrical form. A typical volume for the interior is about one to one and a half litres. A vent 25 is provided in the lid. The lid 23 is placed on the kiln 20 and flames 8 from up to two butane blow-torches are projected through apertures 24 in two opposed side walls of the kiln 20.-After about twelve to fifteen minutes, the metal becomes completely molten.It has been found that the areas of the apertures 24 and of the vent 25 can be critical to the heating time and it is therefore arranged that in particular the area of the vent 25 can be adjusted so as to achieve the optimum desired result. Although the kiln reaches almost 1200"C inside, it will be noted that the outside of the kiln, because of the heat insu latingproperties of the material from which it is made, remains comparatively cool and most parts of it can be handled with bare hands.

Once the metal is molten, the mould is removed from the kiln with tongs and immediately inverted (Figure 8) and placed in a cooling medium.

Figure 9 illustrates a preferred arrangement to achieve this cooling. A canister 15 contains cool dry sand 16 resting on a perforated floor 17 spaced from a base 18 of the canister. An air inlet 19 leads into the space between the floor 17 and base 18 and, as the hot mould is being placed in the sand 16, air is blown into the canister 15 through the inlet 19 by breath or blower (fan), which has the effect of fluidising the sand briefly so that it surrounds the shell 10. After the shell has lost its red heat, cooling can be accelerated if desired by quenching in water. Iron filings could be used instead of sand.

After cooling, the shell is broken away to reveal the casting. The sprue is removed and the casting is finally cleaned if necessary.

Instead of the shell shown in Figure 6, the kiln will also take a standard clay/graphite crucible. Indeed, the crucible may be a standard iron crucible with an insulating sock of ceramic material that fits around the crucible to slow down the heat loss. This can be particularly useful for when the kiln is intended to be used for melting down metal scraps for ingoting, where a small standard crucible, holding about half a kilogram of metal scrap is to be used. It will be appreciated that small amounts of metal lose the heat they need to stay fluid very quickly (due to the rapid increase in the surface area compared to volume of anything as it is scaled down).The insulating sock just discussed slows down this heat loss so that, although it takes slightly longer to melt the metal in an insulating crucible, the heat loss after removal from the kiln is slower, thus allowing more time to pour and/or work with the metal.

Experiments have shown that castings can be made with a high degree of accuracy using the present method. The present method also has the great advantage of requiring a much shorter time to achieve the same effect as the conventional method of casting precious metals. It is also a method which can easily and safely be performed in a home workshop.

It will be further appreciated that, by sealing the crucible 11 to the button cup 2, the metal is poured directly into the hot shell 10 without contact with the atmosphere. In conventional methods, the metal is melted in a separate crucible and then poured into the shell and the contact with the atmosphere cools the metal which is detrimental, particularly when dealing with such small quantities.

Claims (29)

1. A method of casting non-ferrous and precious metals using a lost wax process, including the steps of producing a ceramic shell from a wax model, the shell defining within it the shape of the object to be cast and including an opening through which molten metal can be poured, joining in a sealing manner a crucible containing the metal to be cast to the shell so that the crucible is in communication with said opening, firing the resulting structure to melt the metal, inverting the structure whilst the metal is molten so that it runs into the shell, and after cooling obtaining the cast object from the shell.

2. A method according to claim 1, wherein said wax model is made by pouring or injecting molten wax into a silicon rubber mould.

3. A method according to claim 1 or 2, wherein said shell is produced by joining said wax model onto a wax button cup, coating the assembly so produced with a ceramic slurry and drying it.

4. A method according to claim 3, wherein said ceramic slurry contains carbon, preferably in the form of graphite.

5. A method according to claim 3 or 4, wherein additional coats of ceramic slurry are applied to the dried assembly, these additional coats being without a carbon additive.

6. A method according to claim 5, wherein said slurry for said additional coats comprises molochite or fused silica grit.

7. A method according to any one of claims 3 to 6, wherein bulk heat is applied to said assembly to expel the wax thereby leaving said ceramic shell.

8. A method according to any one of the preceding claims, wherein said crucible is joined to said ceramic shell by applying jointing of thickened ceramic material which is then dried rapidly by firing.

9. A method according to any one of the preceding claims, wherein, if a reducing atmosphere is to be provided for the metal to be cast, then a reducing agent such as charcoal is placed in said crucible prior to firing.

10. A method according to any one of the preceding claims, wherein the step of firing the resulting structure to melt the metal is effected for approximately 12 to 15 minutes.

11. A method according to any one of the preceding claims, wherein the step of firing causes the temperature surrounding said resulting structure to be of the order of 1200"C.

12. A method according to any one of the preceding claims and including the step of cooling said shell after said structure has been inverted.

13. A method according to claim 12, wherein the step of cooling is achieved by placing said shell in a cooling medium.

14. A method according to claim 13, wherein said cooling medium is aerated, cool, dry sand or iron filings, the aeration causing the sand or iron filings to fluidise as said shell is placed into it so that the sand or iron filings surround(s) the shell.

15. A method according to claim 14, wherein said shell is thereafter quenched.

16. A method of casting non-ferrous and precious metals using a lost wax process, substantially as hereinbefore described with reference to the accompanying drawings.

17. An apparatus for casting non-ferrous and precious metals using a lost wax process according to any one of the preceding- claims, the apparatus including a kiln made of compacted or refractory bound self-supporting ceramic fibre construction forming at least one wall, a base and a removable lid, said at least one wall having an aperture in it through which a flame can be passed to heat the interior of the kiln and a lid of the kiln having a vent in it.

18. An apparatus according to claim 17, wherein said wall, base and lid of the kiln can each be approximately 5 cm thick.

19. An apparatus according to claim 17 or 18, wherein said kiln is cylindrical.

20. An apparatus according to claim 17 or 18, wherein said kiln is substantially rectangular.

21. An apparatus according to any one of claims 17 to 20, and comprising a further kiln for the dewaxing stage.

22. An apparatus according to claim 21, wherein the wall thickness of said further kiln is approximately 2 cm.

23. An apparatus according to any one of claims 17 to 22, wherein said vent is adjustable in area.

24. An apparatus according to any one of claims 17 to 23, wherein said kiln is provided with two of said apertures in it through which a flame can be passed, these apertures being on opposed sides of the kiln.

25. An apparatus for casting non-ferrous and precious metals according to any one of claims 17 to 24, substantially as hereinbefore described with reference to the accompanying drawings.

26. A kit of parts for carrying out a method of casting non-ferrous and precious metals using a lost wax process substantially according to the method defined in any one of claims 1 to 16, the kit of parts including a crucible capable of being joined to a ceramic shell, a kiln according to any one of claims 17 to 25 and at least one blow-torch.

27. A kit of parts according to claim 26 and comprising a further kiln.

28. A kit of parts according to claim 26 or 27 and further comprising suspended ceramic slurries.

29. A kit of parts for carrying out a method of casting non-ferrous and precious metals using a lost wax process, substantially as hereinbefore described with reference to the accompanying drawings.