Agitating – Stirrer within stationary mixing chamber – Rotatable stirrer
Reexamination Certificate
2000-03-16
2003-11-25
Cooley, Charles E. (Department: 1723)
Agitating
Stirrer within stationary mixing chamber
Rotatable stirrer
C366S097000
Reexamination Certificate
active
06652137
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to mixers used to mix liquids and solids. More particularly, the invention relates to a stirrer for use with a planetary mixer and a planetary mixer incorporating the stirrer.
2. State of the Art
Planetary mixers can be described as mixers that have at least one stirrer (or mixing blade) that revolves (or orbits) in a mixing vessel (or tank) about a central axis while simultaneously revolving on its own axis. Since planetary mixers move the stirrer or stirrers through all areas of the mixing vessel, they are especially useful when mixing high viscosity mixtures which cannot be adequately mixed with a mixer having a fixed stirrer. Planetary mixers are usually oriented vertically with one, two, or three stirrers revolving about a vertical axis. The stirrers are mounted on a drive mechanism which includes means for lifting the stirrers out of the mixing vessel.
The most commonly known planetary mixer is the type that is typically found in kitchens for mixing dough and various food products, having a single stirrer and somewhat hemispherical bottom mixing bowl. Small machines of this type, such as those made by KitchenAid™, are usually found in residential kitchens as well as some commercial kitchens. Larger machines such as those made by Hobart™ are usually found in commercial kitchens. There are other manufacturers of these machines, but most of the designs are similar. These mixers are commonly made in sizes for processing from 1 gallon through 35 gallons. These mixers are generally not capable of vacuum mixing and they are generally not used in industrial manufacturing or with high viscosity mixtures.
The most common type of industrial planetary mixer has multiple stirrers and a flat bottom mixing vessel. These machines are typically used to process materials such as; adhesives, sealants, coatings, foods, cosmetics, pharmaceuticals, plastics, plastisols, composites, metal powders, magnetic coatings, precious metals, ceramics, ceramic metal products, bulk molding compounds, castable materials, dental composites, etc. They are used in any industry where difficult to mix materials need to be processed. They are used for liquid/liquid mixing, liquid/solid mixing and solid/solid mixing. They are used in laboratories, pilot plants and production plants. They are commonly made in sizes for processing from ¼ pint through 500 gallons. The most typical of these have two stirrers, although there are some designs that have three stirrers. See., e.g., U.S. Pat. No. 4,697,929, the complete disclosure of which is hereby incorporated herein by reference. The most common stirrer design is a substantially rectangular element such as that shown in prior art
FIGS. 1-3
.
As shown in
FIGS. 1-3
, the stirrer
10
has an upper central mounting collar
12
having a vertical bore
14
and two horizontal intersecting bores
16
,
18
for set screws (not shown) and/or drive pins or keys (not shown) for transmitting torque to the stirrer. A pair of arms
20
,
22
extend outwardly and downwardly from the collar
12
. Vertical stirring elements
24
,
26
depend from the arms
20
,
22
and are coupled to each other at their bottoms by a cross bar
28
.
As mentioned above, the stirring element
10
is coupled to a drive shaft in a mixing assembly which causes the stirrer
10
to rotate about the axis of bore
14
and also causes the stirrer to orbit about another axis, typically the central axis of the mixing assembly. The top speed of the outer edge of the stirring elements
24
,
26
can range from less than 5 feet per minute to over 600 feet per minute. This is the combined speed of the rotation of the stirrer on its own axis and the rotation of the stirrer assembly as it orbits about the central axis of the mixer.
Stirrers of the type shown in prior art
FIGS. 1-3
have certain drawbacks. Primarily, they do not have good top to bottom mixing ability. It is therefore necessary to increase the mixing time in order to get a homogeneous mix.
Wetting out powders in liquids as well as adding powders to higher viscosity materials usually requires adding a small amount of powder at a time and waiting until this is mixed in, before adding more. This is because of the poor top to bottom mixing action and because adding too much powder typically brings the level of the powder up higher than the top of the stirrers.
Inserting the rectangular stirrers into high viscosity material as well as lifting them out requires a great deal of force because of the crossbar at the bottom of the stirrers. This requires a heavy duty lifting mechanism to overcome these forces thereby increasing the cost of the machine.
The bottom crossbar typically lifts a large amount of material out of the mixing vessel when the stirrers are raised if the viscosity is high. The material must be scraped off of the stirrers by hand. This is very labor intensive and difficult especially on larger machines.
The rectangular stirrers have stagnant areas on the back (or trailing edges) and the sides of the stirrers that can harbor materials that do not get mixed into the batch. This is because these stirrers have flat areas on the backs of the stirring elements. See, e.g.,
24
a,
26
a
in prior art FIG.
3
.
The rectangular stirrers also allow material to form balls or columns that get knocked around rather than being mixed when the viscosity increases. If the mixing is allowed to continue past this point, the material quite often gets pushed up out of the mixing vessel and into the area around the gearbox. When this happens, the mixing operation is usually considered a failure, and the mixer must be stopped and the material dug out from the gearbox area by hand.
The rectangular stirrers also cause torque spikes as the stirrers come near each other, and as the stirrers come near the side walls of the mixing vessel. This is because the entire length of the stirring elements of the stirrers (from top to bottom) come near each other at one time and the entire length of the stirring elements (from top to bottom) come near the side of the mixing vessel at one time. These torque spikes are one factor that can limit the ability of the mixer to mix high viscosity materials. Overcoming this problem requires a heavier duty drive system, which increases the cost of the machine.
The crossbar (
28
in
FIG. 1
) at the bottom of a rectangular stirrer is needed primarily for strengthening the stirrer rather than for any mixing action. The crossbars cause undue strain on the mixing machine because of the high side loads that they produce at the farthest point away from the gearbox as well as the additional torque required to rotate the bottom crossbars in high viscosity materials. Their presence is another factor that limits the ability of the mixer to mix high viscosity materials. Overcoming this problem requires a heavier duty drive system, which increases the cost of the machine.
In addition, the arms
20
,
22
are normally located below the top of the mixing vessel and are either slightly above or even at the level of the full mixing capacity of the machine. The presence of these arms at this height is a contributing factor to material climbing up out of the mixing vessel. It also limits the amount of powder that can be added at one time and be quickly mixed into the batch.
In order to mix high viscosity materials with a double planetary mixer with rectangular stirrers, it is a common practice to greatly reduce the batch size being mixed, which dramatically reduces the production capacity of the machine. It is a common practice when mixing difficult materials to mix only ½ or even ⅓ of the full working capacity of the mixer. Even by reducing the batch size, problems are still frequently encountered with the material forming balls or columns and not mixing properly.
There are many design alternatives to the rectangular stirrer, each of which overcomes some of the problems discussed above. However, no stirrer design overcomes all of the problems discussed above.
SUM
Bosch Edward T.
Langhorn Kenneth D.
Charles Ross & Son Company
Cooley Charles E.
Galgano & Burke
Sorkin David
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