US2845667A

US2845667A - Molds for casting disc-shaped bodies

Info

Publication number: US2845667A
Application number: US445148A
Authority: US
Inventors: Frischknecht Hans
Original assignee: Sulzer AG
Current assignee: Sulzer AG
Priority date: 1954-07-22
Filing date: 1954-07-22
Publication date: 1958-08-05
Anticipated expiration: 1975-08-05

Description

Aug' 5, 1958 H. FRlscHKNEcH-r 1 2,845,667

` MOLDS FOR CASTING DISG-SHAPED'BODIES Filed July' 22. 1954 INVENTOR HANS FRISCHKN ECHT ATTO R N EYS MOLDS FR CASTING DISOSHAPED BODIES Hans Frischknecht, Winterthur, Switzerland, assignor to Sulzer Freres, Societe Anonyme, Winterthur, Switzerland Application July 22, 1954, Serial No. 445,148

3 Claims. (Cl. 22-113.5)

This invention relates to casting molds and more particularly to casting molds adapted for the casting of discshaped bodies such as bladed turbine rotors.

Casting molds have been used for some time in the centrifugal casting of disc-shaped bodies, such as bladed turbine rotors for axial-flow engines in which the molten metal comprising the blades and the rim is fed into the casting mold at points located at the periphery of the casting mold so that the molten metal required for said parts will be fed directly into the mold at points close to where said parts are located. The molten metal required for the remaining7 part of the turbine rotor (the hub) is fed separately into the central part of the mold. Such casting molds usually consist of a central distributor in which the molten metal is contained which opens not only into the central part of the mold but also has connected to its several runners extending from the central distributor to points at the periphery of the mold. This permits an initial ow of molten metal from the central -distributor through the runners into the portions of the mold in which the blades and rim part are formed and, then directly into the central portion of the mold through a separate conducting means. ln casting with a mold of this type, the conducting means leading from the central distributor into the central part of the mold is kept closed by means of a manually or mechanically operated plug until the blades and the rim portion of the casting mold are filled with molten metal. Thereupon the plug is raised either by the said hand or mechanical operating means to open the conducting means leading directly from the central distributor to the central part of the mold to permit the ow of molten metal directly into the central part of the mold. Fusible metallic plugs have also been used in place of the hand or mechanically operated plugs to close oli the opening from the central distributor to the central portion of the molds. The fusible metallic plugs are of different thicknesses so that by selecting a plug of a proper thickness the operation can be timed by selecting a plug that will fuse through from being in contact with the molten metal in the desired length of time. The reason for using molds of this type is to prevent the molten metal of which the blades `and rim part of the turbine rotor are cast, from having to iiow through the interior of the entire mold cavity Where it will pick up impurities from the mold, such as particles of the mold material. However, this type of mold, because of the relative length of the runners and relatively large diameter of the casting cavity causes the metal to cool before the mold becomes completely lled. This uneven cooling causes shrinkage or contraction defects known as piping in the finished casting. These defects are usually evidenced by microscopically visible pipes present in the hub part of the turbine rotor. At times even macroscopic pipes are present.

Another problem present is caused by slag inclusions in the casting, particularly in the hub part of the turbine rotor. Castings made in the generally accepted type of casting molds having these defects are unsuitable to satis- "atent fy the Very stringent requirements of turbine rotors. In the production of these objects by methods which have been used heretofore, the percentage of unsatisfactory castings due to piping and slag inclusions, has been great.

My invention provides a casting mold which makes it possible to eliminate the aforementioned casting defects. My casting mold comprises a horizontally extending discshaped casting mold corresponding to the shape of the casting, and a molten metal reservoir positioned above said casting cavity. A riser positioned above the central hub portion of the casting cavity having a Haring portion at its uppermost end connects the central portion of the casting cavity with the said reservoir. Additional connecting means, referred to as runners, connect the reservoir to points located at the periphery of the casting cavity. A tapered plug is provided which seats itself by gravity within the flaring portion of the riser and which will automatically become unseated when the pressure in the riser due to the rising level of molten metal in the riser becomes great enough. The molten metal in the reser- Voir will be permitted to iiow directly into the riser when ing portion of the riser by the rising level of the molten metal in the riser which has flowed into the casting mold and riser through the runners.

The use of the mold of my invention makes it possible to eliminate the casting defects which are prevalent when the generally accepted types of casting molds are used. The plug of my casting mold is automatically operated through the rising level of the molten metal in the mold and riser and requires no manual or mechanical operation. When the plug is automatically lifted, thereby permitting the direct ow of molten metal from the reservoir into the riser the molten metal which is then introduced into the central portion of the mold is of a relatively high temperature to that already present in the mold cavity.

This delays the solidficatio-n of the metal in the central portion of the mold and permits the slag particles which are present in the molten metal to rise through the metal into the riser so that no slag particles will be present in the actual casting. Solidiiication will last occur in the riser so that the formation of pipes and shrinkage cavities will occur in the riser, thereby insuring the obtaining of a casting which is free from such defects.

While the mold of my invention may be used in gravity f Reference is now made to the drawing in detail. Thel casting mold itself is produced by the precision casting r process with the aid of a destructible pattern. The mold is formed With a cavity .l corresponding to the shape ofv the bladed turbine rotor. The axis of said cavity coincides with the `aXis of symmetry of the entire casting device. During the casting operation in the centrifugalcasting machine, the mold is caused to rotate on this aXis. Above the cavity 1 is a molten metal reservoir 2 from which uniformly distributed runners 3 lead downwardly into the peripheral part of the cavity 1. Situated above the central part of the cavity 1 is a riser 4 which is connected to the reservoir 2 by a Haring portion 5. The flaring portion 5 connecting the riser 4 to the reservoir 2 comprises an upwardly flaring seat in the manner of a lastenia? truncated cone to receive aplug 6 which is inserted into the ilaring portion from above prior to the start of the pouring operation. The axis of rotation of the cone is the same as the axis of symmetry of the riser 4, the reservoir 2, rthe vflaring .portionS and the plug `6.

Forthe production ofthe casting moldapattern reproducing-the shape of `the bladed :rotor to be ycast is made of .wax :or other readily fusible material. Attached to this pattern are bodies made of the same fusible material, the shape of 'which corresponds to the composite hollow space A(cavity) .formed by the reservoir 2, the runners 3 and the riser 4. The whole is immersed in a uid composition Abath containing a refractory `material in a ver-y nely divided (granular) form, pulverized quartz, for example, and -a suitable bonding agent. After removal the :entire surface of the pattern has a thin coating -of refractory material. Thepattern is Ithereupontransferred into'the mold box 7 and the remaining unoccupied part of the mold box 7 is filled up witha coarser grained refractory substance. After kdrying the casting mold thus formed is `transferred upside down into a calcining -kilnin which the destructible pattern is removed through melting and combustion so Athere is no residue remaining in the casting mold. After the calcination the still hot mold is `mounted on the centrifugal-casting machine and islthen ready for casting.

Ifnecessary the tapered inferior of the ilared portion v5 can be ground'smooth so it will accurately fit the plug whichds inserted from above prior to pouring the metal into the reservoir 2 (as shown in the drawing). ILike;

the casting mold itself, the'plug is made of a -calcined, refractory material. For the casting operation a-predetermined amount of molten` material is poured from the ladle into -the reservoir 12 of the rotating mold, in such a way as to cause thelevel ofthe Ametal in the reservoir 2 to rise `quickly to a high point and to keep it there. In that `way the entrainment 'of specifically lighter slag-particles into the casting cavity 1 -'can be prevented vto a substantial' degree. The liquid metal ows through the runners 3 into the peripheral part of the casting cavity 1 reserved for the bladed rotor and, while under the Ainfluence of centrifugal force, is flung yinto the blade-forming recesses of the casting cavity. After completely filling the aforesaid recesses the casting cavity 1 becomes vfilled in turn from the periphery inwardly whereupon the -level of the metal `moves upwardly into the riser 4. Until then the plug 6 maintains its position as shown. Being subjected 'to the weight of the'metal column in the reservoir 2 itis forced upon its seat in the flared portion 5 and thus cuts off the ow of metal from the reservoir 2 to the riser 4. While the casting cavity 1 is gradually being filled and while the metal is moving upwardly into the riser 4 the pressure of the air trapped in the riser 4 above the level of the metal becomes greater. After exceeding the equilibrium condition established between the forces exerted .from and from below upon the plug 6 the latter ultimately lifts itself off its seat in the ared portion S, thus allowing molten metal to pass directly from the reservoir 2 into the riser 4 bypassing the .runners 3. In the specific embodiment illustrated the unseating of the plug 6 takes place approximately when the level of the metal in the riser 4 has lreached the dot-and-dash line '8 shown in the drawing. The speed of rotation Vof the v.centrifuge must be such that the static head Vof the molten metal where the runners 3 join the periphery of the -cast- 4 ing cavity 1 is sufficient to force the metal into the hub part of the casting cavity 1 and-overcome the centrifugal force exerted upon it. For good results the volume of the riser 4 should be at least as great as that of the casting cavity 1.

The molten metal. held in the reservoir 2 naturally has a slightly higher temperature than the metal rising from below into the riser 4. The metal which now passes directly from the reservoir 2 into the riser 4 retards the cooling of the metal present in the riser 4 upon completion of the casting operation so that the metal in the riser 4 is the last to solidify. Cooling .of the metal in the main casting cavity ll proceeds progressively from the peripheral part of the casting in the direction of the hub part, and thence upwardly in the riser 4. This mode of cooling insures that any microor macro pipes (contraction cavities) formed will occur, not in the actual casting contained within the casting cavity `1 lbut in `the -riser 4-which is subsequently removed from the casting. Simultaneously 'with this result the direct feeding Iof liquid metal -from the reservoir 2 yinto the riser 4 makes lit easier for entrained slag vparticles to migrate upwards for the reason that the addition of hot kmetal Vfrom Vthe reservoir 2 into the riser 4 retards the solidiiication of the melt in said riser 4.

The castings produced with the aid of the aforedescribed apparatus are completely free from contraction pipes, Whether microscopic or macroscopic, and slag inclusions. I have found that very few defective castings are obtained because the automatic unseating of the plug 6 renders'any additional manipulation unnecessary during the casting operation so there is`no Achance Vfor error.

I'claim:

1. Anapparatus for -casting disc-shaped bodies compris'ing-a mold defining a substantially horizontally posi.- tioned disc-shaped castingrcavity corresponding internally to ,the external 'shape ofthe object to be cast, a molten metal reservoir positioned above said casting cavity, a riser positioned above the central portion of said casting cavity having a flaring portion at'its uppermost end connectingsaid'reservoir to the central portion of said casting cavity, at least one runner connecting said reservoir with the casting cavity at points on its periphery, anda nonefusible plug positioned in the flaring portion ofthe riser adapted to seat itself by gravity within `saidflaring portion and to become unseated by pressure from below when the level of molten metal in the riser rises above a predetermined level thereby permitting the free flow of molten metal from the reservoir into the riser.

2. The 'apparatus of claim 1 in which the elements are arranged symmetrically about the axis of rotation of the said` molding apparatus.

3. The 'apparatus of claim l in which the volume of the riser is at least equal to that of the casting cavity.

Y References Cited in the tile of this patent UNITED STATES PATENTS