Twin screw extruder with single-flight kneading disks

Details Twin screw extruder with single-flight kneading disks Twin screw extruder with single-flight kneading disks

1999-12-21

2001-01-30

Cooley, Charles E. (Department: 1723)

Agitating

Rubber or heavy plastic working

Stirrer is through-pass screw conveyor

C366S085000

Reexamination Certificate

active

06179460

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a twin screw extruder comprising a casing with a working direction; two partially intersecting casing bores, which are parallel to each other; two shafts, which are disposed in the casing bores and which are drivable to rotate in the same direction of rotation about an axis of rotation, the axes of rotation having a distance A from each other; and single-flight kneading disks, which are fixed on the shafts and have surface lines extending parallel to the respective axis of rotation, the kneading disks, in a cross-section perpendicular to the axis of rotation, comprising a crest, which is formed as a segment of a circle about the respective axis of rotation and which has a crest angle b and a radius R
A
; a bottom, which is formed as a segment of a circle about the respective axis of rotation and has a bottom angle g and a radius R
I
; two flanks, which join the crest and the bottom; R
A
>R
I
und A≈R
A
+R
I
applying.
2. Background Art
Single-flight kneading disks are known from DE 813 154 B. They have a crest angle greater than 90° , in this regard possessing a comparatively important cross-sectional surface. The kneading disks wipe the casing as well as themselves. In these known kneading disk arrangements, no overall conveyance of the treated material takes place in or against the operating direction of an extruder which lodges the kneading disks. The mixing effect by this type of kneading disks is low. The same is true for the kneading effect. Furthermore, also the free cross-sectional surface is defined and thus the mean dwell time, within a lengthwise section of the extruder, of the material to be treated.
U.S. Pat. No. 5,573,332 teaches a screw element for a screw-type extrusion machine. The screw elements are helical and have varying pitch directions. Lengthwise mixing is obtained by the screwing in opposite directions, whereas crosswise mixing is attained by the elongated wedge of the flank arc. This crosswise flow is a typical continuous shear flow, which is primarily a dispersive mixing operation. Dividing the flow into various partial flows, recirculation and offset combination thereof do not take place, which is why the distributive mix is not optimal.
DE 43 38 795 C teaches a continuously working multi-screw extruding machine for masses to be plastified. Provided between two closely intermeshing conveying screws and a dam-up element is a screw which conveys forwards and adjacent thereto a non-intermeshing screw which conveys backwards. A drawback resides in that the wall of the casing cannot be scraped off, since the screws have a diameter that corresponds to half the center distance. This does not prevent the material to be treated from sticking on the casing wall. Since the casing wall is not wiped, this results in very bad heat transmission. In the case of high differences of temperature between the casing wall and the product, this will result in important inhomogeneities of temperature which will moreover negatively affect the material to be treated.
SUMMARY OF THE INVENTION
It is an object of the invention to improve a twin-screw extruder of the generic type such that given a minimal energy input and high dwell time, the entire material to be mixed is completely distributively mixed and simultaneously the casing wall is entirely wiped.
This object is attained by 0°≦b≦45° applying to the crest angle b. The gist of the invention resides in providing cylindrical kneading disks—i.e. kneading disks having surface lines extending parallel to their respective axis—with a very small crest angle. This reduces the energy input and helps obtain mixing of the entire volume. Furthermore, complete scraping of the walls of the casing bores is ensured, i.e. there is no sticking, and good heat transmission is ensured. The wedge flows along the flanks of the kneading disks prevent neutral zones of reduced mixing to originate. The intermeshing action of the kneading disks ensures intense mixing of the material to be treated. The kneading disks wipe each other at least partially. Given a constant diameter of the casing bores, the free cross-sectional area available to the material to be treated is increased, as a result of which the throughput can be raised.
The extruder, in which the kneading disks of at least one shaft are offset in the working direction by an offset angle e in the direction of rotation for conveyance against the working direction, 0°>|e|>180° applying, has the advantage of return conveying being attained, which strongly increases the dwell times.
The arrangement, according to which the kneading disks of one shaft are offset in the working direction against the direction of rotation by an offset angle |e|>180° and the kneading disks of the other shaft are offset in the working direction in the direction of rotation by an offset angle |e|>180° , has the advantage that especially intense mixing is attained through a circulating way of conveyance.
Additional advantages and features of the invention will become apparent from the ensuing description of an exemplary embodiment, taken in conjunction with the drawing.


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