Centrifugal Casting Mold With Free Flowing Particulate Heat Transfer Means

Abstract

A centrifugal casting mold with free flowing particulate heat-transfer means operable to act as an insulation barrier at high-rotational speeds of the mold and as a high-heat conductor at the static condition or low-rotational speeds of the mold.

Description

This invention relates to centrifugal casting molds. More particularly, the invention relates to centrifugal casting molds having a controlled absorption rate of B.t.u.'s per minute from the molten state of the casting through its solidification temperature.

In centrifugal casting processes, it is desirable and frequently necessary to control the cooling rate. For example, in casting metal (e.g., cast iron) it is necessary to closely control the cooling rate to arrive at the desired microstructure for a given chemical analysis. Generally, the problem is to retard the cooling rate to provide a uniform and fine grain structure. The conventional method is to arrive at a ratio between the mass of the mold and the material being cast. In other words, in order to retard the cooling rate it is necessary to decrease the mass of the mold, particularly the thickness of the mold wall.

A second requirement of a centrifugal casting mold is that it should be capable of being quickly cooled to a desirable temperature for the next casting operation. It is then necessary to return the mold to its original heat-absorption capability after the temperature of the mold cavity surface has been reduced to prevent undesirable surface grain characteristics and to allow the cast material to solidify sufficiently to grip to the mold when the mold is rotated at high speed. This results in a requirement that the mold be capable of being rapidly cooled which is diametrically opposite to the requirement that the mold be capable of retarding the cooling rate.

Accordingly, it is the object of this invention to provide a centrifugal casting mold having a variable controlled heat-absorption rate.

It is an additional object of the invention to provide a centrifugal casting mold with a relatively low heat-absorption rate during its rapid spinning and a relatively high heat-absorption rate during low spinning or static conditions.

Other objects and advantages will become apparent from the following description when taken in connection with the accompanying drawings, in which:

FIG. 1 is a front sectional elevational view of the mold;

FIG. 2 is a cross-sectional view of the mold in the static position or slow rotation condition;

FIG. 3 is a cross-sectional view of the mold in the high-speed rotation condition; and

FIG. 4 is a flow diagram of a typical centrifugal casting process.

Referring to FIG. 1, the centrifugal mold forming the instant invention is shown generally at 10. A first cylindrical shell 12 having a constant internal diameter 14 throughout its entire length forms a casting cavity 15 for the centrifugal mold 10. At the rightward end of this first cylindrical shell 12 is an annular flange 16 extending around the periphery of the first cylindrical shell 12, with this flange concentric with the cavity of the mold and its axis of rotation.

Surrounding the first cylindrical shell 12 and enveloping it is a second cylindrical shell 18, the second cylindrical shell 18 having a joint axis of rotation with the first cylindrical shell 12. Coaxial counterbores 20 and 22 are formed on opposite ends of the second cylindrical shell 18, with counterbore 20 at the rightward end of the second cylindrical shell 18 receiving the annular flange 16 and maintaining the same in a concentric relationship to the axis of rotation of the centrifugal mold 10. The depth of the counterbore 20 serves to axially locate the first cylindrical shell 12 within the second cylindrical shell 18. Supporting the first cylindrical shell 12 at its leftward end is an annular bearing ring 24. The bearing ring 24 has an outside diameter 26 which is received within the counterbore 22, and an internal diameter 28 which receives an outside diameter 30 of the first cylindrical shell 12. The bearing ring 24 is affixed to the second cylindrical shell 18 by means of a plurality of threaded fasteners 32, 32 so as to radially space and center the first cylindrical shell 12 within the second cylindrical shell 18.

At the leftward end of the first cylindrical shell 12 is an end closure 34, permanently secured to the first cylindrical shell 12 by a plurality of threaded fasteners 38, 38 and having a central aperture 36. The diameter of the aperture 36 is less than the inner diameter of the casting so that molten metal will not run out the end of the mold when the molten metal is introduced into the cavity.

The counterbore 20, at the rightward end of the second cylindrical shell 18, has received within it, along with the annular flange 16, a locating ring 40 which abuts against the annular flange 16. A lug ring 42, disposed in a bore 43 communicating with counterbore 20, abuts against the locating ring 40. A plurality of screws 44, 44 secure the flange 16, the locating ring 40, and the lug ring 42 to the second cylindrical shell 18 so that they all will rotate as a unit about the axis of rotation of the centrifugal mold 10.

To prevent the molten metal from running out the rightward end of the mold, a locking ring 46 having a central conical aperture 48 is secured in position by the lug ring 42 within the locating ring 40 and the locking ring 50. A small diameter 53 of the conical aperture 48 is smaller than the inner diameter of the casting so that molten metal will not run out this end of the mold. The locking ring 46 is removable by a twisting action because of its engaging lugs 49 nesting behind portions on the lug ring 42, with this feature explained in more detail in U.S. Pat. No. 2,657,440.

Two circumferentially extending stepped grooves 52, 52 extend completely around an outer peripheral surface 54 of the second cylindrical shell 18 and are used for the purpose of alignment when the centrifugal mold is being spun, rollers on the molding machinery (not shown) engaging the centrifugal casting 10 at these locations to rotate and guide the said centrifugal casting 10.

A chamber 56 of annular configuration is defined by an outer peripheral surface 58 of the first cylindrical shell 12 and an inner cylindrical peripheral surface 60 of the second cylindrical shell 18. Partially filling the chamber 56 is a heat-transfer medium 62 shown herein as copper shot. FIG. 1 shows this heat-transfer medium only partially, it is to be understood that the same extends and is disposed throughout the axial extent of the chamber 56 so as to partially fill it. This heat-transfer medium 62 is shown further in FIG. 2 which depicts the low rotation or static condition of the centrifugal mold 10 and, therein the copper shot is seen as partially filling the chamber 56 and in contact with both the outer peripheral surface 58 of the first cylindrical shell 12, and the inner peripheral surface 60 of the second cylindrical shell 18. This provides a relatively high heat-transfer rate from the first cylindrical shell 12 and the second cylindrical shell 18 through the heat-transfer medium 62.

FIG. 3 shows the centrifugal mold 10 during high-speed rotation and indicates how the heat-transfer medium 62 has uniformly distributed itself about the inner peripheral surface 60 of the second cylindrical shell 18, leaving an air insulating barrier 61 between the heat-transfer medium 62 and the outer peripheral surface 58 of the first cylindrical shell 12.

When the molten metal is first introduced into the cavity 15 of the centrifugal mold 10, (high spin speeds) slow cooling is required, therefore, the first cylindrical shell 12 must not transfer a large amount of heat to the second cylindrical shell 18 since a slow cooling rate must be maintained to enhance the final grain structure of the completed casting. With the heat-transfer medium 62 distributed on the inner peripheral surface 60 of the second cylindrical shell 18, the air barrier that is created acts as an insulation layer causing the molten metal to cool slowly until it has solidified throughout with the proper grain structure. At this solidification point, it is important to speed up the B.t.u. absorption rate of the centrifugal mold 10 to insure proper hardness and good strength characteristics of the casting. This high B.t.u. absorption rate occurs when the heat-transfer medium 56 is in contact with both the first and second cylindrical shells 12 and 18 as indicated in FIG. 2 when the centrifugal mold 10 is rotating slowly or is in a static position. Thus, after the initial solidification of the casting, the centrifugal mold 10 is slowed in rotation, providing a high heat transfer which completes the mold-cooling of the casting at which time it is ejected. The high heat-transfer rate provided by heat-transfer medium 62 then insures that the casting surface of the centrifugal mold 10 (the periphery formed by diameter 14) is cooled sufficiently to provide the next poured casting with the proper surface characteristics and to permit this casting to grip the centrifugal mold 10 so that it may be spun at high rotational speed.

Referring to FIG. 4, the process for centrifugally casting items using the mold of this invention is shown schematically. At point A, the centrifugal mold 10 has been placed on the spinning rollers 11, and is spun to the required speed. The heat-transfer medium 62 is uniformly distributed about the inner peripheral surface 60 at this time (FIG. 3) and the first cylindrical shell 12 is at the required mold temperature. The molten metal is introduced to the cavity 14 of the centrifugal mold 10, and the rotational speed is maintained until the casting has solidified.

The mold is stopped, when the metal has solidified, and raised to point B. Once the mold's rotational speed is slowed down, the heat transfer medium 62 comes into contact with both the first and second cylindrical shells 12 and 18 and rapid cooling occurs. From point B, the centrifugal mold 10 rolls, due to gravity, down an inclined surface to point C where it is then lowered to point D to eject the casting from the mold.

After ejecting the casting, the centrifugal mold 10 again rolls down a sloped surface to await another shot of molten metal at point A. During this wait to be refilled, the centrifugal mold may have to be further cooled to reduce its temperature to the proper temperature for the particular metal being cast. This may be accomplished by letting the centrifugal mold 10 remain in its high heat-transfer condition (FIG. 2) for an extended length of time or by providing supplementary cooling at point A. A more detailed explanation of the centrifugal casting process and an apparatus for carrying it out can be found in the aforementioned U.S. Pat. No. 2,657,440.

From the foregoing description, it is obvious that the centrifugal casting mold described will allow low heat absorption during pouring and initial solidification of the molten metal, and also allow high heat absorption once the metal has solidified, to insure the proper hardness and strength of the casting.

Claims

What is claimed is:

1. A mold for use in centrifugal casting comprising a first shell having an inner wall defining the molding cavity of said mold, a second shell spaced radially outwardly from said first shell, means for maintaining said shells in spaced apart relationship to define a closed chamber therebetween, and a particulate freely flowable heat transfer medium partially filling said chamber and being in contact with both said first and second shells to conduct heat therebetween, said medium being capable of radial movement whereby on rotation of said mold at centrifugal casting speeds said medium will be thrown radially outwardly creating an air barrier between said medium and said first shell.

2. The mold of claim 1, wherein said medium is a particulated material of high heat conductivity.

3. The mold of claim 1, wherein said first and second shells are generally cylindrical.

4. The mold of claim 1, wherein said heat transfer medium is copper shot.

5. A centrifugal casting mold comprising a first cylindrical shell, means for preventing molten metal from flowing out of the ends of said first shell, a second cylindrical shell surrounding said first shell along the axial length of said first shell and spaced radially outwardly therefrom, means for maintaining said first and second shells in spaced apart relationship and defining a closed chamber therebetween, and free flowing particulate means for transferring heat, disposed in and partially filling said chamber, said heat transfer means operable to uniformly distribute itself about the outer peripheral surface of said chamber upon rotation of said shells at centrifugal casting speeds to leave an air space adjacent the inner peripheral surface of said chamber, and operable to contact both said inner and outer peripheral surfaces upon rotation of said shells at speeds below said centrifugal casting speeds.

6. The mold of claim 5, wherein said heat-transfer means comprises a free-flowing particulated material having a high coefficient of thermal conductivity.

7. The mold of claim 6, wherein said material is copper shot.

8. A mold for centrifugally casting a hollow cylindrical member, the mold comprising; (a) a hollow cylindrical jacket having an axis of rotation, (b) a hollow cylindrical insert mounted within said cylindrical jacket and concentric to the axis of rotation of said jacket, said jacket and said insert operable to rotate together about said axis of rotation, (c) an end plug affixed to one end of said cylindrical insert, said end plug having a cylindrical opening concentric to the axis of rotation and smaller than the inner diameter of said cylindrical insert, (d) a locking ring at opposite end of said cylindrical insert from said end plug, and being operable to be removed by a twisting action about the axis of rotation, said locking ring having a cylindrical opening concentric to the axis of rotation and smaller than the inner diameter of said cylindrical insert, (e) an annular gap between the inner diameter of said cylindrical jacket and the outer diameter of said cylindrical insert, said gap being closed by said end plug and said locking ring to define a closed chamber, (f) heat-transfer free-flowing particulate means within said closed chamber and partially filling the same, said heat-transfer means operable to uniformly distribute itself about the inner diameter of said hollow cylindrical jacket upon rotation of said mold about its axis of rotation at centrifugal casting speed to leave an airgap between the heat-transfer means and the outer diameter of said cylindrical insert, and operable to engage both the outer diameter of the cylindrical insert and the inner diameter of the cylindrical jacket upon rotation of the mold about its axis of rotation at speeds less than said centrifugal casting speeds.

9. A centrifugal casting mold comprising; (a) a first rotatable tube, (b) a second rotatable tube received within said first tube and operable to rotate with said first tube, (c) an annular chamber between said first and second rotatable tubes, said chamber having an inner and outer peripheral surface, (d) means for closing the ends of said chamber, (e) free-flow particulate heat-transfer means within said chamber and partially filling the same, said heat-transfer means operable to uniformly distribute itself about the outer peripheral surface of said chamber upon rotation of said first and second tubes at centrifugal casting speeds to leave an air space between the heat-transfer means and said inner peripheral surface of said annular channel and operable to engage both said inner and outer peripheral surfaces of said chamber upon rotation of said first and second tubes at speeds less than said centrifugal casting speed.

10. A centrifugal casting mold comprising; (a) a first rotatable tube having an axis of rotation, (b) first and second coaxial counterbores in opposite ends of said first rotatable tube, said bores concentric with said axis of rotation, (c) a bearing ring received within said first counterbore and fixedly attached to said first rotatable tube to rotate therewith, said bearing ring having a circular opening concentric with said axis of rotation, (d) a second rotatable tube received within said first rotatable tube and rotatable therewith, said second rotatable tube having an annular flange at one end thereof, said flange received within said second counterbore and fixedly attached to said first rotatable tube, and said second rotatable tube radially supported at the end opposite said circular flange by said circular opening within said bearing ring, (e) an end closure fixedly attached to said second rotatable tube at end opposite said circular flange, said end closure having an opening concentric with said axis of rotation, and said opening smaller in size than the internal dimensions of said second rotatable tube, (f) a locking ring received within said second counterbore adjacent said circular flange, and being operable to be removed by a twisting action about said axis of rotation, said locking ring having an opening concentric with said axis of rotation and smaller in size than the internal dimension of said second rotatable tube, (g) an annular chamber between said first and second rotatable tubes, said chamber having an inner and outer peripheral surface and being closed at the ends by said end closure and said locking ring, (h) free-flowing particulate heat-transfer means within said chamber and partially filling the same, said heat-transfer means operable upon rotation of said first and second rotatable tube at centrifugal casting speeds to uniformly distribute itself about the outer peripheral surface of said annular channel, leaving an airspace between the heat-transfer means and said inner peripheral surface of said channel, and upon rotation of said first and second rotatable tubes at speeds less than said centrifugal casting speeds said heat-transfer means being operable to engage both the inner and outer peripheral surfaces of said annular channel.

11. A centrifugal casting mold according to claim 10 wherein said heat transfer means consists of a free-flowing particulated material having a high coefficient of thermal conductivity.

12. A centrifugal casting mold according to claim 11 wherein said particulated material is copper shot having a generally spherical shape.