Double arm mixer extruder

A mixer having a discharge screw centrally located between first and second side walls and disposed in a cavity provided in the bottom. The discharge screw has (a) a shaft with a first end that extends through a first end wall and (b) a driven end opposite the first end of the shaft, with the driven end being connected to a motor shaft for rotating the discharge screw in at least one direction. The driven end has a recessed portion that extends into the second end wall of the vessel. The mixer may include a deflector that is fixed to the second end wall and extends toward the first end wall and is located between the mixing blades and the discharge screw.

The present disclosure describes an improved double arm mixer-extruder that solves the problem of undesired material leakage at the drive end of the auger/discharge screw.

BACKGROUND

Double arm mixer-extruders, of which sigma blade mixers are an example, are typically used for mixing high viscosity materials such as doughs, chewing gum base, rubbers including silicone rubber, butyl rubber and the like. During operation, the mixing blades are counter-rotating and thus always run in the same direction, that is, down into the middle of the mixer bowl. Such mixers may include an auger or discharge screw that rotates in two directions. In one direction, the discharge screw rotates in a discharge direction that forces the mixed product out of one end of the mixer, which may be considered to be the front (first) wall of the mixer. In the other direction, during mixing, the discharge screw rotates in a mixing direction that forces the mixed product away from the outlet of the mixer and toward the end of the mixer opposite the outlet, which may be considered to be the rear (second) wall of the mixer. In other words, during operation (i.e., during the mixing operation), the discharge screw rotates in a direction that forces the mixing product toward the center of the vessel and toward the rear wall of the mixer.

It has been found that, during mixing, the combination of the mixing blades forcing the product down toward the bottom of the mixer and the discharge screw rotating in a mixing direction, the mixing product is driven into the rear wall. As a result, there can be product leakage in the area around the driven end of the discharge screw.

To address this issue, manufacturers have sought to provide seal designs that seek to prevent leakage. While the seals are somewhat effective, leakage still occurs. Thus, there is a need for a solution to the leakage problem.

SUMMARY

In one aspect, the disclosure describes a mixer that includes a generally double-U-shaped vessel (or bowl) with an opening at the top. The vessel (or bowl) has a first side wall, a second side wall spaced from and parallel to the first side wall, with each side wall terminating at a bottom, a first end and a second end spaced from and parallel to the first end. Within the vessel (or bowl) a pair of parallel spaced apart shafts are disposed with each shaft extending from the first end to the second end of the vessel. Each shaft is provided with at least one mixing blade. The at least one mixing blade can be any suitable type of mixing blade that is effective to thoroughly mix ingredients. In one embodiment, the mixing blade is of the sigma type mixing blade.

The described mixer is generally a mixer-extruder type of mixer and, accordingly, a discharge screw is centrally located between the first and second side wall and disposed in a cavity provided in the bottom of the vessel. The discharge screw has (a) a shaft with a first end that extends through the first wall and terminates in a nose and (b) a driven end opposite the first end of the shaft with the driven end being connected to a motor shaft for rotating the discharge screw in at least one direction. The shaft has a root diameter with a plurality of flights extending outwardly from the shaft along a longitudinal length of the discharge screw.

The driven end also has a recessed portion that extends into the second end wall of the vessel. The recessed portion extends into the second end wall of the vessel a distance that is less than a thickness of the second end wall. In some instances, the mixer may have a cylindrical portion with a circular cross section that has a first diameter that is greater than the root diameter. The cylindrical portion may extend from the recessed end toward the first end a distance (a length) from about 2 to about 25 cm.

In some embodiments, the cylindrical portion tapers inwardly to define a conical portion such that at a distal portion of the driven end, the conical portion is coextensive with the shaft and has a diameter substantially equal to the root diameter. As noted above, the plurality of flights may extend from a location adjacent the first end of the shaft to a location between the cylindrical portion and the distal portion of the driven end of the shaft. In some embodiments, the plurality of flights may extend outwardly from the shaft along a longitudinal length of the discharge screw, and may extend from a location adjacent the first end to the conical portion with no flights being present on the cylindrical portion.

In other embodiments, the mixer may include a deflector fixed to the second wall and extending toward the first wall. The deflector may be located vertically between the mixing blades and the discharge screw. In some instances, the deflector has a chevron-shaped cross section. The deflector may have a bottom with a radius of curvature that is substantially the same as the cylindrical portion of the driven end. In some instances the bottom of the deflector may be spaced from the driven end to provide a gap between the deflector and the driven end. The bottom of the deflector may be adjacent to and spaced from the cylindrical portion to define an arc on the cylindrical portion such that an angle subtended by the arc ranges from about 60° to about 75°.

The deflector has a top surface that is spaced from an outer extent of the mixing blades to provide a gap. In some cases, the gap changes from a maximum to a minimum as the at least one mixing blade rotates.

The disclosure also contemplates a method of mixing materials in the above-described apparatus. In this regard, the method may include providing a mixing apparatus with a vessel10containing one or more of the above features, i.e., a discharge screw having a driven end recessed into the rear wall, a discharge screw having a cylindrical portion free of flights, having a diameter greater than the root diameter of the shaft of the discharge screw, and being tapered at a proximal end to define a conical portion that is coextensive with the shaft of the discharge screw, a deflector extending from the rear end wall of the vessel or bowl. With the apparatus in mind, the contemplated method includes introducing material to be mixed through the opening of the vessel, causing the mixing blades to rotate, causing the discharge screw to rotate in a mixing direction at a first speed, and, at a time when the material to be mixed is complete (as understood by the known operator based on the material to be mixed), causing the discharge screw to rotate in a discharge direction at a second speed. The first speed is less than the second speed and in this regard, the first speed may be about 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85% or 90% of the second speed.

While the above features, alone or in combination, may be effective in preventing leakage out the rear end wall of the mixer, it is contemplated that, during the mixing operation, the speed of the discharge screw can be reduced, which should reduce the pressure on the seal arrangement for the discharge screw at the rear wall. The speed can be reduced by any suitable amount from the typical operating speed of about 10 rpm to about 60 rpm. To this end, the speed can be reduced by 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85% or 90% of the typical operating speed.

DESCRIPTION

It is to be understood by one of ordinary skill in the art that the present discussion is a description of specific embodiments only and is not intended to limit the broader aspects of the described apparatus and method.

Turning toFIG.1, a top view of a prior art double arm mixer-extruder10is shown. Typical of such a mixer-extruder, there is a vessel10with an opening at the top, a first end wall26and a second end wall30spaced from and parallel to the first end wall26, a first side wall14and a second side wall16spaced from and parallel to the first side wall14. The vessel has an opening at the top of the vessel for loading the material into the vessel, and the opening is typically closed by a lid (not shown).

Each side wall14,16terminates at the bottom18of the vessel10. The interior of the bottom18has a pair of rotor chambers20,22that have an identical cylindrical shape disposed in the vessel10and are symmetrically connected to each other with their axis lines being arranged horizontally parallel side-by-side so that the two rotor chambers20,22communicate with each other. An open topped chevron-shaped cavity24is formed on the inner bottom18of the vessel10at the boundary of inner peripheral surfaces of the two rotor chambers20,22, such that the cavity24is centrally disposed between the two side walls14,16. This construction is sometimes referred to as a double-U-shaped vessel10. The rotor chambers20,22have a cross section that is uniform in the axis direction.

Two shafts40,50are rotatably supported by the end walls26,30(the first and second walls) of the vessel and each shaft is disposed in the rotor chambers20,22, respectively. The shafts40,50are rotatably disposed and spaced from the inner peripheral surfaces of the rotor chambers20,22. The shafts40,50may rotate at the same rotating speed or at different speeds. Each shaft40,50is provided with at least one mixing blade on their outer periphery42,52respectively. In some instances, the at least one mixing blade is a type known as a sigma mixing blade. Each shaft40,50is connected to a driving source (a motor driven shaft) so that the mixing blades42,52rotate downwardly on the communicating side of the rotor chambers20,22. The geometry and profile of the sigma blade is designed such that the mass of material is pulled, sheared, compressed, kneaded, and folded by the action of the blades against the walls of the vessel10.

An auger or discharge screw60is provided in the cavity24and, as such, is parallel to each of the shafts40,50. It will be appreciated that, in operation, the discharge screw60is located below (in a vertical direction) each of the shafts40,50. Because the cavity24is centrally located between the side walls14,16, the discharge screw60is likewise centrally located between the side walls14,16. The discharge screw60has a nose68at one end66that extends outwardly from the first end wall26of the vessel10and the second end (or driven end)70is connected to a motor shaft for rotating the discharge screw in at least one and preferably in two directions, i.e., a mixing direction and a discharge direction.

Typically, during a mixing operation, the materials to be mixed are introduced into the vessel through the top, the mixing shafts40,50are caused to rotate, which causes the mixing blades42,52to rotate and force the material downward and toward the center portion of the vessel10. At the same time, the discharge screw60is rotated in a mixing direction, which causes the material in the vessel10to move from the first end wall26toward the center of the vessel10and the second wall30; thus, precluding material from exiting the vessel10adjacent the nose68of the discharge screw60. When the material sought to be mixed has been sufficiently mixed, the rotation of the discharge screw60is reversed to that the discharge screw60rotates in a discharge direction forcing the mixed material out of the vessel10through an opening in the first end wall26provided for the discharge screw60.

Referring toFIG.2, a prior art discharge screw60is shown. The prior art discharge screw is characterized by its construction and location within the vessel10. The prior art discharge screw60, at its driven end70, is conically shaped with flights96extending from the first end66the shaft62of the discharge screw60onto the conically shaped portion and nearly to the terminal portion of the driven end70of the discharge screw60. In addition, the terminus of the driven end70is flush with the interior of the second end wall30of the vessel10such that the motor shaft extends through the second wall of the vessel to connect to the discharge screw60.

As noted above, it has been found that, during mixing, the combination of the mixing blades forcing the product down toward the bottom of the vessel10and the discharge screw60rotating in a mixing direction, the mixing product is driven into the rear end wall30. As a result, there can be product leakage out of the vessel10in the area around the driven end70of the discharge screw60.

Turning now toFIGS.3-6, details of the improved mixer, discharge screw, and deflector will now be described. The vessel10has an opening12at the top for loading the material into the vessel10, and the opening is typically closed by a lid (not shown). The vessel10has a first end26from which the mixed product is discharged through a discharge opening28and a second end30spaced from and parallel to the first end26.

The vessel10also has a first side wall14and a second side wall16spaced from and parallel to the first side wall. Each side wall14,16terminates at the bottom18of the vessel. The interior of the bottom has a pair of rotor chambers20,22that have an identical semi-cylindrical shape and are symmetrically connected to each other with their axis lines being arranged horizontally parallel side by side so that the rotor chambers20,22communicate with each other. An open topped chevron-shaped cavity24is formed on the inner portion of the bottom18of the vessel10at the boundary of inner peripheral surfaces of the rotor chambers20,22, such that the cavity24is centrally disposed between the two side walls14,16. This construction is sometimes referred to as a double-U-shaped vessel. The rotor chambers20,22have a cross section that is uniform in the axis direction.

A first and second mixing shaft40,50are rotatably supported by the end walls26,30of the vessel10and each mixing shaft40,50is disposed in the rotor chambers20,22, respectively. The mixing shafts40,50are rotatably disposed and spaced from the inner peripheral surfaces of the rotor chambers20,22. The shafts40,50may rotate at the same rotating speed or at different speeds. Each shaft40,50is provided with at least one mixing blade42,52, respectively on its outer periphery. In some instances, the at least one mixing blade is a type known as a sigma mixing blade.

Each mixing shaft40,50is connected at its driven end to a driving source so that the mixing blades rotate downwardly on the communicating side of the rotor chambers40,50. The geometry and profile of the sigma blade is designed such that the mass of material to be mixed is pulled, sheared, compressed, kneaded, and folded by the action of the blades against the walls of the vessel10.

An auger or discharge screw60is rotatably provided in the cavity24and, as such, is parallel to each of the mixing shafts40,50. It will be appreciated that, in operation, the discharge screw60is located below (in a vertical direction) each of the mixing shafts4050. Because the cavity24is centrally located between the side walls14,16, the discharge shaft60is likewise centrally located between the side walls14,16. The discharge screw60has a shaft62with a first end66that terminates in a nose68and that extends outwardly from the first end wall26of the vessel through the discharge opening28. The discharge screw60has a second end (or driven end)70that is connected through a seal arrangement102to a motor shaft100for rotating the discharge screw60in at least one and generally in two directions, i.e., a mixing direction and a discharge direction.

Referring more particularly toFIG.5, a discharge screw60according to the present disclosure is shown. The discharge screw60has, at its driven end70, and more particularly at a proximal end (proximal portion)74of the driven end, a recessed portion78(best seen inFIG.4) that extends into the second end wall30of the vessel where it is connected with one end of the motor shaft100so that the discharge screw can be rotated. The recessed portion78extends only partially into the second end wall (rear wall)30of the vessel, i.e., between about 10% to about 75% of the thickness of the second end wall30or in some instances between about 25% to about 50% of the thickness of the second end wall30. In some instances, the recessed portion78extends into the second end wall30an amount that ranges from about 10% to about 90% of the total thickness of the second wall, or from about 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, or about 85%. It will be appreciated that the recessed portion78that extends into the rear wall30of the vessel10effectively creates a simple labyrinth, which it is believed should reduce the pressure and flow to and past the seal arrangement102.

The driven end70, at the proximal end74, has a circular cross section that defines a cylindrical portion80of the driven end70of the discharge screw. The cylindrical portion extends80from the proximal end74toward the first end66of the shaft to a transition point88where the cylindrical portion tapers inwardly toward a distal end (distal portion)76of the driven end70to define a conical portion90that, at the distal end76of the driven end, becomes coextensive with the shaft62of the discharge screw60. The shaft62of the discharge screw60has a root diameter64that is less than the diameter84of the cylindrical portion80. As such, it will be appreciated that the distal end76of the driven end70has a diameter92that is the same as or substantially the same as the root diameter64. In addition, it will be appreciated that the root diameter64is less than or smaller than the diameter of the cylindrical portion84.

In some instances, the root diameter is about 25% to about 50% smaller than the diameter of the cylindrical portion84. In some embodiments, the diameter84of the cylindrical portion80may be about 2 cm to about 40 cm.

The length82of the cylindrical portion80may be in the range of about 2 cm to about 25 cm and may be less than, equal to, or greater than the length94of the conical portion90.

A plurality of flights96extend radially outward from the shaft62in a known manner, which as depicted inFIG.5, is in a corkscrew manner. The plurality of flights96may have a pitch98that is equal or substantially equal along the flight length97of the discharge screw70. The plurality flights96extend longitudinally from first end66of the shaft toward the driven end70to define the flight length97. The plurality of flights96begin at a location adjacent to but spaced from the nose68and stop at a location between the distal portion76of the driven end and the transition point88. In some instances, the plurality of flights96end at a location between about 40% to about 60%, or at about 50% of the distance between the distal portion76of the driven end and the transition point88. Importantly, there are no flights provided on the cylindrical portion80of the discharge screw. Without being bound by any theory, the inventor believes that the absence of flights on the cylindrical portion80of the discharge screw70will, during the mixing operation when the discharge screw70is rotating in the mixing direction, reduce the pressure and forces seeking to direct the material being mixed out the second end30in the area surrounding the recessed portion78of the driven end70.

Referring now specifically toFIGS.4and6, a deflector110is provided between the cylindrical portion80of the discharge screw60and the opening12of the vessel or bowl10. In some instances, the deflector110is located between the cylindrical portion80and the outer extent of the mixing blades44,54so that there is a small gap114between the deflector and the cylindrical portion and a small gap116between the deflector and the outer extent of the mixing blades.

The deflector110has a substantially chevron-shaped cross section with a bottom128adjacent and opposed to the outer peripheral extent of the cylindrical portion80. The bottom of the deflector128has a radius of curvature118in a manner to follow a radius of curvature86of the cylindrical portion80to provide a substantially constant gap width between the bottom128of the deflector110and the outer periphery81of the cylindrical portion80. In that regard, the bottom128of the deflector has a radius of curvature118that is substantially the same as the radius of curvature81of cylindrical portion80. Put another way, the bottom128of the deflector defines an arc119on the cylindrical portion80such that an angle α subtended by the arc119ranges from about 60° to about 75°.

As noted above, the deflector110is spaced from the cylindrical portion80a distance such that a small gap114exists between the bottom128of the deflector110and the outer periphery81of the cylindrical portion. The gap across the arc119is substantially constant and is in the range of about 0.25 cm to about 3 cm.

The top120of the deflector110has a first portion122and a second portion124that meets the first portion at a crest126. Each of the first and second portion has a generally concave profile that defines an arc gap116between the outer peripheral extent of the respective mixing blades44,54. The arc gap116is substantially constant when the outer peripheral extent of the respective mixing blades44,54, is adjacent the deflector110. During this time, the arc gap116ranges from about 0.25 cm to about 3 cm.

However, it will be appreciated that when the mixing blades42,52are of the sigma-type mixing blades, the arc gap116will vary from a minimum distance when the outer peripheral extent of the respective mixing blades44,54is adjacent the deflector110to a maximum distance when the outer peripheral extent of the respective mixing blades44,54are opposite from (i.e., 180° from) the deflector110.

Without being bound by any particular theory, the inventor believes that the presence of the deflector110during the mixing operation, when the discharge screw60is rotating in a mixing direction, i.e., in a direction toward the second end wall (the rear wall)30, will operate to reduce the pressure or forces seeking to move the material in the vessel10out toward the seal arrangement102. Put another way, the deflector110will act to block all, nearly all, or a substantial amount of the material being mixed from exiting the vessel10through the seal arrangement102.

The present disclosure also contemplates a method of mixing materials in the apparatus described above. In this regard, the method includes providing an apparatus containing one or more of the above features, i.e., a discharge screw having a driven end recessed into the rear wall, a discharge screw having a cylindrical portion free of flights, having a diameter greater than the root diameter of the shaft of the discharge screw, and being tapered at a proximal end to define a conical portion that is coextensive with the shaft of the discharge screw, a deflector extending from the rear wall of the vessel or bowl. With the apparatus in mind, the contemplated method includes introducing material to be mixed through the opening of the vessel, causing the mixing blades to rotate, causing the discharge screw to rotate in a mixing direction at a first speed, at a time when the material to be mixed is complete (as understood by the known operator based on the material to be mixed), causing the discharge screw to rotate in a discharge direction at a second speed. The first speed is less than the second speed and in this regard, the first speed may be about 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85% or 90% of the second speed.

While the above features, alone or in combination, may be effective in preventing leakage out the rear end wall of the mixer, it is contemplated that, during the mixing operation, the speed of the discharge screw can be reduced, which should reduce the pressure on the seal arrangement for the discharge screw at the rear wall. The speed can be reduced by any suitable amount from the typical operating speed ranging from about 10 rpm to about 60 rpm. To this end, the speed can be reduced by 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85% or 90% of the typical operating speed.

While the concepts of the present disclosure are susceptible of various modifications and alternative forms, specific exemplary embodiments of the disclosure have been shown by way of example in the drawings. It should be understood, however, that there is no intent to limit the concepts of the present disclosure to the particular disclosed forms; the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the claims.