US2985929A

US2985929A - Method and apparatus for support and cooling of shell molds

Info

Publication number: US2985929A
Application number: US860558A
Authority: US
Inventors: Richard T Carter
Original assignee: Individual
Current assignee: Individual
Priority date: 1959-12-18
Filing date: 1959-12-18
Publication date: 1961-05-30
Anticipated expiration: 1978-05-30

Description

May 30, 1961 R. T. CARTER METHOD AND APPARATUS FOR SUPPORT AND COOLING OF SHELL MOLDS 2 Sheets-Sheet 1 Filed Dec. 18, 1959 R. T. CARTER May 30, 1961 METHOD AND APPARATUS FOR SUPPORT AND COOLING OF SHELL MOLDS 2 Sheets-Sheet 2 Filed Dec. 18, 1959 METHOD AND APPARATUS F OR SUPPORT AND COOLING F SHELL MOLDS This invention relates to casting procedures, and more particularly it is concerned with a method and apparatus for the supporting and cooling of a shell mold during and/ or after filling the shell mold with molten metal.

Progress in the sand foundry and the investment foundry has recently brought about the use of comparatively thin refractory shell molds in which material is usually built up in layers to provide a thin Walled, low mass, highly gas permeable ceramic shell. Such a shell mold is fully described, for example, in Patent Nos. 2,806,269 or 2,806,- 270, issued September 17, 1957. This ceramic shell mold has superior filling characteristics as well as enabling more rapid cooling due to the decreased mass of insulating refractory material. The advent of this new concept in metal casting, however, has brought with it new problems, many of which have not been encountered before. Among these are the problems relating to the provision of means for adequately supporting the thin walled mold during and after pouring, and the provision of means for suitably controlling the cooling rate of the casting. In this new field the accent ison the speed of fabrication and on casting at minimum costs. The foundryman therefore wishes to have a mold which is constructed with a minimum amount of material and which can becast Without the need of costly backups and supporting structure and procedures and yet incorporates all the advantages which accrue through the use of thin walled refractory types of molds.

it is therefore a primary object of this invention to provide a method and apparatus by which thin walled shell molds may be cast which provide outward support of theshell without resorting to the costly conventional backup and supporting procedures.

It isa further important object of this. invention to provide a method and apparatus for increasing the rate of-cooling of shell type molds in a manner which is simple and economical.

XStill another object of the invention is to provide a simple and expeditious method of supporting an irregularly shaped mold in the pouring operation.

Another important object of this invention is to provide a method and apparatus by which a variable rate of coolin'gon different parts of the shell mold can be achieved. in. accordance with principles of the invention there is provided a structure having upper and lower chambers which are divided by a porous membrane. The upper chamber is open to the atmosphere and is filled with a bed of fine grain heat-conductive material such as metal grit, zircon, silica sand, etc. The shell mold tobe poured is placed on top of this prepared bed, and during or after pouring, gas underpressure, introduced intothe lower chamber, permeates through the membrane in such a manner as to cause the particles of conductive material in the} upper. chamber to be displaced from each other, substantially in floating condition. The shell mold thus becomes substantially unsupported and lowers itself gradually into the floating particles. When the pressure is removed'the particlesret'urn by gravity to 'a stable conditates Paent tion completely surrounding and supporting the shell inv the form of a gas permeable heat sink. Support and a' controlled cooling environment for the shell are thus' simultaneously provided. After suificient cooling time has been allowed the pressure is again applied through the membrane and causes flotation of, the cooling particles.'

The shell and casting then may be easily removedfrom. the bed. Upon subsequent release of pressure the. bed' is again ready to receive another shell. The invention thus provides a steady support of thin walled shell molds during the cooling period and through judicious selection,

cooling particles the cooling rate of the casting can be} controlled.

Additional objects and advantages of the invention will} Fig. 3 is a sectional view of another container of the v apparatus taken along the line 3-3 of Fig. l;

Fig. 4 is a view of a plurality of the containers utilized in the apparatus, the view being taken along the line 4-4 of Fig. l; and

Fig. 5 is a sectional view of a container illustrating a modified form of cooling bed by means of which selected areas of the casting may be cooled at different rates.

The apparatus shown in Figs. 1-4 enables the provi sion of controlled support and cooling of shell moldsin a substantially continuous casting operation. As shown in Fig. l twelve steel tank structures are mounted on a turntable bed plate 12. Each tank structure is divided into-an upper compartment, 14 and a lower compartment. 16 by a permeable membrane 18 as shownin Fig. 2. A valve 20, associated with an air supply pipe 22, controls introduction of air pressure to the lower charnberiof each tank. The valve 20 is operated by means of an actuating lever 24 pivotally mounted on the tank walland a connecting link 26. A dump valve 28, positioned on the opposite side ofeach lower chamber from valve; 20, is adapted to be operated by a cam 30, two of which are secured to the floor 32 at spaced points adjacent the" turntable 12.

Positioned in the upper chamber is a bed 34 of a finej grained heat-conductive material. Depending on the do-1 sired characteristics this may be an iron grit, azircon, sand or other similar material in discrete particle form.

The turntable 12 is bolted to a central member36 which is mounted for rotation on a suitable pedestal sup-r port 38 which is mounted on the floor 32 as shown in; Fig. 3. A set of bearings 40, positioned between the sup:

port 38 and central turntable member 36 permit rotation" of the turntable as driven by suitable means (not shown): Air or another suitable fluid medium is supplied under pressure through conduit 44 to the interior of the pedestal support and through a horizontal passageway 46 to a can? nelure-48. An aperture 50 on thero'tating central mern; ber 36 communicates with distributing pipe 52 whic is; in turn connected. to the circular conduit '22. A valve, operating mechanism 54 is also secured to the; pedestal support 38 and includes two

cam units

56, 58 which operate the valve levers 24. When a cam engages a leverit: operates the associated valve 20 through linkage 26 so that air pressure is supplied to the lower chamber 16 and through the membrane 1 8 so that the particles of the bed 34 in the upper chamber 14 rise and become slightly sepa; rated from each other. The volume occupied by'lthd particles increases slightly and theytend to beha a manner similar'to that efafiuid;

Patented May 30, 1961 The operating stations adjacent the turntable 12 include a mold loading station at which there is provided a conveyer 60 for shell molds 62, a pouring station at which there is a pouring ladle 64 and a removing station at which there is provided a conveyer 66 for removing the cast and cooled shell molds.

In operation a shell mold 62 which has been preheated to temperatures ranging from about 400 to 1800 F. is placed on top of the heat-conductive bed 34 from the conveyer 60 shown in Fig. 2. The turntable carrying this shell is then moved to the pouring position and the metal is poured into the mold from the ladle 64. On completing this pouring operation air is introduced into the lower compartment 16 as the associated valve lever 24 of the tank contacts the cam 56. As the air permeates through the membrance 18 the particles in the upper chamber 14 become slightly separated and the cast shell sinks into the bed into a position indicated generally in Fig. 3. When the cast shell reaches the desired level in the bed 34 the air pressure is turned off as by release of the lever from the cam 56 and the dump valve is operated by cam 30 to remove any excess pressure in the lower chamber. The bed of material particles then has returned to a stable condition completely enveloping and supporting the cast shell. The turntable 12 continues to rotate and passes through several positions which provides sufficient time for the casting to cool the required degree and then the lever 24 is again operated, this time by cam 58, to introduce pressure into the lower chamber so that the bed of material is returned to a fluid condition and the cast shell is removed from the bed and placed on the conveyer 66 for suitable subsequent handling. The turntable 12 continues to rotate with air pressure remaining applied to the bed 34 so that the particles which absorbed heat from the cast mold in the cooling cycle are then themselves returned to suitable cool condition. The valve lever 24 rides off cam 58 just before the loading position, the air pressure valve is closed and the bleed valve 28 is opened by the cam 30 for a short time to remove any pressure in the lower chamber thus preparing the bed for the loading of another shell.

Under some conditions it may be desirable that the cooling of the base of the shell mold should be more rapid than the cooling of the remainder of the casting. This may be achieved by only partially immersing the shell in the heat conductive bed while it is in a fluid state and securely positioning the shell in that position for pouring by removing the application of pressure. In this way the portion of the shell immersed in the material bed will be at a lower temperature than the remainder of the shell and will impose a more rapid chill on that portion of the casting. Further there are many instances in which it is not convenient to construct a shell mold in such a manher that the shell will stand stably on a level surface for pouring. Accordingly the method and apparatus of the invention can be utilized so that shells of any shape or configuration can be supported securely by the bed in the desired pouring position without the necessity of other supporting means. Thus, air under pressure may be introduced into the bed to cause the particles to be displaced from each other before the pouring operation to permit the lower portion of the shell to sink into the bed, after which the air under pressure to the bed is removed to permit the particles to fall by gravity to rigidly support and position the shell in the bed. Thereafter, the molten material is poured into the shell, and air under pressure is again introduced to the bed to again displace the particles and permit the filled shell to descend to a predetermined depth in the bed. Thereafter, the air under pressure is again removed so that the bed supports and cools the filled shell.

At the present time, there is considerable limitation with regard to the cross section of and the static head that can be retained by certain bonded type shell molds. By presently known techniques these problems can be overcome to some extent by carefully packing the shell mold in sand. However this procedure has disadvantages of time and cost and it is frequently difiicult to pack this type of mold so that the pressure is evenly distributed. The invention provides a ready solution to this problem by permitting the sinking of the shell into the supporting medium prior to pouring. Side wall movement can be completely eliminated and at the same time an excellent gas permeable heat sink is provided. The shell mold is removed in a similar manner as that described above.

Fig. 5 shows a significant modification of the invention. For the best economy in necessary feed metal it is often expedient to cool different sections of the casting at different rates. Such results can be obtained economically by the following procedure. The casting is oriented on the sprue in a manner so that the lower section thereof should preferably freeze first and so on progressively up to the top of the casting. The particles of the cooling bed comprise several different materials, each having a different specific gravity and each possessing different thermal transfer coefiicients. Materials of different specific gravity will be suspended in the bed in layers by the pressure and will remain in the configuration on release of the pressure. As an example, referring to Fig. 5, there is provided in the upper chamber 14 of the container 10 a layer 68 of mesh iron grit, a layer 70 of mesh zircon sand and a layer 72 of a suitable insulating material. The materials remain in this general relationship throughout the entire operating procedure. Thus a shell may be cast at a given temperature and then introduced into supporting thermally conductive media which provide varying rates of heat transfer throughout the depth of the bed. In the illustrated case an insulating material has been used in the upper layer to prevent the sprue from freezing, thus improving the feed time for this section.

A further example, indicative of still other advantages of the invention, may be understood by considering the problems currently encountered with respect to the shell mold strength. Preferably the shell should be cast without additional external support because of the cost of supporting techniques known in the prior art. On the other hand all known shells are subject to failure for one of two reasons when large castings are considered. Shells generally fall into two classes, those using binders and refractories which break down at relatively low temperatures, causing distortion of large castings; and those using a higher refractory with surface sintering of the particles functioning as a binder. This latter type of shell is less likely to distort due to its greatly increased strength but it is likely to cause hot tearing in the casting. Neither type of shell therefore completely solves the problem of casting the heavier class of castings.

The invention enables the use of either type of shell in effecting a consistent cast dimension without the above enumerated problems. In the case of the low refractory shell, side wall bulging is prevented due to the cooling support. The sintered shell may be cast with less wall thickness, thus avoiding the hot tearing problem due to the strength of the shell.

Thus while a preferred embodiment of the invention has been shown and described, various additional modifications thereof will be obvious to those skilled in the art and it will be understood that the invention is not intended to be limited to the preferred embodiment or to details thereof and departures may be made therefrom within the spirit and scope of the invention as defined in the claims.

I claim:

1. The method of supporting and cooling a refractory shell mold comprising the steps of placing the shell on a bed containing a multiplicity of discrete particles of heatconductive material, pouring molten metal into said shell, subjecting the bed to gas under pressure from below to cause the particles in said bed to be displaced from one another so that said shell mold is permitted to sink into said bed, and reducing said pressure so that the particles tend to fall back into surrounding and supporting relationship about the mold to provide a controlled cooling environment for the mold.

2. The method of supporting and cooling a refractory shell mold comprising the steps of providing a bed comprising a multiplicity of discrete particles of heat-conducting medium, introducing a supply of compressed air into said bed to cause the particles to be displaced from each other, lowering a shell mold partially into said displaced particles, removing the air supply that said particles tall by gravity to rigidly support and position the shell mold, pouring molten material into the shell mold so supported, applying air to said bed to displace the particles, permitting the filled shell mold to descend to a predetermined depth, and removing the air pressure so that said bed supports said shell in a gas permeable heat sink medium for cooling said filled shell mold.

3. The method as claimed in claim 1 wherein said bed comprises a plurality of difierent chilling media having various specific gravities whereby they tend to be displaced vertically in layers throughout the bed under the influence of pressure applied to the bed and to remain in such layers after the removal of pressure.

4. The method of supporting a thin walled gas permeable refractory shell mold filled with molten metal for cooling comprising the steps of placing the shell mold on a bed containing a multiplicity of particles of heatconductive material, subjecting said bed to gas under pressure to cause said particles to be displaced so that said bed tends to act like a fluid permitting said shell mold to sink therein, and removing the pressure from said bed so that said particles tend to closely surround said mold in supporting and cooling relationship.

5. Apparatus for supporting shell molds and con trolling the cooling rate thereof comprising a plurality of containers mounted on conveying means for movement past a mold loading station, a pouring station and a mold unloading station, each container being open to the atmosphere at the upper side thereof and bounded on the lower side by a gas permeable membrane, a bed formed of a multitude of discrete particles disposed in each said container and supported on said membrane to provide a gas permeable heat sink, and means to apply a gas under pressure through said membrane to said bed for causing the particles therein to rise and separate slightly so that a mold placed on said bed at said loading station will tend to sink therein, said mold being securely supported by said particles upon removal of said pressure from said membrane such that a mold filled with molten material at said pouring station and sunk in said bed may cool at a controlled rate.

6. The apparatus as claimed in claim 5 wherein the particles in said bed comprise several different materials, each material having a difierent specific gravity and each possessing different thermal transfer coefllcients such that the different particle materials tend to be displaced vertically in layers throughout the bed under the influence of said gas pressure and to remain in such layers after the removal of said pressure.

References Cited in the file of this patent UNITED STATES PATENTS 2,660,770 Davis Dec. 1, 1953 2,718,041 Georgen Sept. 20, 1955 2,778,077 Andrews Jan. 22, 1957 2,800,692 Cooke July 30, 1957 2,815,550 Volyi Dec. 10, 1957