Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02312917 2000-OS-30
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EXTRUSION DEVICE
The invention relates to an extrusion arrangement, in particular
an extrusion arrangement for thermoplastic or pasty compositions
or for high-viscosity melts, and to the use of such an
arrangement.
Known extrusion arrangements mostly have an essentially
cylindrical housing in which at least one screw is arranged. In
the vicinity of one end of the screw, the housing has an inlet
through which the extrusion arrangement can be charged with the
composition which is to be extruded and which usually is
initially in the form of a powder or granules. The screw is
driven by a motor shaft. The front of the housing has a die
orifice in the vicinity of the screw end remote from the drive.
It is frequently possible to control the temperature of sections
of the extrusion arrangement. The composition to be extruded is,
during conveyance by the screw, melted, mixed and subsequently
forced through the die orifice where it emerges in the form of a
continuous strand. The die orifice can be designed, for example,
as a perforated plate or perforated strip with a plurality of
fine orifices which produce correspondingly fine product strands.
Extrusion arrangements of these types can be used, for example,
to produce granules. A particularly important area of application
is moreover the production of pharmaceutical granules by melt
extrusion. The use of an extrusion arrangement in the
pharmaceutical sector is described, for example, in US 4,880,585.
In this case, a polymeric binder and the actual pharmaceutical
active ingredient are melted in the extrusion arrangement, where
appropriate together with other additives, mixed together and
forced through the perforated plate of the extrusion arrangement
in the form of numerous continuous product strands. Directly
downstream of the extrusion arrangement there is a granulation
unit, for example what is called a hot-cut arrangement which
divides the product strands continuously emerging from the
extruder with rotating knives into small cylindrical pieces
(granules) which then fall into a collection unit. At this time,
the granules are still thermoformable and may therefore change'
their shape while falling or in the collection unit. It is
possible by process control for example to round off the edges of
the cylindrical granules to result in what are called pellets.
Granules or pellets containing active ingredient are interesting
variants of conventional formulations. They can be used, for
example, directly as animal feed additives or as plant treatment
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compositions. Direct intake of active ingredients in granule form
is also conceivable in the pharmaceutical sector. However, in
pharmacy, granules and pellets are more often packed into
capsules or compressed to tablets conventionally. Of particular
interest in this connection is the possibility of providing
incompatible active ingredients in a single dosage form, for
example as capsule or tablet, by mixing the various granules or
pellets. Granules and pellets also have the advantage of making
dust-free further processing possible.
However, problems are associated with the conventional production
of granules by extrusion of the melt and subsequent hot-cut.
After the reduction in size by the knives of the cutting
arrangement, the granules are still hot and thermoformable. If
granule particles come into contact with one another or with a
wall of the apparatus at this time, bonding or adhesion together
may occur and eventually leads to the need to switch off the
extruder and the hot-cut and clean the apparatus. The production
process then has to be started off again.
The adhesion together and bonding of the particles can in the
final analysis be prevented only by reducing the temperature of
the extruded product strands. Reducing the temperature of the
extrudate can, for example, be achieved by lowering the extruder
temperature, in particular in the region of the perforated plate,
or, if the extruded temperature is unchanged, by raising the
output of the extruder. However, the flow properties of the
polymeric melt change as the temperature falls. In particular,
the viscosity of the melt increases so that, as a consequence,
some perforations in the perforated plate may be blocked. This
problem arises in particular in the production of pharmaceutical
granules, where the perforated plates used on the production
scale typically have 50-500 perforations, and the perforations
normally have diameters in the range from 0.5 to a few
millimeters. The blockage of individual perforations results in a
nonuniform melt throughput, with a high melt throughput in the
case of unblocked perforations leading to a heating of the melt
through shear, while radiation cooling of the melt occurs when
the perforations are virtually blocked and accordingly the melt
throughput is low. This results in different granule sizes, and
the large hot granules produced at perforations with a high
throughput are particularly prone to bonding, and the small
granule particles produced at perforations with a low throughput
lead to an increase in the proportion of fines produced. In this
case too it is eventually necessary to switch off the production
process and clean the apparatus.
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It is an object of the present invention to indicate a novel
extrusion arrangement with which it is possible in particular to
extrude thermoplastic or pasty compositions or high-viscosity
melts reliably even through fine die orifices. It is moreover
intended that the arrangement be suitable in particular for
producing granules or pellets, and the temperature of the product
strands should be lower than with conventional extrusion
arrangements so that bonding of the granules or pellets both with
one another and with the wall of the extrusion arrangement after
the hot-cut is reliably prevented.
We have found that this object is achieved by an extrusion
arrangement having a housing, having at least one screw arranged
in the housing and having a die orifice arranged at the front of
the housing in the vicinity of the end, remote from the drive, of
each screw, wherein at least one stripper device sweeping over
the inside of the die orifice is arranged between the end, remote
from the drive, of each screw and the die orifice.
With the novel extrusion arrangement, the composition to the
extruded, for example a polymeric binder, a pharmaceutical active
ingredient and further additives are first melted in a
conventional way and mixed together during conveyance by the
screw. Active ingredients, binders and additives which can be
used are disclosed, for example, in DE 195 39 363 A1. The melt is
extruded through a die orifice and the extrudate is finally
reduced in size to individual granule particles. In contrast to
conventional extrusion of strands, however, the extrudate is not
discharged by the pressure generated in the extrudate, but is
conveyed by a special stripper device through the die orifice of
the extrusion arrangement. The novel arrangement therefore
differs from known extrusion arrangements only by the additional
stripper device so that it is essentially no more costly to
produce and even extrusion arrangements which are already
installed can easily be retrofitted with the stripper device
proposed by the invention. Retrofitting of this type can be
implemented relatively simply, in particular because even in
conventional extrusion arrangements the screw does not extend
right up to the die orifice, for example the perforated plate,
and a space is almost always provided in between, and the
stripper device proposed by the invention can now be arranged
therein.
Numerous advantages are associated with the novel arrangement.
The temperature of the extrudate and/or of the die orifice of the
extruder can be reduced so that bonding or adhesion together of
the granules which have been reduced in size is reliably
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prevented. Unlike conventional extrusion arrangements, however,
lowering the temperature of the extrudate does not, despite the
increased viscosity of the extrudate, lead to blockage of the
perforations of the die orifice. On the contrary, such blockage
is reliably prevented by the continuous stripping of the inside
of the fine perforations of the die orifice. Since the stripper
device acts only on the extrudate located immediately on the
inside of the die orifice, there is virtually no additional
energy input into the melt so that there is also no unwanted
increase in temperature due to the conveying operation. It is
even possible with the novel arrangement to extrude pasty
compositions and high-viscosity melts without difficulty even on
use of perforated plates with very small diameter perforations.
The optimal process parameters for this are normally determined
using a laboratory-scale extruder with 1-5 perforations. Scale-up
to an industrial production scale with some hundreds of
perforations entails no additional problems.
The extrusion at comparatively low temperature markedly extends
the range of applications of melt extrusion. For example, it is
now possible also to extrude melts which contain active
ingredients and which comprise heat-sensitive consituents which
would be damaged by the temperatures used in conventional
extruders.
In order to take account of the unavoidable play in the
individual components of the extrusion arrangement and possible
changes in temperature and pressure during the process, but
especially when the extrusion process starts off, the stripper
device is preferably pushed elastically against the inside of the
die orifice. This can be achieved, for example, by a compression
spring acting on the stripper device.
The stripper device is advantageously connected to rotate with
the assigned screw so that the inside of the die orifice is
continuously stripped. The stripper device may, for example, have
a shaft which is fastened to the tip of the screw. The screw
preferably has for this purpose a recess in which this shaft
engages in order to rotate therewith. The abovementioned
compression spring can be arranged between the base of the recess
and the shaft of the stripper device. The novel arrangement can
therefore be implemented by a simple modification of the tip of
the screw of a conventional extrusion arrangement.
The novel arrangement operates without dead space so that there
is no risk of degradation of the polymeric active ingredient
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melt.
The stripper device is advantageously designed as a traversing
knife. The knife can in this case consist, for example, of a
5 hardened material, preferably a hardened metal.
The die orifice of the extruder is advantageously designed as a
perforated plate, the perforated plate typically having between
50 and 500 perforations whose diameter is in the range 0.5 -
10 mm. It is advantageous for the temperature of the perforated
plate itself to be controllable.
If the extruder is, for example, a twin screw extruder, each
screw is preferably assigned a separate stripper device.
The novel arrangement is particularly suitable for producing
granules, in which case a granulating unit, for example a cut-off
unit with rotating knives, is located downstream of the extrusion
arrangement. The novel arrangement is particularly preferably
used to produce granules containing active ingredients, for
example pharmaceutical granules, plant treatment compositions,
animal feed additives and supplements or human food supplements.
The present invention is described in more detail below by means
of an example described with reference to the appended drawing.
In the drawing,
Figure 1 shows part of an axial cross-section of a first
embodiment of the novel extrusion arrangement;
Figure 2 shows a front view of a second embodiment, designed as
twin-screw extruder, of the novel extrusion arrangement
in detail.
Figure 1 depicts part of a cross-section of a preferred
embodiment of the novel extrusion arrangement. Like a
conventional extrusion arrangement, the novel extruder 10
comprises a cylindrical screw housing 11 in whose interior a
screw 12 is rotatably arranged. The screw shaft 13 of the screw
12 terminates in a shaft tip 14 before a perforated plate 18. The
temperature-controllable perforated plate 18 is arranged in a
perforated plate holder 19 and forms the front termination of the
extruder 10. The perforated plate 18 has orifices 20 through
which the screw 12 conveys the molten extrudate. Not depicted are
the charging units which are known from conventional extruders
and which can be used to feed, for example, a powdered polymeric
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binder and a powdered active ingredient to the extruder, which
are then melted by heating elements (likewise not depicted) to
give the extrudable melt. The screw 12 in this case
simultaneously acts as mixing device for homogeneous mixing of
the individual components of the melt.
To produce pharmaceutical granules, the orifices 20 in the
perforated plate 18 have a diameter of only a few mm. The
perforated plate 18 may have a few 100 orifices 20.
A stripper device designed as a traversing knife 21 is fixed in
the shaft tip 14 of the screw 12 in order to rotate therewith. In
the depicted example, the traversing knife 21 has two essentially
diametrically opposite knife blades 22, 23. The knife shaft 24 of
the traversing knife 21 is held by a fastening screw 26 in a hole
15 bored in the shaft tip 14. The hole has an internal thread 16
which cooperates with a complementary external thread on the
fastening screw 26. A compression spring is arranged on the base
of the central recess 27 of the fastening screw 26 and acts
against the shaft 24 of the traversing knife 21 and presses the
latter against the inside of the perforated plate 18.
Figure 2 shows a partial depiction of a front view of a second
embodiment of the novel extruder 10. The variant depicted in
Figure 2 is a twin-screw extruder with each of the two screws 12
being provided with a traversing knife 21. The perforated plate
18 with orifices 20 arranged circularly therein is depicted in
partial detail.
To carry out the novel process, the extruder 10 is charged in a
conventional way with polymeric binder and active ingredient, and
these components are mixed and melted and conveyed by the screw
12 into the intermediate space 28 bounded by the shaft tip 14 and
perforated palte 18. The rotating traversing knife 21 in this
intermediate space 28 forces the extrudate through the orifices
20 in the perforated plate 18 and thus prevents these orifices
becoming blocked even if the temperature of the molten extrudate
is lowered in order to prevent later bonding of the granule
particles.
The extrudate leaves the orifices 20 in the form of a continuous
product strand which is reduced in size by a (not depicted)
hot-cut device known per se into small cylindrical granule
particles. Depending on the process control, the granule
particles can solidify in this cylindrical shape or, as long as
they are still thermoformable, be rounded off to pellets.
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The advantages of the novel extrusion arrangement are also clear
from the comparative investigation described below:
Examples:
The following formula was processed using a Werner & Pfleiderer
ZSK 40 twin-screw kneader:
Component ~ by weight
Theophylline 50
Eudragit RS 10
Kollidon VA-64 40
A Werner & Pfleiderer MWG 260/90 pelletizer was used to cut the
extrudates.
Comparative Example 1:
In order to obtain a homogeneous discharge from the perforated
strip (15x1 mm}, the extruder of conventional design was run with
the following temperature profile:
Zone 1 Zone Zone 3 Zone Zone 5 Head Die
2 4
105 125 155 155 160 180 180
The product came uniformly out of the perforated strip but bonded
to the knives and to the housing so strongly that the process had
to be stopped after a short running time.
Comparative Example 2:
In order to couteract the bonding, the processing temperature was
reduced:
Zone Zone 2 Zone 3 Zone Zone 5 Head Die
1 4
90 110 140 140 150 170 170
The tendency to bond decreased markedly, but the throughput
through the perforations varied greatly so that the resulting
granule particles varied in size. After a short time, some
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perforations were completely blocked.
Example 3:
The extruder was fitted with the novel stripper device. Retaining
the temperature profile described in Comparative Example 2,
nonadhesive, uniform granule particles were obtained. The
throughput through the individual perforations was very uniform,
and no perforations became blocked even after a lengthy time.
15
25
35
45
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