GB2234926A - Casting into a gas-permeable mould - Google Patents

Abstract

A method of casting a metal article comprises the steps of forming a shape to be moulded in particulate moulding material. A reduced pressure is applied to the moulding material, and molten metal is then drawn by the reduced pressure into the shape so as to form the article in metal. The shape may be provided by embedding a sacrificial former (e.g. foam pattern) in moulding sand; alternatively, the shape may be provided by a shell mould.

Description

CASTING OF METALS This invention relates to the casting of metal articles.

Conventionally casting methods involve the creation of a simple or complex cavity shape formed in a refractory material such as silica, zircon, chromite olivine or other sands or in metal shot or grit. Some methods known as die casting use a metal mould in which the cavity has been formed usually (but not exclusively) in a higher melting point metal than that being used to produce the casting.

One method of producing a cavity involves the use of a foamed plastic pattern, for example expanded polystyrene or Polymethyl methacrylate in the shape of the final casting required.

The plastic foam is coated with a refractory paint and buried in either a type of sand eg, silica sand although sometimes metal shot is preferred. Molten metal is then poured onto the metal entry point.

The heat from the metal burns or vaporises the low density foam and reproduces faithfully the shape of the foamed pattern.

In an alternative method, the polystyrene pattern is treated in the same manner as a lost wax pattern in the well known investment casting process. The expanded polystyrene is moulded with a metal entry system, known as a sprue, runner and ingate alternatively the metal entry system is formed separately and glue assembled to the pattern. The completed replica (or multiple tree of replicas) is then coated in an aqueous or alcohol based ceramic slurry of a suitable fine refractory filler with, for example, an average grain size of 0.075 millimetres. A coarser granular refractory material such as zircon sand with an average grain size of 0.02 millimetres may be added to the slurry coated pattern while the slurry is uncured to cause adhesion when cured in air by chemical means.

The coated pattern is further coated with the ceramic slurry followed by coarser particular lower refractory material such as Molochite. The layering process is repeated until a suitable thickness of shell is obtained. This may involve several further coats of coarse Molochite. When the desired thickness is obtained the coated pattern is placed in a furnace, eg. at 1000 degrees or more.

The result of this firing is that the expanded polystyrene is vaporised and completely removed while the ceramic shell is fused together to form a hard but permeable shell the inside of which faithfully reproduces the shape and dimensions of the original foamed plastic pattern.

To produce the casting the ceramic shell is surrounded in a refractory sand or metal shot to give the ceramic shell support, metal is poured into the shell to form the casting, and the metal is allowed to solidify.

In both casting methods described it has been established that it may be desirable to apply a reduced pressure in the sand or metal shot bed. In both methods the reduced pressure has the advantage of removing the air displaced by the incoming molten metal. In the case where the metal is poured onto a foam pattern the reduced pressure additionally removes the fumes and decomposition products from the vaporised or burnt foam pattern.

It has been established that it is undesirable for metals to be transferred to the casting cavity in a turbulent stream, such as may occur from an insulated ladle or a furnace. This undesirable method of metal transfer is generally used with casting moulds having the metal entry point to the cavity at the top of the mould since the metal stream becomes turbulent as it is tipped through the air into the metal entry system and subsequent mould cavity.

One proposal to overcome this turbulent casting method is to introduce the metal to the mould cavity via a metal entry system which is formed in the base of the mould. The method involves the mechanical or electromagnetic pumping of the molten metal upwardly into the mould, normally situated above the furnace. As yet such methods have been restricted to the lower melting point metals such as aluminium because of the limitations of the refractory parts of such a pump could not deal with the temperatures required for casting ferrous metals such as iron and steels.

Furthermore, the method of pouring or pumping molten metal onto a combustible pattern as yet can only be used in non-critical components because of the risk of internal inclusions in the resultant casting from for example the decomposition products of the combustible pattern. These products of the pattern arise due to the irregular flow pattern of the metal onto the pattern. The pattern decomposes to gaseous products but if the products of combustion are not given time to decompose then for example and expanded polystyrene pattern may turn to carbon in the form of soot.

According to the present invention there is provided a method of casting a metal article comprising the steps of forming a shape to be moulded in particulate moulding material, applying a reduced pressure to the moulding material, and causing molten metal to be drawn by said reduced pressure into said shape to form the article in metal, and allowing the molten metal to solidify to form the cast article.

The method of the invention may be used for moulding operations where the pattern is provided by a permeable, particulate shell mould but is more particularly applicable to the use of solid sacrificial formers (eg. of a foam material) which may be destroyed by the molten metal. It is however also possible to effect destruction of such a sacrificial former in the particulate material (by the application of heat) before the introduction of the molten metal.

The present invention has the advantage that the molten metal is drawn into the shape to be moulded under the effect of the reduced pressure. As the metal enters the mould shape, any gaseous decomposition products formed (eg. by destruction of the material of a sacrificial former) will tend to provide an increase in pressure which will reduce the rate at which the metal is being "sucked in".

This rate may therefore be automatically controlled by the rate of pattern decomposition, allowing the pattern to decompose fully to the gaseous state avoiding solid carbon decomposition products. It is also possible to adjust the degree of reduced pressure being applied at any particular stage of the casting process to ensure the correct rate of decomposition of the pattern.

The fact that the molten metal does not need to be mechanically or electromagnetically pumped means that a wide variety of metals with different melting points may be cast, eg. aluminium, cast iron, steel.

It is also possible to assist passage of the molten metal into the mould shape. For example, it is possible to apply (at least during some stage of the casting process) a force on the molten metal either mechanically or by gas pressure to maintain the metal in the casting cavity. The degree of this force may also vary the rate at which the molten metal is introduced into the cavity, thereby controlling decomposition of a pattern.

Alternatively, the casting cavity may be connected to the molten metal source by a flexible tube and (at some stage during the casting operation) lowered below the level of the molten metal so as to cause this metal to siphon into the mould.

The method of the invention is particularly effective in the case where the shape to be moulded is provided in a binder free particulate material.

In a particularly preferred embodiment of the invention, the casting method comprises the steps of embedding a shape (preferably a solid sacrificial former) into a particulate binder free refractory material so as to define therein the shape to be cast. The particulate material is covered by a membrane (preferably a flexible membrane) and the arrangement is then inverted and reduced pressure applied to the particulate material. This reduced pressure may be applied before or after the inversion. Communication is the established between the shape to be cast and a lower source of molten metal. This communication may be effected by means of a tube which passes through the membrane. Molten metal is sucked upwardly into the shape to form the article being cast.

The method described in the preceding paragraph has advantages compared to the use of a foamed pattern in a binder free particulate material in a situation where the metal entry point is in the base of the mould. In such a prior arrangement, a seal has to be effected to prevent particulate material leakage, particularly during a vibration step to compact the particulate material. This disadvantage is avoided in the method of the preceding paragraph since (during such a vibration stage) the metal entry point may be at the "top" of the mould, and it is only after the inversion that the metal entry point becomes lowermost.

The invention is described by way of example, with reference to the accompanying drawings, wherein: Figure 1 is a diagrammatic cross-sectional view through part of an apparatus for performing the method of the present invention.

Referring to Figure 1, the mould is prepared by taking a combustible pattern 1, made from a low density foam such as expanded polystyrene or polymethyl methacrylate complete with metal entry system 2, formed in the same material and coating the pattern in a ceramic slurry, having a typical particle size of 0.75 millimetres. The point at which the metal enters onto the pattern is not coated. After suitable drying the pattern is placed in a container 3, having one or more permeable panels 4 connected to a pumping system 5 capable of a high volume air flow typically 12 cubic meters per minute and a potential vacuum of 300 torr. The container is partly filled with a refractory material 6, for example sand or zircon or silica sand.The coated combustible pattern is placed on the sand bed in the container and the container is completely filled with the refractory particulate material (sand).

The complete container is subjected to a vibration which has a typical frequency of 3000 cycles per second and an amplitude suitable to cause an acceleration of below the acceleration due to gravity, although in some circumstances the box may be required to be vibrated with an acceleration above that caused by gravity. The vibration is applied for typically 5 seconds to obtain the highest bulk density of the particular material.

A thin impermeable sheet of material 7 is used to cover the top of the container, for example Ethylene Vinyl Acetate having a typical thickness of 0.05mm. The container is then subjected to a reduced pressure typically 300 torr causing the film to seal the flow of air into the bed of sand in the container. The container is not fully sealed but typically the leak rate of air into the box should be less than one tenth of the pumping rate of the system.

The complete container is then turned through 180 degrees while maintaining the reduced pressure, making the metal entry point at the bottom of the container 3.

The container is then conveyed to a position above the vessel holding the molten metal 8. A connecting refractory tube 9 is placed into the molten metal and the other end is introduced into the metal entry system where a small piece of the film has been removed.

The air in the refractory tube 9 is pumped away by the pumping system drawing air up into the tube through the pattern and the sand bed into the chamber through the permeable wall.

The reduced pressure causes the metal to rise in the tube, the radiant heat burning or vaporising the combustible pattern and filling the volume which was originally occupied by the pattern 1. The metal is allowed to solidify in the mould while that in the connecting refractory tube is kept molten for example by thermal insulation or by an induced electric field from an inductive coil 10.

When sufficient time has passed for the casting to solidify the vacuum is removed and the casting and sand fall out of the container 3 under the influence of gravity. In the case of large section castings the cooling time may be long and the container 3 may be required to be tipped back through 180 degrees to enable the pumping system to be disconnected for the extended cooling period.

In this present example the casting cavity is formed from a combustible pattern which is destroyed by the heat of the incoming metal. However the process may equally be performed by partly or fully removing the pattern before sucking in the molten metal. This process is again more easily accomplished using a flame 11 situated adjacent to the metal entry system 2, Figure 2. The casting cavity remains stable in its shape due to the differential pressure either side of the coating originally used on the foam pattern.

The process described may be equally performed using a permeable particular ceramic shell mould, backed with the same binder free particular material.

In the example given a single casting cavity has been used but more than one casting cavity may be used from one or more metal entry systems.

Claims (13)

1. A method of casting a metal article comprising the steps of forming a shape to be moulded in particulate moulding material, applying a reduced pressure to the moulding material, and causing molten metal to be drawn by said reduced pressure into said shape to form the article in metal, and allowing the molten metal to solidify to form the cast article.

2. A method as claimed in claim 1 wherein the shape is defined by sacrificial former which is destroyed by said molten metal.

3. A method as claimed in claim 1 wherein the shape is defined by a heat destructible former which is at least partially destroyed by a source of heat prior to the introduction of the molten metal.

4. A method as claimed in claim 1 wherein the shape is defined by a permeable, particulate shell mould.

5. A method as claimed in any one of claims 1 to 4 wherein the molten metal is sucked upwardly into said shape.

6. A method as claimed in any one of claims 1 to 5 wherein the particulate material is binder free.

7. A method as claimed in claim 1 comprising the steps of embedding a shape into a particulate binder free refractory material so as to define therein a shape to be cast, covering the particulate material with a membrane (eg. a flexible membrane), inverting material/former assembly, applying (before or after the inversion) the reduced pressure to the particulate material, effecting communication between the shape to be cast and a lower source of molten metal, causing molten metal to be sucked upwardly into said shape to form the article being cast, and solidifying the molten metal.

8. A method as claimed in any one of claims 1 to 7 wherein a force is additionally applied to the molten metal to maintain the metal in the casting cavity.

9. A method as claimed in claim 8 wherein the force applied to the molten metal is a gas pressure.

10. A method as claimed in claim 1 wherein the casting cavity is lowered below the level of the source of molten metal to cause the metal to siphon into the mould during or after casting to maintain the casting cavity full of molten metal.

11. A method as claimed in any one of the preceding claims wherein the particulate moulding material is a refractory sand.

12. A method as claimed in claim 11 wherein the refractory sand is selected from zircon, silica, chromite, quartz.

13. A method as claimed in any one of claims 1 to 10 wherein the particulate moulding material is a metal shot.