US2806269A - Moulds for precision casting - Google Patents

Description

amazes MOULDS FOR PRECISION CASTING Adam Dunlap, Gosforth, Neweast-le-upon-Tyne, England, assignor to Rolls-Royce limited, Derby, England, a British company No Drawing. Application August 24, 1953,, Serial No.. 376,243

Claims. priority, application Great Britain September 11, 1952 8 Claims. (l. 22-496) In what has now become recognised as the conventional method of making moulds for investment casting using an expendable pattern such as wax or plastic, the mould is produced by a two stage process. The first involves a relatively thin coating of the pattern assembly by spraying, painting or dipping and the second the final investment of the coated pattern in a suitable container by a slurry or refractory suspended in a liquidbinding medium, the final consolidation of the mould being obtained by vibration.

When the liquid binding medium has hardened by chemical action, the expendable pattern is melted out by heating at 100150 C. for several hours. Following this, the mould is gradually raised to a temperature in the range 850-1100 C. to remove completely the last trace of wax and volatile constituent of the binder medium. This requires several hours heating.

The process of consolidating the slurry by vibrating requires from 1 to 4 hours, thus where a high mould output is required, many vibrating tables are necessary. 8 to 10 hours are generally required for both the removal of the main bulk of the expendable pattern and for the heating to the high temperature. Again where a large output is being handled, large ovens and mould firing furnaces are needed.

I am aware that there has been previously described a process for precision casting by the lost wax. process in which a wax pattern of the object to be cast is provided with an initial refractory coating conveniently applied by spraying, and then with a multiplicity of superimposed dip coatings such as to result in the pattern being invested with a substantial shell of refractory material namely of the order of from A inch to /2 inch thick (according to the size and shape of the wax pattern): after formation of the shell investment it is contemplated to remove the wax pattern, preferably by the use of a suitable hot solvent, and then to complete the forming of the mould by surrounding the shell with a supporting packing of dry sand.

According to this invention a multi-layer shell mould is formed around an expendable pattern by a process which includes dipping alternately in an acid and an alkaline solution of a sodium silicate, and including a finely ground refractory material suspended in the solution, the normality of the alkaline solution being greater than that of the acid solution, each successive, single dipping being followed by the application of a stucco material to the Wet mould.

By this means a sufiiciently rigid shell can be. formed around the pattern to form a mould which can be used with an investment which is much simpler, cheaper and more quickly provided than is the conventional method and will have other advantages as below stated.

The normality of the alkali solution must be considerably greater than the normality of the acid solution to ensure rapid gelling when the two are brought together on the shell. I prefer to keep the ratio about 2-311.

ited States Patent Preferably before the application of the layers of, the shell proper the pattern is given an initial refractory coating bydipping in or spraying with a; solution of the type used in the conventionalmethod.

The shell mould may be built: up as follows: An initial coat is formed on the pattern by spraying with or dip.- ping in a refractory solution ofknown composition. Suitable solutions to form the initial coat are as follows;

(a) Applied by spraying.

(1) Soap solution (1%) ml' 3-20 Sodium silicate ml I60 Hydrochloric acid, 10% ml 36 Flint powder "gm" 1000 (Z) Ethyl silicate containing piperidineuml u 160 Methylated spirits ml-' 320 Flint-powder gn1' 1000 (3) Ethyl silicate ml 70 Methylatedspirits ml Q5 Water m- 5 Flint powder gm... I000 (b) Applied by dipping (1)- Water ml 2160 Sodium. silicate ..mL.. 1800 Glycerine ml' 600 Hydrochloric acid, 10% ml 650 Ammonium alginate, 10% sol m1. 150 Octyl alcohol ml.. 5: Sulphucrex (wetting agent) ml'. Flint powder lb 23 (2) Water ml.- 4530 Gelatine gm 40' Gl-ycerine ml 350 Sodium silicate ml 2000 Hydrochloric acid, 10% ml 950 Octyl alcohol ..ml Sulphucrex (sodium lauryl sulphate). ml... L50 Flint powder lb 32 (3) Ethyl silicate mI 1350 Methylated. spirits ml 3.720 Hydrochloric: acid, 10% m1- 250 Water ml 80 Flint powder lb* 23 Sodium silicate ..ml 790 Water ml 0: Concentrated nitric acid ml 260 Flint powder lb 10.-

This; mixture. is 0:44 normal acid and. is made by mix ing some of the water and acid. and pouring the diluted.

silicate slowly into the acid solution with vigorous stir ring to prevent: it gelling Instead of nitric acid. other mineral acids: such as hydrochloric and'. sulphuric: could beused.

A suitable alkali suspension is the following:

Sodium solicate m1 790 Water ml 17"I0' Flint powder." lbs 10 This mixture is 0.88' normal alkali;

In the acid and alkali suspension mentioned above the flint powder may be replaced partly or wholly by other refractories such as finelyground alumina, sillimanite, zirconia, zircon and the like. a 1

The silicate content of both solutions may be the same and with a grade of sodium silicate commonly used the final silica content of each dipcoat liquid will be about 7.5 percent, but the silica content of these solutions can be varied and useful results have been obtained in a range from 3 to percent. When an adjustment to the silica content is made, it is done in such a way that the first solution is always acid and the second always alkaline. The acid normality of the former should be in the range of O.2 to Q.85 normal andthe alkaline normality of the second should be in the range 0.4 to 1.7 normal, to preserve the desired ratio of 1;23.

Each layer or coat of the shell is produced by one dip in each separate solution, each dip being followed by the "stuccoing operation.

Any time up to about one hour may elapse between the dips in the acid and alkali solutions as'long as the former has not dried because it must be wet in order that the gelling reaction will follow. The dipping may, however, take place without any interval of time at all and I prefer that that interval be not more than two or three minutes.

. The second and succeeding clips in acid solution can be made at any convenient interval after the preceding alkali dip, from A to 3 hours being generally most suitable.

A variety of refractory materials can be used for the stuccoing operation such as silica, sillimanite, firebrick grog (crushed firebrick), zirconia, zircon and the like. I prefer firebrick grog.

The grain size of the refractory used is important and the following range is suitable:

Percent retained by weight Sieve size each followed by the appropriate stuccoing," until a.

shell about /1 inch thick is produced. (This normally requires 3 or 4 clippings in each solution.) The expendable pattern is now removed by causing it to liquify (by melting or preferably by a suitable solvent).

The shell mould with pattern removed is next placed in a suitable box and invested with dry sand, care being taken to prevent any loose sand from entering the shell mould.

The dry sand will be a sufficient backing up of the shell for normal casting. If, however, pressure casting is to be used a firmer support may be required and in that case the same may be caused to set by incorporating in it a fritting material. Various fritting materials may be used such as anhydrous sodium silicate, sodium nitrate, sodium silico-fluoride. These frit at different temperatures and one should be chosen which will frit at the temperature at which the mould is to be heated.

When the shell has been suitably invested the metal may be poured in (or forced in) and the casting completed.

The advantages of this invention are in the saving of time and expense in comparison with the conventional method, and in the production of better castings. The materials employed are much cheaper thanin the conventional method. The vibrating equipment necessary in that method to consolidate the investment material is unnecessary. If the pattern is to be removed by heating it, the heating time required is much less (of the order of one hour or less as compared with 8 to 10 hours for the conventional method.) Alternatively if the pattern is removed by a solvent the time for removal is reduced owing to the rapid permeation of the solvent through the wall of the shell. Finally the shell is much more permeable to air than is the mould backed by a solid investment. Air can therefore escape from the mould through the shell when casting is taking place and this enables thinner sections to be cast and also avoids faulty castings due to air being trapped in the mould.

I claim:

1. The method of forming a multi-layer shell mould around an expendable pattern which includes preparing two solutions of sodium silicate each containing a finely ground refractory material suspended therein and one being alkaline and the other having an acid material added to make the same acid but of a normality less than the normality of the alkaline solution, and then applying to the pattern alternately the acid and the alkaline solutions and stuccoing a finely ground refractory material to the wet shell after each application, said alkaline solution always being applied While the shell is still wet with said acid solution.

2. The method of forming a multi-layer shell mould around an expendable pattern which includes preparing two solutions of sodium silicate each containing a finely ground refractory material suspended therein and one being alkaline and the other having an acid material added to make the same acid but of a normality less than the normality of the alkaline solution forming an initial refractory coat on the pattern according to the conventional practice of precision casting, then applying to the coated pattern alternately the acid and the alkaline solutions and stuccoing a finely ground refractory material to the wet shell after each application, said alkaline solution always being applied while the shell is still wet with said acid solution.

3. The method of forming a multi-layer shell mould around an expendable pattern which includes preparing two solutions of sodium silicate of a silicate content ranging from 3% to 15% each containing a finely ground refractory material suspended therein and one being alkaline with an alkaline normality ranging from 0.4 to 1.7 normal and the other having an acid material added to make the same acid with an acid normality ranging from 0.2 to 0.85 normal but of a normality less than the normality of the alkaline solution forming an initial refractory coat on the pattern according to the conventional practice of precision casting, then applying to the coated pattern alternately the acid and the alkaline solutions and exposing the Wet shell after each application to a shower of finely ground refractory material said alkaline solution always being applied while the shell is still Wet with 1 said acid solution.

4. The method of claim 3 wherein ground flint powder is the refractory material suspended in said solutions.

5. The method of forming a multi-layer shell mould for precisioncasting which comprises: forming an initial refractory coat around an expendable pattern; then alternately applying to the coated pattern an alkaline and an acid solution of sodium silicate, each solution having a finely ground refractory material suspended therein, said acid solution being of an acid normality less than the alkaline normality of said alkaline solution, said alkaline solution always being applied while tnc shell is still wet with said acid solution; stuccoing a finely ground refractory material to the wet shell after applying each solution; after each solution has in turn been applied at least three-times causing the pattern to liquify and how out of the shell; and investing theshell in dry sand.

6. The method of claim 5 wherein the alkaline normality of the alkaline solution is in the range of 0.4 to 1.7 normal; wherein the acid normality of the acid solution is in the range of from 0.2 to 0.85 normal; and wherein the alkaline normality of the alkaline solution is at least twice the acid normality of the acid solution. 5

7. The method of claim 5 wherein the sand has incorporated therein a material capable of fritting at elevated temperatures.

8. The method of claim 5 wherein each solution is applied by dipping and wherein the stuccoing operation 10 comprises showering dry material on the wetshell.

References Cited in the file of this patent UNITED STATES PATENTS Collins Aug. 7, 1945 Feagin May 18, 1948 FOREIGN PATENTS Great Britain Nov. 7, 1951 Great Britain Apr. 1, 1953

Claims (1)

1. THE METHOD OF FORMING A MULTI-LAYER SHELL MOULD AROUND AN EXPENDABLE PATTERN WHICH INCLUDES PREPARING TWO SOLUTIONS OF SODIUM SILICATE EACH CONTAINING A FINELY GROUND REFRACTORY MATERIAL SUSPENDED THEREIN AND ONE BEING ALKALINE AND THE OTHER HAVING AN ACID MATERIAL ADDING TO MAKE THE SAME ACID BUT OF A NORMALITY LESS THAN THE NORMALLY OF THE ALKALINE SOLUTION, AND THEN APPLYING TO THE PATTERN ALTERNATELY THE ACID AND THE ALKALINE SOLUTION AND STUCCOING A FINELY GROUND REFACTORY MATERIAL TO THE WET SHELL AFTER EACH APPLICATION, SAID ALKALINE SOLUTION ALWAYS BEING APPLIED WHILE THE SHELL IS STILL WET WITH SAID ACID SOLUTION.