Agitating – Rubber or heavy plastic working – Stirrer is through-pass screw conveyor
Reexamination Certificate
2001-04-19
2003-07-08
Cooley, Charles E. (Department: 1723)
Agitating
Rubber or heavy plastic working
Stirrer is through-pass screw conveyor
C366S304000
Reexamination Certificate
active
06588925
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in one aspect to a single screw extruder for extruding combinations of materials such as thermoplastics polymers, rubbers, waxes and solid additives, and in another aspect to a mixer for such materials. The mixer could however be employed in the manufacture of inks, paints and other materials where one or more of the components is liquid at the room temperature.
BACKGROUND OF THE INVENTION
Single screw extruders are very widely used in the plastic industry for producing compounds of rubber and thermoplastic polymers with solid additives. They are simple to construct and therefore relatively inexpensive; however that they have limited distributive and dispersive mixing capacity has been long recognised and is well documented (cf “Single Screw Mixing: Problems and Solutions” Martin Gale, a paper presented at a RAPRA Technology Ltd. seminar Aug. 6, 1995).
Further background as to the mixing limitations of single screw extruders is given in an article in Plastics Additives and Compounding August/September 1995 pages 21-23 entitled “New dispersive mixers based on elongational flow” and the associated patent, U.S. Pat. No. 5,932,159, published Aug. 3, 1999. This stresses the need for a variety of dispersive forces including elongation flow and multiple passes through regions of high stress, conditions which are normally difficult to generate within a single screw machine.
There are many devices that can be used to improve the distributive mixing capacity of single screw extruders; however these devices offer only marginal improvements in dispersive mixing. A good example of this is the cavity transfer mixer (U.S. Pat. No. 4,419,014) where the process melt is transferred repeatedly between cavities in a rotor and opposed cavities in the barrel wall. The rotary motion of the rotor means that the material is constantly subdivided and re-orientated. However this does not generate a high shear rate since the walls of opposing cavities are quite widely separated. In practical devices this low shear rate also limits the maximum cavity size since there is a tendency for the melt to stagnate.
Pins can also be used to improve mixing either protruding radially from the barrel or from the surface of a rotor or the screw itself. Whilst pins do generate chaotic flow, improving distributive mixing, they do little in the way of dispersive mixing since the pins do not move relative to a complementary shear surface. In the case of pinned barrels, gaps in the helical flight sweep over the pins generate high sheer events and also allow significant re-circulation of the polymer melt improving distributive mixing. However the proportion of material subjected to high shear is quite small.
OBJECT OF INVENTION
A basic object of the invention is the provision of an improved single screw extruder, and to a mixer for use with such extruder.
Summary of a First Aspect of the Invention
According to a first aspect of the invention there is provided a single screw extruder comprising a drivable screw, with at least one flight, located within a static barrel so as to define an annular, material flow gap between the exterior of the screw and the interior of the barrel, a mixer associated with the screw, whereby material passes from an upstream portion of the flow gap, into the mixer and is then either returned to a downstream portion of the flow gap or is discharged, with the mixer comprising a rotor driven by the screw and a stator, the rotor and the stator each carrying mutually facing interengaging rings of teeth whereby the material is urged outwardly from the annular gap along a first tortuous mixing path, and then returned inwardly along a second tortuous mixing path, the teeth extending axially, or generally so, with respect to the longitudinal axis of the screw.
SUMMARY OF A SECOND ASPECT OF THE INVENTION
According to a second aspect of the invention there is provided a mixer for mixing solids with liquids, liquids and liquids e.g. polymer alloys, for use with other devices or combinations of devices capable of driving the rotor and introducing material in a fluid state into the mixer under sufficient pressure to cause the material to be mixed to flow through the mixer, the mixer comprising:
(i) a cylindrical stator chamber having opposed, radially extending faces provided with axially projecting, radially spaced-apart rings made up of alternating teeth and ridges,
(ii) a rotor rotatably fitted within the stator and provided on its opposite side faces with axially projecting, radially spaced-apart rings of alternating teeth and ridges, and
(iii) the rotor and stator rings interengaging with both radial and axial clearance so as to define a tortuous material flow path.
ADVANTAGES OF THE INVENTION
The extruder in accordance with the first aspect has been found to provide significant improvement in the extruder performance and the quality of extruded product compared with prior art single screw extruders, whilst the mixer in accordance with the second aspect has been found to be particularly advantageous and to provide a fundamental improvement in the mixing of solids and liquids—such as a liquid thermoplastics material and solid additive and improve the manufacture of polymer alloys. The rings on the stator may be as thin as possible whilst maintaining mechanical integrity since their only functions are to provide a barrier to melt flow and complementary shear surfaces to the rotor. This arrangement limits possible melt stagnation in the gaps between stator teeth. In addition, the screw may serve as a main bearing for the mixer, whilst because the teeth are concentric around the barrel, there is no constraint on the length of the teeth that may be provided.
PREFERRED, OR OPTIONAL, FEATURES OF THE EXTRUDER
The mixer is located intermediate the ends of the screw.
The mixer is located at the discharge end of the screw/extruder.
As the extruder will be used for extruding a range of materials, it is clear that a suite of mixers exhibiting differing geometrical properties to provide different mixing capabilities, is desirable for optimum mixing. Thus, to permit reasonably expedient changing of a mixer, the extruder is provided with readily releasable means eg a pair of releasable flanges, within which the mixer is housed.
The internal diameter of stator chamber is larger than the internal diameter of the extruder to which it is attached and the rotor is relatively short.
PREFERRED, OR OPTIONAL, FEATURES OF THE MIXER
Clearly, the mixer can be used in combination with any apparatus capable of introducing the materials to be mixed under sufficient pressure to cause these materials to flow through the mixer. One such apparatus is a single screw extruder. Thus it is necessary to provide the mixer with an entry aperture and an exit aperture for material feed under pressure into, through, and out of, the exit aperture of the mixer. In the mixer the materials undergo four actions namely (i) a radial movement under pressure suitably from a central feed port to an outlet port, (ii) an orbital movement involving division of the radially moving material into portions some of which go one way while vicinal portions go the opposite way and (iii) a shearing action (iv) an elongational deformation.
This mixer differs from that of a conventional extruder configuration—with a long thin screw and any ancillary mixers are contained in a narrow cylinder, i.e. the primary internal barrel diameter—since the mixer has, in it is preferred configurations, a short broad rotor within a chamber with an internal diameter greater than that of the barrel to which it is attached. Looking at this basic geometry two significant advantages become clear. Firstly the shortest path length through the mixer increases only linearly with rotor radius whilst the volume available for mixing increases with the square of that radius. Secondly angular velocity rises linearly with the rotor radius. This means that the largest mixing volume coincides with the highest potential shear rates.
By interengage is mean
Cooley Charles E.
Kaltor Limited
King & Schickli PLLC
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