US2752653A - Method of and dies for forming hollow expendable patterns for casting - Google Patents

Description

y 1956 H. s. EMBLEM ETAL 2,752,653

METHggPggDAND DIES FOR FORM ING HOLLOW ABLE PATTERNS FOR CASTING Filed Feb. 24, 1955 2 Sheets-Sheet 1 F I 16 '11 ?/l/ F l G. 2

I 15b C' INVENTORS M M BY -W, M+M ATTORNEY5 2 W we mm HT cw Mm M R EOR ED E HF N O E up 0 H E m M Jul 3. 1956 2 Sheets-Sheet 2 Filed Feb. 24, 1955 FIG.7

INVENTOR5 M-K-M ATTORNEYS WIT-D OF AND DIES FOR FOG HGLLOW EXPENDABLE PATTERNS FOR CASTING Harold Gar-ton Emblem, Melbourne, Desmond Joseph Cloherty, Chellaston, and Ralph Harold Hancock, @oxbench, England, assignors to Rolls-Royce Limited, Derby, England, a British company Application February 24, 1955, Serial No. 490,370 Claims priority, application Great Britain March 9, 1954 8 Claims. (Cl. 22-196) This invention relates to casting processes, and in particular to processes of the kind in which an expendable pattern is invested in a refractory material to produce a casting mould. Processes of this kind are commonly referred to as lost wax, the expendable pattern being normally of wax, which is melted out of the mould after investment of the pattern. In place of wax other mate rials such as low melting point metals may be used.

In certain cases the article to be cast is of hollow form, and processes of the kind referred to have been used to produce hollow cast articles from hollow patterns which are obtained by the provision of inserts in the pattern, the inserts being in certain cases shaped as to be removable from the pattern prior to investment. Thus metal inserts have been used which are supported in a die in which the pattern is formed, such inserts being of uniform section or slightly tapered to facilitate their withdrawal from the pattern. In this manner a pattern of hollow form is provided, which when invested is filled with refractory material, whereby after melting out of the pattern'the mould is so formed as to provide for the casting of an article reproducing the hollow formation.

In other cases an insert of fired water-bound clay refractory is supported in the pattern-forming die prior to the introduction of wax or equivalent therein, so that the pattern is formed round the insert in a single operation, said insert being invested with the pattern, whereby after the pattern is removed the insert becomes part of the mould.

The present invention seeks to provide a method of producing hollow cast articles by casting processes of the kind referred which will present accuracy in the finished part and which will facilitate the production of patterns therefor and series of die parts for carrying out the said method.

According to the present invention a method of producing an expendable pattern for precision casting having provision for a hollow formation in the part to be cast includes providing a refractory insert conforming to the desired hollow formation, supporting same in a die, part of which conforms with and contacts the external contour of said insert and part of which conforms with the external contour of desired pattern to provide a space between said latter part and said insert, introducing expendable material into said space and thereafter carrying out at least once the sequence of removing a part of the die conforming to the external contour of the insert, substituting therefor a further die portion conforming with the contour of the desired pattern to provide a further space between said further die portion and said insert and introducing expendable material into said further space until the complete pattern has been formed around the insert.

Normally the moulding of the pattern around the insert will be completed in two stages, but in the case 2,752,653 Patented July 3, 1956 of certain intricate patterns it may be desirable for the moulding to be carried out in three or more stages.

Thus a preferred method according to the invention includes forming an insert of refractory material conforming to the hollow shape required, supporting same in a split die one part of which conforms with and contacts the external contour of the said insert and the other part conforms with the external contour of desired patf tern to provide a space between said other part and said insert; introducing expendable material into the said space whereby the pattern is formed in part around said insert, thereafter removing said first part of said split die and replacing said part by a further die part having a surface conforming with the remaining external surface of the pattern to provide a further space between said further part and said insert and introducing further expendable material into said further space thereby to complete the moulding of the pattern around the insert.

Preferably the pattern is formed by a casting process in a split die, portions of which are removed in succession to be replaced by further die portions having surfaces defining part of the external surface of the pattern, wax or equivalent being introduced after each such replacement.

If desired the refractory insert may be preformed, in which case it may be prepared from vitreous silica, or sintered alumina. The refractory insert may also be prepared by hardening with carbon dioxide a mixture containing silica sand or zircon and sodium silicate.

The invention is illustrated diagrammatically in the accompanying drawing in which:

Figures 1 to 3 illustrate the method of producing a wax pattern having a refractory insert as shown in crosssection in Figure 4.

Figure 5 shows the pattern in vertical section supported for investment in a refractory material.

Figure 6 illustrates the application of the process to the production of a pattern having refractory inserts arranged to present a plurality of passages in a cast article, and

Figure 7 illustrates a turbine blade having a plurality of passages therein which may be cast by the method described with reference to Figure 6.

Referring to Figure 1 a refractory insert is formed in a split die comprising portions A and B. The refractory insert may include if desired one or more rods of strengthening refractory material 10 around which a refractory slurry is poured to provide the main body of the insert 11. If desired the rods of strengthening refractory material ltl may be hollow. Conveniently the split die provides an open end through which the slurry in introduced and at the lower end venting passages are preferably provided for the exhaustion of air; the rods 10 may be located in recesses at the closed end of the die and if desired by a suitable jig at the open end. Thus, as shown in Figure 5, they are slightly longer than the overall length of the main body 11 of the insert. The rods it! may be silica, alumina, mullite or porcelain.

One suitable material for forming the refractory in the split die is a slurry prepared from Sillimanite and acid hydrolysed ethyl silicate solution. This slurry may be made to gel rapidly by adding ammonia solution. A suitable hydrolysed ethyl silicate solution may be prepared as follows:

1600 cc. of ethyl silicate, having a silica percentage of 40, and an acidity of under 0.1% as HCl, sold under the British registered trademark Silester as Silester are mixed with 840 cc. of industrial methylated spirits (64 over proof ethyl alcohol), cc. water, and 6 cc. of N/l hydrochloric acid solution. This mixture should be allowed to stand overnight before use.

Sillim ani te (all passing a sieve of mesh No. 10, all retained on a sieve of mesh No. 30) gms Sillimanite (all passing a sieve of mesh No. 100) p I V gms 7O Hydrolysed ethyl silicate solution prepared as described above ccs 55 Ammonia solution prepared as described above ccs 2.5

The above mesh numbers are according to British standard specification No. 410 of 1943.

Such a refractory slurry gels rapidly and preferably the moulder refractory insert is fired by burning off the alcohol present in the gelled slurry. For this purpose the-refractory insert can be removed from the split die after. gelling.

Referring now to Figure 2, it will be seen that the die portion A is replaced by a die portion C which has an internal surface conforming with part of the external surface of the desired pattern. Where for example the pattern corresponds to the external shape of a turbine blade, as indicated in Figure 5, the length of the pattern will be shorter than the refractory insert, and the internal surface of the die portion C will conform at its ends to the external surface of the refractory insert, wax or equivalent pattern-forming material being introduced by an injection device shown diagrammatically as a syringe 13 into the cavity 12 through an injection passage 15 constituted between the die portions B and C along the parting line. In this manner the pattern is moulded in part around the insert. During injection of the patternforming material air normally exhausts along the parting line, but separate exhaust passages may be provided.

Referring now to Figure 3, the die portion 3 is replaced by a further die portion D. The latter portion presents an internal surface conforming with the remaining external surface of the pattern. Wax or equivalent is introduced into cavity 14 through injection passage 15A constituted between the die portions C and D along the parting line whereby the moulding of the pattern around the insert is completed.

Figure 4 shows a section through such a completed pattern, the wax or equivalent pattern-forming material being indicated at 16 enclosing the refractory insert 11. A suitable known wax may be prepared by melting together a mixture of ll lbs. of American gum rosin WW grade; 6 lbs. of parafiin wax, 2 lbs. of candelilla wax, and 1 lb. of carnuba wax. Other suitable pattern-forming materials are ceilulose acetate, polystyrene and frozen mercury.

Figure illustrates the investment of the pattern in a suitable refractory. Such investment is carried out in known manner by mounting the pattern on a wax support 17 carried on a base-plate 18. The wax support 17 forms the cavity in the final mould by which metal is introduced in the casting operation. According to known practice in casting processes of the kind specified the investment container 19 may be attached to the base-plate by means of wax at 20. The investment material is poured into the container 19 in slurry form, and may be of any known or convenient kind, for example those bound with ethyl silicate or sodium silicate. Other refractory investments such as a water-based material with concrete as a binding medium may be used. It will be seen that the investment bonds with the ends 11A and 11B of the insert, projecting beyond the end of the wax of the pattern. Thus when the wax is melted from the mould after gelling of the investment, the insert remains supported within the mould to constitute a part thereof presenting a hollow passage within the turbine blade when cast;

It is generally advantageous to vibrate the refractory 4 investment slurry before gellation occurs, in order to pack the refractory material round the pattern and the refractory insert, and consolidate the body of the mould. In this case, the investment container 19 may be extended by a sheet of waxed paper. When packing is complete, the mould is trimmed level with the top of the investment container 19, to remove any accumulation of fine refractory particles at the top of the mould. When the mould is to be used for casting metals or alloys of high melting point, for example high alloy steels, oralloys sold under the British registered trademark Nimonic, it 'is desirable that the mould be fired to a temperature of at least 1000 C. prior to the casting operation, which should be performed while the mould is hot.

It is frequently advantageous to give the pattern a fine refractory coating, prior to investment, by coating it with a finely divided refractory suspended in a liquid binder, and then dust a refractory of relatively coarse particle size on to the coated pattern. When dry, the coated pattern and uncoated wax support 17 are attached to the base-plate 18.

After casting the insert may be removed from the part by mechanical means such as sand-blasting.

Whilst in the embodiment described above the insert is of acomparatively simple shape, the process may also be used'for the production of parts having more intricate passages for hollow shapes therein. For example in Figure 7 there is shown diagrammatically a turbine blade having three passages for the flow of cooling air. These passages are indicated at 21 and may extend through the full length of the blade. Alternatively in certain cases they may terminate at a point adjacent one end and be interconnected by cross passages, as indicated at 21A. Where the passages extend through the whole length of the cast part, simple refractory rods may be used. Alternatively, where the passages interconnect and do not extend through the full length of the blade, an insert in the form of a trident is used. Such insert rods or insert trident may be cast in a split die having portions corresponding to A and B in Figure 1. Alternatively, they may be preformed and supported in the first stage of the pattern moulding process in a support die corresponding to portion B of Figure 2. Thereafter the support die B is removed and replaced by a die portion corresponding to portion D of Figure 3. The final stage of the process is illustrated in Figure 6 where die portions D and C correspond to similar parts in Figure 3, there being an injection passage 15B constituted between the parts D and C.

Adoption of the inv ntion presents accuracy in the finished part, it being appreciated that during the process of moulding the wax or equivalent pattern-forming material around the insert, the insert is supported in the first stage by direct contact with a die portion having a surface conforming with the contour of the insert, i. e. die portion B in Figure 2, whilst in the second stage the insert is directly supported through an intermediate layer of wax or the equivalent by a die portion having a surface conforming with part of the external surface of the desired pattern, i. e., portion C, Figure 3. In this manner distortion of the insert due to inequalities of pressure loading which may arise during injection of the wax or equivatent is avoided. Further, the insert is accurately located with respect to the pattern surrounding it, whereby when invested, as illustrated in Figure 5, to form part of the castingmould, it is accurately positioned with respect to the internal surfaces of the mould. Further, the use of a preformed insert avoids difficulties of packing of the investment which arise where the investment material is required to flow into hollow spaces within a wax or equivalent pattern.

Whilst in the above embodiments the moulding of the wax or equivalent around the insert is completed in two stages of the process, it will be appreciated that in certain cases where patterns are intricate by virtue of intricacy of the shape of inserts and/or by virtue of their external shape, it may be desirable to utilise a split die having three or more portions, whereby the moulding of the pattern is completed in three or more stages.

What we claim is:

1. A method of producing an expendable pattern for precision casting having provision for a hollow formation in the part to be cast which includes providing a refractory insert conforming to the desired hollow formation, supporting same in a die, part of which conforms with and contacts the external contour of said insert and part of which conforms with the external contour of desired pattern to provide a space between said latter part and said insert, introducing expendable material into said space and thereafter carrying out at least once the sequence of removing a part of the die conforming to the external contour of the insert substituting therefor 21 further die portion conforming with the contour of the desired pattern to provide a further space between said further die portion and said insert and introducing expendable material into said further space until the complete pattern has been formed around the insert.

2. A method of producing an expendable pattern for precision casting having provision for a hollow formation in the part to be cast which includes forming an insert of refractory material conforming to the hollow shape required, supporting same in a split die one part of which conforms with and contacts the external contour of the said insert and the other part conforms with the external contour of desired pattern to provide a space between said other part and said insert; introducing expendable material into the said space whereby the pattern is formed in part around said insert, thereafter removing said first part of said split die and replacing said part by a further die part having a surface conforming with the remaining external surface of the pattern to provide a further space between said further part and said insert and introducing further expendable material into said further space thereby to complete the moulding of the pattern around the insert.

3. A method of producing an expendable pattern for precision casting having provision for a hollow formation in the part to be cast which includes casting a refractory slurry in a die to a shape conforming to the hollow formation desired, causing the same to set, supporting said cast and set refractory member in a split die one part of which conforms with and contacts the external contour of the said insert and of which the other part conforms with the external contour of desired pattern, to provide a space between said latter part and said insert, introducing expendable material into the said space whereby the pattern is formed in part around said insert, thereafter removing said first mentioned part of said split die and replacing said part by a further die part having a surface conforming with the remaining external surface of the pattern to provide a further space between said further part and said insert and introducing further expendable material into said further space thereby to complete the moulding of the pattern around the insert.

4. The method claimed in claim 3 in which said slurry is prepared from sillimanite and acid hydrolysed ethyl silicate solution.

5. The method claimed in claim 3 in which the refractory has cast into it strengthening members formed of material selected from the group consisting of silica, alumina, mullite and porcelain.

6. A method of producing an expendable pattern for precision casting having provision for a hollow formation in the part to be cast which includes casting a refractory slurry in a split die comprising first and second die parts and providing an internal shape conforming to the hollow formation desired, thereby to provide a cast insert, removing one of said die parts and replacing it by a third die part conforming with the external contour of the desired pattern, to provide a space between said third die part and said insert, introducing expendable material into the said space whereby the pattern is formed in part around said insert, removing the second of said die parts and replacing it by a fourth die part having a surface conforming with the remaining external surface of the pattern to provide a further space between said fourth part and said insert in introducing further expendable material into said further space thereby to complete the moulding of the pattern around the insert.

7. A series of die constituting members for producing an expendable pattern for precision casting a blade for an internal combustion turbine engine with provision for a hollow formation therein comprising a die part adapted to support a refractory insert conforming to the desired hollow formation, a second die part adapted to fit on to the first and to leave a space between itself and the insert corresponding to the desired pattern to be forme over the insert, passage means for the introduction of expendable material into said space and a third die part adapted to be substituted for the first die part leaving a space between itself and said insert corresponding to the desired pattern to be formed over the insert and passage means for the introduction of expendable material into said latter space.

8. A series of die constituting members for the precision casting of a blade for an internal combustion engine with provision for a hollow formation comprising two die parts adapted to fit together to define a space for casting a refractory member corresponding to the desired hollow formation with extensions at each end beyond said hollow formation and with passages for the introduction of refractory material, a third die member adapted to be substituted for one of said first two die members and to fit onto the other of said two die members and to leave a space between itself and the insert corresponding to the desired pattern to be formed over the insert, passage means for the introduction of expendable material into said space, a fourth die member adapted to be substituted for the other of the said first two die members leaving a space between itself and said insert corresponding to the desired pattern to be formed over the insert and passage means for the introduction of expendable material into said latter space.

No references cited.

Claims (1)

1. A METHOD OF PRODUCING AN EXPENDABLE PATTERN FOR PRECISION CASTING HAVING PROVISION FOR A HOLLOW FORMATION IN THE PART TO BE CAST WHICH INCLUDES PROVIDING A REFRACTORY INSERT CONFORMING TO THE DESIRED HOLLOW FORMATION, SUPPORTING SAME IN A DIE, PART OF WHICH CONFORMS WITH AND CONTACTS THE EXTERNAL CONTOUR OF SAID INSERT AND PART OF WHICH CONFORMS WITH THE EXTERNAL CONTOUR OF DESIRED PATTERN TO PROVIDE A SPACE BETWEEN SAID LATTER PART AND SAID INSERT, INTRODUCING EXPENDABLE MATERIAL INTO SAID SPACE AND THEREAFTER CARRYING OUT AT LEAST ONCE THE SEQUENCE OF REMOVING A PART OF THE DIE CONFORMING TO THE EXTERNAL CONTOUR OF THE INSERT SUBSTITUTING THEREFOR A FURTHER DIE PORTION CONFORMING WITH THE CONTOUR OF THE DESIRED PATTERN TO PROVIDE A FURTHER SPACE BETWEEN SAID FURTHER DIE PORTION AND SAID INSERT AND INTRODUCING EXPENDABLE MATERIAL INTO SAID FURTHER SPACE UNTIL THE COMPLETE PATTERN HAS BEEN FORMED AROUND THE INSERT.

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