Note: Descriptions are shown in the official language in which they were submitted.
21298S6
Apparatus for granulating synthetic plastics materials
The invention relates to an apparatus for granulating synthetic
plastics materials, in particular thermically instable, highly viscous
synthetic plastics material, for example polyvinyl chloride or
polyvinylidene chloride,comprising at least one rotating granulating knife
which rotates in front of outlet openings for the synthetic plastics
material through which the plasticized synthetic plastics material is
pressed by a plasticizing screw bearingly supported for rotation within a
housing immediately in front of the outlet openings and rotated by a drive
means, the synthetic plastics material passing through these outlet
openings with a component of motion directed in axial direction of the
plasticizing screw,and a cooling means for the synthetic plastics material
grains cut off by the granulating knife.
The apparatus of this kind known until now comprise in the most
cases a granulating housing in which the outlet end of a screw extruder
discharges by a nozzle plate creating the synthetic plastics material
cords. The screw extruder is charged at the other front end and is driven
by an elongation of the screw shaft which extends beyond the charging
location. As a rule, the grains cut off in the granulating housing are
cooled by cooling water directed into the region of the knife and are
conveyed off together with the cooling water running off. Such
constructions have the disadvantage within such plants which are thought
for treating sensitive synthetic plastics materials, in particular such
synthetic plastics materials which are thermically instable and tend to a
reduction of the molecule chains if they remain in the hot region for a
longer time. Polyvinyl chloride or polyvinylidene chloride are examples
for such sensitive synthetic plastics material kinds. The longer dwell
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times result from deviations in the flow of the treated synthetic plastics
material which must be feeded in direction perpendicular to the
longitudinal direction of the screw, and from back-up spaces and frictions
in connection which such deviations, as well as from flow channels increa-
sing the dwell time.
A further disadvantage consists in that such screw extruders which
are driven in the region of the inlet end of the screw, cannot be used in
cases in which it would be desirable to feed the screw at its front end in
substantially axial direction, thus for example within processing plants
for recycling goods in which the screw housing is connected in radial
direction to the exit opening of a receptacle in which the synthetic
plastics material is comminuted by means of rotating knives.
It is also known (EP-A 373,372) to use a screw protruding into a
filling hopper for the purpose to convey the synthetic platics material to
be plasticized into a plasticizing chamber in which the synthetic plastics
material is plasticized by means of rotating pressing wings and is pressed
out of the plasticizing chamber in radial direction through exit openings
disposed on the periphery of the plasticizing chamber. Granulating knives
rotate outside these exit openings and are connected to a hub surrounding
the driving shaft of the conveyor screw. This construction is disadvan-
tageous because the synthetic plastics material introduced into theplasticizing chamber must be plasticized in a very short time if it should
be ensured that the plasticized material can be pressed through the exit
openings. This requires a high power demand to drive the pressing wings.
Further it is of disadvantage that the knives which rotate on the outer
periphery of the exit openings formedby a perforated ring, together with
their holding devices constitute a considerable mass, and again an
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increased power demand is required for its motion.
The invention has at its object to improve an apparatus of the
initially described kind so that the synthetic plastics material
processed at a low power expenditure is gently treated and that the
applicability of the apparatus is made more versatile. In particular, it
should be possible to use an apparatus of the initially described kind
also within the mentioned recycling-processing plants. The invention
solves this task by the features that the exit openings are disposed
adjacent to the drive-side front end of the tip-driven plasticizing screw
and that the drive means for the plasticizing screw comprises a shaft
which is disposed coaxially to this drive-side front end and is
surrounded by the circulation path of the knife disposed between the
drive means and the exit openings and which shaft constitutes a member
separate from the plasticizing screw that is coupled for common rotation
and detachably to the front end of the plasticizing screw.
Unlike the initially described known construction, the screw of the
inventive apparatus is therefore provided with a tip drive means, that
means, it is driven at that front end towards which the screw conveys.
Thereby the other front end of the screw not facing the exit end of the
synthetic plastics material is made free and the synthetic plastics
material can be supplied there without any problem. Therefore, it is
possible to connect the inventive apparatus with the screw housing at
least substantially radial to a processing receptacle for the synthetic
plastics material and thereby to make use of the centrifugal force for
the purpose of filling the screw, which centrifugal force is exerted to
the comminuted synthetic plastics material by the knives revolving within
the processing receptacle. Further, a compact construction having short
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flowh paths for the plasticized synthetic plastics material is obtained
so that the dwell times of this material in the hot region are comparati-
vely low and therefore the danger of thermic deteriorations of the
processed synthetic plastics material is decreased. Since the exit
openings are disposed immediately in front of the front end of the screw,
its greatest distance from the screw axis is not greater than the screw
diameter so that also the knives must only show this maximum distance
from the screw axis. Thereby, if compared with the above described known
construction, a substantially reduced moved mass is obtained for the
knives and thereby a reduced energy expenditure. Further, plasticizing of
the synthetic plastics material to be processed by means of the
plasticizing screw can be made in a more energy-saving and
gently-treating manner than with the pressing wings of the known
construction. The detachable connection between the driving shaft and the
plasticizing screw facilitates disassembly of the screw for the purpose
of cleaning or if worn.
It is known for granulating of polymer fibre ropes (SU-A-1,452,682)
to introduce these polymer fibre ropes into peripheral grooves of a screw
housing extending in axial direction, and to convey them there by means
of a conveyor screw to a granulating knife revolving around the screw
axis, this granulating knife revolving at the front-side end of the screw
housing around the axis of the screw shaft. Within this, no plasticizing
of the supplied synthetic plastics material takes place so that the
problems solved by the invention do not occur here.
It would be possible within the spirit of the invention to let the
screw convey towards a separate perforated plate, the knife revolving at
the exit side thereof. Such a separate perforated plate, however, can be
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avoided within the spirit of the invention by the feature that the exit
openings are confined by the screw and its housing. Therefore, the screw
with its housing constitutes at the same time the nozzle channels through
which the synthetic plastics material supplied by the screw is extruded.
A preferred embodiment of the invention consists in that the shaft
is formed as a hollow shaft and is coupled by means of a screw in the
front end of the screw for common motion in axial direction and axially
adjustable and also coupled for common rotation with the screw by at
least one wedge or a toothed connection. This is a particular simple
construction which enables one to exactly adjust the axial position of
the extruder screw by rotating the screw in order to ensure that the
front end of the extruder screw and the front end of its housing are
positioned exactly in one common plane to that the knife can exactly cut
off the synthetic plastics material cords discharging from the nozzle
channels.
As already mentioned, it is suitable to dispose the exit openings
between the screw and its housing. For this, there exist substantially
two main variants: It would be possible to machine the exit openings or
the channels forming them into the inner periphery of the screw housing.
So formed channels would stand still and the knife could rotate together
with the screw. As a rule, however, the revolution speed of the screw
will not be great enough to obtain the desire cut-off of the grains by
the knife. Therefore, it is more favourable within the spirit of the
invention to form the exit openings by longitudinal channels at the
periphery of a flange at the drive-side front end of the screw, the
flange immediately neighbouring the last volutions of the screw and
contacting the inner surface of the housing, the knife being bearingly
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supported for rotation on the shaft and being connected with a drive
means for relative rotation with respect to the shaft. Thus, so formed
longitudinal channels leading to the exit openings rotate together with
the screw what requires the separate drive means for the knife, however,
this does not constitute a disadvantage in comparison with the initially
described known construction because also there a separate drive means
for the knife is necessary. The separate drive means for the knife
enables one to rotate the knife with the desired speed without any
problem so that a perfect cut-off of the granules is ensured. A preferred
further embodiment of the lastly described construction consists within
the spirit of the invention in that the knife is fastened to the front
end neighbouring the screw of a sleeve rotatably supported on the shaft,
which sleeve is driven preferably by means of a toothed belt, for
rotation of the knife.
Cleaning of the channels leading to the exit openings is
facilitated if within the spirit of the invention the section of the
housing covering the longitudinal channels constitutes a ring
separate from the remaining housing, however detachably connected with
it, preferably by means of screws. In such a manner, the longitudinal
channels leading to the exit openings can be made free without that for
this the screw must be disassembled.
According to a further embodiment of the invention, the cooling
means is formed by a blower which at the same time constitutes a conveyor
means for the granulate particles cut-off and comprises an air channel
leading to the knife and preferably being disposed radially with respect
to the shaft, and a discharge channel leading from the knife to a col-
lecting station for the granules and preferably also being disposed
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radially with respect to the shaft. Such an air cooling for the cut-off
particles in the most cases is sufficient for sensitive synthetic
plastics material kinds ,the extrusion temperature of which is not so
high. In operation, the air cooling is more inexpensive than the water
cooling and separate means for drying the granulate particles can be
saved. A separate granulating housing can be saved within the spirit of
the invention, if the air channel and the discharge channel on the one
side are confined by a wall supportingthe housing of the screw and on the
opposite side by a disk connected to the shaft or to the sleeve
positioned on it.
In the drawing, the object of the invention is schematically shown
by way of an exemplative embodiment. Fig.1 shows a granulating plant
provided with the inventive apparatus, in a side view. Fig.2 is a top
view to Fig.1. Fig.3 shows in an enlarged scale and in section the front
end of the screw provided with the screw drive means. Fig.4 is a front
view seen in direction of the arrow IV - IV of the front end of the
screw.
The plant according to Figs.1 and 2 comprises a receptacle 1 for
the synthetic plastics material to be processed which is filled into the
receptacle 1 from above. A knife 2 rotates around the vertical axis of
the receptacle in the bottom region of the receptacle 1 and comminutes
the synthetic plastics material within the receptacle 1. The comminuted
material is pressed through an opening in the wall of the receptacle 1
into the intake opening of a screw extruder 3 disposed radially with
respect to the receptacle 1, by which screw extruder the material is
plasticized. Heating zones and, respectively, or cooling zones 4 can be
related to the screw extruder 3. By means of the heating zones the
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desired processing temperature can be obtained. If this temperature is
exceeded, for example by the influence of friction heat, the desired
temperature can be ensured by means of the cooling zones. The front end 5
of the screw extruder 3 protruding from these means 4 leads to a granula-
ting head 6 in which a granulating knife driven by a granulating drive
means 7 cuts-off the grains. The grains cut-off are conveyed off the
granulating head 6 through tubes 8 to a sack filling station 9. The
conveyance is made by an air stream created by a conveyor blower 10. The
drive means 12 for the screw extruder 3 is also provided on the bottom
plate 11 of the plane, which drive means drives the tip-side end of the
screw.
In Fig.3 this end of the screw is shown more in detail. The screw
13 is bearingly supported in a cylindrical housing 14 which is connected
to the said exit opening of the receptacle 1. The drive means 12 for the
screw drives a shaft 15 which extends coaxially to the screw 13 and is
bearingly supported in the frame 16 of the apparatus. This shaft 15 is a
hollow shaft through which extends a rod 17 which on its end facing the
screw 13 carries screw threads constituting a screw 18 which can be
screwed into a corresponding threaded bore 19 of the screw 13. On the
other end, the rod 17 is supported in any desired manner on the hollow
shaft 15, for example by means of a flange, not shown, which engages the
end of the hollow shaft 15. Thereby not only a detachable connection
between the screw 13 and the shaft 15 is obtained, but also the
possibility to adjust the position of the screw 13 in axial direction in
a simple and easy operable manner by rotation of the rod 17, as long as
the shaft 15 is secured against displacement in its longitudinal
direction, for example in its drive means 12. Thereby it can be ensured
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that the front end of the screw 13 and the front end of the housing 14
are always exactly positioned in a common plane. By two wedges 20 or by a
toothed connection it is ensured that there is a connection for rotation
between the driving shaft 15 and the screw 13 and simultaneously, that
these members can be shifted in longitudinal direction relative to each
other. The screw 13 has a flange 22 immediately adjacent to its last
volutions 21, which on its periphery 23 is provided with some
longitudinal or spiral channels 24 spaced apart from each other in equal
distances and extending in longitudinal direction of the screw 13, which
channels immediately neighbour the plasticizing screw 13. These channels
24 are covered by a ring 25 forming a section of the screw housing 14 and
being detachably fastened to the body member of the screw housing by
means of screws 26, so that an easy exchange of the screw 13 or of the
ring 25 in case of wear is possible and for the purpose that the longitu-
dinal channels 24 can be easy made free for cleaning without the
necessity to disassemble the screw 13. The longitudinal channels 24 with
their ends disposed on the outer front end of the ring 25 constitute the
exit openings 27 through wich the plasticized synthetic plastics material
supplied by the screw 13 is pressed out of the longitudinal channels or
spiral channels 24. The front end of the housing 14 together with the
ring 25 is supported by a vertical wall 28 of the frame 16. One or more
granulating knives 29 provided with cutting edges 30 revolve in front of
the exit openings 27, which knives with their path disposed between the
drive means of the screw 13 and the exit openings 27 concentrically
surround the shaft 15 and are fixed to the front side 31 of a sleeve 32
which is bearingly supported on the shaft 15 by means of rolling bearings
33. The sleeve 32 carries Qn its outer side a toothed ring 34 driven
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from the granulating drive means 7 (Fig.1) by means of a toothed belt 35.
The speed of revolution of the sleeve 32 and therefore of the knife 29 is
substantially greater than that of the shaft 15 so that a sufficient
relative velocity between the revolving exit openings 27 and the also
revolving knife 29 is obtained. Suitably, the knife 29 revolves in a
direction which is opposite to the revolving direction of the screw 13.
The granulate particles cut-off by the cutting edges 30 of the
knives 29 are conveyed off by means of an airstream created by a conveyor
blower 10 (Fig.2) which simultaneously provides for a sufficient cooling
of the cut-off synthetic plastics material particles. For this, a curved
tube 36 (Fig.2) leads the supplied cold air from the conveyor blower 10
to a lateral inlet opening 37 (Fig.3) of the frame 16, and a cooling
means can be provided for the air. The opening 37 leads into an annular
space 38 which on the one side is confined by the wall 28 of the frame 16
and on the other side by a disk 39 fixed on a step 40 of the sleeve 32
and rotating therewith. An exit opening 41 is disposed on the side of the
frame 16 that is opposite to the side of the inlet opening 37, through
which exit opening the conveyor air together with the granulate grains
conveyed by it enters the tube 8 (Fig.2). Suitably, the frame 16 has a
circular! cross section in the region of the shaft 15 so that as less as
possible dead corners are formed for the conveyor air entering through
the opening 38 and escaping through the opening 41, so that it is ensured
that the cut-off synthetic plastics material particles are conveyed off.
Such a cooling means 42 (Fig.2) has proved to be good, inasmuch just at
processing of polyvinyl chloride no very high temperatures and
glueinesses occur.
The drive means 12 for the screw 13 can be provided with a control
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gear means 43 (Fig.2) for the purpose to adjust at choice the revolution
speed of the screw 13.
