US3542344A - Method and apparatus for mixing flowable materials in closed containers - Google Patents

Description

O Un ted States Patent [1l13,542,344

[72] lnventor Charles J. Oberhauser [50] Field of Search 259/75, 72,

Watertown, Massachusetts 76, 77, 78, 79, 80, 81, 13, 56 21 A 1. No. 840 254 in} Fi lg d Jul 9, 1969 [5 6] References Cited Continuation-impart of Ser. No. 743,835, UNITED STATES PATENTS J ly 1 1 bandone 1,629,762 5/1927 Woodrow 68/174 [45] Patented Nov. 24,1970 2,516,655 7/1950 68/174 [73] Assignee Dynatech Corporation 3,430,927 3/1969 259/88 3,432,149 3/1969 Stalberg 259/75 Cambridge, Massachusetts [54] METHOD AND APPARATUS FOR MIXING FLOWABLE MATERIALS IN CLOSED CONTAINERS 13 Claims, 15 Drawing Figs.

[52] U.S. Cl 259/75 [51] Int. Cl 1301f 11/00 MIN!" I:

Primary Examiner-Robert W. Jenkins Attorney-Cesari and McKenna ABSTRACT: Materials capable of flow are mixed in a closed container by rapidly accelerating and decelerating the container about an axis. Through the combination ofshear forces and the recurrent creation and destruction of a vortex in the material, rapid, thorough mixing results.

Patented Nov. 24, 1970 I POWER INVENTOF? CHARLES J. OBERHAUSER 5M0, Qdw'a JZ'JC ATTORN 3 Patented Nov. 24,- 1970 3,542,344

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ATTORNE METHOD AND APPARATUS FOR MIXING FLOWABLE MATERIALS IN CLOSED CONTAINERS RELATED APPLICATION This application is a continuation-in-part of application Ser. No. 743,835, filed July 10, 1968, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to the mixing of materials that are capable of flow. Such materials include liquids, powders, colloids and suspensions. More particularly, the invention relates to mixing a flowable material by exerting mechanical forces on a closed container of the material. The invention is well suited for mixing paint. And although not limited to this use, it

will be described in this context The conventional paint mixer used by a retailer shakes a closed cylindrical paint can back and forth. Thorough mixing requires at least several minutes per can provided there is significant ullage or head space in the can. Considerably more time is required when the ullage is small.

During this time, the services which the salesman can offer other customers are restricted, even though the purchase of the paint he is mixing is, practically speaking, complete. If he does tend to other customers, he often has to interrupt dealings with them to remove the paint can from the mixer and give it to the purchaser thereof. Alternatively, he may wait until he is through with the new customers, in which case, the purchaser of the paint may have to wait an inordinate length of time to obtain the mixed paint. Thus, the conventional retail store paint shaker is costly in terms of time, both salesman time andcustomer time. These problems are compounded when a customer purchases several cans of paint, as is often the case.

There are other conventional machines which agitate material in a container as part of a process of cooking or sterilizing its contents. For example, there is apparatus which rotates the container about its axis, first in one direction and then in the other, in'a hot environment. In such apparatus, the rotational speed of the container is relatively slow and the container rotates for a relatively long time in each direction. While such devices do enhance the cooling or sterilizing of some foods, they cannot thoroughly mix highly thixotropic or viscous material such as paint.

Other conventional devices which reciprocate the container endwise are equally unsuitable for the purpose intended here.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an improved method and apparatus for mixing flowable material.

Another object of the invention is to provide a method and apparatus for mixing flowable material without introducing a moving impeller or like element into the material. A further object is to provide an improved method and apparatus for rapidly redistributing flowable material within a closed container.

Further, it is an object that the method and apparatus produce rapid mixing. Also, the mixing action should be suitable for use with thixotropic material.

It is also an object of the invention to provide a mixing method and apparatus of the above character that does not require ullage in the container of the material being mixed.

Further objects of the invention include the provision of mixing apparatus of the above character which can be manufactured at relatively low cost and which is safe to use.

Another object is to provide an improved mixer which is inherently in balance so that it vibrates little in use.

Still another object is to provide mixing apparatus which is compact and requires minimum shelf space.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others, and the apparatus embodying the features of construction, combinations of elements and arrangements of parts adapted to effect such steps, all as exemplified in the following detailed disclosure, and the scope of the invention is indicated in the claims.

According to the invention, an inhomogeneous flowable material in a container is mixed by subjecting the container to a rotational movement that exerts considerable angular acceleration on the material about an axis. This causes the material inside the container to undergo rotational flow and as a result ofthis flow, the material becomes thoroughly mixed and redistributed within the container.

A particular advantage of the invention is that it can mix flowable materials rapidly. In addition, the invention can readily be practiced by nonskilled personnel with reliable and low cost equipment. Hence, it enables a clerk or salesman in a paint store to mix'cans of paint in a small fraction of the time required by the paint shakers in common use.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERRED EMBODIMENTS The paint mixer shown in FIG. 1 rotates a closed container 10 to mix the paint 1! (FIG. 2) thereinThe illustrated container has a round cross section and may be a conventional metal paint can or be blow molded of plastic material.

The container 10 is seated in a mating socketlike support 12 carried on a turntable 14. The support 12 can be sufficiently deep to hold additional containers 10' (FIG. 2) stacked on the container 10. Container 10 also has conventional brackets 16 and I8 protruding outwardly from the sidewall to receive a bail-type wire handle (not shown). Support 12 has a pair of diametrically opposed slots 20, 22 in its inner wall to receive these protruding brackets 16, 18, respectively. This seating engagement of the handle brackets in the support slots constrains container 10 to rotate with support 12.

An electric motor 24 through its shaft 26 rotates turntable 14. The motor is secured to a fixed base 28 on which is mounted a shelf 30 fitted with a shaft-supporting bearing 32. A power supply 34 is connected to the motor 24 through a double-pole, single-throw, on-off switch 36 in series with a .reversing switch 38 to operate the motor in either direction.

Supply 34 may, if desired, be adjustable so that the rate of acceleration can be controlled by the setting of a control knob 34a. In one embodiment, a capacitor-start, induction-run electric motor is used, with a brake to aid in decelerating the motor and container. The reversing of the motor and the actuation of the brake are effected by a cycle timer which may be electromechanical, electronic, or any combination thereof.

The mixer is operated simply by closing the on-off switch 36 which starts container 10 rotating in one direction about its longitudinal axis 40. After the motor 24 has rapidly accelerated the container 10 to top speed, the reversing switch 38 is reversed. In response, the motor 24 rapidly decelerates the container and then accelerates it in the opposite direction.

On the other hand, the inertia of the material within the 'container 10, together with its fluidity, prevents the material from accelerating at the same rate as the container. Accordingly, the fluid undergoes a rotational flow relative to the container and this results in mixing of the fluids.

It is believed that rapid, thorough mixing occurs principally because of three phenomena. First, there is a circular shearing action caused by the rapid acceleration of the container relative to its contents. FIGS. 3A to 5B illustrate this effect. Starting from rest, when the mixer is started, the container rotates, e.g.clocltwise, but its contents remain stationary clue to its inertia. The resulting relative motion imparts torsional forces F to the material which gradually brings all the contents up to the same rotational speed W as the container, as illustrated in FIGS. 3A--5A. Thus, one may consider the liquid as being divided into a series of concentric cylindrical layers. The layer adjacent container reaches the angular velocity W of the container almost immediately, followed then by the next layer radially inwards and so on. Actually, a radial line L made on the top surface of the liquid would appear as a spiral when all the liquid has reached the same rotational speed as the container. (See FIG. 5A.)

Secondly, as seen in FIGS. 3B5B, if there is ullage, the rapid rotation of the container causes the gradual development of a vortex V (FIG. 5B) coaxial with the axis of rotation 40. This results in a downward movement of liquid at the center of container 10; radially outward movement of liquid at the bottom of the container and movement of liquid upward along the sides of the container, all of which enhance the mix ing action.

Finally, when the vortex V is fully developed, the next phase of the mixing cycle occurs and the container is rapidly decelerated. As seen in FIGS. 6A--8A, the inertia of the liquid contents of container 10 causes it to continue rotating in the same direction initially. However, viscous shear forces gradually cause the liquid to follow the counterclockwise rotation of the container. This results in a violent collapse of vortex V as seen in FIG. 7B. This, in turn, mixes the liquid vigorously before another vortex V forms as seen in FIG. 8B.

In practice, the above conditions are repeated several times in accordance with a mixing schedule to achieve complete and homogenous blending of the liquid contents of container 10. FIG. 9 is a graph showing the variation ofW with time ofa typical schedule. The container is accelerated linearly to 1,000 rpm. in .5l.5 seconds. This time corresponds to the elapsed time between FIGS. 3A-5B. Then the container is accelerated linearly in the opposite direction for .5- l .5 seconds. Destruction of the vortex (FIG. 78) occurs during this second time period while the liquid is decelerating and is repeated with each reversal thereafter.

This mixing schedule is critical. First, the maximum rotational speed of the container in each direction must be within well-defined limits, i.e. 600- 1 ,800 rpm. If this speed is below this range, the paint does not mix thoroughly. Also, surprisingly, the same result occurs if the speed goes appreciably above this range. 4

Secondly, the period between reversals should be in the range of 13 seconds in order to form a well-defined vortex V during each cycle. Otherwise, mixing efficiency suffers.

By way of example, one mixer constructed in accordance with the invention achieved complete mixing of a gallon of highly thixotropic paint when, for a period of seconds, the container was accelerated alternatively in one direction and then the other to a speed of about 1,200 rpm. with a period of 2 seconds between reversals. Drawbar tests, conventional tests for determining the extent to which paints are mixed, established that the paint so mixed was free of streaks and flocculation. In addition, the paint gave uniform coverage and was completely blended. This same degree ofmixing with conventional, prior art paint shakers usually requires in the order of5 or 6 minutes.

Rather than operate the mixer of FIG. 1 by accelerating motor 24 from zero to a certain speed and then to a certain speed in the opposite direction, one might simply accelerate the container in one direction and then suddenly stop it and then accelerate it in the same direction again. Alternatively, one might modulate the motor speed about a selected nonzero value. These are equivalent to rectifying and shifting the FIG.

9 curve. The important feature is that the angular acceleration be relatively large.

If desired, the mixer can incorporate a conventional clutchbrake coupling (not shown) to connect motor 24 to turntable 14 to change the direction of turntable acceleration without changing the direction of the motor. Also, to minimize the power requirements of the mixer, a conventional energy storage device such as a fly wheel may be used with a smaller motor to rotate the container.

A further feature of the present paint mixer is that container 10 can be completely full of paint, as shown in the enlarged detail FIG. 2. That'is, mixing in accordance with the invention does not require ullage in the container.

With some of the smaller containers, i.e. pint and quart contents, the mixing action is improved if the can is mounted in its support 12 so that it is rotated about the diagonal axis of the can, rather than its longitudinal axis. For this purpose, the interior of support 12 can easily be countoured to hold the can on edge so that its diagonal axis coincides with the axis of rotation.

The mixing method and apparatus provided by this invention thus produces complete mixing of flowable material in a closed container in a relatively short time. No ullage is required in the container. Further, mixers embodying the invention operate without vibratory or shaking action common in prior mixers and which generally causes high wear.

The mixer described herein should not be confused with prior mixers and churns using a splashing or dashing type of action. These may be classified into several categories: devices in which rotation of a container causes paddles therein to splash into and out of a liquid, devices in which rotation of a liquid around two axes causes theliquid to splash about inside the container; and devices, similar to the first of these, in which the paddles are intended to splash about inside the container; and devices, similar to the first of these, in which the paddles are intended to stir the liquid as they pass through it. All of these devices require ullage in the container to mix the liquid.' The present invention does not require ullage, although, of course, ullage will not prevent operation of the mixer in the intended manner. Nor should it be confused with those mixers which simply rotate the container in one direction or the other relatively slowly which do not rapidly accelerate the can or its contents about an axis or create or destroy a vortex formed in the contents.

It should be understood that while the present application specifically describes a device that provides relative rotation of a container and its contents about a single axis, operation is essentially unchanged if thecontainer is simultaneously wobbled so that a group of axes are involved or rotated simultaneously about a plurality of axes, i.e. like a gyroscope. Accordingly, reference to an axis includes such a group. Other variations of like consequence are similarly included within the definitions of the terminology used herein.

In addition, while the description is in the context of a paint mixer, the same technique can also be used to mix, blend or redistribute other flowable liquids. For example, it can be used to achieve a uniform distribution of-an additive in a batch of ceramic material in the form of a slurry. It can also be used to obtain a uniform temperature distribution of a liquid in a closed container. That is, the acceleration ofliquid contents as described above militates against the presence of localized hot or cold regions on or in the container.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in carrying out the above method and in the constructions set forth without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween. l

lclaim:

1. A method of mixing paint comprising the steps of:

A. containing the paint in a generally cylindrical container having no internal mixing paddles or impellers;

B. rapidly accelerating the container about an axis of rotation for a plurality of revolutions until a vortex is created in the paint; and

C. after the vortex is formed. accelerating the container in the opposite direction about the axis quickly enough to abruptly destroy the vortex.

2. A method of mixing paint as defined in claim 1 and including the additional steps of:

A. continuing accelerating the container in said opposite direction about the axis until a completely new vortex .is created in the paint;

B. after the vortex is formed, accelerating the container in the original direction quickly enough to abruptly destroy the new vortex; and

C. timing a plurality of these acceleration steps in alternate directions so that a succession of new vortexes are created in the paint and then abruptly destroyed.

3. The method as defined in claim 2, wherein the container is rotated first in one direction and then the other to a speed of 600- l,800 r.p.m. with a period of 1-3 seconds between reversals.

4. The method as defined in claim 2, wherein a cylindrical container is rotated about its diagonal axis.

5. The method as defined in claim 1, wherein the container is accelerated to 600l,800 r.p.m. and brought back to rest all in l-3 seconds.

6. Apparatus for mixing paint comprising:

. A. means for holding a generally cylindrical paint container having no internal paddles or impellers for rotation about an axis;

B. a motor connected, to rapidly accelerate the holding means about the axis for a plurality of revolutions until a vortex is created in the paint inside the container; and

C. means for braking the holding means to a stop quickly enough to abruptly-destroy the vortex.

7. Apparatus as defined in claim 6. wherein said holding means is arranged to hold a cylindrical container so that the container rotates about its diagonal axis.

8. Apparatus as defined in claim 6, wherein said motor accelerates the container from rest to 600 l .800 r.p.m. and the brake means decelerates it again to rest all in l-3 seconds.

9. Apparatus according to claim 6 in which said motor is arranged to move said holding means so as to exert angular acceleration for a period of .10 to .75 seconds until the holding means rotates at a speed of from 600 to l ,800 r.p.m.

10. Apparatus according to claim 9, wherein the braking means brakes the holding means so that the holding means is stopped in .05 to .75 seconds.

11. Apparatus for mixing paint as defined in claim 6 and further including:

A. means for reversing the motor so that it accelerates the container in the opposite direction until a completely new vortex is created in the paint; and

B. control means for sequencing the motor. braking means and reversing means so that a succession of new vortexes are created in the paint and then abruptly destroyed.

12. Apparatus as defined in claim 11, wherein the control means controls so as to rotate the container in opposite directions to 600 1,800 r.p.m. for a plurality of cycles with a period of l-3 seconds between reversals.

13. Apparatus for mixing paint comprising:

A. a generally cylindrical paint container having no internal paddles or impellers;

B. means for supporting the paint container for rotation about an axis;

C. drive means connected to rapidly accelerate the support means through a plurality of revolutions about the axis so that a vortex is created in the paint inside the container; D. means for accelerating the support means in the opposite direction so as to abruptly destroy the original vortex and create a completely new vortex in the paint; and

E. means for timing these acceleration steps in alternate directions for a plurality of cycles so as to create and destroy a succession ofnew vortices.

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