US3250518A - Investment mixer - Google Patents

Description

May 10, 1966 w. J. VAN ROSSEM INVESTMENT MIXER Filed March 16, 1964 II III'IIIIVI IIJ I NVEN TOR.

United States Patent 3,250,518 INVESTMENT MIXER Walter J. Van Rossem, 1917 Thayer Ave, Los Angeles, Calif. Filed Mar. 16, 1964, Ser. No. 352,068 5 Claims. (Cl. 259107) The present invention relates generally to mixers for the plaster or investment material for investing wax patterns; and the present invention is more particularly concerned with the design of an improved bearing for the rotating shaft entering the mixer and carrying the mixing arm inside the mixing bowl.

Apparatus of the type with which we are here concerned is used for investing a wax pattern, under vacuum, with a suitable composition which will harden and form a mold into which molten metal is poured to make a casting, for example in dental work, the manufacture of jewelry, and other fine arts. In this process, the materials to be mixed are a dry powder and water. The water and powder are mixed under vacuum to form a plastic mix which is then poured into the flask around the pattern. The flask with the mixture in it is normally then vibrated on a vibrating table.

These operations are normally carried out under a very high degree of vacuum which is applied in order to eliminate any air bubbles that are liable to incur in the investment material and which, by adhering to the outer surface of the wax pattern, result in a rough or imperfect casting. These bubbles are largely air which was originally dissolved in the water added to the investment powder or mechanically included before or during mixing. By subjecting the investment mix to a high degree of vacuum these occluded and dissolved gases are expanded and boiled out of the mix at a low temperature below the melting point of the wax pattern. Vibrating the investment material, before hardening, on a vibrating table is a further aid to the elimination of air and vapor bubbles and to obtain an accurate pattern by causing the investment material to conform faithfully to the outlines of the wax pattern.

One of the problems previously encountered in mixers for investment material is maintenance of the bearing for the shaft which passes through the wall of the mixing bowl and carries inside the bowl the arm with which the investment material is stirred and mixed. The plaster mix is very highly abrasive and rapidly wears relatively moving parts. When the vacuum is released at the end of the mixing operation, the pressure differentials may be in a direction to drive this abrasive mixture into the shaft bearing. While this problem can be alleviated to a large extent by cleaning the mixer thoroughly after each use, it will be realized that this discipline may not be observed in a commercial laboratory where the mixer is in more or less continuous use. As a consequence, the abrasive material is frequently allowed to get into the bearings. Once wear starts, clearances are increased and access of the abrasive mixture is easier,.thus increasing wear at a higher rate.

This condition creates a vicious cycle which not only wears the hearing but eventually makes it impossible to maintain within the mixing bowl the high degree of vacuum required because air can leak around the rotating shaft. When this happens highly accurate work of the investment process is no longer possible.

In some designs of mixer, the problem has been largely avoided by a top-mounting of the shaft bearing. That is, the shaft passes downwardly through the top wall of the mixing bowl and carries the stirring arm at its lower end, a design which is common in various mixing devices for paint, salad dressing, and other familiar uses. However, in these cases there is no problem of inverting the mixing Patented May 10, 1966 bowl in order to discharge by gravity the fluent contents of the bowl into the molding flask.

In a mixer for investment material it is greatly preferred to have the shaft extend upwardly through the bottom wall into the bowl, thus requiring the bearing to' be bottom-mounted. The rotating shaft is then connected at its lower external end to the drive shaft of a motor or other means for rotating the shaft and the stirring arm. Known structures of this type have suffered from the fact that the thrust bearing surfaces on the shaft are brought together by the downward pressure exerted in holding the mixing bowl on the motor drive shaft, with the result that this thrust accelerates wear on the thrust bearing surfaces, especially when investment material reaches these surfaces.

Thus it becomes the general object of the present invention to provide a mixer suitable for investment material in which the shaft bearing is mounted at the bottom of the mixing bowl but is so constructed that little or no investment material reaches the relatively movable metal surfaces and consequently these'surfaces have a long life.

More particularly it is an object of the present invention to provide a mixer for investment material in which the shaft bearings are adequately sealed against entry of abrasive material so that the bearing, even though submerged in the investment material, is not accessible to the investment material.

A further object of the present invention is to provide in an investment mixer a bearing construction of novel design in which the thrust bearing surfaces exposed to the investment material are not pressed together, but rather tend to be separated by the normal thrust exerted on the rotating shaft as it is held upon the drive shaft of the motor or other means rotating the shaft in the mixer.

These objects of the present invention are achieved in a mixer for investment material by providing a bowl, which is adapted to hold the investment material to be mixed, with a fixed sleeve projecting through the bowl wall, and a shaft rotatably mounted in the sleeve and projecting beyond the sleeve at each end thereof. A stirring arm is carried on the shaft at the end inside the bowl while the end of the shaft outside the bowl is adapted for connection to suitable means for rotating the shaft. Fluid sealing means between the shaft and the sleeve near the outer end of the sleeve provides a seal against entrance of air into the bowl, thus permitting a high degree of vacuum to be maintained .in the bowl and a lubricant retaining device is placed inwardly of the sealing means from the shaft, the lubricant providing a seal against access of investment material to the fluid sealing means. Outside the bowl, the sleeve-and shaft are provided with cooperating means, in the form of a pair of engaging thrust surfaces, which resist displacement of the shaft inwardly of the bowl by forces directed axially thereof. A pair of mutually opposing surfaces on the shaft and sleeve are located inside the bowl; but at this position the members of the latter pair of surfaces tend to be separated by the thrust on the shaft so that investment material between these surfaces produces a minimum of wear. With this design, the shaft bearing can be mounted at the bottom of the bowl, allowing the outward end of the shaft to be below the bowl.

How the above objects and advantages of the invention, as well as others not specifically referred to herein, are attained will be better understood by reference to the following description and to the drawings, in which:

FIG. 1 is a longitudinal median view through an investment mixing device embodying the present invention.

FIG. 2 is a fragmentary view similar to FIG. 1 showing a portion of the investment mixing device in the inverted position occupied while the flask is being vibrated.

FIG. 3 is an elevation of the mixer shaft, alone.

Referring now to FIG. 1 of the drawing, the mixer illustrated therein comprises a bowl 110 which is adapted to hold a quantity of investment material while it is being mixed with water. Bowl is normally open at one side or end as shown, the open side of the bowl being closed by housing 12 within which flask 14 is mounted and by gasket 15 which closes one end of housing 12 and supports therein flask 14. Gasket 15 also holds in place at one end of the flask the combined base and sprue former 16. Gasket 17 between the bowl and the housing and gasket 15 provide airtight seals between the separable parts of the mixer.

It will be noted that housing 12 provides an internal funnel structure 18 which is designed to direct the investment mix into the upper end of the flask when the mixer is in the inverted or pouring position (FIG. 2) in contrast with the upright mixing position shown in FIG, 1. A suitable fitting 20 is provided on one wall of housing 12 for connection to a vacuum line, not shown, in order that a suitable degree of vacuum can be applied to the interior of the mixing device, including bowl 10.

Sleeve 22 passes through the wall of the bowl 10 and is preferably located at such a position that the sleeve passes through a portion of the bottom wall of the bowl when the mixer is in the upright mixing position illustrated. Sleeve 22 is stationary and may be held in place on the bowl wall by any suitable means, as by cement or the like, that also provides an airtight joint between the bowl and sleeve. Sleeve 22 is metal in order to provide the desired strength and accuracy of dimensions.

Inside sleeve 22 is shaft 23 which is freely rotatable within the sleeve and. which projects beyond the sleeve at both ends. At the lower or outer end of shaft 23 there is carried coupling 24 which is connected to the shaft in any suitable manner, as by screw-threaded joint 25 so that the coupling may, for practical purposes, be regarded as a part of the shaft. The lower end of coupling 24 is provided with a transverse slot 24a, or is otherwise suitably formed, in order to engage tongue 26 on the upper end of the vertically extending shaft 27 which is driven by a motor, not shown in the drawing, either directly or through a suitable speed-reducer. Shaft 27 serves as means for rotating shaft 23 of the mixer.

Shaft 23 is provided with a shoulder 28 engaged by coupling 24 to limit the travel of the coupling where it is screwed onto the shaft. The distance between shoulders 28 and 36, mentioned later, is such that the coupling does not tighten and bend the shaft in sleeve 22.

At its upper end inside bowl 10, shaft 23 is provided with stirring arm means 29. The stirring arm means may have any configuration desired and normally consists of one or more lengths of wire bent into such a shape that the wires thoroughly agitate and mix the investment material as the arm means passes through the material upon rotation of shaft 23.

First fluid sealing means between the shaft and sleeve are provided in the form of O-ring 30 located near the outer or lower end of sleeve 22. Above and inwardly along shaft 23 from O-ring 30, the shaft is provided with lubricant retaining means. This lubricant retaining means takes the form of a shallow, axially elongated recess cut in the peripheral surface of shaft 23, as indicated at 31. For this purpose a recess depth of .005 inch has been found to be quite satisfactory as a recess of this depth holds an adequate amount of a reasonably stiff grease in a film that is strong enough to resist the pressure differential across it when the interior of the bowl is under a high degree of vacuum.

Above and inwardly of recess 31, there is preferably located a second fluid sealing means in the form of O-ring 33 which is located near the upper end of sleeve 22. This second O-ring is preferably added to the bearing construction, although it is not necessary, in order to provide a seal between the body of lubricant in groove 31 and the interior of the bowl. Thus the second fluid sealing means keeps the pressure differentials throughout the bearing down to sufiiciently low values that there is no tendency for the lubricant to be pulled out of recess 31 into the interior of the bowl and become mixed with the investment material.

It has been found experimentally that the first sealing means in the form of O-ring 30 located near the outer end' of the bearing sleeve combined with the lubricant film inwardly of the sealing means, are highly effective for two different purposes. In the first place, these two means prevent the entrance of outside air into the bowl as the vacuum is applied to bowl interior. Also, the body of lubricant is highly effective 'in preventing investment material that gets into the bearing from passing through the film of lubricant and reaching the fluid sealing means at 31 Thus the lower O-ring is efliciently protected by the lubricant against contamination by the investment material and the clearances below or outwardly of the body of lubricant can be maintained at all times sufiiciently small that the bearing is well lubricated and operates free of any abrasive material.

It will be noticed that the sleeve and the shaft rotating therein cooperatae to provide a pair of engaging thrust surfaces at 35. One of these surfaces is the lower end surface of the sleeve While the other is the opposing end face on coupling 24 which, as mentioned above, may be considered as a portion of the shaft since it rotates with the shaft. conform in shape to each other and preferably lie in a substantially radial plane. It will. be noted that the thrust surfaces are located outside of the bowl below sealing means 30 where the thrust surfaces may be lubricated and are entirely free from contamination by the investment material. When the mixer is pressed downward on driveshaft 27 to maintain a driving connection between shaft 27 and coupling 24, the downward thrust on the bowl and the upward and inward reaction on shaft 23 brings these thrust surfaces together. 1

Cooperating means on the shaft and sleeve provide two opposed surfaces at the inner end of the sleeve as indicated at 36; but it will be seen that these same thrust forces just mentioned tend to separate the two surfaces at 36 rather than to press them together. At the upper end of the sleeve, the end face of the sleeve is opposed by a downwardly facing shoulder formed on the shaft near its upper end, as shown in FIG. 3, these two surfaces preferably being in a radial plane; The axial length of sleeve 22 is such that the two surfaces at 36 are spaced well above the bottom of the bowl and are near the average level of the surface of the investment material during the mixing operation.

Actually there is no fixed position for the level of the surface of the investment material since if only a small amount is being mixed, the mix level is normally down below the surfaces at 36, whereas if a large quantity of investment material is required and is being mixed, the level of the mix will be somewhat above surfaces 36.

- However, on the average the amount of investment material being mixed in the bowl at any time brings the surface level approximately at the location of opposed surfaces 36.

This location of these opposedsurfaces which between them provide access to the bearing interior, has several advantages. In the first place, any air which may work out of the bearing into the investment material passes through only a small portion of the. investment material, if at all. Also there is very little tendency for any of the investment material to be forced into the bearing when the joint 36 is near the top of the body of investment material. In any event any investment material working into this joint between the surfaces on the end of the sleeve and on the bearing has a minimum abrasive effect because the axial forces on the sleeve are in a direction such that these surfaces tend to separate rather than to approach one another with the result that the abrasive Thesetwo engaging surfaces indicated at 35 compound does not abrade the opposing surfaces to any significant extent.

Also the level of the surface of the investment material at the center of the bowl depends upon the speed with which the stirring arm means is rotated. A speed in the vicinity of 400 rpm. may be considered as typical or average; but sometimes drive shaft 27 is rotated at speeds as high as four to five times this figure, as a speed of 1725 r.p.m. is now commonly used. At this considerably higher speed, the investment material is thrown outwardly against the walls of the bowl and there is a corresponding lowering of the level of the material at the center of the bowl.

After the investment material is thoroughly mixed, the mixer is disconnected from driveshaft 27 by lifting it off the driveshaft and the entire mixer is inverted, allowing the mixed investment material to flow by gravity through funnel 18 into flask 14 and around the pattern 40 mounted in the bottom of the flask, all as is well known in the art. Although it will be understood that the mixer may be in this position at the time the vacuum is broken, the investment material adhering to the shaft and sleeve end portions thereof is in a position to he forced into the bearing at the time that normal pressure is restored inside the bowl, if the design of the bearing structure permits.

It is generally recognized in the investment casting art that the mixer is the weakest link in the chain of equipment required for this process. The bearing for the rotating shaft is, as has been already pointed out, the sensitive point of this piece of equipment; and it has been particularly characteristic of submerged or bottom-mounted bearing structures that they wear out rapidly. This results in unnecessary expense to the user or poor technique, or in some cases both. On the contrary, the present design has been tested and developed under continuous laboratory operation and conditions similar to those encountered in commercial laboratories and has been found to possess an unexpectedly long life. This is attributed to the continuous lubrication of the bearing and the lack of any abrasive material between surfaces that are caused to approach and bear against one another during the mixing operation. As a result the bearing tolerance has remained close and the fluid tight seals remained efiicient and able to hold a high degree of vacuum. All this contributes not only to the longevity of this piece of equipment but .to the improved accuracy and satisfaction of operation of the entire technique of the investment casting.

It will be realized that various changes may be made in the details of design and arrangement of the elements of the present invention by persons skilled in the art without departing from the spirit and scope of the present invention. Accordingly, it is to be understood that the foregoing description is considered as being illustrative of, rather than'limitative upon, the invention as defined by the appended claims.

I claim:

1. Vacuum investing apparatus, comprising:

a bowl adapted to hold a quantity of investment material to be mixed and open at one end;

closure means for said one open end including investment discharge means and means for connecting to a vacuum line to maintain the interior of the apparatus under a partial vacuum;

a flash positioned by engagement with the closure means to receive investment material from said discharge means when the apparatus is inverted;

a fixed incompressible sleeve passing through the bowl wall at a position located at the bottom of the bowl when mixing is carried on, said sleeve extending upwardly into the bowl to a location near the average surface level of investment material in the bowl during mixing;

a generally upright rigid shaft rotatably mounted in the sleeve and projecting beyond the sleeve at each end thereof, said shaft being adapted at its lower end for connection to means for rotating the shaft;

stirring arm means on the shaft at the upper end thereof;

said sleeve and said shaft having cooperating means providing a pair of engaging thrust bearing surfaces located at the lower end of the sleeve outside the bowl, said surfaces resisting displacement of the shaft upwardly relative to the bowl by forces directed axially thereof and inwardly of the bowl;

and fluid sealing means between the shaft and the sleeve.

2. A mixer as in claim 1 in which the fluid sealing means comprises a pair of separate sealing elements spaced along the shaft and a shallow annular recess between and spaced from each of said sealing elements providing a lubricant retaining means between the sealing elements.

3. A mixer as in claim 1 in which the shaft has an outwardly extending surface opposing an end surface of the sleeve, said two mutually opposing surfaces being so arranged that they tend to separate under the influence of a downward axial force on the shaft maintaining engagement of the shaft with means for rotating the shaft.

4. Vacuum investing apparatus, comprising:

a bowl adapted to hold a quantity of investment material to be mixed and open at one end;

removable closure means for said one open end including investment discharge means and means for connecting to a vacuum line to maintain the interior of the apparatus under a partialvacuum;

a flask positioned by engagement with the closure means to receive investment material from said discharge means when the apparatus is inverted;

a continuous incompressible sleeve passing through the bowl wall at a position located adjacent the bottom of the bowl when mixing is carried on, said sleeve extending upwardly into the bowl through investment material in the bowl during mixing;

a generally upright rigid shaft rotatably mounted in the sleeve and projecting beyond the sleeve at each end thereof, said shaft being adapted at its lower end for connection by relative axial movement to mean for rotating the shaft;

stirring arm means on the shaft;

fluid sealing means between the shaft and the sleeve;

said sleeve and shaft having means providing a first pair of opposed surfaces adjacent the upper end of the shaft separated by relative upward movement of the shaft;

and said sleeve and shaft also having means providing a second pair of engaging thrust bearing surfaces located at the lower end of the sleeve outside the bowl, said surfaces resisting displacement of the shaft upwardly relative to the bowl and separation of the first pair of surfaces by forces directed axially of the shaft and inwardly of the bowl.

5. A mixer for investment material as in claim 4 in I which the first pair of opposed surfaces are located near or above the average surface level of investment material in the bowl during mixing.

References Cited by the Examiner UNITED STATES PATENTS 1,480,914 1/ 1924 PoplaWski 259-108 2,660,210 11/ 1953 Berglind 259-107 X 2,696,022 12/ 1954 Steinbock et al 22-35 X 2,777,177 1/ 1957 Steinbock et a1 22-35 2,794,627 6/ 1957 Rodwick 259-108 2,930,596 3/ 1960 Waters 259-108 X 2,992,715 7/ 1961 Blachly 146-68.1 X

CHARLES A. WILLMUTH, Primary Examiner.

E. L. ROBERTS, Assistant Examiner.

Claims (1)

1. VACUUM INVESTING APPARTUS, COMPIRISING: A BOWL ADAPTED TO HOLD A QUANTITY OF INVESTMENT MATERIAL TO BE MIXED AND OPEN AT ONE END; CLOSURE MEANS FOR SAID ONE OPEN END INCLUDING INVESTMENT DISCHARGE MEANS AND MEANS FOR CONNECTING TO A VACUUM LINE TO MAINTAIN THE INTERIOR OF THE APPARATUS UNDER A PARTIAL VACUUM; A FLASH POSITONED BY ENGAGEMENT WITH THE CLOSURE MEANS TO RECEIVE INVESTMENT MATERIAL FROM SAID DISCHARGE MEANS WHEN THE APPARATUS IS INVERTED; A FIXED INCOMPRESSIBLE SLEEVE PASSING THROUGH THE BOWL WALL AT A POSITION LOCATED AT THEBOTTOM OF THE BOWL WHEN MIXING IS CARRIED ON, SAID SLEEVE EXTENDING UPWARDLY INTO THE BOWL TO A LOCATION NEAR THE AVERAGE SURFACE LEVEL OF INVESTMENT MATERIAL IN THE BOWL DURING MIXING; A GENERALLY UPRIGHT RIGID SHAFT ROTATABLY MOUNTED IN THE SLEEVE AND PROJECTING BEYOND THE SLEEVE AT EACH END THEREOF, SAID SHAFT BEING ADAPTED AT ITS LOWER END FOR CONNECTION TO MEANS FOR ROTATING THE SHAFT; STIRRING ARM MEANS ON THE SHAFT AT THE UPPER END THEREOF; SAID SLEEVE AND SAID SHAFT HAVING COOPERATING MEANS PROVIDING A PAIR OF ENGAGING THRUST BEARING SURFACES LOCATED AT THE LOWER END OF THE SLEEVE OUTSIDE THE BOWL, SAID SURFACES RESISTING DISPLACEMENT OF THE SHAFT UPWARDLY RELATIVE TO THE BOWL BY FORCES DIRECTED AXIALLY THEREOF AND INWARDLY OF THE BOWL; AND FLUID SEALING MEANS BETWEEN THE SHAFT AND THE SLEEVE.

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