Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The invention relates to a screw for a single
screw extruder with a conically shaped feed zone which
undergoes transition into a decompression zone which is
followed by a compression zone.
Such screws are used for the extrusion
processing of pulverous, highly dispersed or granulated
polymer materials with large bulk volumes.
There is already known a screw for a single
screw extruder with a conically shaped feed zone or entry
zone and a cylindrical compression zone in which the pitch
angle of its screw line is constant.
Furthermore a screw for a single screw extruder
is known which has a decompression zone which is arranged
in the compression zone. With this, the pitch angle of the
screw line is constant and lies within the limits of 17 to
27, the compression ratio along the screw is likewise
constant and lies between 1.5 and 5.
Disadvantageous with the known screws is the
small diameter of the screw core at the beginning of the
feed zone, where the forces are greatest and where there
occurs most frequently a fracturing of the screw, the loss
of compression off-centre of the feed region to the screw
and the outer cylinder belonging to it, the failure of a
decompression zone or compression relief zone at the
beginning of the compression zone which is necessary for
the deaeration of the moulding composition, the non-optimal
pitch angle of the worm screw line and the compression
ratio in the feeding and compression zone, so that the
productivity is small, the difficult removal of the screw
from the outer cylinder and the fact that they are not
suitable for the direct processing of pulverous materials
wi1:hout prior granulation.
The object underlying the invention consists
therefore in so constructing the screw of the initially
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indicated type that error free large yields can be produced
with high productivity, with maintenance of a rapid and
simple disassembly and with direct feeding with pulverous
material without pre-granulation.
According to the present invention, there is
provided a screw for a single screw extruder, with a
conically shaped feed zone which undergoes transition into
a decompression zone which is followed by a compression
zone, characterised in that the compression ratio in the
feed zone of the screw is greater than four, with a pitch
angle to the screw line of 15 to 23, the decompression
in the decompression zone amounts to 1.5 to 2.0 and the
compression ratio in the compression zone is less than 4.5
with a pitch angle to the screw line of the screw of 25
to 32.
The compression zone of the screw may be
cylindrical, or it may be conically tapered in its forward
region.
With the screw according to the invention, the
smallest pitch angle to the screw line of the screw is
l`ocated in the feed region of the extruder. On rotation of
the screw, this makes possible, the ready uptake of
pulverous highly dispersed and other materials with large
bulk volumes. Since the compression ratio of the screw is
large in the feed zone, a high degree of compaction and a
high degree of the degasification of the material are
achievable. The largest pitch angle to the screw line of
the screw is in the exit zone, as a result of which a
maximum productivity of the extruder is guaranteed and
products with large cross-section can be produced. The
provision of a decompression zone and possibly a vacuum
zone makes possible a separation of air, moisture and other
fluid substances contained in the material or separated off
in the processsing. Furthermore, the working volume of the
screw duct of the screw can get bigger in the feed zone of
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the extruder, corresponding to the bulk volume of the
material being worked. ln this way, the compression ratio
of the screw may be varied in the compression zone of the
S extruder in dependence on the variation in the specific
volume of the material on melting and the desired degree of
additional homogenisation as a result of production of the
differential return flow.
The disassembly of the screw from the outer
cylinder is capable of being carried out quickly and
effectively, which is especially important in the case of
the possible thermosetting three-dimensional cross-linking
of the moulding composition during the extruding. Finally,
the shearing forces and correspondingly the residence time
of the material in the extruder can be regulated within
wide limits which is of especial significance in the
processing of thermostable polymer moulding compositions.
Preferred embodiments of the invention are
explained hereinafter as examples without limitative
manner, having reference to the attached drawings, wherein:
Figure l shows a first constructional form of
the screw, and
Figure 2 shows a second constructional form of
the screw.
The screw 2 for single screw extruders shown in
Figures 1 and 2 has a conically shaped feed zone I, a
decompression zone II connecting thereto and a compression
zone III following thereon. The compression ratio in the
feed zone I is, with a pitch angle '~l to the screw line of
the screw 2 of 15 to 23, greater than four. The
compression release in the decompression zone II amounts to
1.5 to 2Ø The degree of compression in the compression
zone II is less than 4.5 with a pitch angle '~2 to the screw
line of the screw 2 of 25 to 32.
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The compression ratio in the feed zone I forming
the first region of the screw is determined by the larger
diameter of the screw 2 and the larger duct depth at the
beginning of the feed zone I in comparison with the
diameter of the screw 2 and the duct depth at the end of
the feed zone I.
The second region of the screw 2 is formed by
the decompression zone II, a transition zone and the
compression zone III. It has an essentially smaller
variation in diameter which remains unaltered in the case
of a cylindrical compression zone III. In the
decompression zone II, the depth of the screw channel is
greater. The decompression zone II can be connected with
opening 4 in the outer cylinder 3 for the drawing off of
gaseous products carried forward in the feeding or
separated off in the working. The compression ratio in the
compression zone III amounts to 1.5 to 4.5 and extends,
with the working of hard polyvinylchloride, to from 2.7 to
3.1, with soft polyvinylchloride to from 3.0 to 3.5, with
high density polyethylene from 3.70 to 4.35, with low
density polyethylene to from 4.00 to 4.50, with
polypropylene to from 3.70 to 4.35, with polyamide and,
polystyrene to from 4.00 to 4.35. A storage chamber 1 is
arranged at the beginning of the outer cylinder 3.
In the embodiment shown in Figure 1, the
compression zone III is cylindrical, being shown conically
in Figure 2.
The operation of the single screw extruder
according to the invention is as follows:
The polymer material is fed into the feed zone I
from the chamber 1. Resulting from the rotation of the
screw 2 in the outer cylinder 3, the screw web takes up the
polymer material from the chamber 1 and conveys it forward
into the screw channel of the screw. The polymer material,
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is moreover, compacted as a consequence of the reduction of
the duct's cross-section. In the compression zone II, the
air carried forward from the polymer material, the moisture
and other gaseous products are separated off, after which
the polymer material, as a result of the rotation of the
screw, is compressed and homogenised and then is pressed
out to the shaping tool at the end of the outer cylinder 3.
