Apparatus For Centrifugal Compaction

Abstract

A method and apparatus for the centrifugal compaction of particulate material in which the particulate material is admixed with a liquid to form a slurry and is then placed in a chamber and centrifuged about a main axis to cause the material to compact into a discrete body. A particular feature of the invention is to be found in the fact that the chamber in which the slurry is contained during compacting is caused to rotate about an axis inclined to the main axis so that the particulate material is agitated and thereby nests intimately together during the centrifuging operation thereby resulting in workpieces significantly more dense than can be arrived at by simple centrifuging or even by pressing.

Description

The present invention relates to a method and apparatus for the centrifugal compaction of granular or particulate materials.

Granular or particulate or powdered materials are employed for the making of ceramic articles and also for the making of articles from metals and the like. The use of granular material is of benefit, in some cases, for reasons of economy, because the workpiece formed by the compacting operation can be formed nearly to size and shape thereby eliminating expensive machining operations.

In other cases, as in the case of some ceramics and cermets and certain metals, the product is so difficult to machine that it is highly uneconomical to attempt to form the article by any other method than by the compaction of powders. Ceramic articles fall into this class, as do articles formed of hard metallic carbides.

A characteristic of the harder materials is that the particles or powders, which are employed for the manufacture thereof, are quite often jagged in nature, having points and sharp edges thereon, so that no simple compacting method will bring the powders into such intimate relation that all bridging of the particles on each other, which produces voids and flaws, is greatly reduced and, in most cases, completely eliminated.

Lubricants are usually employed in respect of such powders to assist in the free movement of the particles relative to each other during compaction, but even in the presence of a lubricant, the jagged particles from which carbide materials and ceramic materials are made will tend to form bridges between particles, with voids and flaws resulting.

With the foregoing in mind, it is a particular object of the present invention to provide an improved method and apparatus for effecting the compaction of powdered materials which will result in the formation of workpieces which are more uniformly compacted and more dense than have been attainable heretofore by known practices.

A particular object of the present invention is the provision of a method and apparatus for compacting particulate materials in which the compacting force on the materials is developed in a centrifuge.

Still another object is the provision of a method and apparatus for centrifuging particulate material into a condition of compaction which develops simultaneous controlled agitation of the materials together with motion between particles relative to each other at the time of compaction and which results in a superior uniformity and density of the final product.

These and other objects and advantages of the present invention will become more apparent upon reference to the detailed specification taken in connection with the accompanying drawings in which:

FIG. 1 is a somewhat schematic perspective view showing one form of centrifuging apparatus according to the present invention utilizing two flasks with one shown in outwardly swung position and the other in vertical position.

FIG. 2 is a plan view of the centrifuge shown in FIG. 1 with one of the flasks thereof hanging vertically and the other shown in swung out position.

FIG. 3 is a partial vertical section through the centrifuging apparatus showing one side thereof and is indicated by line III--III on FIG. 2.

FIG. 4 is a vertical section through one of the flasks of the apparatus showing the construction thereof and the member inside the flask forming the material receiving chamber.

FIG. 5 is a fragmentary perspective view showing one form which the clutch can take for connecting a drive to the chambers in the flasks.

FIG. 6 is a fragmentary sectional view showing one form which the brake can take which is associated with the drive for the chambers in the flasks.

FIG. 7 is a sectional view like FIG. 4 but shows a modified arrangement of the flask and chamber.

FIG. 8 is a schematic view illustrating the manner in which the arrangement of FIG. 4 functions.

FIG. 9 is a schematic view like FIG. 8 but shows the manner in which the modification of FIG. 7 functions.

FIG. 10 shows a modification in which the cross sectional area of the cavities is relatively small and the cavities are inclined to the axis of the flask to obtain substantial action in the liquid contents in the cavities;

FIG. 11 is a sectional view schematically illustrating a flask having a mold member therein for producing a complex shape.

BRIEF SUMMARY OF THE INVENTION

The present invention discloses a centrifuge in which flasks are mounted on a structure rotatable on a main axis by means of trunnions so that the flasks will be swingable radially with respect to the main axis. When the main axis is vertical, as is usually the case, the flasks will swing outwardly at the bottom when the rotating structure is driven in rotation. This arrangement permits the flasks to be loaded from the top and for the centrifugal action to impel the material being compacted toward the bottoms of the flasks which, when the rotary structure is rotating, will be radially outermost.

The material to be compacted is granular and is admixed with a liquid to form a slurry. The liquid has a lower specific gravity than the particles admixed therewith and, when the rotary structure rotates, the centrifugal force acting on the particles will cause them to migrate in the radially outwardly direction and thereby to become compacted.

The liquid contains a lubricant and a binding agent so that the particles will slide on one another during compaction and remain in position at the end of the compacting operation thereby to form a discrete body which can be removed from the flask and manually handled.

A particular feature of the present invention is in the provision of an inner chamber in the flask which is rotatable therein and which, when the flask is swung outwardly to its outermost position, is inclined to the horizontal and is driven in rotation. By rotating the inner chamber as the rotary structure rotates, a sort of tumbling action is produced on the particles within the chamber which effectively breaks down bridges in the material and assists in causing the particles to rest closely together so that a higher degree of compaction of the material is obtained that can be obtained by simply centrifuging the material.

After the material is compacted into a discrete body, it is removed from the chamber and is then dried and sintered to form a solid member. It has been found that articles made according to the invention disclosed herein are markedly more free of pits and voids than compacted granular workpieces made by other methods, including straight centrifuging methods and dry pressing methods, and are more dense thereby leading to a superior end product for most purposes.

As mentioned, the main axis about which centrifuging takes place is usually vertical, as a matter of convenience, but may be disposed at any angle. When the main centrifuging axis is at an angle other than vertical, the angle of the axis of rotation of the flasks is inclined at an angle to a plane perpendicular to the main axis. In any case, the angle of inclination of the axis of rotation of the flasks can be quite small or quite large, say, as small as 1.degree. or as large as 89.degree.. In practice, an angle of about 15.degree. has been found to be satisfactory.

The speed of rotation of the chamber in the flask is preferably only a fraction of the speed of the rotation of the rotary structure and is sufficient to provide for the aforementioned tumbling action of the particles until the particles commence to compact in the radially outer end of the chamber. The speed of rotation of the chamber is not sufficient to disturb the compacted particles.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings somewhat more in detail and with particular reference to FIGS. 1 to 3, the centrifuging apparatus will be seen to comprise a drive mechanism generally indicated at 10 which is disposed inside a large circumferential shield member 10a. Mounted on the upper end of the output shaft of the drive mechanism 10 is a plate 11 forming the central member of a rotary structure which also comprises laterally extending spaced parallel bars 12 secured to plate 11, as by cap screws 14, and notched at 16 so as to engage the edges of the plate 11. Bars 12 form the arms of the centrifuge. The rotary structure rotates on a vertical main axis but, as mentioned previously, the main axis could be at any other angle.

Toward their outer ends, the arms are provided with laterally aligned bores in which bushings 18 are mounted and which bushings rotatably receive trunnions 20 of support rings 22. Each support ring 22 is adapted for receiving a cylindrical flask member 24 from above with each flask member having a flange 26 at its upper end to engage the upper side of the respective ring 22.

It will be appreciated that the centers of gravity of flasks 24 are disposed substantially lower than the horizontal pivot axes defined by the respective trunnions 20 of the pertaining support rings so that, when the rotary structure of the centrifuge rotates, the flasks will swing outwardly at their lower ends.

As the centrifuge slows to a halt, however, the flasks return to a vertical position thereby providing for free access to the upper ends thereof for receiving the material to be centrifuged. In FIGS. 1 and 2, the right hand flask is shown in the dependent position it will occupy when the rotary structure is at rest whereas the left hand flask is swung outwardly to the position which it will occupy when the rotary structure is rotating.

Each of the bars 12 of the centrifuge arms on top thereof near the outer end and radially outwardly from the position of trunnions 20 is provided with an elongated upwardly opening notch 28 that receives the downwardly thickened region at the inner end of a respective one of a pair of spaced parallel arms 30 which are fixedly secured to bars 12 as by cap screws 32. On the lower sides of arms 30, at the outer ends thereof, are formed the downwardly opening inclined slots 34 in which pair of which is seated a member 36 extending transversely of the said arms and fixed thereto as by cap screws 38.

The central portion of each member 36 is recessed, as at 40, and mounted therein is an arcuate cushion member 42 made, for example, of Teflon or the like. As will be seen at the left side of FIGS. 1 and 2, the cushion member 42 is arranged to engage the lower finished circumference 44 of the respective flask when the flask swings outwardly thereby to stop the respective flask with the longitudinal axis thereof inclined to the horizontal and, specifically, downwardly with respect to the horizontal. More generally, the flasks are halted with the axes thereof inclined to a plane perpendicular to the main centrifuging axis.

As will be seen in FIG. 4, each flask comprises a relatively thick walled cylindrical member 50 closed at the bottom by a plate 54. Resting on plate 54 is a bearing support plate 56 having a cavity in which is mounted the outer race 58 of a thrust bearing which has an inner race 60 and a tapered rollers 62 interposed between the races. Inner race 60 is adapted for receiving a pintle or stub shaft 64 formed on the bottom of a further support plate 66 which has a cylindrical bore 68 in the upper side thereof.

Cylindrical bore 68 is adapted for receiving the pintle or stub shaft portion 70 formed on the bottom of a cylindrical member 72 disposed in flask 24 and coaxial therewith and forming the chamber which is to receive the material to be compacted. Cylindrical member 72 extends to near the top of the flask and is adapted for having the upper end thereof closed by a closure cap 74 held in place by cap screws 76.

The upper end of flask 24 comprises a bearing support ring 78 held in place in the upper end of member 50 as by radial set screws 80. Ring 78 supports the radial bearing 82 which fits about the enlarged portion 84 formed on the upper end of cylindrical member 72.

It will be noted that between the lower end of enlarged portion 84 and the upper end of an upstanding annular lip 86 of ring 78 there is a substantial amount of axial clearance so that cylindrical member 72 is always supported on the thrust bearing at the bottom thereof and even conditions of high load at high centrifuging speeds will not close the aforementioned gap.

The inside of cylindrical member 72 may itself receive the material to be compacted but it is more advantageous to form the member 72 to a size larger than the piece to be made and to insert in member 72 a mold member 88, which can be formed of epoxy resin, for example.

Mold members 88 is preferably provided with a film-like liner member 90 of any suitable material, such as polyvinyl alcohol, and it is into this member that the slurry 92 consisting of the granular or particulate material to be compacted and the liquid vehicle therefor is placed. The mold member 88 may have a single cavity therein or it may have a plurality of cavities.

Further, two or more mold members may be placed in member 72 in end to end relation. Thus, a single article can be made in each flask or a plurality of members can be made in each flask.

The particulate material may consist of any sort of granular material including that which is used to form ceramics as well as any type of powdered metal and may also consist of such material as hard metal carbides and a binder metal therefor.

In respect of the hard metal carbides, for example, the carbide may be tungsten carbide or titanium carbide or tungsten titanium carbide and the binder material may consist of any of several types of metals, or alloys, or combinations thereof such as cobalt, nickel, molybdenum, iron and other known binder materials. All such materials are finely ground prior to compaction according to practices well known in the art.

A particular characteristic of carbide particles is that they are often somewhat dendritic in nature with sharp edges and points and thus do not slide free upon one another. Accordingly, in the compacting of such materials, whether by dry pressing, or by centrifuging while suspended in a slurry, the particles tend to engage each other in less than fully nested position and form bridges which can show up as pronounced flaws in the finished product.

The liquid in which the particles are suspended to form the slurry includes a lubricant such as paraffin wax to facilitate the sliding of the particles on one another but even in the presence of a lubricant, as the particles begin to compact into a small space, the aforementioned bridging effect will begin to take place. The disrupting of such bridges during the compacting operation is, therefore, a highly desirable objective to achieve.

The structure of the present invention is specifically designed to prevent the formation of such bridges in the material during compacting by creating controlled agitation in the material during compaction which will promote the nesting of the particles together but without, however, in any way interfering with the formation of a discrete body which can be handled when it is removed from the centrifuge.

As was pointed out above, the flask in its outwardly swung position has the axis inclined to a plane perpendicular to the main axis and for this reason the axis of inner member 72 is also so inclined and by rotating this member in the flask while the rotary structure of the centrifuge is rotating, sufficient agitation of the particles within the chamber is created to cause the intimate nesting together of the particles and to prevent the formation of the bridges referred to.

Rotation of the inner member in the flask is accomplished by a clutch arrangement provided on the inner member which couples with a drive therefore when the flask is swung to its outermost position. This clutch arrangement can take any of several forms but for the purpose of the present disclosure will be seen to comprise a pair of drive pins 100 extending axially outwardly from the upper end of top closure member 74 of inner member 72. Pins 100 are tapered on the end and are yieldably supported in the axial direction.

Drive pins 100 are adapted for cooperation with a pair of drive lugs 102 mounted on the end of a shaft 104 supported by bearings 106 in a bracket 108 carried by plate 11 and fixed thereto by screws 109. The lugs 102 are tapered inwardly to permit pins 100 to cam thereover, if necessary.

Lugs 102 are spaced apart so as to define a central slot 112 and, when starting the centrifuge, shaft 104 is rotated till lugs 102 are horizontal and the respective inner member 72 is rotated so that drive pins 100 will be in the vertical radial plane of the slot so that, when the flask swings outwardly on its trunnions, one of the drive pins will pass through slot 112 and, thereafter, when shaft 104 is set into motion, the lugs 102 will engage drive pins 100 and drive inner member 72 in rotation in flask 24.

It will be apparent that other types of clutching arrangements could be employed and that the important thing in respect of the clutch is to establish a driving connection to the inner member 72 for each flask, at least, when the respective flask is swung to outermost position.

Each shaft 104 is connected by a universal joint 110 (FIGS. 1, 2) with a respective shaft 113, and each of which is connected by a further universal joint 114 with a respective slow speed output shaft 116 of a speed reducer 118 which has an input shaft 120 arranged on the axis of rotation of the rotary structure of the centrifuge.

Shaft 120 carries one element 122 (FIG. 6) of a brake mechanism which has another element 124 slidably keyed to a shaft 126 stationarily supported by a member 128. Element 124 of the brake is connected to a lever 130 which is normally biased by spring 132 in a direction to disengage the brake elements.

Also connected with lever 130 is the armature of a solenoid-armature arrangement 134 which, upon energization, will overcome the bias of spring 132 and move the brake elements 122 and 124 into an operative engagement with each other.

When the centrifuge is started, the solenoid is de-energized and the brake elements are disengaged and input shaft 120 rotates with the rotary structure of the centrifuge. However, after flasks have swung to their outermost position thereby bringing drive pins 100 into operative relation relative to lugs 102, the solenoid is energized and this will cause the elements of the brake to engage and to hold input shaft 120 against rotation which will, in turn, cause rotation of the output shafts 116 relative to the rotary structure of the centrifuge whereby the inner members 72 of the flasks will be driven in rotation.

In an arrangement wherein the distance from the center of rotation of the rotary structure of the centrifuge to the trunnions for the flasks is about 10 1/2 inches, and the bottoms of the flasks are spaced about 10 inches from the trunnions, the rotary structure might rotate at a speed of from, say, 600 to 800 revolutions per minute while the drive to the chambers or inner members in the flasks will cause rotation thereof at, say, from about 15 to 25 revolutions per minute.

The inclination of the flasks from the horizontal is illustrated in the drawings at 15.degree., but the exact amount of inclination of the flask can be varied considerably, say, from 1.degree. to 89.degree., and the benefits derived from the practice of the present invention may be realized. A suitable angle of inclination of the flasks which can easily be arrived at might, for example, be as little as 10.degree. or as much as 20.degree..

Further, the particular angle of inclination of the flask is, at least in part, determined by the manner in which the inner member forming the material chamber is arranged therein. In FIG. 4, the inner member 72 is coaxial with the outer member 50 forming the body of the flask and under these circumstances an angle of inclination of 15.degree. has been found to be satisfactory.

In a modified form which the apparatus can take, as shown in FIG. 7, the trunnion mounted flask, indicated generally at 150, can be permitted to swing outwardly to a substantially horizontal position because the inner chamber member therein, indicated generally at 152, is inclined relative to the axis of the flask. This is accomplished by mounting inner member 152 in a frame consisting of an upper plate 154, a lower plate 156, tie bolts 158 connecting plates 154 and 156 and with member 152 supported on the plates in inclined relation, on suitable support bearings which permit rotation of inner member 152.

The plates 154 and 156 are receivable in flask 150 and will support inner member 152 in the illustrated inclined position therein. As before, inner member 152 has an element 160 of a disengageable clutch thereon so that when the flask is in its outwardly swung position, the inner member is automatically coupled to a drive so that the inner member can be driven in rotation thereby.

FIG. 8 schematically illustrates the manner in which a flask 200 is stopped at an inclined position with the axis about 15.degree. below the horizontal. Flask 200 has an inner container 202 therein containing slurry 204. When the centrifuge is rotating at high speed, the radially inner surface of the body of slurry in inner container 202 is substantially vertical as shown by line 206. As the inner container is rotated within the flask, the slurry will continuously shift therein to maintain the surface plane at 206 thereby causing relative motion of the particles in the slurry and the nesting together thereof described previously and leading to the high degree of compaction referred to.

FIG. 9 illustrates the manner in which the modification of FIG. 7 can operate and wherein flask 208 is swung out substantially horizontal position while inner container 210 is in an inclined position due to its support within the flask. When inner container 210 is rotated on its axis, the same agitating action on the slurry therein will be obtained as was described in connection with FIG. 8.

In FIG. 10, flask 212 has therein an inner container 214 within which is mounted mold members having the relatively small diameter cavities 216 therein. Flask 212 is stopped with its axis about 15.degree. below the horizontal and with the flask stopped at this angle and the centrifuge in operation, rotation of the inner container will bring about a substantial movement of the slurry within the individual cavities 216.

FIG. 11 illustrates somewhat schematically one of the particular advantages to be realized from the centrifugal compacting of particulate material according to the present invention. In FIG. 11, a flask 230 is provided with an inner member 232 which may, for example, consist of epoxy resin or the like.

Within the cavity in member 232 is a further member 234 having a relatively complex cavity therein and in which the slurry to be compacted is placed. After centrifuging the slurry to its desired degree of compaction of the particulate material therein, member 234 is removed from member 232 and the free liquid on top of the compacted body in member 234 is poured off.

By forming member 234 of a material such as polyvinyl alcohol which can be dissolved completely away from the body therein by water and the body can then be sintered according to standard practices.

Polyvinyl alcohol has been mentioned because it will dissolve in the water and can be recovered for reuse, but it will be understood that other materials having different solvents can be employed for this purpose. The important thing is that the material forming the mold cavity be strong enough to maintain the shape of the cavity during centrifuging and thereafter be releasable from the compacted body without imposing any strains on the body, as well as being impervious to solvent or chemical action of the liquid used to form the slurry.

In respect of powders pressed by conventional pressing techniques, and even when hydrostatically compacted at pressures up to 50,000 psi, the shrinkage of the compacted body which takes placeupon sintering will range from about 19.4 to 19.6 percent. The same carbide powders when compacted as disclosed in the present application had shrinkage factors as low as 14.5 percent without hydrostatic compacting and as low as 13.6 percent when hydrostatically compacted at about 35,000 pounds per square inch.

Further, the pits and flaws in sintered compacts made as disclosed in the present application were extremely low as compared to the pit and flaw count in compacts made according to standard practices, for example, by dry compacting.

Specific examples of the practice of the present invention are given below.

EXAMPLE I

One typical mix of tungsten carbide powder was made as follows:

Tungsten Monocarbide -- 93.68 percent by weight

Cobalt -- 5.81 percent by weight

Paraffin Wax -- 0.51 percent by weight

The starting materials in powder form were ground together in a jar mill in naptha containing paraffin wax in solution until reduced to about 1.06 Fisher subsieve particle size. The resultant powder was dried and then contained about 0.51 percent by weight of paraffin wax.

A liquid vehicle solution was then prepared for making up the powder slurry. It contained the following:

Perchloroethylene 100.00 milliliters (Sunoco 3420) A Paraffin Wax having a melting point of about 137 degrees Fahrenheit (Sun Oil Co.) 0.90 grams (Elvax 260) Vinyl Resin in the form of a Copolymer of Ethylene and Vinyl Acetate. (E. I. DuPont de Nemours and Co.) 1.80 grams (Arquad 2C-75) A Dialkyl Quaternary Ammonium Chloride derived from a fatty acid. (Armour Industrial Chemical Co.) 3.40 grams

This vehicle solution has a gel point of about 65.degree. F.

The above liquid vehicle solution was then mixed with the tungsten carbide powder in the porportion of 90 milliliters of solution to 1 kilogram of powder. The resulting slurry was then charged to a tungsten carbide lined jar mill and milled for 3 days at 115.degree. F. It was then ready to be centrifuged to form compacts.

The centrifuging flask used is illustrated in FIG. 4 of this application. The mold member shown (88 in the drawings) was made of epoxy resin and the cavity therein was 2 1/8 inches diameter by 10 inches in depth. The film-like liner member (90 in the drawings) with which the cavity was lined was made of polyvinyl alcohol.

Upon completion of milling, the slurry was poured into the polyvinyl alcohol liner member contained inside the cavity and centrifuged for 20.75 hours total time at 95.degree. F. During centrifuging, the rotational speed about the main axis of rotation was 530 revolutions per minute producing a force of 177 times G, the acceleration of gravity. During this period the flask was rotated about its axis at 53 revolutions per minute for a period of 20 hours and was not rotated during the final period of centrifuging.

Upon completion of centrifuging, the powder compact together with the polyvinyl alcohol liner member in which it was encased was removed from the cavity of the mold member. The compact and polyvinyl alcohol liner member were then moistened with water and allowed to stand until the liner member softened and disintegrated sufficiently to be stripped from the compact without causing any surface damage or "pluck out." This was done at ambient temperature and took about 1/2 hour.

The compact was then dried at 160.degree. F. for about 36 hours. The compact was then cut into test pieces; in some cases it was isostatically hydropressed before cutting into test pieces.

Test pieces were then sintered at about 200 microns absolute pressure at 2,200.degree. F. for 1/2 hour.

For comparison, the same powder batch used in making the centrifugal compacts described above was made into compacts by conventional isostatic hydropressing at 50,000 pounds per square inch. Test tips cut from these compacts were sintered under the same conditions used for the centrifugal compacts.

Differences in properties are shown in the tables which follow:

TABLE I ______________________________________ TRANSVERSE RUPTURE STRENGTH IN LBS. PER SQ. IN./1000 Method Used Conventional Centrifugal Compaction Compaction ______________________________________ Lowest Value 271 340 Highest Value 384 445 Average Value 337 403 ______________________________________

TABLE II ______________________________________ MACRO PIT AND FLAW COUNT Method Used Conventional Centrifugal Compaction Compaction ______________________________________ Total Pits and Flaws per 100 square inches .002 inches and larger in diameter 41.8 0.9 .004 inches and larger in diameter 9.0 0.0 Total Area Examined -- square inches 134.0 115.5 ______________________________________

It will be noted that the transverse rupture strength of the samples made by centrifugal compaction according to the present invention and the pit and flaw count are both substantially improved over the values obtained when compacting by conventional methods.

In the table which follows, Shrinkage Factors are expressed as the ratio of a given dimension before and after sintering.

TABLE III ______________________________________ SHRINKAGE FACTORS Method Used Conventional Centrifugal Compaction Compaction ______________________________________ Isostatic pressure used in pounds per square inch No pressure used -- 1.134 to 1.146 35,000 -- 1.132 to 1.137 50,000 1.194 to 1.196 1.138 to 1.140 ______________________________________

The Shrinkage Factor data above illustrates the very substantial improvement in density obtained by centrifugal compaction of the present invention. With conventional compaction the lowest shrinkage factor was 1.194 corresponding to 58.7 percent of theoretical density. With Centrifugal Compaction densities ranged from 66.4 to 68.9 percent of theoretical.

EXAMPLE II

A mix of tungsten carbide powder was made up of the following powdered ingredients:

Tungsten Monocarbide 87.50% by weight Cobalt 12.15% by weight Paraffin Wax 0.35% by weight

The powdered ingredients were ground together in a jar mill containing naphtha with paraffin wax in solution until reduced to about 1.18 Fisher subsieve particle size and then dried. It contained about 0.35 percent by weight of paraffin wax after drying.

The dried powder was then mixed with liquid vehicle solution of the composition described under Example I, in the proportion of 85 milliliters of solution to 1 kilogram of powder. The resulting slurry was milled in a tungsten carbide lined jar mill for 3 days at 115.degree. F. after which it was ready for centrifuging to form compacts.

The centrifuging equipment was the same as that used in Example I, and a polyvinyl alcohol liner member was used as in Example I. Rotational speed about the main axis of rotation was 530 revolutions per minute producing a force of 177 times G and the flask was rotated about its own axis at 53 revolutions per minute. Centrifuging time was about 20 hours and temperature was 95 to 105.degree. F.

Upon completion of centrifuging, the compacts were processed as in Example I except that sintering temperature was 2,575.degree. F.

Conventional hydropressed compacts were made from the same composition for comparison as described under Example I.

Properties obtained by the centrifugal method of this invention as compared to properties obtained by conventional hydropressing are given in the tables which follow. The conventional compact was hydropressed at 50,000 pounds per square inch; the Centrifugal Compact was not hydropressed.

TABLE I ______________________________________ TRANSVERSE RUPTURE STRENGTH IN LBS. PER SQ. IN./1000 Method Used Conventional Centrifugal Compaction Compaction ______________________________________ Lowest Value 422 452 Highest Value 481 504 Average Value 462 478 Hardness -- Rockwell A 88.0 88.1 ______________________________________

TABLE II ______________________________________ MACRO PIT AND FLAW COUNT Method Used Conventional Centrifugal Compaction Compaction ______________________________________ Total Pits and Flaws per 100 square inches .002 inches and 10 larger in diameter 37.7 2.7 .004 inches and larger in diameter 6.8 1.8 Total Area Examined -- Square Inches 103.4 110.0 ______________________________________

TABLE III ______________________________________ SHRINKAGE FACTORS Method Used Conventional Centrifugal Compaction Compaction ______________________________________ Isostatic Pressure used in pounds per square inch No Pressure Used -- 1.142 to 1.150 35,000 -- 1.137 to 1.141 50,000 1.180 to 1.187 -- ______________________________________

With conventional isostatic compaction the lowest shrinkage factor was 1.180 corresponding to 60.9 percent of theoretical density. With triaxial centrifugal compaction densities ranged from 65.8 to 68.0 percent of theoretical.

Claims

What is claimed is:

1. In an apparatus for the centrifugal compaction for granular materials, a support, means engaging said support and defining a first axis of rotation thereof, flasks pivotally mounted on said support in circumferentially distributed relation and radially spaced from said first axis, each flask being swingable outwardly at the bottom in response to rotation of said support, chamber means carried by each flask and having a cavity adapted to receive a slurry consisting of the said granular material to be compacted and a liquid, a drive shaft on said first axis for rotating said chamber means about respective second axes extending angularly to said first axis as said support rotates about said first axis, said second axes being inclined at a predetermined angle to a plane perpendicular to said first axis, drive means responsive to outward swinging movement of the lower ends of said flasks on said support for drivingly connecting said chamber means to said drive shaft, and means on said support for engaging and halting said flasks at said predetermined angle of inclination to said plane.

2. An apparatus according to claim 1 in which said chamber means comprises a hollow cylindrical metallic shell adapted to receive therein mold elements defining the size and shape of the article to be made.

3. An apparatus according to claim 1 in which said drive means includes output shaft means coupled at one end to said drive shaft and extending radially along said support to the region of said chamber means.

4. An apparatus according to claim 1 which includes a speed reducer having an input shaft on said first axis and forming said drive shaft and an output shaft for each chamber extending on said support to near the respective chamber, and cooperating elements of a brake connected to said input shaft and to a stationary point and operable when actuated to hold said input shaft stationary during rotation of said support thereby to cause said output shafts to rotate and drive the said chamber means on said second axis.

5. An apparatus according to claim 4 which includes means selectively operable for actuating said brake means.

6. An apparatus according to claim 1 which includes a ring for supporting each said flask, trunnions on each ring pivotally connecting the ring to said support on a horizontal axis which is perpendicular to a radius extending therefrom to said first axis, each said flask being generally cylindrical and being insertable into the respective ring from above and including flange means near the top to engage the top of said ring, each said chamber means being insertable into the respective said flask from above, and bearing means interposed between each said chamber means and the pertaining said flask to permit free relative rotation of the chamber means in said flask.

7. An apparatus according to claim 6 in which said bearing means include thrust bearing means interposed between the bottom of each said chamber means and the bottom of the pertaining said flask and radial bearing means disposed between an upper region of each said chamber means and an upper region of the pertaining said flask.