GB2404354A - A rapid casting method - Google Patents

Abstract

A method of casting low volume metal or alloy components comprises the steps of 1) making a full-size model 2 of the component, 2) using the model as a master pattern, 3)create at least a two piece mould 4 and 14, 4) inject molten wax 24 into the mould to create a pattern, 5) casting a component using the pattern in a lost wax casting process and then repeating the steps to cast multiple components. The mould may be formed of a fibre-reinforced composite material and the full-sized model may be formed using a stereolithography process.

Description

XA1 814GB - 1

RAPID CASTING

This invention relates to methods of casting, in particular to the casting of low-volume batches of metal or alloy components.

Low-volume cast components are typically required where the design of the component has not been finalised (i.e. in prototype production), or where a relatively small number of products incorporating the components are involved, such as in the manufacture or modification of specialised products, such as in the aerospace and defence industries, or in the automotive industry for the production of prototypes, or of low volume (and usually high quality) cars.

These low-volume components may involve a run of as few as ten or twenty components or up to as many as a few thousand components.

Conventionally there are two methods for producing low volumes of cast components. In the "old style" method a model of the component is produced (usually by traditional mechanical engineering processes), and this model is analysed in order to design a metal mould; this mould is then created from a solid billet of refractory metal, using traditional machining techniques. The completed mould is then used to create wax patterns which are used in the lost wax" casting process to cast the total number of components required. This "old style" method is capable of producing components to high standards of dimensional accuracy, but is expensive and time-consuming because of the time required to manufacture moulds (lead times of up to 18 months are typical in practice). Because of the long time required for the "old style" process, it has the added disadvantage of design inflexibility; where the component is found to require re-design or modification, because of the length of time required to s create a new mould, designers are reluctant to and/or dissuaded from changing the component design, and instead may consider re-designing other adjacent components and thereby compromise the design integrity and/or the functionality of the eventual product.

The second conventional method of producing low volume runs of cast components is the so-called "quickcast" process. In this process, a fullsize model of the component is created using stereolithography; the resulting XA1814GB - 2 "stereolith" is then used in a manner similar to the well-known "lost wax" casting process - the stereolith is encased in a matrix of refractory material, such as hard-packed sand and molten metal/alloy is poured into a sprue leading through the sand to the stereolith. The molten metal melts or burns off the stereolith, so that the metal takes up the shape of the stereolith inside the matrix or mould of refractory material (with the residual stereolith material escaping by way of the sprue).

The "quickcast" method is expensive, because it requires one stereolith for every component to be cast because the stereolith itself is sacrificial.

Nevertheless, the "quickcast" method is often preferred because it is much quicker than the "old style" process, provided that only a small number of castings is required.

Accordingly, the present invention provides a method of producing cast metal or alloy components comprising the successive steps of: a) making a substantially full-size model of the components to be produced; b) creating a mould of at least two pieces using the model as a master pattern; c) injecting molten wax into the mould to create a wax pattern; so d) casting a component using the wax pattern in a "lost wax" casting process; e) iterating steps c) and d) to cast multiple components.

Such a method has been shown to produce more accurately dimensioned castings with good surface finish and more quickly than as conventional methods (typically the entire process takes no more than a week, whilst minor modifications to components can be carried out in a matter of days) and also cheaper (significantly so compared to the "old style" method).

Moreover, because each step of the method is a fairly mature technology in itself, the overall process is reliable and does not require highly skilled so operations.

XA1 814GB - 3 Advantageously the mould is formed of a suitable fibrereinforced composite material. Such materials have been found to be capable of producing several thousand castings of metals/alloys used in the aerospace industry without deterioration in the quality of the casting produced. Preferably the mould is formed in accordance with the teachings of WO 01/91984 and WO 00/30784.

Conveniently, the creating of the mould incorporates the design steps of establishing the optimum number of pieces of which the mould should consist, and of establishing the parting planes between these pieces. In many cases it will be possible to make the mould in two pieces, so that there is only a single parting plane between the two halves of the mould. In such cases, the model would be placed on the surface of the viscous mould material and pressed down into it. The mould material would then be solidified and the process either repeated with the model inverted and pressed into a second quantity of viscous mould material, or the surface of the mould material is covered with a release coating and then the protruding part of the model covered with viscous mouid material, the second quantity of mould material then being solidified to produce the two mould halves.

In order to allow molten wax to be injected into the mould, preferably one or more sprues are provided; this can easily done by machining or drilling into the mould so as to communicate between the mould cavity and the outside of the mould.

The full-size model may conveniently be formed by stereolithography.

As we describe in our WO 01/14127, stereolithography involves exposing a photosensitive polymer resin to UV light so that the resin cures into a solid object. To achieve this, a laser is directed at the resin surface, and where the laser beam touches the resin, it cures to approximately 0.1 - 0.2 mm deep. A thin layer of liquid resin is applied to the surface of the cured resin and this is then cured using the laser. In this manner, layers of cured resin can be built up into a desired solid object. The laser is controlled by a computer system which is fed CAD data representing the object to be produced. The laser is controlled )<A1 81 4GB - 4 such that it only operates at the locations indicated by the CAD data to produce the desired object, the full size model.

Curing whole layers of resin to form a completely solid object is time consuming and expensive. Alternatively it is possible to cure only a part of one layer of the object, which results in the object having a honeycomb structure.

This method is less time consuming and is cheaper, but the object will shatter if drilled.

Both stereolithographic techniques described above may be combined to produce an object incorporating both solid and honeycomb structures, with no discernible bond line between these structures. This produces a full size model with good strength characteristics which can be machined in the solid region if necessary, having a solid skin and a honeycomb interior, and which can be formed relatively quickly and cheaply.

It may be desirable to iterate step b) in order to create multiple moulds.

In this way a single stereolithographic model can be used to make several moulds, and thus increase the maximum number of castings capable of being produced, although this is clearly subject to the robustness of the stereolith.

The invention will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a schematic crosssectional view of a part of a mould making process forming a part of the present invention, and Figure 2 is a schematic cross-sectional view of the mould shown in Figure 1 being used to create a wax pattern.

Figure 1 illustrates a later stage in the process of forming a mould in accordance with the invention. A stereolithographic model 2 (for ease of illustration the model shown is a sphere) sits upon a pourable mould material which is cured to form a hardened first mould portion 4. The model 2 and mould portion 4 are enclosed within a cylindrical retaining collar 6 and a bottom plate 8. The bottom plate 8 forms the base of a rigid container and is adapted to be fastened to the lower plate of a wax injection machine (not shown).

XA1 81 4GB - 5 The upper surface 10 of the mould portion not in contact with the model is to be the interface between mould portions when the complete mould comes to be assembled, and this surface is coated with a non-stick material as described below. Clearly when the mould is for forming a simple spherical shape the position of the interface is easily evaluated, however the design of a mould for complex shapes is more difficult. Moulds of more than two parts may be required, and it may be necessary to design mould inserts, etc. as is known in the art - this process per se is not inventive.

In Figure 1 it can be seen that a pourable mould material 12, which consists of a curable resin pre-mixed with a curative that results in hardening to a solid flexible material, is being poured into the collar 6 above the first mould portion 4 and the model 2. This material is poured in sufficient quantity to completely cover the model 2, and eventually hardens to form a second mould portion 14 (see Figure 2). Because the interface 10 has been coated with a non-stick material (not shown), the two mould portions 4, 14 do not adhere.

Accordingly the two hardened mould portions can be split quite easily to remove the model 2 (not shown).

Referring now to Figure 2, the mould portions 4, 14 have been taken apart, the model removed, and re-assembled within collar 6. A top plate 16 has now been fixed to the collar to hold the assembly together. A sprue 18 has been drilled through the mould portion 14 to the mould cavity 20 and communicates with a hole 22 in the top plate 16; this is for the injection of molten wax 24, which fills the mould cavity 20, hardens in the shape of the model 2 and can then be ready for use in a "lost wax" coasting process to produce a component.

The entire process according to the invention therefore comprises the steps of: forming a stereolithographic model of the component to be cast, using the techniques described in our earlier international application, WO 01/14127; using the model as a master pattern to create a mould, following the processes of WO 00/30784 and WO 01/91984; injecting molten wax into the mould to create a wax pattern, and using the wax pattern in a "lost wax" casting process XA1 81 4GB - 6 to produce a cast metal or alloy component. The latter steps of the process are repeated to produce multiple castings.

Claims (7)

1. XA1 81 4GB / Claims 1. A method of producing cast metal or alloy

   components comprising the successive steps of: a) making a substantially full-size model of the components to be produced; b) creating a mould of at least two pieces using the model as a master pattern; c) injecting molten wax into the mould to create a wax pattern; d) casting a component using the wax pattern in a "lost wax" casting process; e) iterating steps c) and d) to cast multiple components.
2. 2. A method according to Claim 1 wherein the mould is formed of a fibre reinforced composite material.
3. 3. A method according to Claim 1 or 2 wherein steps b) comprises establishing the optimum number of pieces of which the mould must consist and establishing the parting planes there between.
4. 4. A method according to Claim 1, 2 or 3, wherein step b) comprises creating at least one sprue in the mould.
5. 5. A method according to any preceding Claim wherein the full-size model go is created using a stereolithography process.
6. 6. A method according to any preceding Claim comprising iterating step b) in order to create multiple moulds.
7. 7. A method of producing cast metal or alloy components in low volume runs substantially as hereinbefore described.