Title of Invention

EXTRUDER

Abstract An extruder comprises a displacement part with a housing, inside of which at least one screw shaft is housed and which is provided, at one end, with a material entry opening for the material to be processed and, at the other end, with a material outlet opening for the processed material, and with an intermediate opening (5) between the material entry opening and the material outlet opening for discharging gas from the displacement part. A material retaining device is connected to said intermediate opening and has at least one screw shaft (7, 8) inside an intermediate housing (6). This screw shaft extends from the intermediate opening (5) to a gas discharge opening (13) in the intermediate housing (6) and conveys material entering the intermediate housing (6) back into the displacement part. To this end, the screw section (9, 10) of the screw shaft (7, 8) facing the displacement part has a direction of thread that returns into the displacement part, and the screw section (11, 12) of the screw shaft (7, 8) facing away from the displacement part has an opposite direction of thread.
Full Text Extruder
[0001] This invention relates to an extruder according to the preamble of claim 1.
[0002] Extruders are often used for conveying and processing substances. To
separate gases during processing of plastics in an extruder, a gas outlet opening is
provided in the housing of the process section of the extruder in which the solid or
viscous plastic is processed. To prevent plastic from being discharged from the process
section through said opening, the material retaining device is provided, also known as
the "stuffer". It has one or two screw shafts whose screw has a direction of thread over
the total length by which it forces plastic entering said opening back into the process
section, while gas passing through the screw can be drawn off.
[0003] As has turned out, however, plastic extruded with an extruder having such a
retaining device shows a great number of black specks, which are extremely disturbing
in particular in products made of transparent plastic, such as PET.
[0004] The problem of the invention is to provide an extruder having such a
retaining device at the gas outlet opening which largely eliminates the formation of
black specks.
[0005] This is obtained according to the invention by the extruder characterized in
claim 1. Advantageous embodiments of the invention are rendered in the subclaims.
[0006] As has turned out, the formation of black specks is due to the fact that the
screw shaft and the inside wall of the retaining device have the same high temperature
as the process section. In the known extruder, plastic residues adhering to the hot
screw shaft or inside wall of the retaining device can thus decompose. Since the
decomposition depends on the duration at which a plastic residue adheres to the hot
metal surface of the screw or the inside wall of the retaining device, a plastic residue
transported by the screw over its total length as far as the gas outlet opening in the
intermediate housing of the material retaining device will naturally undergo
particularly great decomposition. The solid products of decomposition are then, in the
known extruder, conveyed by the material retaining device screw backfeeding over its

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fuil length back to the process section and mixed with the plastic in the process section
to form the black specks in the extruded plastic product.
[0007] Starting out from this finding, the invention is based on giving only the
portion of the material retaining device screw adjacent the process section a
backfeeding design. The plastic is in contact with said portion only a short time.
Moreover, in said portion the material is continually replaced on the hot screw surface
and inside wall as in the process section. This prevents decomposition of the plastic
and thus a formation of black specks in said portion.
[0008] In contrast, plastic residues passing through said portion of the material
retaining device reach the screw portion with the opposite pitch according to the
invention, i.e. they are no longer conveyed back to the process section but away
therefrom and are thus separated from the process section. In this area they are thus
exposed to the hot temperature of the screw and the inside wall of the intermediate
housing for some time and are decomposed. The solid black decomposition specks can
be drawn off together with the separated gas through the gas outlet opening in the
intermediate housing, or an additional opening on the underside of the intermediate
housing can be provided for the residues to drop through.
[0009] The process section of the inventive extruder can have only one screw shaft.
However, there can also be provided two or more, in particular eight or more, axially
parallel screw elements which can closely intermesh, the screw shafts being received
and guided by axially parallel concave circular segments on the inside wall of the
extruder housing. A plurality of said screw shafts can be disposed in a plane or form a
ring, as described in EP 0 788 867 B1.
[0010] The material retaining device can likewise have only one screw shaft.
However, it is preferable to provide two or more axially parallel screw shafts whose
screws closely intermesh and which are in each case received and guided by axially
parallel concave circular segments in the inside wall of the extruder housing.
[0011] It is possible to use corotating or counterrotating screws in the material
retaining device. Since counterrotating screw shafts form an axially closed system

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unlike the axially open system of corotating screw shafts, i.e. counterrotating screw
shafts cause forced conveyance, counterrotating screw shafts are generally preferred in
the material retaining device since they prevent the occurrence of black specks even
more rigorously.
[0012] Moreover, the self-cleaning of counterrotating screws is fundamentally
better than of corotaling screws, since with counterrotating screws their crests touch
each other and lie against the inside wall of the intermediate housing, while corotating
screws have a clearance between the crests of the screws and the inside wall. Wear can
be combated by hardening the screws and the intermediate housing.
[0013] With counterrotating screw shafts it is furthermore possible to completely
omit drive gears on the screw shafts if need be, since only one shaft needs to be driven
which drives the other shaft by form-fitting engagement of the screws. This
substantially simplifies the installation, assembly and maintenance of the screw shafts.
[0014] The screw shafts of the material retaining device are preferably formed in
one piece with their drive, for example a drive gear. There is thus no coupling between
screw shaft and drive gear as provided according to the prior art. The split gear box
driving the screw shafts can thus be built small, light and economically.
[0015] So that no toxic gases can escape from the extruder unchecked, but also to
prevent air from passing into the extruder, the intermediate housing with the material
retaining device must be connected vacuumtight to the extruder housing and to the
split gear box, whereby the split gear box must in turn be of vacuumtight form.
|0016] Since the split gear box contains bearings, seals and similar parts that can be
damaged at the high temperature of the process section of for example approx. 250°C,
it is advantageous to dispose heat insulating means, for example an insulating plate,
between the intermediate housing with the material retaining device and the split gear
box.
[0017] The inventive extruder is particularly suitable for processing plastics, such as
PVC, PE and others. Since the formation of black specks is largely prevented, it is

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particularly suitable for producing transparent plastic products, in particular water-
clear products, for example made of PET.
[0018] Embodiments of the inventive extruder will hereinafter be explained more
closely with reference to the enclosed drawing. The figures are described as follows,
each showing a perspective view:
Figure 1 a segment of the process section of the extruder with the material retaining
device connected thereto;
Figure 2 a material retaining device with two counterrotating screw shafts, the
housing of the material retaining device being whole and a part of the
housing of the split gear box being omitted;
Figure 3 a view corresponding to Figure 2, but of a material retaining device having
two corotating screw shafts;
Figure 4 a view corresponding to Figure 2, but of screw shafts without drive gears;
and
Figure 5 the material retaining device removed from the process section.
[0019] According to Figure 1, only a housing segment 1 of the process section of
the extruder is shown. The process section consists of a plurality of such juxtaposed
housing segments which can be clamped together by pull rods (not shown) guided
through the bores 2 in the housing segment 1.
[0020] The conveying direction of the process section is shown by the arrow 3. On
the upstream conveying side of the segment 1 the housing segment (not shown) with
the material inlet opening of the extruder is disposed, and on the downstream
conveying side the housing segment (not shown) with the material outlet opening
thereof.
[0021] The process section is formed as a ring extruder, as described e.g. in EP 0
788 867 B1. That is, extending axially through the housing segment 1 and the other
housing segments (not shown) of the process section is an inside core (not shown).

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Between the inside core and the housing segments there is an annular space with
axially parallel rotatable shafts extending therein, disposed along a circle at equal
central-angle distance, each of which carries a number of tandem-arranged screw and
other processing elements with which adjacent shafts closely intermesh, the inside core
on the interior side of said space and, as shown in Figure 1, the inside wall of the
housing segments 1 being provided with channel-shaped axially parallel depressions 4
in which the particular shaft with its processing elements is received and guided.
[0022] The housing segment 1 shown in Figure 1, disposed between the material
inlet opening and the material outlet opening of the process section, is provided with
an intermediate opening 5 for removing gases from the plastic being processed in the
process section.
[0023] For this purpose, the segment 1 has connected thereto an intermediate
housing 6 having two axially parallel screw shafts 7, 8 disposed therein, which are
shown in Figure 2 and extend perpendicular to the longitudinal axis of the process
section. Each screw shaft 7, 8 has a short screw element 9, 10 facing the process
section, and a long screw element 11,12 facing away from the process section. The
screw elements 9, 11 and 10, 12 of the two shafts 7, 8 intermesh closely, the
intermediate housing 6 being provided on the inside wall with axially parallel channel-
shaped depressions (not shown) which receive the screw elements 9 to 12 and thus
guide the screw shafts 7, 8. The screw elements 9 to 12 are disposed rotationally fast
on the screw shafts, e.g. by a splining.
[0024] The screw shafts 7, 8 extend with their screw elements 9 to 12 from the
intermediate opening 5 in the housing segment 1 of the process section as far as the
gas outlet opening 13 in the intermediate housing 6 through which gases separated
from the plastic in the process section are drawn off.
[0025] The screw elements 9, 10 have a direction of thread so as to convey plastic
material penetrating from the process section into the intermediate housing 6 back to
the process section in the direction of the arrow 14. This device is accordingly called
the material retaining device. In contrast, the screws 11,12 have the opposite direction
of thread, thereby causing plastic material reaching the screw elements 11, 12 via the

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screw elements 9, 10 to be conveyed away from the process section according to the
arrow 15. Due to the high temperature of the surface of the screw elements 9 to 12 and
the inside wall of the intermediate housing 6, plastic material adhering to these
surfaces for some time is decomposed. While plastic is continually replaced in the area
of the backfeeding screw elements 9,10 and thus decomposition is prevented, plastic
residues reaching the feeding elements 11, 12 are decomposed. The decomposed
plastic residues can either be drawn off through the gas outlet opening 13, or a further
opening (not shown) can be provided on the underside of the intermediate housing 6 in
the area facing away from the process section, through which the plastic residues can
be removed.
[0026] In the embodiment according to Figure 2, the screw element 9 on the screw
shaft 7 is of right-hand form and the screw element 11 of left-hand form, while on the
screw shaft 8 the screw element 10 is of left-hand form and the screw element 12 of
right-hand form.
[0027] While Figure 2 shows counterrotating screw shafts 7, 8, the screw shafts 7, 8
in Figure 3 are driven in corotating fashion.
[0028] Both the screw shafts 7, 8 according to Figure 2 and the screw shafts 7, 8
according to Figure 3 can be formed in one piece with their drive gears 18,19; 20, 21.
[0029] As shown in Figure 4, if there are two counterrotating screw shafts 7, 8 a
gear drive can be completely omitted, since the driven screw shaft 8 drives the shaft 7
by the screw elements 10, 12 of the former engaging the screw elements 9,11 of the
latter. This means that in the embodiment according to Figure 4 it is only necessary to
mount the shafts 7, 8 in the split gear box 22.
[0030] To seal the process section vacuumtight, the intermediate housing 6 is
flange-mounted vacuumtight laterally on the housing segment 1 according to Figures 1
and 5. Further, the intermediate housing 6 is connected vacuumtight to the split gear
box 22. To reduce heat transfer from the hot intermediate housing 6 to the split gear
box 22, a plate 23 made of thermal insulating material is disposed between opposing
faces of the intermediate housing 6 and the split gear box 22.

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[0031] The screw shafts 7, 8 of the material retaining device are driven by the
electromotor 24 via a reduction gear 25. The housing 26 connecting the reduction gear
25 to the split gear box is, as apparent from Figures 2 to 4, penetrated only by a shaft
27,28 which can be sealed vacuumtight with a magnetic clutch. While the shaft 27 is
disposed coaxially to the shaft 8 in the embodiment according to Figures 2 and 4 with
counterrotating shafts 7, 8, the shaft 28 in the embodiment according to Figure 3 with
corotating shafts 7, 8 is provided with a gear wheel 29 which meshes with the gear
wheels 20,21 on the shafts 7, 8.

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Ciaims
1. An extruder having a process section with a housing in which at least one screw
shaft is disposed and which has at one end a material inlet opening for material to
be processed, and at the other end a material outlet opening for processed
material, and between the material inlet opening and the material outlet opening,
for the escape of gas from the process section, an intermediate opening (5)
having connected thereto a material retaining device which has, in an
intermediate housing (6), at least one screw shaft (7, 8) extending from the
intermediate opening (5) to a gas outlet opening (13) in the intermediate housing
(6) and conveying material penetrating into the intermediate housing (6) back
into the process section, characterized in that the screw portion (9, 10) of the
screw shaft (7, 8) facing the process section has a direction of thread backfeeding
into the process section, and the screw portion (11, 12) of the screw shaft (7, 8)
facing away from the process section has an opposite direction of thread.
2. The extruder according to claim 1, characterized in that the screw portion (9, 10)
with the backfeeding direction of thread is formed shorter than the screw portion
(11, 12) with the opposite direction of thread.
3. The extruder according to claim 1 or 2, characterized in that the intermediate
housing (6) has an outlet opening for solid products of decomposition at the end
facing away from the process section.
4. The extruder according to claim 1, characterized in that the material retaining
device has at least two intermeshing screw shafts (7, 8).
5. The extruder according to claim 4, characterized in that the intermeshing screw
shafts (7, 8) are counterrotatable.
6. The extruder according to any of the above claims, characterized in that the drive
of at least one screw shaft (7, 8) is formed in one piece with the screw shaft (7,
8).

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7. The extruder according to claim 6, characterized in that the drive is formed by a
drive gear (18 to 21).
8. The extruder according to claim 6 or 7, characterized in that the drive is disposed
in a split gear box (22), and heat insulating means (23) are provided between the
split gear box (22) and the intermediate housing (6) of the material retaining
device.
9. The extruder according to any of the above claims, characterized in that an
annular space is formed between the housing of the process section and an inside
core extending axially therein, said space having axially parallel rotatable shafts
extending therein disposed along a circle at equal central-angle distance, each of
which carries a number of tandem-arranged screw and other processing elements
with which adjacent shafts closely intermesh, the inside core on the interior side
of said space and the inside wall of the housing being provided with channel-
shaped axially parallel depressions (4) in which the particular shaft with its
processing elements is received and guided.
10. Use of the extruder according to any of the above claims for extruding
transparent plastic products.

An extruder comprises a displacement part with a housing, inside of which at least one screw shaft is housed and
which is provided, at one end, with a material entry opening for the material to be processed and, at the other end, with a material
outlet opening for the processed material, and with an intermediate opening (5) between the material entry opening and the material
outlet opening for discharging gas from the displacement part. A material retaining device is connected to said intermediate opening
and has at least one screw shaft (7, 8) inside an intermediate housing (6). This screw shaft extends from the intermediate opening
(5) to a gas discharge opening (13) in the intermediate housing (6) and conveys material entering the intermediate housing (6) back
into the displacement part. To this end, the screw section (9, 10) of the screw shaft (7, 8) facing the displacement part has a direction
of thread that returns into the displacement part, and the screw section (11, 12) of the screw shaft (7, 8) facing away from the
displacement part has an opposite direction of thread.

Documents:

http://ipindiaonline.gov.in/patentsearch/GrantedSearch/viewdoc.aspx?id=hUTG+TZgArI+6/bAV1AWRA==&loc=wDBSZCsAt7zoiVrqcFJsRw==


Patent Number 268358
Indian Patent Application Number 1415/KOLNP/2007
PG Journal Number 35/2015
Publication Date 28-Aug-2015
Grant Date 27-Aug-2015
Date of Filing 20-Apr-2007
Name of Patentee BLACH VERWALTUNGS GMBH & CO, KG
Applicant Address HOHER STEG 10, D-74348, LAUFFEN
Inventors:
# Inventor's Name Inventor's Address
1 BLACH, JOSEF, A. HOHER STEG 10, D-74348, LAUFFEN
PCT International Classification Number B29C 47/76
PCT International Application Number PCT/EP2005/010233
PCT International Filing date 2005-09-22
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 10 2004 051 203.5 2004-10-20 Germany