Agitating – Rubber or heavy plastic working – Stirrer is through-pass screw conveyor
Reexamination Certificate
2002-08-13
2004-03-16
Cooley, Charles E. (Department: 1723)
Agitating
Rubber or heavy plastic working
Stirrer is through-pass screw conveyor
C366S083000, C366S089000
Reexamination Certificate
active
06705753
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an extruder, in particular for processing polymer melts, with an extrusion space which is arranged in an extruder barrel and through which at least one extruder screw shank extends in such a way that it can be rotated by a motor, said screw shank having on it at least one section in the form of a circumferential bulge, the extrusion space having at least one barrel section which surrounds in an essentially equidistant manner the at least one extruder screw shank in the area of the at least one circumferential bulge to form an annular gap as a restricted flow zone (blister), in particular to set the pressure of the melt.
2. Description of the Related Art
An extruder of this type is disclosed by U.S. Pat. No. 4,332,481.
Such a restricted flow zone formed as an encircling gap, for example as an annular gap, in the extrusion space is also referred to as a blister and serves in particular for setting the pressure of the melt processed in the extruder. Downstream of this restricted flow zone, entraining agents may be added for example, being incorporated into the expanding polymer melt and serving as stripping agents. The effect of the blister is strongly dependent on the clearance in the annular gap between the cylindrical part of the extruder screw and the inside surface of the cylindrical extrusion space. In practice, the clearance is often of the order of approximately 3 mm. The optimum clearance is dependent less on the size of the respective extruder than on the properties of the material to be processed (temperature, viscosity). According to the prior art, changing of the clearance often involves a corresponding change of the screw shank or the cylindrical part of the screw shank in the region of the restricted flow zone. The processing of different materials therefore frequently requires the screw shank to be changed in order that best possible processing conditions can be set. In the case of non-steady-state processes during processing, the optimum size of the annular gap of the blister changes. Until now there has been no possible way of optimizing the clearance in such a situation. In particular during start-up operations, in which the extruder has not yet reached a constant operating temperature, changes in the effective clearance of the blister may be brought about by different degrees of thermal expansion. It has so far also not been possible to deal with this in an optimum way.
DE 198 31 540 A1 discloses a device for partially changing a closed flow channel cross section, the inner contour of which is formed by a displacement element, for example of a cylindrical shape. The flow channel is preferably part of an extrusion die for producing a tube from flowable compositions. The device has external adjusting elements in the form of pins which can be adjusted radially from the outside inward and at least one region of a flow channel wall which comprises a sleeve with a wall thickness of at most 1 mm, the sleeve being locally deformable inward by means of the adjustable pins. The displacement element arranged in the flow channel may likewise be provided with a sleeve of a small wall thickness, which is locally deformable outward by means of supporting elements that are axially displaceable on conical supporting surfaces. In this way, the flow channel cross section can be reduced in a specifically selective manner in the region of the thin flow channel wall. A disadvantage of this known device is that the adjustable pins and displaceable supporting elements do not allow uniform deformation over the cross section of the flow channel, since the introduction of the deforming forces can only ever take place in a punctiform or locally limited way. The greater the number of elements used for the deformation, the more uniformly the deformation can be set, but the greater the actuating effort for these elements as well. Another disadvantage is that the thin walls of the sleeves are unavoidably deformed by the melt pressure prevailing in the flow channel counter to the force of the elements used for the adjustment. Consequently, it is impossible for the device to be operationally used under geometrical conditions of the entirely unloaded state (zero position).
Further possibilities emerge from JP-60002329, where regions of an extruder barrel which can be elastically deformed by adjusting devices and interact with screw segments to produce an adjustable restricting gap are provided, or from U.S. Pat. No. 3,122,789, which shows a membrane which can be elastically deformed by a pressure fluid to adjust the flow channel height in a slot die.
SUMMARY OF THE INVENTION
It is an object of the invention to develop an extruder of the type stated at the beginning to the extent that a largely optimum mode of operation in the region of the blister is possible even if different materials are to be processed or the material properties change, without any major modification work being required for the extruder. The effort for the adjustment of the blister is to be as little as possible.
This object is achieved in the case of an extruder of the generic type by the clearance of the encircling gap at the blister being able to be changed in a specifically selective manner by externally adjustable force, with elastic deformation of the surface of the extrusion space and/or of the screw shank. For this purpose, the invention provides a hydraulic pressure unit which supplies the force for the surface deformation in the region of the restricted flow zone either on the inside surface of the extrusion space or on the circumferential bulge of the at least one screw shank or else on both, a hydraulic pressure chamber being provided in the region of the restricted flow zone in the wall of the barrel section where the blister is to be arranged and/or in the circumferential bulge of the at least one screw shank. The pressure chamber has on the side respectively facing the extrusion space a considerably smaller wall thickness in comparison with the wall thickness of the extrusion space. This wall is referred to hereafter as a membrane wall because of its small thickness.
The pressure chamber is expediently provided respectively in said barrel section of the extruder barrel and not on the screw shank. The reason for this is primarily to be seen in the fact that, because of the associated sealing problems, it is more difficult to provide a supply of hydraulic pressure medium to the pressure chamber on the screw shank than on the extruder barrel, which unlike the screw shank is completely immovable.
The invention can be realized particularly advantageously in the case of a single-screw extruder, which has a purely cylindrical extrusion space, with the result that the encircling gap at the blister is formed as an annular gap. The hydraulic pressure in the pressure chamber allows the membrane wall to deform to the respectively desired dimensions in a geometrically very uniform shape (circular shape). The desired settings can be controlled extremely quickly and easily, since all that matters is the difference in pressure between the hydraulic pressure and the pressure in the extrusion space.
However, the invention can also be applied very advantageously in the case of twin-screw extruders. In these extruders, the extrusion space is formed by two longitudinal bores which overlap partially in cross section, at least over part of the axial length of the extruder barrel, and because of their shape are also referred to as a “spectacle-bore”. Arranged in this spectacle-bore are two co-rotatable or counter-rotatable motor-driven screw shanks which lie parallel to each other and each have at least one circumferential bulge. The circumferential bulges of the two extruder screw shanks in this case respectively lie directly next to each other. If the pressure chamber is not arranged respectively on the two extruder screw shanks but is provided on the inside surface of the barrel of the extrusion space, that is to say in the spectacle-bore, it is recom
Berstoff GmbH
Cohen & Pontani, Lieberman & Pavane
Cooley Charles E.
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