Agitating – Rubber or heavy plastic working – Stirrer is through-pass screw conveyor
Reexamination Certificate
1999-04-21
2001-01-09
Cooley, Charles E. (Department: 1723)
Agitating
Rubber or heavy plastic working
Stirrer is through-pass screw conveyor
C366S085000
Reexamination Certificate
active
06170975
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to multi-shaft extruder kneading discs and kneading blocks for kneading material in a multi-shaft extruder and to a multi-shaft extruder containing the same. More particularly, the present invention relates to kneading discs and kneading blocks wherein each of the kneading discs has a plurality of lobes with at least one of the lobes having a radius less than another lobe.
2. Background Art
Kneading discs for twin-screw extruders are well known. Generally, the kneading discs have 2 to 3 kneading lobes the length of the bushing. The crests of the kneading lobes are generally flat in the radial direction, but do have an arcuate shape in the circumferential direction. These prior art kneading discs are usually manufactured such that side by side kneading discs interact with each other when mounted in a twin screw extruder as kneading blocks. The number of discs in a kneading block are generally 4, 5 and 7. That is, the typical two diametrically opposed lobe kneading discs has the discs oriented at 60 degrees to each other in a 4 disc block; at 45 degrees or 90 degrees to each other in a 5 disc block, and at 30 degrees to each other in a 7 disc block.
The kneading discs have an upper face and a parallel lower face which are substantially mirror images of each other. Lateral side walls join the upper and lower faces. The discs generally have an annular central region with two or three lobes radially extending from the central region. The crests of the lobes are generally truncated. The kneading blocks generally have the discs arranged thereon so that they are in balance.
By increasing torque capacity and available rpm (speed), the capacity of an extrusion production line has been increased without increasing extruder size (diameter). However, an increase in speed has resulted in higher material temperature. (For a more detailed commentary on this issue, please refer to the '97 Antic paper “Understanding High Rate and High RPM Compounding on co-Rotating Twin-Screw Extruders.) As rpm is increased, the material will, at some point, reach a temperature where the polymer will start to degrade.
Temperature is generated by the deformation of material as a consequence of energy input. This energy input results from the shear generated as: (1) two discs (each disc is on a different screw shaft) pass each other going in opposite directions, (2) the lobe crest of one element disc wipes the root of the other, and (3) the lobe crests of two discs approach each other in the area of the screw channel known as the apex region. In the apex, an open surface of a small but not inconsequential area is reduced to virtually zero area during the disc rotation. In the standard full diameter kneading block element, all three of these actions, as well as the pushing of material in a circumferential motion by the disc lobe crest, contribute to material deformation and an associated temperature rise.
SUMMARY OF THE INVENTION
It is an aspect of the invention to provide a disc geometry that would reduce temperature rise and allow even higher rpm and throughput rates.
It is another aspect of my invention to relieve all of the apex area reduction in 3 of the 4 occurrences during a full revolution. My kneading discs are preferably in a fixed series for ease of assembly on the screw shafts. The number of discs in the series or kneading block is typically the amount needed so that the rotation between each disc results in a progression through 180 degrees or 360 degrees from beginning to the end of the disc. The number is typically 5 or 7. Five if the stagger angle is 45 degrees or 90 degrees, and seven for 30 degrees.
It is still another aspect of the present invention to provide a multi-shaft extruder kneading disc having a disc shape with a central section defining an extruder shaft mounting bore and at least one lobe extending radially from said central section and having a flat crest end. The at least one lobe having a radius X which defines an outer diameter of the kneading disc. The central section having a radius Y which defines an inner diameter of the kneading disc. At least one other lobe extending radially from the central section and having a radius Z which defines the outer diameter of the other lobe. The radius Z of the other lobe which defines the outer diameter of the other lobe is equal to Y+⅞(X−Y) to Y+¼(X−Y).
My kneading disc is one where one lobe has a reduced length or radius such that it was a height above the root diameter approximately equal to ¼ to ⅞ of the channel depth. The shape of the tip of this reduced lobe is still rounded, and in a preferred embodiment, an arc of a circle of a screw diameter reduced by approximately 2 times the depth of the disc reduction. This, however, is not a requirement. The progression of the tip reduction can proceed uniformly in the direction of disc stagger, opposite to disc stagger. This is not a preferred embodiment since it creates an unbalanced geometry for rotation around an axial center. No matter what the stagger angle, the progression of reduced tips should be such that a reduced tip at an angle has a corresponding tip reduction at 180 degree rotation at the same point on the element. This is best achieved with a random pattern. With an odd number of discs, as is typically required, there will always be one disc of unbalance, but this is a minimal value.
The above has referred to two lobe, lens shape, screw cross-section geometry. The same can be applied to three lobe, triangular shape, screw cross-section geometry.
The kneading discs have an outer diameter and an inner diameter with the disc having a bore to attach the disc to a drive shaft. The upper and lower faces are substantially mirror images of each other and generally extend parallel to each other. The central annular section and two lobes are joined by two lateral side walls that taper towards each other and are joined at both ends of the disc. The first lobe has a truncated or flat crest and the second lobe has a rounded crest. Generally, two or more of my kneading blocks are placed end to end on each side by side extruder drive shaft in a 90 degree orientation.
My kneading disc preferably is constructed to have at least one normal size lobe extending from the central region and at least another lobe having a radius reduced by ⅛ to ¾ of the channel depth with respect to the radius of the normal size lobe. That is, in a two lobe kneading disc, the first lobe has the normal radius and the second lobe has a radius equal to Y+¼ (channel depth) to Y+⅞ (channel depth). In a three lobe kneading disc having my configuration, the first and second lobes have the normal radius and the third lobe the reduced radius; or the first lobe has the normal radius and both the second and third lobes have the reduced radius.
The kneading discs are described as being applicable for a twin-screw extruder. However, the kneading disc may be used with an extruder having more than two side by side screws or shafts (multi-screw or multi-shaft extruder).
My kneading discs are constructed such that when they are mounted on the shafts of the multi-shaft extruder, they are non-rotatable relative to their respective shaft. This is done by splines or key joints on the bushing and the corresponding shaft. Some of the known construction has 24 splines, 6 splines, two keys and a single key.
Another aspect of the invention is to provide a multi-shaft extruder having a plurality of kneading discs each having a disc shape with a central section defining an extruder shaft mounting bore and at least one lobe extending radially from said central section and having a flat crest end. The at least one lobe having a radius X which defines an outer diameter of the kneading disc. The central section having a radius Y which defines an inner diameter of the kneading disc. At least one another lobe extending radially from the central section and having a radius Z wh
Conte Robert F. I.
Cooley Charles E.
Krupp Werner & Pfleiderer
Lee Mann Smith McWilliams Sweeney & Ohlson
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