US2877523A - Vacuum casting - Google Patents

Description

March 17, 1959 JQs. TURNBULL 2,877,523

VACUUM CASTING Filed Aug. 29, `1955 E I INyr-:fqTc-mj ATTORNEYS VACUUM CASTING John Stratton Turnbull, Bowdon, Metropolitan-Vickers Electrical British company Application August 29, 1955, Serial No. 531,028

Claims priority, application Great Britain September 3, 1954 England, assignor to Company Limited, a

2 Claims.

This invention relates to vacuum casting, that is to say, to methods of casting wherein the mould issubjected to suction or evacuation to a desired degree during the pouring of the metal or other material to be cast.

In the casting art, it is in general necessary that the gas, or air Ebe removed from the mould cavity at a higher rate than that at which the molten metal enters the;.cavity,

siderations in mind, it has been proposed to subject the.

mould to suction or vacuum but the use of such vacuum casting has been limited because in general it is only possible to apply the suction to one or two faces of the` usual rectangular mould and the evacuatng equipment is required to withdraw the air or gas from the mould cavity through a considerable thickness of the material forming the mould.

The present invention has for its object to provide a method of vacuum casting wherein the disadvantages set forth above ymay be obviated. The invention is eminently, but not exclusively, applicable to the so-called precision casting.

The methodof-.vacuumcasting according-tov the present invention comprises supporting a permeable shell mould within a container by means of permeable -supporting means between the outer surface of the shell and the inner surface of the container, connecting the container with a vacuum accumulator and evacuating pump with valve means between the container and the accumulator, evacuating the accumulator with thevalve means closed, and subsequent to said evacuationV of the accumulator; opening said valve means and pouringv the metal or other casting material into the mould as a combined operation, that is to say, more or less simultaneously.

The supporting means referred to preferably comprises packed granular material of angular form and of large size,k such as hereinafter indicated. The compositev materialformed by the shell and the supporting means is designed for maximumr or high flow of air or gas. from the mould'cavity or cavities within the shell directly from the inner surface of each cavity through the permeable wall of the shell. Since a permeable shell mould is employed, that is to say, a mould which has an outer shape conforming substantially to the general shape of the mould cavity, or cavities in the case of a complex casting, the arrangement is therefore such that the air or gas can pass freely through the shell wall on all or substantially all surfaces of the mould cavity or cavities. Since the accumulator is evacuated prior to the casting operation and with the valve means closed, a comparatively small evacuating pump is enabled rapidly to evacuate ICC 2., the. mould cavity or cavities of air or gas, it beingdmderstood. that. the accumulator has acapacity many times that of the mould cavity or cavities.

Preferably the pump is left running during the4 cast.- ingr operation and solidiiication of the casting, whereby` to. balance leakage of air into the container when the mould is full ofthe poured metal. By this means also decar.-

burisation of the surface metal of the casting and forma?v The shell may be formed by various mihods, but is.

preferably inl accordance withl the invention set forth and claimed in copending United States application Ser. No. 319,776, tiled November 10, 1952, now U. S. Patent No. 2,815,552, issued December 10, 1957 toy John Stratton Turnbull and Richard Glyn Nicholas, assignors to Metropolitan-Vickers Electrical Company Limited.

The invention also includes Within the scope thereof apparatus for carrying out the method hereinbefore set forth', said apparatus comprising a permeable shell mould,

arranged within a container and supported thereiny by permeable supporting means between thev outer surface; ofv the. shell and the inner surface ofthe container, a vacuum accumulator and an evacuating pump for evacuating said accumulator, and a connection betweensaid' accumulator and said container, said connection including valve means for controlling the passage of air or gas from the container into the accumulator.

Reference will now be made byA way of example to the accompanying drawing, in which:

Fig. lis a cross-sectional elevation showing diagrammatically one arrangement of mould for carrying outthe method of casting according to the. invention, and

Fig. 2 is a diagram illustrating the mould of Fig. 1 arranged in apparatus according to the invention.

In Fig. 1, a shell mould 1 of the desired object to vbe cast is made-such as by themethod according to the aforementioned U. S. applicationNo. 319,776/ 52, now U. S.

Patent No. 2,815,552, issued December 10, 1957. This.

mould is positioned in a metal container 2 and invested therein with a coarse dry refractory aggregate of angular nature indicated by the cross hatching 3. The aggregate may for example have a particlev sizey of between onequarter and one-half of an inch. v

The mould is provided with a pouring throat 4 whichL extends through the open top. of the container. The top of the mould, with the exceptionof the pouring throat 4, has built up thereon a collar 5 of solid investment ex' tending from the external surface of the throat to the sides of the container 2, thereby completing `the enclosurev of the mould 1 and the refractory aggregate 3. The collar maybe formed from a thickviscous ceramic slurry poured or moulded roundthe throat. 4 of the mould onto the top of the` refractory investment, the slurry'beingsd,

Patented Mar. 17, 1959 welded into one side of the container and connected such as by a rubber tube or hose to the evacuating pump or accumulator.

Y In Fig. 2, the rubber hose is indicated at 7 and connects' -the pipe 6 with a pipe 8 in turn connected with a vacuum accumulator 9, a valve 10 being included in the pipe 8. The accumulator 9 is connected directly with an evacuating pump 11. Alternatively, the container 1 may be provided with a bottom wall having a number of orilices therein and the mould may be placed on a vacuum stillage incorporating a pump accumulator, and valve means, so that in operation the air or gas may be drawn through said oriiices.

The vacuum accumulator 9 with which the pipe 6 or the vacuum stillage is connected may have for example a volume of twenty to thirty times that of the mould cavity.

When casting, the mould may be either hot or cold according to conditions and requirements. When the mould is employed hot, then with the second method of evacuation just above indicated, the metal tube protruding from the container is chilled-oft' before being connected with the rubber tube or hose.

ln making the casting, the accumulator is iirst evacuated to the desired degree without drawing air from the mould, valve means between the accumulator and the container being closed at this time. At, or approximately at, the instant when pouring of the metal into the mould is commenced, said valve means are opened and the vacuumpreviously built up in the accumulator is then applied to. the container so as to obtain a high rate of extraction of air and gas from the mould cavity as the molten metal begins to flow down the throat. It will be understood that the use of the accumulator and opening of the valve means only at the time at which the casting is commenced enables a relatively small and economical evacuating pump to be employed. The use of a shell mould and highly permeable backing enables the air or gas to be extracted rapidly by the vacuum from the mould cavity or cavities. t

The valve means is left open for the necessary-time required for the cast metal to solidify, which time may for example be of the order of tive minutes. The pump is left running during this time so as to balance any leakage of air through the wall of the pouring throat and the collar by which the throat is sealed to the walls of the container at the upper end thereof.

. The process according to the invention, whilst having general application, is eminently applicable for the precision casting of long and thin objects of simple or complex shape, and enables a high surface iinish or definition to be obtained.

As well understood in the art, long castings of relatively small sectional thickness or area are difficult to ll completely with poured metal without trapping gas, obtaining miss-run edges and other defects. The invention also enables alloys which might otherwise be regarded as uncastable to be cast with considerably improved results. In casting alloys, which in general provide inferior surface iinish by the normal casting processes as a result of high viscosity of the molten metal and consequent tendency to the trapping of small gas pockets on the surface of the casting, the invention overcomes these defects and in general enables Acastings f having good-definition to be obtained where not possible Y- permeable material, a one-piece elongated readily gas` by the usual methods.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. An apparatus for use in the precision vacuum casting of relatively long thin objects comprising a substan tially air-tight container confining a body of readily gas permeable shell mold mounted in said container and conformably embedded in said body of material, said mold forming at least one relatively narrow cavity and having a pouring throat projecting above said container lthrough an opening provided therein and including a relatively narrow throat section communicating with said cavity whereby poured-casting material in molten state ills said throat to seal the cavity Vsubstantially air-tight in said container, an exhaust conduit extending through l the container into said body of material, an accumulator having a volumetric capacity at least many times that ofsaid mold and'said container, a vacuum pump connected to said accumulator, and a valve in the conduit between said container and the accumulator said valve lbeing normally closed in said apparatus to permit said accumulator to be evacuated and conditioned for sudden and continuing application of its vacuum to the interior of said container when said valve is opened.

2. In a vacuum casting process for use in producing Vrelatively long thin objects the steps of providing an upright relatively narrow one-piece gas permeable shell mold embedded in a coniined relatively coarse conformable gas permeable support medium in a substantially air-tight container and having a relatively narrow pouring throat section projecting beyond the container, ac-

cumulating vacuum in a source having a volumetric capacity many times that of the cavity in the mold and container, sealing the cavity within the container by pouring molten casting material into the mold to fill up the lrelatively narrow throat section while substantially at the same time rapidly and substantially evacuating said mold through substantially all faces of said cavity by suddenly applying the accumulated vacuum of said source to said coniined support medium, and `continuing to apply a vacuum to evacuate the interior' of said container as'molten casting material iills said mold to substantially evacuate leakage of air into said container.

References Cited in the le of Vthis patent UNITED STATES PATENTS OTHER REFERENCES Fiat Final Report No. 1168 entitled The C Process I of Making Molds and Cores for Foundry Use, by W. W.

McCullough, published by the Giice of Technical Services, Dept. of Commerce, Washington, D. C,

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