Multi-screw extruder

[0001] The invention relates to a multi-screw extruder for the continuous processing and/or working of a bulk material, in particular a powder, grain or flaked product, and to a method for filling the processing spaces of such a multi-screw extruder.

[0002] Multi-screw extruders for processing and/or working bulk material are known in the art. A special group of multi-screw extruders has parallel abutting screws divided into a first process space and second process space for processing the product inside the extruders. In areas where the processing elements are tightly meshing conveying elements, there is essentially no connection to enable an exchange of material between the first process space and the second process space at least in this area. At most gases and traces of the most finely distributed product can get from one to the other process space. The impossibility of any product exchange between the two process spaces is particularly disadvantages in the feed zone of such a multi-screw extruder with several process spaces if only one of the process spaces can be filled with product via the feed hole of the extruder. The other process space(s) in this feed zone is/are filled with extremely small quantities of the product, if any. This translates into a waste of process space volume and a limitation of product feed capacity of the extruder. This is particularly disadvantageous in the case of a ring extruder with several screws arranged as a rim in a hollow space of its casing, which run parallel to the axial direction of the casing and form an inner process space inside the rim along with an outer process space outside the rim. Only the outer process space outside the rim is here filled with product via the feed hole, while the inner process space remains unused in the area of the feed hole. In the end, this feed limitation allows only a fraction of the potential throughput to be realized with this ring extruder.

[0003] One potential solution to this is offered by DE-196 04 228, which discloses such a ring extruder for the continuous processing of free-flowing materials. In this case, the aforementioned problem of feed limitation is resolved by forming at least one opening in the screw rim by omitting the conveying properties of at least one processing element in the area of the material feed hole in the process space of the ring extruder. Instead of a conveying screw element, a spacer sleeve with a smooth outer cylinder wall is used in this area on the mandrel of at least one of the screws of the rim in the area of the material feed hole. While this enables a material exchange between the outer and inner process space in the area of the feed hole, it does so at the cost of the conveying properties in this area. As a result, a portion of the product passed through the material feed hole remains in the dead spaces of the feed zone, so that optimum use is still not made of the volume of the inner process space.

[0004] The object of the invention is to solve the problems of prior art mentioned at the outset, and in particular to overcome the feed limitation for a ring extruder of the design mentioned above.

[0005] This object is achieved with a device according to the characterizing features of claim 1, and with a method according to the features of claims 19 and 20.

[0006] The underlying idea of this solution according to the invention is to at least partially remove a web in at least double-threaded elements. This is because the continuous self-cleaning and tight meshing of the Erdmenger profiles can be omitted in the feed zone. However, this enables a product exchange between the inner process space and the outer process space of the ring extruder mentioned at the outset. As a result, the entire volume of both the inner and outer process space of the extruder can be utilized over its entire processing length. In addition, the conveying effect of the only partially removed conveying elements still present in the entire feed zone makes it possible to prevent dead volumes in the feed zone. The product is hence uniformly distributed in both process spaces from the very start, i.e., the same product quantity per volume is fed to the inner and outer process space and continuously removed from this area. This eliminates the need to use non-tightly meshing elements away from the conveying area of the feed zone to balance out the product quantity in the inner and outer process space.

[0007] The significant advantage according to the invention that there is no dead space in the fill zone means that the product finds no space in which it can become deposited. This 100% removal of the product from the fill zone ensures that the entire available volume can be used to accommodate newly added product. There is no impediment by residual product, and no unnecessary retention time of the product as the result of downstream “compensation elements”. The later are hence omitted entirely, which decreases the investment costs for such an extruder, and reduces the overall retention time of the product in such a process. Eliminating such “compensation zones” for the method according to the invention, which are completely contained in the modified fill zone based on the invention, makes it possible to use the process space of the extruder for other purposes, or omit it entirely, which yields an overall shorter processing zone, and hence reduces investment and space requirements for the extruder according to the invention. Another potential disadvantage of such “compensation zones” of prior art is that they lie in an area of the method used for other processing purposes, e.g., the melting zone. Omitting these “compensation zones” in this case makes it sufficient to use only tightly meshing elements for melting, for example, which results in a shorter overall retention time of the product in the extruder and, in the case of PET, in less damage to the product.

[0008] It makes sense to only use conveying screw elements in the area of the feed hole, wherein in particular the at least one conveying screw element has a gap toward the adjacent screw element alongside the screw elements whose projected surface lying in the plane in which run the two longitudinal axes or rotational axes of the screws arranged on either side of the gap, a radial dimension ΔR and a dimension ΔL alongside the screw elements.

[0009] This gap formed by the at least one conveying screw element toward the adjacent screw element is preferably dimensioned in such a way that its radial dimension ΔR ranges between about 1/30 and ½ of the screw shank outer diameter Da, and in particular between 1/10 and ¼ of the screw shank outer diameter, wherein the axial dimension ΔL of the gap alongside the screw elements is derived from dimension ΔR and the pitch of the screw elements, and in particular measures about 2ΔR. This enables both a sufficiently conveying effect along the product conveying direction and a sufficient exchanging effect between the outer and inner process space of the extruder.

[0010] The pitch of the webs of the screw elements in the feed zone best measures at least 0.5 times, preferably at least 1 times the outer diameter Da of the screw elements. This permits a swift removal of the supplied product from the area of the feed hole. This is particularly advantageous for preparing recycled PET (RPET) in such a ring extruder.

[0011] The ratio between the outer diameter Da and inner diameter Di of the screw element best lies between 1.4 and 1.9.

[0012] The leading edge of the conveying screw elements preferably runs perpendicular to the axial direction of the extruder, at least at the radial edge area of the webs of the screw elements. This facilitates the conveying effect of the screw elements.

[0013] The trailing edge of the conveying screw elements also preferably runs perpendicular to the axial direction of the extruder, at least at the radial edge area of the webs. This also helps increase the holding capacity of the ring extruder according to the invention for loose bulk material like RPET flakes or RPET chips.

[0014] Other advantageous embodiments of the leading edge at the radial edge area of a web of conveying elements are characterized in that the leading edges run in the conveying direction overlapping the perpendicular to the axial direction, or that the leading edges are concave at least at the edge area of the webs. As an alternative or in addition, the leading edges (active edges) can also be back cut at the edge area of the webs. All of these measures also help improve the conveying capacity of screw elements designed in this way.

[0015] In a particularly advantageous embodiment of the invention, the axial partial area of the hollow space containing the screws located in the area of the feed hole is radially expanded, and this radial expansion extends along a portion of the screw rim in its circumferential direction. This step increases the process space volume in this axial partial area at the feed hole, which is particularly advantageous for the feed behavior of the extruder according to the invention for loose bulk material. In particular when preparing RPET, which is compacted and later melted when processed, this has a particularly advantageous effect.

[0016] The expansion here preferably extends along the circumference of the screw rim in the circumferential direction on either side away from the feed hole, and extends between the respective radially outer surface of the hollow space and the screw rim. This enables a particularly effective feeding of the ring extruder both in its outer process space via expansion along the circumference of the screw rim, as well as in its inner process space through the gap or gaps. A stuffing screw can be attached to the feed hole to increase the feed capacity. In addition, the extruder casing can still have vent holes in proximity to the feed hole, which are preferably exposed to a pressure below atmospheric pressure. This makes it possible to increase the feed capacity for loose bulk material even further for the extruder according to the invention.

[0017] In another preferred embodiment, at least one web is removed in the extruder according to the invention with at least one multi-threaded conveying element in the fill zone. One of the webs can be completely removed in a two-web conveying element, or even two of the three webs in a triple-threaded conveying element can be removed. It is sufficient for one of the webs to be present throughout for each conveying element, and interact with its adjacent conveying element in a tightly meshing and mutually stripping manner. The free space obtained by omitting the webs also has a positive effect on the feed behavior of the ring extruder according to the invention.

[0018] In the method according to the invention in claim 19, the multi-screw extruder according to the invention described at the outset can be filled with the bulk material to be processed and/or worked, in particular a powder, grain or flaked product, wherein the product is supplied on the outside of the screw rim and distributed in the area of the feed hole on the inner process space and the outer process space of the multi-screw extruder. In other words, a portion of the supplied product stream is drawn into the inner process space and axially conveyed by the at least one screw element that is not tightly meshing along the axial direction in at least one partial area. A portion of the product stream is here drawn into the inner process space through the at least one gap that forms between the at least one conveying screw element and an adjacent screw element. The entire inner process space is here constantly evacuated by the partially removed but yet continually conveying screw elements in the area of the feed hole. The product can be drawn to the outside of the screw rim by gravitational force and/or with a stuffing screw. The feed capacity can be further increased by keeping the process space of the ring extruder at a pressure below atmospheric pressure in the area of the feed hole.

[0029] FIG. 1 is a partial section through a 12-screw ring extruder perpendicular to axial direction A along a sectional plane 1—1 (see FIG. 2). This sectional view describes both the section through a ring extruder according to prior art as well as through a ring extruder according to this invention. The extruder casing 2 consists of a core 2a and outer casing 2b. Extending between the core 2a and outer casing 2b is a rim-like hollow space 1, which is divided into an inner process space 1a and an outer process space 1b by screws 3 arranged in a rim in this hollow space 1, which each carry a processing element or conveying element 4. The processing elements 4 shown here are double-threaded conveying elements each with a first flight 41 and a second flight 42. The screw profile (along axial direction A) is preferably designed in such a way that adjacent conveying elements 4 always contact each other, so that the first web 41 and second web 42 of a conveying element 4 is in contact with the core 43 or 44 of a respective adjacent conveying element 4. This profile (Erdmenger profile) ensures that all conveying elements 4 always strip each other. At the same time, this also separates the inner process space 1a from the outer process space 1b at least in this axial area of the ring extruder, not allowing an exchange of product between the two process spaces. Only very small quantities of product and gas can be exchanged between the inner process space 1 a and the outer process space 1b. A continuous self-cleaning also takes place between the concave inner cylinder segments 5a of the radially inner surface 5 of the hollow space 1 and the webs 41, 42 of the conveying elements 4. In the same way, the concave outer cylinder segments 6a of the radially outer surface 6 are always contacted by the webs 41, 42 of the conveying elements 4 and freed of any adhering product. The processing elements (e.g., conveying elements) 4 are each secured to their respective screw 3 by a positive, fixed connection.

[0030] FIG. 2 is a diagrammatic top view of a ring extruder according to the invention shown on FIG. 1. Discernible through a feed hole 21 in the extruder casing 2 is the screw rim with its processing elements/conveying elements 4. A total of six screws, i.e., the upper half of the screw rim, are visible. A radial expansion 22 of the outer process space 1b is provided between the radially outer surface 6 of the outer process space 1b and the outer surface of the screw rim formed by the processing elements 4. In addition, the two uppermost processing elements 4 are “removed” in such a way as to generate a slit S between them, through which the outer process space 1b with the inner process space 1a (see FIG. 1) is connected. Of course, corresponding slits can also be provided between the other adjacent processing elements/conveying elements 4 instead of the one shown slit S. When combined, this slit or these slits and the radial expansion 22 of the outer process space generate a considerable increase in the feed capacity of the ring extruder according to the invention for bulk material. The increase in feed capacity is particularly pronounced for loose bulk material, e.g., recycled RPET present in the form of chips or flakes.

[0031] FIGS. 3A and 3B show the screw elements in the feed zone of a multi-screw extruder according to the invention. For the sake of simplicity, the screws with the conveying elements 4a and 4b are not shown in their rim-like configuration, but rather shown in a flat arrangement that borders the outer process space 1b open to the outside (see FIG. 1 and FIG. 2) relative to the inner process space 1a. FIG. 3A shows the feed zone in a section perpendicular to the axial direction A along section plane 3A—3A of FIG. 3B. FIG. 3B shows the feed zone of FIG. 3A from the viewing direction denoted on FIG. 3A.

[0032] The conveying elements shown on FIGS. 3A and 3B are each double-threaded conveying elements 4a, 4b, 4a, 4b, wherein the conveying elements 4a each have two complete, unremoved webs 7 and 8, while the conveying elements 4b each have a partially removed web 9 and a completely unremoved web 10. The partially removed web 9 of the conveying elements 4b is partially removed, so that a gap S is formed between the unremoved conveying elements 4a and the partially removed conveying elements 4b in the areas 91, 92, 93, 94 and 95 in which the removed web 9 lies opposite the core of the respectively adjacent conveying element 4a. The slit S has a radial dimension ΔR and axial dimension ΔL. If the conveying elements 4a and 4b rotate during operation, the slits S formed between the adjacent conveying elements 4a and 4b move to and fro in axial direction A. In the case shown on FIGS. 3A and 3B, the ratio of screw outer diameter Da to screw inner diameter Di measures about 1.3.

[0033] FIGS. 5A and 5B show a portion of the introduction zone of a multi-screw extruder according to the invention based on a third exemplary embodiment of this invention. As opposed to FIGS. 3A and 3B as well as 4A and 4B, only two conveying elements 4e and 4f from the feed zone are shown. As on FIGS. 3A and 3B as well as FIGS. 4A and 4B, FIG. 5A shows the two conveying elements 4e and 4f in a section plane 5A—5A of FIG. 5B perpendicular to axial direction A, while FIG. 5B shows the two conveying elements 4e and 4f from the viewing direction shown by the arrow P on FIG. 5A. The two conveying elements 4e and 4f are double-threaded conveying elements each with flights 17 and 18 or 19 and 20. The flight 19 of conveying element 4f is removed in the partial areas 191, 192, 193, 194, 195, 196 and 197, so that gaps S with a radial expansion ΔR and axial expansion ΔL are also formed here in the removed areas between the conveying element 4e with unremoved webs 17, 18 and the conveying element 4f with the removed web 19. The Da/Di ratio in this case measures about 2.7. In the present exemplary embodiment, the gaps S have a particularly high radial expansion ΔR, which his particularly well suited for feeding RPET chips. While this diminishes the strength (maximum transferable torque), it does yield lots of space for transporting RPET chips between the conveying elements. In the case of a ring extruder, the power transmission from the drive will in this case be designed in such a way that the torque transmission is relayed by these “small-grained” (Da/Di large) feed elements and parallel to these “small-core” feed elements via the “large-core” conveying elements arranged primarily on the bottom of the rim, so that the “small-grained” conveying elements/feed elements.

[0034] FIG. 6 shows the fill zone of a ring extruder according to the invention in a side view perpendicular to axial direction A. This exemplary embodiment involves a ring extruder with ten screws 3, on which conveying elements 4′ with web capping and conveying elements 4 without web capping area arranged in the shown fill zone. In addition, the extruder casing 2 is expanded in the feed zone shown. The overall expansion 22 of the casing 2 consists of an expansion 22b of the outer process space 1b (see FIG. 1) and an expansion of 22a of the inner process space 1a (see FIG. 1). Both the inner expansion 22a and the outer expansion 22b of the extruder casing 2 come about as the result of having removed a portion of the material from both the radially inner surface 5 and radially outer surface 6 of the extruder casing 2. The inner “flower” (see FIG. 1) formed out of the inner concave cylinder segments 5a was removed from the radially inner surface 5, while the outer “flower” (see FIG. 1) formed out of outer concave cylinder segments 6a was removed from the radially outer surface 6. Only on the left side of the radially outer surface 6 on FIG. 6 was a portion of the outer concave cylinder segment 6a retained. A conveying element 4′, 4, 4″ is allocated to each of these three concave outer cylinder segments 6a. A conveying element 4′ with web capping is allocated to the uppermost of the three outer concave cylinder segments 6a, while a conveying element 4 without web capping is allocated to each of the two lower outer concave cylinder segments 6a. Gaps were formed between adjacent conveying elements 4′ with web capping and between conveying elements 4′ with web capping and conveying elements 4 without web capping. Since these gaps S move to and fro along the axial direction (perpendicular to the plane of projection) during operation, only the gaps S that fall into the sectional plane in the instantaneous rotational setting of the conveying elements 4′ and 4 are shown on FIG. 6. Of course, the gaps would shift given a further rotation of all conveying elements 4′ and 4 in the sectional view on FIG. 6, so that a gap S comes about at least once during a total rotation of the conveying elements 4′ and 4 while completely rotating the conveying elements 4′ and 4′ between all adjacent conveying elements 4′ and between all adjacent conveying elements 4′ and 4. The capping of conveying elements 4′ is dimensioned in such a way that the arising gaps S are large enough to allow RPET chips to easily get in the inner expanded process space 22a. In addition, the radial expansion 22a of the inner process space 1a is formed along the entire periphery of the core 2a, while the radial expansion 22b of the outer process space 1b is expanded along a large portion of the rim periphery along circumferential direction U. Only the aforementioned “remainder” of the outer flower formed by the three outer concave cylinder segments 6a of the radially outer surface 6 ensures a seal of the outer process space 1b on the left side of the outer screw rim circumference on the drawing.

[0035] The exemplary embodiment of FIG. 6 is especially advantageous, since it achieves an increase in feed behavior relative to the ring extruder known from prior art as the result of a four measures:

• • [0036] 1. The gap S formed between the conveying elements in the area of the feed hole 21 and in the area of the outer radial expansion 22b of the outer process space 1b allow for an easier transfer of product from the outer process space 1b to the inner process space 1a.
  • [0037] 2. The inner expansion 22a constitutes an enlargement of the inner process space 1a due to the removal of the radially inner surface 5 (removal of inner flower, see FIG. 1).
  • [0038] 3. The outer expansion 22b constitutes an enlargement of the outer process space 1b (by removing the outer flower, see FIG. 1).
  • [0039] 4. The fact that all conveying elements 4 and 4′ in the feed zone continue to convey despite the partial removal of screw elements, the product accommodated in these expanded process spaces 1a, 22a and 1b, 22b is always conveyed away immediately, thereby achieving a significant increase in feed capacity. The feed zone expanded according to the invention on FIG. 6 increasingly narrows along axial direction A (see FIG. 2), so that the initially loose, incoming product is increasingly compressed as it is fed through the gap S and along the circumferential direction U, and later along axial direction A and, if necessary (in the case of RPET), melted. In this way, the extruder according to the invention can be operated at an efficient fill level (and hence a sufficient throughput) even when loaded with initially very loose (e.g., chips) bulk materials.