Note: Descriptions are shown in the official language in which they were submitted.
CA 02399743 2002-08-26
i
Extruding device.
The present invention relates to extruders for plastic material.
More particularly, but not exclusively, the invention is intended for use in
processing a mixture of a thermoplastic and a thermosetting material. In a
particularly preferred application, the thermosetting material is presented by
crumbled pieces made from used tires.
In one application to which the present invention relates, the crumbled pieces
of
rubber from used tires are to be mixed with a thermoplastic material to form
and
alloy-like mixture capable of being injected into a mold or the like.
One problem with attempting to mix a thermoplastic material with thermosetting
particles such as crumpled used tire particles is that the mixing can only be
effected at elevated temperatures. In order to achieve the desired temperature
regime, the mixture from which an alloy is to be made is to be subjected to a
shear force generating environment that is efficient enough to simultaneously
create sufficient additional heat and to homogenize the components.
A disc extruder for extruding plastic has been described long ago, but to the
best knowledge of the present applicant, never put into practice. It basically
comprised a rotary disc or rotor which co-operates with a stator having a
heated
counter-face forming a narrow gap with the rotor. The working mixture was
proposed to be introduced between the disc and the stator and subjected, by
the
stator, to a shear force.
Reference may be had, for instance to US Patent 3,689,181 (Maxwell), to USP
3,712,783 (Maxwell), to US Patent 3,790, 328 (Maxwell) and US Patent
6,352,425 (Lupke et al.).
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The basic principle of a disc extruder utilizes the principle that when a
visco-
elastic material is subjected to a shearing force between two disc surfaces
rotatable relative to each other, a centripetal force results which forces the
material towards the centre of the discs and can be discharged through a
discharge opening provided in one of the discs.
Viewed from the standpoint of the present invention, the extruders referred to
above have several disadvantages and are therefore not suitable for operating
with a mixture of thermoplastic and thermosetting particles. In particular,
the
devices described do not provide exit pressures which are often required for
injection molding of the thermoplastic/thermosetting mixture. This is mainly
due
to the fact that the flow of molten material has to change the direction of
its
flow from a generally radially inward direction to an axially outward
discharge.
In the Maxwell ' 181 patent there are two radial discs rotatable relative to
each
other, of which one is coincident with a wall of a cup-shaped chamber and
serves as a stator, while the opposed disc is rotatably driven relative to the
stator. Even though the reference purports to increase the centripetal effect,
the
discharge pressure at the outlet provided at the centre of the chamber is
insufficient, particularly when working with a mixture of a thermoplastic and
thermosetting material.
The Maxwell '783 patent provides a conical rotor and a conical stator co-
operating much in the fashion of the previously described reference. The
conical
rotor requires more axial space than the flat disc-shaped arrangement of the
previous reference. Also, the Maxwell '783 patent, like the previous
reference,
does not provide means for a sufficient exit pressure for injection molding of
the
processed material.
Maxwell '328 suffers from the same drawbacks as the previously described
embodiments. Besides, it unduly increases the axial width of the disc portion
by
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including at its periphery and near the feeding opening, a spiral member which
is
adapted to force the incoming mixture axially, towards the mutually rotated
disc
surfaces.
The Lupke et al. reference, which issued relatively recently, provides a
further
indication that the conical disc-shaped processing surfaces movable relative
to
each other. The device is further increased in axial length by an integrally
formed
hollow tubular portion which provides an outlet for the material which has
passed between adjacent conical processing surfaces.
It is also known, e.g. from US Patent 3,756,573 (Sponseller) to locate a
conical
rotor and a stator between two conveyor screw - type feeding mechanism and
another screw disposed downstream of the rotor/stator assembly. The device is
very complex in structure and requires a relatively large space since the
entire
downstream screw is disposed axially away from
It is an object of the present invention to provide a device which is capable
of
securing the heating/mixing regime desired for the above purpose while at the
same time permitting a generally simultaneous high pressure extrusion of the
resulting mixture out of the device. It is another object of the present
invention
to provide the device which is simple in structure and thus less expensive to
produce, while requiring a relatively small space in a production plant.
According to the invention, an injection device is provided which comprises a
radial disc-shaped stator and a rotatably driven radial disc-shaped rotor
facing
the stator to define an annular mixing gap between the two. A hopper is
provided above the mixing gap for feeding into the gap a mixture which is
comprised of a selected thermoplastic material, for instance, in the form of
pellets, and crumbled pieces of rubber produced from used tires or some other
thermosetting material. At least the stator surface is heated. The rotary
motion
of the rotor relative to the stator subjects the mixture to an intensive
shearing
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force which increases the temperature of the processed mixture substantially
above the temperature provided by the heated stator and/or rotor. Thus, the
increased temperature is sufficient to provide a thorough interaction and
homogenization of the two components to produce a thermoplast-thermoset
alloy which is at the liquid state at the discharge part of the disc, the
discharge
part being the inner part of the disc/stator due to the centripetal forces
developed by the shearing action between the stator and rotor. In accordance
with the invention, an axial force developing mechanism is disposed at the
center of the stator/rotor section for an immediate further injecting the
processed material into a form or the like.
The axial force developing mechanism is, in one embodiment, an axially
reciprocable ram adapted to force the alloy through a discharge port into a
dye.
In another embodiment, the axial force developing mechanism is a single or
multiple feed screw arrangement which may be operatively associated with a
perforated end disc. The processed material is then forced to flow through the
openings in the disc to form the desired pellets or other form of solid
particles of
the final alloy.
In the drawings:
Figure 1 is a diagrammatic sectional view showing one embodiment of the
inventive apparatus; and
Figure 2 is a diagrammatic sectional view showing another embodiment of
the inventive apparatus.
It will be appreciated that the embodiments shown present only exemplary
versions which can be modified to a greater or lesser degree as may be
required.
Turning now to Fig. 1, the apparatus shown comprises a cup shaped chamber 1
provided with particulate matter feeding means, in the embodiment shown, a
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hopper 2 at the upper part of the chamber, which communicates, via a feed
inlet
3 in the cylindric wall of the chamber 1, with the interior of the chamber 1
as
will be described. The chamber 1 is provided, at its right-hand end with a
heating element 4. The opposite end of the chamber 1 has a flange 5 which
provides connection means for a cover 6 which is integrally formed with a
rearwards extending tubular portion 7. The above described elements are
fixedly
secured and stationary relative to a frame (not shown) of the device.
Threaded into the tubular portion 7 is an outwardly threaded sleeve 8. Mounted
rotatably with respect to the sleeve 8 on bearings 9, 10 is a tubular shaft 1
1
provided, in the embodiment shown, with a drive sprocket 12. The sprocket 12
is to be understood to present only one of many other equivalent drive means
for rotatably driving the disc 13. The right-hand end of the shaft 1 1 is
fixedly
secured to a radial disc 13 comprising a short cylindric section 14 and a
generally radial, annular face 15 which is parallel with the inside annular
face 16
coincident with the bottom of the cup-shaped chamber 1. It is preferred that
both the annular face 15 and the annular face 16 be radial, but small
deviations
to say, a shallowly conical shape having an apex angle of about 175° is
also
acceptable.
The radial disc and the bottom wall of the chamber thus form a first and a
second mixing disc. The mixing discs present an embodiment of the
arrangement wherein the discs are rotatable relative to each other since the
chamber 1 is stationary and the disc 13 is rotatable about the axis 17. The
faces
15 and 16 of the mixing discs define therebetween a mixing gap 18 which, like
the faces 15 and 16, is annular and thus has a radially outer portion near the
feed inlet 3, and a radially inner portion 19 near an outlet tube 20 having,
at its
discharge end a flange 21 compatible with a flange 22 of a back flow valve 23
and connected to it by bolts (not shown). The valve 23, in turn, is connected
in
similar fashion to the downstream mold die 24.
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As already mentioned, the tubular portion 7 is threaded at its inner surface
to
threadably receive the sleeve 8 having an enlarged diameter end portion 25.
The
right hand end of the tubular shaft 1 1 is fixedly secured to the rotor 13. At
the
right hand end portion of the shaft 1 1, an inner lining 27 guides a ram 28.
The
inside of the outlet tube 20 is likewise provided with a lining 29. The rod 30
of
the ram 28 is connected by a coupling 31 with the piston rod 32 of a hydraulic
cylinder 33. The cylinder 33 (which is a preferred embodiment of what
generally
can be referred to as "second driving means" additional to the "first driving
means" presented by the sprocket 12) is fixedly mounted on a base plate 34
supported by a plurality of supports 35 at a predetermined distance from the
outer face of the cover 6.
The outer diameter of the mixing discs 13, 16 may vary within a large range,
depending on the desired throughput. Typically, the width of the mixing gap18
is determined by a formula
B = Ki R (1)
wherein Ki is a constant for a given relative rotation velocity and
R is the radius of the mixing gap.
It follows from the above that the optimum width of the gap 18 having an outer
diameter of about 30" would be about 2.1 ". The typical speed of a device of
this size would be slightly over 100 rpm which would result in a throughput of
about 1000 Ibs per hour.
In operation, the apparatus is started by driving the rotor 13 by the first
drive
means (the sprocket 12) at a given speed. At the same time, the heating
element 4 is actuated. The mixture of a thermoplastic and thermosetting
material is fed through the hopper 2 into the gap 18 between the rotor 13 and
stator 16 where it is subjected to high heat and a shear force. The shear
force
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still further increases the heat of the mixture. At the same time, the working
mixture is intermixed and homogenized into an alloy, while advancing, due to
the
centripetal motion developed by the relative movement between the rotor 13
and stator 16, toward the center of the rotor/stator assembly. Reciprocating
motion of the ram 28 then further forces the alloy past the back flow valve 23
and into a mold die 9.
The second embodiment, shown in Fig.2 presents a virtually identical portion
of
the mutually rotatable discs in that the two discs are again formed by a rotor
and a stator. Reference number 1 1 1 shows the face of a stator 1 12. The
rotor
is designated with number 1 13.
The stator 1 12 is heated by a heating element 1 14. The inner portion of the
gap
131 between the rotor and stator terminates at an extrusion port 1 15 which
coincides with an inlet port of a screw barrel 1 16. The barrel 1 16 is
provided
with a cylindric heating element 1 14a. The outer portion of the gap 131 is in
the
region of bevel 132 provided in the face of the rotor 1 13. An annular nut 1
17
at the discharge end of the barrel 1 16 secures a perforated plate 120 to the
barrel 1 16 and thus to the the port 1 15. The plate 120 has a central opening
which supports the discharge end of a screw 1 18. The opposed end of the
screw 1 18 is threaded into the rotor 1 13 at 133 near the port 1 15 The
discharge end of the screw 1 18 carries, inside of the plate 120, a cutter 1
19
which frictionally engages the inner surface of the perforated plate 120. The
entire feed screw and barrel assembly is provided with insulation 121. Number
122 designates a base to which the particular device is secured.
The stationary barrel housing of the rotor 1 13 is designated with number 123.
The rotor 1 13 is fixedly secured to a drive shaft 124 suitably mounted in
bearings 125. The securement is effected via a flange 130 and bolts which are
not specifically shown in the drawing. The shaft 124 is provided, at its end
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remote from the disc 1 13, with a sprocket wheel 126 which forms a part of a
drive mechanism not shown in detail.
It will be appreciated that, in this embodiment, the sprocket 126 drives both
the
rotor 1 13 and, through the threaded connection 133, the feed screw 1 18.
Therefore the single sprocket provides both means for driving the rotor and
means for driving the axial feeding means, i.e. the feed screw.
The hopper is designated with number 127. In this embodiment, both the stator
1 12 and the rotor 1 13 are heated, the heating system of the rotor being
shown
with number 128 and being provided with a suitable thermal insulation layer
129.
The operation of the rotor/stator portion of the second embodiment is
virtually
identical with that of the first embodiment. The difference is in that the
axial
feed of the homogenized mixture is effected by a feed screw and that, instead
of forcing the alloy into a mold, the mixture is forced through openings in
the
perforated disc 120 which co-operates with the cutter 1 19 to produce pellets
of
the homogenized mixture.
It will be appreciated that more or less substantial modifications of the
embodiments described can be made without departing from the scope of the
present invention. As an example, more than one feed screws can be used or
the ram mechanism may have a different configuration.
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