Moulding process

Abstract

A process for the manufacture of an investment casting mold which comprises coating an expendable pattern in a thixotropic dip slurry, drying the coating, removing the pattern and firing the mold.

Description

This invention relates to a process and composition for the production of investment casting moulds.

In the known process of investment casting a shell is built up around an expendable pattern by dipping the pattern into a slurry of a binder and a refractory material and drying the resulting adherent coating. This operation is repeated a number of times until a shell of desired thickness is formed on the pattern. Generally there are at least five dips, usually with the application of intermediate coats of refractory powder between each dip. The expendable pattern is subsequently removed and the mould fired before use.

We have now found that by using a thixotropic slurry the moulds can be formed with a reduced number of processing steps.

Accordingly, the present invention provides a process for the manufacture of an investment casting mould comprising the step of coating a pattern with a thixotropic mixture comprising a refractory material, a binder, a liquid dispersant for the binders and a thickening agent which renders the mixture thixotropic.

A thixotropic mixture, of course, is a mixture whose viscosity is reduced when the mixture is subjected to shear conditions but which returns to substantially its original value after the shear conditions are removed. In the process of the invention therefore, it is necessary to subject the mixture to shear conditions, at least in the region of the pattern for at least a portion of the time during which the pattern is immersed in the mixture. It is also important that the rate at which the thixotropic mixture sets-up or regains its non-fluid condition after shearing is sufficiently rapid that after removal of the pattern from the mixture excessive flow-off of the mixture from the pattern does not occur.

We have found that it is convenient to employ a thixotropic mixture of such a consistency that a shear stress of at least 1000, preferably at least 3000 and more preferably at least 5000 dynes/cm.sup.2 (as measured on a Contraves Rheomat-15 rheometer fitted with a D cup and bob) is required to break the structure of the material from a solid to a liquid. The upper limit obviously will be fixed by the necessity of the material to be useful as described.

Shearing of the mixture may be effected by agitating the mixture directly, for example, by stirring the mixture or vibrating the vessel containing the mixture, and/or by moving the pattern within the mixture. Thus, for example, the pattern may be oscillated within the mixture; if desired, such movement of the pattern may be continued whilst the pattern is being withdrawn from the mixture, so that the oscillatory motion is superimposed on the withdrawing motion. It will be apparent that movement of the pattern during withdrawal from the mixture should not be so violent that the adhering mixture is displaced; preferably such displacement is avoided by careful removal of the pattern from the mixture. The frequency of vibration of the thixotropic mixture is not critical; the degree of agitation required in any particular case will depend upon the viscosity and degree of thixotropy of the mixture and the complexity of the pattern. Agitation such that the viscosity of the mixture in the immediate vicinity of the pattern is of the order of 10-25 poises, preferably about 15-20 poises is usually adequate.

It is particularly important that the thixotropic mixture be subjected to these conditions if the pattern is of a complex shape and/or the mixture is highly thixotropic since accurate reproduction of the finer details in the pattern is thereby facilitated.

It is usually desirable before applying the thixotropic mixture, to invest the pattern first with a primer coating of comparatively low viscosity, for example a conventional primer containing a refractory, binder and a dispersing liquid. The purpose of such coatings is inter alia the provision of a fine grain lining layer to the mould. To this end the primer coat often contains a particularly finely divided refractory which may be of chemically resistant material since it is this layer in particular which must withstand the chemical degradative effects of contact of molten metal with the mould.

Subsequent coat(s) of the mould are derived from the thixotropic mix according to the invention and suitable refractory materials for use therein include silica; alumina, e.g. tabular alumina and bauxite; magnesium, calcium and titanium oxides; zinc and tin oxides; magnesite; mag-chrome grog, zirconium silicate; zirconia; zircon; aluminium silicates, e.g. sillimanite, and alusite, kyanite, mullite and molochite; porcelain and china clays; carbides, e.g., silicon and tungsten carbide; nitrides, e.g., silicon and boron nitride; boron; asbestos; ferric oxide; chromium oxide; chromite; mica; aluminium phosphate and mixtures thereof.

Generally the refractory material is in the form of a powder. It is preferred to use a refractory powder at least 50% by weight of which has particle size of less than 0.15mm.

A wide variety of binders may be used, although the binder chosen should be one which will set satisfactorily in a thick layer. We have found that air-drying binders (which do not form an impermeable skin on curing) are very effective; if desired, a mixture of two or more compatible binders may be used. Examples of suitable binders include silicates, e.g., alkyl silicates, such as ethyl or isopropyl silicate, alkali metal silicates such as sodium and/or potassium silicate, silica sols, aluminium oxychloride, aluminium phosphate, gypsum/silica mixes and cements such as aluminous or Portland cement.

The liquid dispersant for the binder is preferably a solvent for the binder. The choice of a solvent clearly depends on the type of binder to be used; examples of solvents which may be used include polar solvents, for example, water; mineral acids; alcohols, e.g., aliphatic alcohols, especially methanol, ethanol, isopropanol and butanol; glycols and glycol ethers, e.g. ethylene glycol, monoethyl ether. Mixtures of two or more solvents may be used. In practice the commercially available binder is commonly in the form of a solution or suspension and the solvent of this solution may be sufficient to form the thioxtropic mixture without the addition of further solvent.

The thickening agent can be any which will give a thixotropic mixture of the desired properties. Suitable thickening agents include fumed silica, for example the fumed silica available in commerce under the trade name Cab-o-Sil, treated clays, for example highly beneficiated hydrous magnesium silicate, for example that sold under the trade name Ben-a-gel and Benaqua and the organically modified magnesium silicate clays, for example those sold under the trade name Bentone of which the grades LT; 18C; 34;38 and 27 are particularly suitable.

In some cases we have found it advantageous to include a small amount of a dispersing agent, for example diethanolamine or the thixotropic mixture.

The relative proportions of the components of the thixotropic mixture can vary over a wide range. Suitable percentages by weight of the components in the mixture may be as follows:

Refractory material 10-90% preferably 50-80% Binder (dry solids 1-15% preferably 5-15% content) Dispersant 5-50% preferably 10-30% Thickening agent 0.1-10% preferably 0.5-5% Dispersing agent 0.1-5% preferably 0.5 to 2% (optional)

As noted above, binders are commonly marketed in solution form and it may therefore be unnecessary to add separate solvent as dispersant.

The green mould may be dried before firing in a stream of air or by heating at a moderate temperature, for example up to 50.degree.C. Since a thick shell is generally formed in only one or two operations, it is usually desirable to dry slowly, e.g., for 4 hours to 3 days, at temperatures of 10.degree. to 30.degree.C.

If an expendable pattern of sufficiently low melting point, is used it may be drained in its molten state from the mould when this is quite hard. Removal of the pattern may be also achieved by shock heating the mould, e.g., to a temperature of at least 750.degree.C preferably 900.degree. to 1200.degree.C. (Such heating may be continued to cure the mould). Alternatively the mould may be treated with steam, preferably dry steam, at above atmospheric pressure, e.g., from 2 to 10 atmospheres and then fired. Firing of the mould is continued until it is quite hard- usually 5-60 minutes at about 1000.degree.C.

The invention is illustrated in the following examples in which percentages are by weight. In the examples, ACPE refers to the complex aluminium phosphate of formula AlPClC.sub.8 H.sub.25 O.sub.8 obtained by the procedure of Example 1 of UK Application No. 29862/69 (Dutch Application No. 7008594).

EXAMPLE 1

The following mixture was formulated: ACPE 6.2 gm. Water 35 ml. Molochite, having an average particle size 100 gm. less than 0.044mm Bentone LT 1.5 gm. Teepol 0.1 ml. Octanol 0.3 ml.

A similar mixture as formulated with the Bentone omitted, as a primer dip.

A wax pattern was dipped in the primer dip, a stucco coat of Molochite 60 mesh was then applied and the coating allowed to dry. The coated pattern was then dipped in the first mixture which was vibrated at 200 cycles/sec., by applying an oscilator to the base of the vessel in which the mixture was placed.

The pattern was withdrawn coated with a shell having a thickness of 0.5 cm, which was allowed to dry at 30.degree.C for 4 hours. The wax pattern was then removed by dewaxing in trichloroethylene vapour. The resulting green mould was then sufficiently strong to be handled and fired at 800.degree.C.

EXAMPLE 2

Example 1 was repeated using the following mixture for the coating:

ACPE 3.1 gm Ethyl Silicate solution in isopropanol contain- 3.2 gm. ing 25% solids based on SiO.sub.2 Water 0.8 ml. Isopropanol 41.5 ml. Molochite having an average particle size 100 gm. less than 0.044 mm Bentone 27 3 gm. Toluene 9 gm. Teepol 0.1 ml.

A primer dip consisting of the aforementioned mixture minus Bentone 27 and toluene was applied in Example 1.

The wax pattern was removed by flash dewaxing at 1000.degree.C for about 30 seconds. The resulting mould was strong and suitable for casting after firing.

EXAMPLE 3-7

The following mixtures were formulated:

Silica sol N 1030 1 litre (Ex-Nalfloc) Molochite 120 (Ex-English China 2000 gm. Clays) Benagel EW 30 gm. Water 200 ml. Lissapol NDB 20 ml. Octanol 40 ml.

EXAMPLE 4

The composition as Example 3 was prepared except that the Molochite used was Molochite 200.

EXAMPLE 5

The composition of Example 3 was prepared except that Benaqua was used instead of Benagel.

EXAMPLE 6

The composition was as Example 1, with the addition of 35 ml. of diethanolamine.

EXAMPLE 7

A composition was formulated as follows:

ACPE (80 w/w aqueous 100 ml solution) Water 500 ml Benagel EW 40 gm Octanol 20 ml Molochite 120 1000 gm Lissapol NDB 10 ml

The above compositions 3 to 7, were used in a similar manner to those of Examples 1 and 2, after an initial coating of a primer coat to the pattern.

Extra thick moulds were obtained by employing a second immersion in the thixotropic mix to build up the coating.

In preparing the compositions it is usual to mix all the inepedients and then to subject the mixture to high shear stirring for, say, 5 minutes or low shear stirring for, say 20 minutes until a smooth creamy consistence is attained. These slurries we have found are usually immediately usable in the process of the invention, although their full non-sheared consistency was not reached until about 12 hours after mixing.

Claims

What we claim is:

1. In a process for the manufacture of an investment casting mould which comprises the steps of coating an expendable pattern by immersion in a fluid mixture comprising a refractory material, a binder, and a liquid dispersant; drying the coating; removing the pattern and firing the mold; the improvement comprising adding to said mixture a thickening agent in an amount sufficient to render the mixture thixotropic, and requiring a shear stress of at least 1000 dynes/cm.sup.2 to break its structure from a solid to liquid.

2. A process according to claim 1 in which the thixotropic mixture requires a shear stress of at least 3000 dynes/cm.sup.2 to break its structure from a solid to liquid.

3. A process according to any of claim 1, in which the thixotropic mixture is subjected to shear conditions at least in the region of the pattern for a least a portion of the time for which the pattern is immersed in the mixture.

4. A process according to claim 3 in which the mixture is subjected to shear such that its viscosity in the immediate vicinity of the pattern is within the range 10-25 poises.

5. A process as claimed in claim 1 wherein the pattern is coated with a primer coating before application of the thixotropic mixture.

6. A process as claimed in claim 1 wherein a single coating of the thixotropic mixture is applied.

7. A process as claimed in claim 1 wherein the thixotropic mixture contains a treated clay as thickening agent.

8. A process as claimed in claim 7 wherein the treated clay is a modified magnesium silicate clay.

9. A process as claimed in claim 1 wherein the thixotropic mixture contains 0.1 to 10% by weight of the thickening agent.

10. An investment casting mould produced by the process as claimed in claim 1.