WO2024056513A1

WO2024056513A1 - Twin screw extruder and method for processing a rubber material

Info

Publication number: WO2024056513A1
Application number: PCT/EP2023/074582
Authority: WO
Inventors: Frank Reineke; Gerhard Scheel
Original assignee: KraussMaffei Extrusion GmbH
Priority date: 2022-09-14
Filing date: 2023-09-07
Publication date: 2024-03-21
Also published as: DE102022123406A1

Abstract

Between the inlet opening (9) and the outlet opening (not shown) of the counter-rotating twin screw extruder the screws (1a, 1b) are arranged such that they mesh tightly, i.e. each crest portion (2a, 2b) reaches up to the surface of the screw shaft (3b, 3a) of the respective other screw (1b, 1a) while ensuring the usual tolerance range. In the region of the axial extent of the inlet opening (9), the crest portions (2a, 2b) are partially reduced relative to the main diameter of the screws (1a, 1b) so that in this region the screws do not mesh tightly.

Description

TWIN SCREW EXTRUDER AND METHOD FOR PROCESSING A RUBBER MATERIAL

Technical area

The present invention relates generally to screw extruders, particularly twin screw extruders. The present invention further relates to a method for processing a rubber material.

State of the art

A large number of different designs of screw extruders are known to those skilled in the art, which are differentiated according to the number of screw(s) used, the arrangement of the screws relative to one another and screw geometries. Common screw extruder designs can be divided into single-screw, twin-screw and multi-screw extruders, twin-screw extruder designs into co-rotating and counter-rotating twin-screw extruders, namely non-combing and meshing, in particular tightly meshing.

In non-combing or tangent twin-screw extruders, which play only a minor role in the field of plastic extrusion, the screw webs do not engage in the screw flights of the other screw. In the case of a meshing arrangement, the screw webs of the two screws engage with one another, ie the screw web or the screw webs of one screw protrude into the screw flight or the screw flights of the other screw. Intermeshing twin-screw extruders are usually designed to mesh tightly. With a tightly combing arrangement the The screw base of one screw is scraped from the flank of the screw web of the other screw, that is, the web height of the screw web approximately corresponds to the distance between the screw base of one screw formed by the surface of the respective screw shaft and the screw base of the other screw. The screw base is the location of the surface of the screw shaft in the screw flight between two revolutions of the screw web (base web) for single-start screws or between two adjacent screw webs (base webs) for multi-start screws.

Depending on the area of application, different designs of screw extruders are prevalent. While co-rotating, tightly meshing twin screw extruders are very important in many areas of plastics preparation and processing, counter-rotating, tightly meshing twin screw extruders are particularly prevalent in the area of rigid PVC extrusion.

In counter-rotating twin-screw extruders, the sealing mesh results in a closed chamber volume, which means they pump volumetrically (similar to a gear pump). In contrast to friction delivery, this results in good energetic pump efficiency, temperature-friendly delivery and hardly any axial mixing. On the one hand, these properties would make tightly meshing, counter-rotating twin-screw extruders interesting for the processing of rubber materials; on the other hand, the sealing mesh prevents the insertion of rubber lining strips, and pre-plasticization cannot be implemented here in a conventional manner. Single-screw extruders are therefore generally used in the area of rubber processing. Here, the energetic pump efficiencies are still higher than in co-rotating twin-screw extruders, but as a friction conveyor, in addition to undesirably high heat dissipation, there can also be problems with pressure build-up, which is why single-screw extruders often have to be connected to a gear pump to build up sufficient pressure, especially in order to overcome high extrusion resistance. Presentation of the invention

Against this background, it is the object of the invention to create an apparatus alternative to the prior art, particularly for the processing of rubber materials, but possibly also for the processing of other plastic masses, which at least reduces problems that arise in connection with conventional screw extruder designs.

According to one aspect of the present invention, this object is achieved by a twin screw extruder according to claim 1.

According to one aspect, the invention thus relates to a twin-screw extruder which has two counter-rotating screws and a housing in which the screws are arranged, each of the screws having a screw shaft and at least one base web wound around the screw shaft, the screw housing having an inlet opening and an in Conveying direction of the screws has an outlet opening arranged downstream of the inlet opening, and the screws are arranged in a tightly meshing manner at least in sections between the inlet and outlet openings. Within the axial extent of the inlet opening, based on the screw shafts, at least between a base web of one of the screws and the screw shaft (and thus the screw base) of the respective other screw, at least in one section there is a minimum gap width of, for example, at least 5%, preferably at least 10%, particularly preferably at least 25% of the distance between the surfaces of the screw shafts from one another (i.e. the distance between one screw base and the other screw base) is maintained in said section, so that in this section there is preferably combing and no sealing combing. Preferably, there is a meshing arrangement, especially of the screws, in this section, i.e. no non-combing and (yet or equally) no tight meshing or no close meshing and no non-combing arrangement. Compared to a conventionally designed counter-rotating twin-screw extruder, according to the invention, at least one base web is at least partially removed in the feed area or designed with a reduced web height (= radial distance between the outer radius of the base web and the screw base).

Due to the non-tightly meshing design in at least part of the feed area along the axial direction, fully continuous conveying of the supplied product stream into the inner process space of the extruder is possible, so that even conventional rubber feeding strips can be safely retracted, whereas with a continuously tightly meshing design, this occurs with every screw revolution Depending on the flow, there are one or more points in time at which the product flow is interrupted at the intake. Even if no feeding strips are fed, the present invention enables more uniform product feeding in many cases. In addition, by appropriately designing the web geometry in the catchment area, the shear conditions there can be adapted to the rheological properties of the products used or to the desired process properties. This means that, compared to a continuously meshing design, locally increased shear forces can be introduced into the product flow.

The further material transport to the extruder's injection molding tool can be carried out in a very temperature-friendly manner thanks to the volumetric conveying provided by the sealing mesh, and this can be achieved with higher throughputs than conventional single-screw extruders. An improvement in the achievable throughputs results in particular when conveying against a high extrusion resistance, although in applications that required the use of a gear pump when using a single-screw extruder, the latter can also be omitted according to the invention.

The housing can essentially be designed like extruder housings known per se from the prior art. In cross section, the housing interior therefore has an essentially eight-shaped basic shape, with the respective base web in the respective axial area in which sealing meshing is present, also scrapes along the housing wall correspondingly close to the usual tolerances.

While in the following description versions with a parallel screw arrangement and a corresponding partially cylindrical basic shape of the two concave parts of the housing interior space are primarily described, the invention can advantageously also be carried out with conical double screws and thus screw axes that are at an angle to one another. The basic shape of the two concave parts of the housing interior is then also conical. With a parallel screw arrangement, the basic shape of the screw is cylindrical; the diameter of the cylindrical envelope surface is called the main diameter. Particularly advantageous for the implementation of the present invention is a pitch (pitch) of the at least one base web in the range from 0.5 times to 3 times, preferably 1.2 times to 1.5 times, in particular 1.33 times. times the main diameter.

When choosing the materials for the production of the screws and the housing, the person skilled in the art can rely on the materials known per se from the prior art in extruder construction. The storage of the screws and the screw drive can also be carried out according to the invention as is known per se from the prior art. The specialist is not forced to undertake complex new designs.

Advantageous embodiments of the invention can be implemented in particular according to one of the subclaims.

In an advantageous embodiment, an interrupted screw web is arranged in said section on the outer circumference of at least one of the base webs. This increases the number of degrees of freedom for the execution of even more complex screw geometries in order to adapt the conveying and shearing behavior of the extruder to the desired process conditions in the feed area to align. This means that in the area radially outside the base web, a different screw geometry can be implemented than in the radial area of the base web. The respective outer screw web is interrupted in particular where it overlaps with a screw flight of the base web geometry.

In particular, it is advantageously possible to arrange several interrupted screw webs on the outer circumference of at least one of the base webs in said section, so that in the radial region of these screw webs, i.e. radially outside the base web, a different number of turns results than in the radial region of the at least one base web. Preferably, the ratio of the number of screw flights formed by the at least one base web to the number of screw flights formed by the interrupted screw webs arranged on its outer circumference is in a range from 1:2 to 1:8, particularly preferably this ratio is 1 :3, i.e., for example, a six-flight screw geometry of the interrupted outer screw flights is carried out radially outside of a two-flight base web geometry.

Preferably, the pitch of the interrupted screw web or webs differs from the pitch of the at least one base web on which the interrupted screw web or webs are arranged. Preferably, the ratio of the pitch of the at least one base web to the pitch of the interrupted screw web or the interrupted screw webs is in a range from 1:1.5 to 1:5, particularly preferably in a range from 1:2 to 1:3, in particular at 1:2.5.

While the interrupted screw web(s) can advantageously extend to the base of the screw of the other screw (in the case of parallel screws, the outer diameter of the interrupted screw web(s) can therefore correspond to the main diameter), it can also advantageously extend between the interrupted screw web or the interrupted one A minimum gap width must also be maintained between the screw webs of one screw and the screw shaft of the other screw. In particular, in the case of parallel screws, the outer diameter of the interrupted screw web(s) can be slightly reduced compared to the main diameter.

According to a particularly preferred embodiment, the screws each have two or more base webs wound around the screw shaft, so that the screws each have two, three or more flights, particularly preferably two flights.

According to a further particularly preferred embodiment, the minimum gap width is maintained at least in one section within the axial extent of the inlet opening, based on the screw shafts, at least between a base web of each of the screws and the screw shaft of the respective other screw. This means that in both screws the base web is at least partially removed or the outer radius is reduced compared to a completely tightly meshing arrangement. In particular, both screws can advantageously have a screw geometry that is exactly the same only with the opposite direction of rotation but otherwise at least over an axial region, in particular the region in which the sealing mesh is removed, and can therefore be designed to be mirror-symmetrical in the longitudinal section at least in sections.

According to a further advantageous embodiment, the screws are also not tightly meshed in sections between the inlet and outlet openings. While the volumetric conveying effect is maintained in sections, the specialist has the option of arranging shearing, mixing, kneading, homogenizing, plasticizing or other areas with tailor-made flow mechanical conditions in between, depending on the desired process conditions in the individual case. According to an advantageous development of the invention, one of the screws can be made longer than the other screw, so that it projects beyond the other screw on the output opening side. Such an arrangement essentially corresponds to a single-screw extruder that is connected downstream of a twin-screw conveyor. In the axially projecting part of the longer screw, the geometric conditions of conventional single-screw extruders can be replicated, with the upstream area of tightly meshing twin screws ensuring the pressure build-up with sufficiently high energetic pump efficiency.

According to a further aspect, the object on which the present invention is based is achieved by a method according to claim 13.

According to the invention, rubber material can therefore be processed by feeding it to a twin-screw extruder as described above. In addition to natural rubbers, rubber materials in particular include synthetic rubbers including silicone rubbers.

Remaining process steps can be carried out as in conventional processing processes for rubber materials. In particular, the rubber material can advantageously be supplied as a rubber lining strip.

However, the use of a gear pump can be omitted compared to conventional processes with single-screw extruders. Under certain circumstances, cooling of the extruder shell, which may be necessary in a conventional process, can be omitted or can be done with less use of coolant, since the present invention ensures more temperature-friendly conveying in the extruder.

The invention is explained in more detail below using the accompanying schematic drawings. The drawings are not to scale; In particular, for reasons of clarity, the ratios of the individual dimensions to one another do not necessarily correspond to the dimensional ratios in actual technical implementations.

Preferred exemplary embodiments are described, but the invention is not limited to these. In principle, any variant of the invention described or suggested in the context of the present application can be particularly advantageous, depending on the economic and technical conditions in the individual case. Unless stated otherwise, or as far as technically feasible in principle, individual features of the described embodiments can be exchanged or combined with each other and with features known per se from the prior art.

Short description of the characters

Fig. 1 shows the detail of a twin-screw extruder according to the invention in longitudinal section, the area where the screws are mounted and the area of the outlet opening are not shown.

Fig. 2 shows the detail of a further twin-screw extruder according to the invention in a longitudinal section similar to Fig. 1, whereby the area of storage of the screws and the area of the outlet opening are not shown.

Fig. 3 shows the detail of a further twin-screw extruder according to the invention in a longitudinal section similar to Fig. 1, whereby the area of storage of the screws and the area of the outlet opening are not shown.

Fig. 4 shows a perspective view of the arrangement of the screws of a further twin-screw extruder according to the invention indicated housing, the position of the input opening being indicated by a bold dashed line.

Preferred embodiment of the invention

Corresponding elements are designated by the same reference numerals in the drawing figures.

Fig. 1 shows an exemplary embodiment of a twin-screw extruder according to the invention in longitudinal section, the screws 1a, 1b being shown uncut as usual. Figures 2 and 3 similarly show variants of the exemplary embodiment from Fig. 1. As explained below, the variant from Fig. 2 differs from the variant from Fig. 1 in that in Fig. 2 only one of the two screws 1a, 1 b the web height of the base webs 2a, 12a is reduced in sections. In Fig. 3, the housing 10 is changed compared to the variant from Fig. 1 in that the base webs 2a, 2b, 12a, 12b have reduced inner diameters in the area of the reduced web height.

Only an axial section of the extruder is shown in Figures 1-3; the area of the output or injection mold (further left in the drawing plane) and the screw drive (further right in the drawing plane) are not shown. Both the injection tool and the drive with which the screws 1a, 1b are set in opposite rotation to one another, and the storage of the screws 1a, 1b are carried out in a conventional manner.

The screws 1a, 1b are arranged in parallel in a common housing 10, the housing interior of which has an eight-shaped cross section, the partially cylindrical areas of the housing inner wall 11 each having a cylinder diameter which essentially corresponds to the main diameter D of both screws 1a, 1b, but with the usual tolerances that prevent the screws 1a, 1b from grinding on the inner wall 11 of the housing. The position of the inlet opening 9, through which rubber material is fed to the screws 1a, 1b, is indicated by a dashed line. The position of the entrance opening 9 is symmetrical with respect to the position of the two screws 1a, 1b.

The geometry of both screws 1a, 1b is in opposite directions but is otherwise identical at least between the inlet opening 9 and the outlet opening, in Figures 1 and 3 also in the area of the inlet opening 9. Each of the screws 1a, 1b has two base webs 2a, 12a or 2b, 12b, so it has two flights. The pitch (pitch) of each of the base webs 2a, 12a, 2b, 12b is 1.5 times the main diameter D.

Between the input opening 9 and the (not shown) output opening, i.e. to the left of the input opening 9 in Fig. 1, the arrangement of the screws 1a, 1b is tightly meshed, i.e. the respective base web 2a, 2b, 12a, 12b is sufficient, while maintaining the usual tolerances in order to avoid grinding of the screws 1a, 1b against each other, to the surface of the screw shaft 3b, 3a of the other screw 1b, 1a. The outer diameter of the base webs 2a, 12a, 2b, 12b in this section therefore corresponds to the main diameter D of the screws 1a, 1b. Downstream of the inlet opening 9, the twin screw extruder thus functions as a volumetric screw conveyor. The screw base 4a or 4b of one screw 1a or 1b is scraped from the flank of the base web 2b, 12b or 2a, 12a of the other screw 1b or 1a, i.e. the web height of the base web 2a, 2b, 12a, 12b corresponds approximately to the distance of the screw base 4a or 4b formed by the surface of the respective screw shaft 3a or 3b of one screw 1a or 1b from the screw base 4b or 4a of the other screw 1b or 1a.

In the area of the axial extent (with respect to the screws 1a, 1b) of the inlet opening 9, the base webs 2a, 2b, 12a, 12b are opposite Main diameter D of the screws 1a, 1b removed, so that the sealing combing is eliminated in this area. A minimum gap width remains between the respective base web 2a, 12a or 2b, 12b and the screw shaft 3b or 3a (the screw base 4b or 4a) of the other screw 1b or 1a, in the present example a minimum gap width of approximately 25% of the Distance between the surfaces of the screw shafts 3a, 3b from one another.

Furthermore, with an axially continuous, partially cylindrical design of the housing inner wall 11, there is also a gap between the respective base bar 2a, 2b, 12a, 12b and the housing inner wall 11 in the axial section in which the base webs 2a, 2b, 12a, 12b are removed Gap width essentially corresponds to the above minimum gap width. In some applications, such a gap between the inner housing wall 11 and the base web 2a, 2b, 12a, 12b can influence the conveying effect in the feed zone too strongly, i.e. reduce it, by creating an axial backflow or a dead space zone in the gap area. In other applications, such axial mixing may be desirable.

If a gap between the inner housing wall 11 and the base webs 2a, 2b, 12a, 12b is undesirable, the inner housing wall 11 can also, as shown in FIG Areas above each

While the base webs 2a, 12a, 2b, 12b in the exemplary embodiment in FIG , 12b varies over the area of the axial extent of the input opening 9.

While in the exemplary embodiment of Figure 1 the base webs 2a, 2b, 12a, 12b of both screws 1a, 1b are removed in the area of the axial extent of the inlet opening 9, designs can also be implemented are in which the corresponding base webs 2a, 12a are removed in this area on only one of the two screws 1a, 1b, as shown in Fig. 2 for the upper screw 1a in the figure.

Figure 4 shows the two screws 1a, 1b of a further twin-screw extruder according to the invention in a perspective view. The opposite direction of rotation of the screws 1a, 1b is illustrated by arrows. The wall of the housing 10 is cut and only partially shown. The arrangement of the entrance opening 9 is again indicated by a broken line.

The screws 1a, 1b are arranged in parallel in a common housing 10, the housing interior of which has an eight-shaped cross section, the partially cylindrical areas of the housing inner wall 11 each having a cylinder diameter which essentially corresponds to the main diameter D of both screws 1a, 1b, but with the usual tolerances that prevent the screws 1a, 1b from grinding on the inner wall 11 of the housing. The position of the entrance opening 9 is again symmetrical with respect to the position of the two screws 1a, 1b.

Between the inlet opening 9 and the (not shown) outlet opening, the arrangement of the screws 1a, 1b is tightly meshed, i.e. the respective base web 2a, 2b is sufficient, while maintaining the usual tolerances, to avoid the screws 1a, 1b grinding against one another, to the surface of the screw shaft 3b, 3a of the other screw 1b, 1a. Downstream of the inlet opening 9, the twin screw extruder thus functions as a volumetric screw conveyor.

In the area of the axial extent (with respect to the screws 1a, 1b) of the inlet opening 9, the base webs 2a, 2b are partially removed compared to the main diameter of the screws 1a, 1b, so that the sealing mesh is eliminated in this area. The removal of the material from the base webs follows a helical, spiral geometry, so that on the outer circumference of the removed base webs 2a, 2b result in interrupted screw webs 5a, 5b.

Two different screw geometries are formed on the screw shafts 3a, 3b in the area of the axial extent of the inlet opening in a radially inner and a radially outer position. The interruptions in the outer screw webs 5a, 5b result from the fact that the pitch or pitch in the radially outer screw geometry is larger, i.e. steeper, than the pitch or pitch of the radially inner screw geometry formed by the base webs 2a, 2b Example shown is approximately by a factor of 2. The interruptions occur where the screw webs 5a, 5b of the radially outer screw geometry overlap with the screw flights of the radially inner screw geometry formed between the base webs 2a, 2b. These interruptions in the outer screw webs 5a, 5b eliminate the sealing in these areas.

The possibility of creating a second, superimposed screw geometry with the selective partial removal of the material of the base webs 2a, 2b results in additional degrees of freedom with which the twin-screw extruder can be adapted to its respective intended use. So, as shown in Fig. 4, the radially outer screw geometry can differ from the radially inner screw geometry not in terms of their respective pitch or pitch, but also in terms of their number of flights. While in the example shown, the base webs 2a, 2b form a two-flight screw geometry. The radially outer screw geometry in Fig. 4, on the other hand, has four flights.

With regard to the pre-selection of the respective screw geometries in the catchment area, the expert can generally rely on experience from the field of single-screw extruders, which concerns fundamental aspects of shear behavior. The detailed design of the screw geometries can be optimized by means of design tests using the parameters mentioned such as pitch/gear height, number of gears, minimum gap width can be varied gradually.

List of reference symbols a, 1 b screws a, 2b base webs a, 3b screw shafts a, 4b screw base a, 5b interrupted screw webs a, 6b conical transition a, 7b, 8a, 8b partially cylindrical wall sections inlet opening 0 housing 1 housing inner wall 2a, 12b base webs

main diameter

Claims

Expectations

1 . Twin-screw extruder comprising two counter-rotating screws (1a, 1b) and a housing (10) in which the screws (1a, 1b) are arranged, each of the screws (1a, 1b) having a screw shaft (3a, 3b) and has at least one base web (2a, 2b, 12a, 12b) converted around the screw shaft (3a, 3b), the screw housing (10) has an inlet opening (9) and an inlet opening (9) in the conveying direction of the screws (1a, 1b) downstream of the inlet opening ( 9) has arranged exit opening, the screws (1a, 1b) are arranged at least in sections in a tightly meshing manner between the entrance (9) and the exit opening, and within the axial extent of the entrance opening (9, based on the screw shafts (3a, 3b). ) a minimum gap width is maintained at least in one section between a base web (2a, 2b, 12a, 12b) of one of the screws (1a, 1b) and the screw shaft (3a, 3b) of the respective other screw (1a, 1b), so that there is no sealing combing in this section.

2. Twin screw extruder according to claim 1, wherein in said section on the outer circumference of at least one of the base webs (2a, 2b, 12a, 12b) an interrupted screw web (5a, 5b) is arranged.

3. Twin screw extruder according to claim 2, wherein in said section a plurality of interrupted screw webs (5a, 5b) are arranged on the outer circumference of at least one of the base webs (2a, 2b, 12a, 12b), so that in the radial region of the screw webs (5a, 5b) results in a different number of turns than in the radial area of the at least one base web (2a, 2b, 12a, 12b).

4. Twin screw extruder according to claim 3, wherein the ratio of the number of screw flights formed by the at least one base web (2a, 2b, 12a, 12b) to the number of interrupted screw webs (5a, 5b) arranged on its outer circumference Screw flights are in a range from 1:2 to 1:8, in particular 1:3.

5. Twin screw extruder according to one of claims 2-4, wherein the pitch of the interrupted screw web (5a, 5b) or the interrupted screw webs (5a, 5b) is different from the pitch of the at least one base web (2a, 2b, 12a, 12b) on which or which the interrupted screw web (5a, 5b) or the interrupted screw webs (5a, 5b) are arranged.

6. Twin screw extruder according to claim 5, wherein the ratio of the pitch of the at least one base web (2a, 2b, 12a, 12b) to the pitch of the interrupted screw web (5a, 5b) or the interrupted screw webs (5a, 5b) is in a range of 1 :1.5 to 1:5, in particular in a range from 1:2 to 1:3.

7. Twin screw extruder according to one of claims 2-6, wherein in said section between the interrupted screw web (5a, 5b) or the interrupted screw webs (5a, 5b) one of the screws (1a, 1b) and the screw shaft (3a, 3b) the respective other screw (1a, 1b) also maintains a minimum gap width.

8. Twin-screw extruder according to one of the preceding claims, wherein the screws (1a, 1b) each have two or more base webs (2a, 2b, 12a, 12b) wound around the screw shaft (3a, 3b), so that they each have two, are three or more courses.

9. Twin screw extruder according to one of the preceding claims, wherein within the axial extent of the inlet opening (9), based on the screw shafts (3a, 3b), at least between a base web (2a, 2b, 12a, 12b) of each of the screws (1a, 1 b) and the screw shaft (3a, 3b) of the other respective screw (1a, 1 b) the minimum gap width is maintained at least in one section.

10. Twin screw extruder according to one of the preceding claims, wherein the minimum gap width is at least 5%, preferably at least 10%, in particular is at least 25% of the distance between the surfaces of the screw shafts (3a, 3b) in said section.

11. Twin-screw extruder according to one of the preceding claims, wherein the screws (1a, 1b) are not tightly meshed in sections between the inlet and outlet openings.

12. Twin screw extruder according to one of the preceding claims, wherein one of the screws (1a, 1b) is longer than the other screw (1a, 1b), so that it projects beyond the other screw (1a, 1b) on the outlet opening side.

13. A method for processing a rubber material, wherein the rubber material is fed to a twin-screw extruder according to any one of claims 1-12.

14. The method according to claim 13, wherein the rubber material is supplied as a rubber lining strip.

15. The method according to any one of claims 13 or 14, wherein the use of a gear pump is omitted.