EXTRUDER FOR THE VISCOSITY-INCREASING PROCESSING OF MELTABLE POLYMERS

Abstract

An extruder for the viscosity-increasing preparation of meltable polymers, wherein an extruder screw with at least one helical extruder screw flight is positioned in a housing having an inner housing recess. A diameter central region has a larger outer diameter than at least one of the other diameter regions, and a conical transition is formed in each case between regions of different diameters. Two degassing zones are provided in the diameter central region, each of which has at least one associated suction opening in the housing, one degassing zone being designed in the region of the satellite screws and an additional degassing zone being designed upstream thereof in the flow direction. The thread depth of the screw threads, formed between the extruder screw flights is greater in both degassing zones than in at least one sealing and compression section formed therebetween.

Claims

1. An extruder for viscosity-increasing processing of meltable polymers, the extruder comprising: an extruder screw with at least one helical extruder screw flight, the extruder screw being subdivided with respect to its outer diameter into a diameter start region, diameter center region and diameter end region, the diameter center region having a larger outer diameter than at least one of the other diameter regions; a housing with an inner housing bore, in which the extruder screw is rotatably arranged; a transition cone formed between regions of different diameter; at least one degassing zone formed in the diameter center region, which has a housing bore from which at least one suction opening extends to an outer side of the housing; a plurality of rotationally driven satellite screws surrounding the extruder screw are provided in the region of the degassing zone, the satellite screws each rotate individually about a satellite axis and together with the extruder screw; at least two degassing zones provided in the central diameter region, each having at least one associated suction opening in the housing, one degassing zone being formed in the region of the satellite screws and a further degassing zone being formed upstream of it, wherein a depth of the screw channels formed between the extruder screw flights is greater in both degassing zones than in at least one sealing and compression section formed between the degassing zones.

2. The extruder according to claim 1, wherein a feed zone is formed for drawing in solid plastic particles and for melting and homogenization, and wherein the flow channel formed between the extruder screw and an inner wall of the housing bore tapers at the end of the feed zone immediately upstream of the transition cone.

3. The extruder according to claim 1, wherein the flow channel formed between the extruder screw and an inner wall of the housing recess first widens and then tapers after the transition cone in a discharge zone.

4. The extruder according to claim 1, wherein in the center zone the inner diameter of the housing bore and the outer diameter of the extruder screw flights are larger than in the discharge zone.

5. The extruder according to claim 1, wherein in the center zone the inner diameter of the housing bore and the outer diameter of the extruder screw flights are larger than in the feed zone.

6. The extruder according to claim 1, wherein in the center zone the inner diameter of the housing bore and the outer diameter of the extruder screw flights are the same as in the feed zone.

7. The extruder according to claim 1, wherein the depth of the screw channels formed between the extruder screw flights in both degassing zones is greater by a factor of 3 to 10 than for the screw channel in the sealing and compression section formed between the degassing zones.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein:

[0048] FIG. 1 is a perspective view of an extruder from the outside;

[0049] FIGS. 2A, 2B are each parts of an extruder screw in perspective view;

[0050] FIG. 3 is a detail of the extruder screw in side view;

[0051] FIG. 4 is a detail of the extruder in perspective view;

[0052] FIG. 5 is a detail of the extruder in a side, partially cut view, and

[0053] FIG. 6 is a schematic sectional view of the extruder according to FIGS. 1 to 5; and

[0054] FIG. 7 is a schematic sectional view of an extruder according to a further embodiment.

DETAILED DESCRIPTION

[0055] In FIG. 1, an extruder 100 according to the invention is shown in perspective view from the outside, whereby end bearing and drive elements are not shown. In particular, the housing 10 with an inner housing bore 18 in the form of a bore in which an extruder screw 20 is rotatably mounted is visible. The housing 10 has an inlet area 11 with an intake opening 12 for solid polymer particles. Connected via a connecting flange 13 is an intermediate region 14 with an enlarged diameter, which has two housing openings 15.1, 15.2, each of which extends into the inner housing bore 18. A suction device, in particular a vacuum pump, is connected to each of the housing openings 15.1, 15.2.

[0056] A further connecting flange 16 connects to an end region 17 of the housing 10, the diameter of which is again reduced, and which corresponds approximately to that of the initial region 11. At the end of the end region 17, the housing recess opens so that the processed polymer melt can be discharged from this point for further processing.

[0057] FIGS. 2A and 2B each show parts of the extruder screw 20 from the same perspective view. In FIG. 2A, the extruder screw 20 is shown with its satellite screws 50. In FIG. 2B, only their receiving grooves 51 are visible so that the view of the geometry of the extruder screw 20 remains unobstructed.

[0058] A feed and metering zone 21 has a helical extruder screw flight 31. Extruder screw 20 also has a discharge zone 25 of the same or similar diameter as the feed and metering zone 21 and also has only one extruder screw flight 35.

[0059] Between them is a degassing zone 23, which in turn is divided into a first pre-degassing zone 23.1, a sealing and compression zone 23.2 and a post-degassing zone 23.3. In the degassing zone 23, the screw shaft core, whose diameter varies along its length, is surrounded by a total of three intertwined extruder screw flights 32, 33, 34.

[0060] In FIG. 3, the section of the extruder screw 20 essential to the invention is shown in an enlarged, lateral view, with the respective outer diameters D1, D2, D3 also indicated. Exemplary dimensions and geometrical relations are as follows: In the feed and metering zone 21, the extruder screw flight 31 has a relatively small outer diameter D1 of 110 mm, for example; In the discharge zone 25, the extruder screw flight 35 has an outer diameter D3 which is 0.8 to 1.2 times the outer diameter D1, i.e. approximately equal to D1, but may be 20% larger or smaller; and/or In the degassing zone, 23 the extruder screw flights 32, 33, 34 have a uniform outer diameter D2 which is at least 1.5 times D1, and in particular twice as large. In the example, D 2=190 mm.

[0061] The outer diameters D1, D2 and D3 thus vary only between the zones, but are constant in each case within the respective zone 21.2, 23, 25. Tapered transition zones 22, 24 are formed in between.

[0062] The shaft core diameter is largely constant both in the feed and metering zone 21 and in the discharge zone 25. Small variations in the shaft core diameter and/or the pitch of the extruder screw 20 are provided, as is usual in extrusion technology, in order to achieve homogenization and compaction and/or to influence the flow rate locally.

[0063] Immediately in the transition from the degassing zone 23 to the discharge zone 25, for example, the shaft core diameter of the discharge zone is reduced compared to the diameter in the further course, so that the melt pressure in the discharge zone 25 can be built up again after it was at approximately zero in the post-degassing zone 23.3 due to the high vacuum present there.

[0064] It is essential to the invention that the shaft core diameter is abruptly reduced twice within the degassing zone 23. At the transition from the transition zone 22 to the pre-degassing zone 23, as well as in the sealing and compression section 23.2 that follows, the shaft core diameter is large; the height of the extruder screw flights 32, 33, 34 and thus the height of the screw flights 41, 43 formed therebetween is small in relative terms.

[0065] Between these points, the shaft core diameter is significantly smaller, resulting in a greater flight depth between the screw flights. In the example given, the flight depth for screw flights 41, 43 is, for example, 4 mm, corresponding to between 10% and 20% of the outer diameter D2. The flight depth of screw flights 42, 44 in the degassing zones 23.1, 23.3 is 32 mm in the example, so that the height of screw flights 42 there has increased by a factor of 3 to 10 compared with screw flights 41 in the initial zone 23.1 or in the sealing and compression section 23.2.

[0066] The dashed double lines in FIG. 3 serve to indicate the course of the extruder screw flights along the length of the extruder screw shaft. In the feed and metering zone 21 and in the discharge zone 25, there is only one helical extruder screw flight 31, 35 in each case. In the degassing zone 23, the dashed double lines indicate the course of a first extruder screw flight 32. It can be clearly seen that these lines cross two further extruder screw flights 33, 34 in each case. Thus, a total of three intertwined extruder screw flights 32, 33, 34 are formed in the degassing zone 23.

[0067] FIG. 4 shows a perspective view of the transition from the feed and metering zone 21 to the degassing zone 23 within an intermediate area 14 of the housing 10. For this purpose, the housing parts 11 and 13 (see FIG. 1) are removed so that there is a clear view of the conical transition zone 22. The extruder screw flight 31 of the feed and metering zone 21 runs out in front of the transition zone 22. Already within the transition zone 22, the three extruder screw flights 32, 33, 34 of the degassing zone 23 have their beginnings, whereby in FIG. 4 only the beginnings of the extruder screw flights 32, 33 are visible. The termination of the extruder screw flight 31 before the transition zone 22 and the beginning of the three extruder screw flights 32, 33 and 34 within the transition zone 22 result in an early division of the melt stream into three partial streams.

[0068] FIG. 5 shows the essential part of the extruder 100 according to the invention in a partially cut view. Herein, the intermediate portion of the housing 10 is shown in section. It can be seen that the outer edges of all three extruder screw flights 32, 33, 34 end very close to the inner wall 19 of the housing bore 18. It is easy to see in FIG. 5 the course of the diameter of the extruder shaft core, which is greatly reduced in the pre-degassing zone 23.1 and then again behind the sealing and compression zone 23.2 at the beginning of the post-degassing zone 23.2. As a result, large-volume, deepcut screw flights 42 are formed between the inner wall 19, the parallel sections of the extruder screw flights 32, 33, 34 and the extruder shaft core.

[0069] FIG. 6 is a schematic, highly exaggerated representation of the dimensional relationships on the extruder screw 20. The direction of flow is from left to right. Shown are the shaft core diameter and the outer diameter measured over the outer edges of the flights; the screw flights themselves are not shown in detail. Through this illustration, the variation of the flight depth between the shaft core and the inner wall of the housing 10 over the length of the extruder screw 20 becomes particularly clear.

[0070] At the end of the metering zone 21, the shaft core diameter increases for the first time. The outer diameter D1 remains constant. This reduces the channel depth. Compression of the conveyed melt occurs. In the transition zone 22, the flowable volume expands because the outside diameter increases to D2. This is compensated for by a further reduction in the passage depth in the conical transition zone 22. The aim is to convey the melt to the pre-degassing zone 23.1 in such a way that the flow channels formed are initially fully filled. The narrow gap there also increases the shear in the polymer melt.

[0071] At the beginning of the degassing zone 23, the corrugated core diameter is abruptly reduced significantly, while the outer diameter D2 of the flights remains constant. The suddenly increasing volume of the flow channel there can no longer be filled by the melt supplied. There is a sudden expansion of the melt, which was previously strongly sheared in the transition area 22 and thus also has been strongly heated. As a result of the pressure drop, the volatile substances contained in the melt dissolve particularly well during the expansion. The preceding heating by shear also contributes to this. The volatile substances, namely the majority of the load in the polymer melt, can thus already be extracted from the pre-degassing zone 23.1 via the extraction opening 15.1, as indicated by the block arrow.

[0072] This is followed by a flow channel constriction in the sealing and compression zone 23.2. On the one hand, this serves to collect the melt gas-free again and to convey it homogeneously. On the other hand, the flow channel should be filled without gaps over the entire circumference so that the flowing melt itself effects a gas seal and separates the degassing zones 23.1, 23.3 from each other in a gas-tight manner.

[0073] This is followed by renewed expansion in the melt, because in the post-degassing zone 23.3 the flow channel height increases abruptly due to a renewed reduction in the shaft core diameter with the same outer diameter D2 of the undrawn screw flights. The associated churning of the melt is further intensified by the rotating satellite screws 26 in this section. In the post-degassing zone 23.3, a high vacuum is applied to remove any remaining volatile impurities after passing through the pre-degassing zone 23.1. This achieves a high purity of the treated polymer and the desired increase in intrinsic viscosity.

[0074] Behind the post-degassing zone 23.3, the flow channel tapers towards the transition zone 24. In the transition zone 24, the webs and the corrugation core each have a different cone angle, which causes an enlargement of the flow channel. Between the transition zone 24 and the beginning of the discharge zone 25, a short constant channel depth is provided before the shaft core diameter increases again and the channel depth is consequently reduced while the outer diameter D3 of the extruder screw flights remains constant.

[0075] FIG. 7 shows a schematic view similar to FIG. 6 of a further form of extruder 100′. This deviates from the previously described embodiment only with regard to the design of an extruder screw 20′ in a feed zone 21′. The degassing zone 23′ with the pre- and post-degassing zones 23.1, 23.3 and the intermediate sealing and compression zone 23.2, the transition zone 24 and the discharge zone 25 are designed in exactly the same way as in the first embodiment.

[0076] The feed zone 21′ has a diameter D1′ which is the same size as the diameter D2 in the degassing zone 23′ and is thus significantly larger than the diameter D3 in the discharge zone. The flow channel has a greater height in the feed zone 21′ than in the transition zone 22′. In this embodiment compression therefore also takes place in the transition zone 22′ before a strong expansion occurs in the pre-degassing zone 23.1′.

[0077] The differently formed feed zone 21′ is advantageous, for example, if the polymer does not need to be drawn in there as a solid, melted and homogenized, but is already taken over in molten form from an upstream process.

[0078] The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.