EP1401628A1

EP1401628A1 - Device for extruding a thermoplastic polymer into a tube, and an extruder head therefor

Info

Publication number: EP1401628A1
Application number: EP02743982A
Authority: EP
Inventors: Jan Visscher
Original assignee: Machinefabriek de Rollepaal BV
Current assignee: Machinefabriek de Rollepaal BV
Priority date: 2001-07-03
Filing date: 2002-07-02
Publication date: 2004-03-31
Also published as: WO2003004247A1; NL1018450C2

Abstract

The invention relates to a device (1) for extruding a thermoplastic polymer into a tube, comprising means for annular feed of tube material for extrusion to an extruder head which comprises an extruder head gap with an inner wall and an outer wall, wherein the inner wall (6) and/or the outer wall (5) of the diverging conical gap (4) and/or of the converging part of the compression gap is provided with guide structures (8, 9) running in axial direction for tube material. The invention also relates to an extruder head in accordance with the above stated device.

Description

DEVICE FOR EXTRUDING A THERMOPLASTIC POLYMER INTO A TUBE, AND AN EXTRUDER HEAD THEREFOR

The present invention relates to a device for extruding a thermoplastic polymer into a tube, and to an extruder head for use in such an extruding device .

In this extruder head the thermoplastic polymer undergoes an extrusion process (under the influence of changes in pressure and orientation) such that the tube formed by extrusion meets set quality standards. These quality standards relate to the correct diameter and wall thickness, waviness (internal smoothness of the tube) and the impact resistance. During the extrusion process the polymer is stretched in axial as well as peripheral direction at one or more positions (for instance in the conical gap and/or compression gap) . During this stretching in peripheral direction the polymer does not react uniformly, which can be caused by material differences as a result of for instance welding (joining together) at an earlier stage before or during the extrusion and/or as a result of temperature differences in axial, and particularly peripheral direction.

The European patent application 153,511 describes such a device .

In the extrusion of such tubes, in particular tubes of relatively large diameter (up to 600 mm and more) , it has been found that differences in material properties occur in the extruded tube along the periphery. This occurs particularly in the case a tube is extruded which consists of two or more layers . For instance a tube with a wall consisting of a relatively thin inner and outer shell and a foamed layer located therebetween. It has been found that as a result of welding and/or temperature differences the innermost and/or outermost layer may have a reduced thickness along the periphery, and the material of the intermediate layer can possibly also penetrate locally to the inner or outer surface of the tube .

The invention relates to providing a solution to this problem and provides for this purpose a device for extruding a thermoplastic polymer into a tube, comprising means for annular feed of the tube material for extrusion to an extruder head which comprises an extruder head gap with an inner wall and an outer wall, wherein the inner wall and/or the outer wall of the diverging conical gap and/or of the converging part of the compression gap is provided with guide structures running in axial direction for tube material.

The invention is based on the insight that, by making use of these guide structures in the inner and/or outer wall of the extruder head gap, flow of the extruding material in axial direction is enhanced and decreased or slowed in tangential direction. This enhanced flow in axial direction and reduced flow in peripheral direction while the material is passing through the extruder head gap results in the above stated problems being alleviated or avoided.

The guide structures according to the invention running in axial direction can be applied at one or more locations in the extruder head gap.

It is recommended that the extruder head gap comprises a diverging conical gap which is bounded by an inner cone and an outer cone, in which a wall of the inner cone and/or the outer cone bounding the conical gap is provided with guide structures running in axial direction for the tube material. Arranging of these guide structures in the conical gap is particularly recommended because stretch and flow in peripheral direction can occur to a greater extent because of the diverging conical gap .

Another advantageous location for arranging the guide structures according to the invention is the compression gap of the extruder head. The material for extruding does after all have a relatively high speed in the compression gap, whereby an axial guiding is very advantageous in avoiding the described adverse effects.

The guide structures according to the invention can consist of recessed parts. This means that in the wall of the inner cone and/or the outer cone there are arranged grooves through which extruding tube material can pass. The material in the recessed parts themselves and the adjoining material will thus be able to flow to a lesser extent in peripheral direction. The guide structures can also consist of raised parts, i.e. ribs and the like standing away from the wall of the inner cone and/or outer cone . These raised parts prevent material flowing between the raised parts from flowing in peripheral direction. Combinations of recessed and raised parts are also possible. According to a preferred embodiment the guide structures extend along the whole axial length of the conical gap. It is however also possible for the guide structures to extend along only a part of this axial length of the conical gap. If the guide structures extend along only a part of the axial length of the conical gap, successive guide structures can then be applied which are then preferably disposed mutually offset along the periphery. If the guide structures herein connect to each other with mutual overlap, any form of peripheral . flow is practically avoided. An even and uniform flow is obtained along, around or through the guide structures if in further preference the guide structures have a rounded form in cross-section.

The tube of thermoplastic polymer to be produced with the device according to the invention may in some conditions not possess a uniform thickness along the periphery. An improved uniform peripheral thickness is obtained if, as proposed in EP 153,511, heating/cooling means are arranged locally, and particularly at the position of the so-called mould land, along the periphery. By heating the extruding material to a greater or lesser extent locally along the periphery, the flow behaviour at this position is thereby influenced and can be controlled toward a more homogeneous peripheral thickness . When the device according to the invention is used to extrude a tube with^' a relatively great wall thickness (for instance of 10 mm) , this automatic temperature control is then found to act less effectively in obtaining a uniform peripheral thickness. According to the present invention it is proposed to arrange these separately controllable heating/cooling means at the position of the so-called compression gap of the extruder head. There is in any case a relatively large pressure drop in this compression gap and the extruding material will have a relatively high transporting speed, whereby, under the influence of different temperatures prevailing along the periphery, the thickness of the tubes can be controlled in more effective manner. It is therefore proposed according to the present invention that the device of the type stated in the preamble further comprises a compression gap which follows the conical gap of the extruder head and which is bounded by a compression inner wall and a compression outer wall, wherein the compression inner wall and/or compression outer wall is provided with separately controllable heating/cooling means distributed over the periphery and received in peripheral segments.

The peripheral wall thickness of the tube can be controlled more precisely if in further preference an end gap of the extruder head which is bounded by an end gap inner wall and an end gap outer wall is provided with separately controllable heating/cooling means distributed over the periphery in the end gap inner wall and/or end gap outer wall and received in peripheral segments .

The heating/cooling means can be arranged in the inner wall, outer wall or both walls of the compression gap. For simplicity of construction however, it is recommended that the heating/cooling means are arranged in the outer wall .

The number of peripheral segments which are provided with separately controllable heating/cooling means depends on the desired result, and particularly on the wall thickness of the tubes. The number of peripheral segments generally amounts to 2-12, preferably 4-10, and in practice 8 peripheral segments suffice .

An optimum control of the peripheral wall thickness of the tube can be obtained if in further preference the peripheral segments at the position of the compression gap are arranged offset over the periphery relative to the segments at the position of the end gap. It is hereby possible to influence the flow behaviour along the length of the extrusion device at two locations, but along the periphery at a plurality of locations, by adding heat or extracting heat. It is thus possible, particularly in the production of polyolefin tubes, for instance PE or PP tubes, to guarantee a relatively highly uniform wall thickness at relatively large wall thicknesses .

In the device for extruding a thermoplastic polymer according to the invention it has further been found that at the exit from the extruder head and the transition to the calibration tube at the position of the outlet gap of the extruder there occurs so-called burr growth, i.e. an accumulation occurs of substances from the tube material, contamination and also filler. These accumulating materials have an adverse effect on the appearance and quality of the outer surface of the extruded tube. The invention provides a solution for this burr growth problem. This is achieved in that an outlet gap of the extruder is provided with means for avoiding burr growth. According to a first embodiment the means for avoiding burr growth comprise a constriction arranged in the outer wall of the outlet gap. According to a second embodiment the means for avoiding burr growth comprise an inner wall extended axially relative to the outer wall . The means for avoiding burr growth are therefore intended to ensure that the extruded tube still makes contact with the wall as it exits the outlet gap. Because this contact is ensured, the formation of burr growth is also reduced or avoided.

Although the use of the separately controllable heating/cooling means at the position of the compression gap and the use of the means for avoiding burr growth at the position of the outlet gap are described in combination with the guide structures in the conical gap, it will be apparent to the skilled person that these inventions can each be applied separately to achieve the result intended for each invention. Finally, the invention relates to an extruder head evidently intended for use in the above stated device for extruding a thermoplastic polymer into a tube. Mentioned and other features of the device according to the invention will be further elucidated hereinbelow on the basis of a number of embodiments which are given only by way of example without the invention being deemed limited thereto. Reference is herein made to the annexed drawings, in which: figures 1, 2 and 5 each show a schematic and broken-away view of a device for extruding a thermoplastic polymer into a tube; figure 3 shows a cross-section of guide structures according to the invention; figure 4 shows a detail IV of figure 1; and figure 6 shows a variant of detail VI of figure 5. The device according to the invention is described in general sense in EP 153,511. Only details relevant to the understanding of the present invention will be further described hereinbelow.

Figure 1 shows a device 1 for extruding thermoplastic polymer into a tube. This thermoplastic polymer can consist of PVC, polyolefin, such as polyethylene and polypropylene, and the like. Device 1 comprises a feed block 2 for adding material as according to arrow 3. From the feed block the tube material is fed to an extruder head gap of an extruder head, and in particular to a conical gap 4 diverging in axial direction. Conical gap 4 is bounded by a conical outer wall 5 and a conical inner wall 6. Conical outer wall 5 has a smooth surface. Conical inner wall 6 is provided with guide structures 7 running in axial direction. These guide structures consist in this case of recessed parts 8 and 9 (see figure 3b) . Recessed parts 8 and recessed parts 9 each extend respectively over an upstream and a downstream part of the axial length of conical gap 4. Recessed parts 8 and^' 9 overlap each other and are arranged offset in peripheral direction. Inner cone 10 connects onto a breaker plate section 11 which comprises breaker plates 22 which are arranged distributed over the periphery and between which the material for extruding flows. The material then passes through a compression section 13 and the extruded tube leaves device 1 via a mould land section 14.

Figure 2 shows another device 15 according to the invention. In this case the conical inner wall 6 has a guide structure 16 which runs in axial direction and takes the form of a raised part as shown in figure 3a. Conical inner wall 6 is provided with guide structures 17 which have the same form as guide structures 16 on the conical outer wall. Both guide structures 16 and 17 extend over the whole axial length of the conical gap. Figure 5 shows another device 17 according to the invention. Use is made in this case of a feed block 18 for feeding three tube materials. Materials 19 and 20 for the outer wall and material 21. This material 21 can optionally be provided with a foaming agent, whereby the tube will obtain a foamed intermediate layer. Feed block 18 is adapted in known manner such that the three materials enter the conical gap 22 as three layers. In the case the conical outer wall 5 and conical inner wall 6 are both provided with guide structures 23 and 24 which each extend over a part of the axial length of conical gap 22 and connect to each other mutually offset along the periphery. Use is made in this case of raised parts, the cross-section of which is shown in figure 3c. It will be apparent that in each of the devices 1, 15 and 17 according to the invention any possible guide structure can be applied, in particular the guide structures with a cross-section as shown in figure 3. These guide structures are recessed or raised parts and have a rounded form over the cross-section which enhances guiding of the tube material flowing past and does not disturb it.

Although not described, other diverging walls of the extruder head gap of the guide structures according to the invention can be provided. For instance walls in the sections 14 and the converging part 26 of compression gap 13 (figures 1 and 2) .

Figure 4 shows detail IV of the device 1 of figure 1 on larger scale. In compression section 13 the extruding thermoplastic polymer passes through a compression gap 23. A high axial pressure drop occurs in this gap, whereby the material has a relatively high axial speed. Compression gap 23 is bounded by a compression inner wall 24 and a compression outer wall 25. Compression outer wall 25 is constructed from eight peripheral segments 26 which are each provided with electric heating means 27. These heating means are connected to a control unit (not shown) . This control unit drives the heating means 27 separately depending on the peripheral thickness of the tube being extruded in device 1. The flow through compression gap 23 can hereby be controlled axially over the periphery subject to the on-line produced tube. It will be apparent that cooling means, optionally arranged in compression inner wall 24, can be applied instead of or in combination with heating means 27. Although not explicitly described, compression gap 23 can also be provided with guide structures as described for the conical gap with reference to figures 1, 2 and 3. Reference is made in this respect to figure 6. Figure 6 shows the end of a device 28 according to the invention at the position of its end gap 29. At that position in the mould land section 14 there are arranged segments 30 as according to figure 4 which are distributed over the periphery and which are each provided with heating means 31. In this case also there are eight peripheral segments 30 present. The orientation of peripheral segments 30 is chosen such that they are arranged offset over the periphery relative to the peripheral segments 26 at the position of compression gap 23. Figure 6 further shows that at the position of end gap 29 the inner wall 32 is extended axially relative to outer wall 33. Extruded tube material will therefore lie against the outer wall 33 until it leaves device 28. The formation of burr growth is hereby avoided.

Figure 5 shows similar means 34 for avoiding burr growth in the form of an annular constriction 35.

Claims

1. Device for extruding a thermoplastic polymer into a tube, comprising means for annular feed of tube material for extrusion to an extruder head which comprises an extruder head gap with an inner wall and an outer wall, wherein the inner wall and/or the outer wall of the diverging conical gap and/or of the converging part of the compression gap is provided with guide structures running in axial direction for tube material.

2. Device as claimed in claim 1, wherein the extruder head gap comprises a diverging conical gap which is bounded by an inner cone and an outer cone, in which a wall of the inner cone and/or the outer cone bounding the conical gap is provided with guide structures running in axial direction for tube material.

3. Device as claimed in claim 1 or 2, wherein the extruder head gap comprises a compression gap which is bounded by a compression inner wall and a compression outer wall, wherein the compression inner wall and/or the compression outer wall is provided with guide structures running in axial direction for the tube material .

4. Device as claimed in claims 1-3, wherein the guide structures comprise recessed parts and/or raised parts .

5. Device as claimed in claims 1-4, wherein the guide structures extend along the whole axial length of the conical gap.

6. Device as claimed in claims 1-5, wherein the guide structures extend along a part of the axial length of the conical gap.

7. Device as claimed in claim 6, wherein guide structures offset along the periphery of the conical gap extend along a part of the axial length of the conical gap.

8. Device as claimed in claim 7, wherein guide structures extending along a part of the conical gap connect to each other with overlap in axial direction.

9. Device as claimed in claims 1-8, wherein the guide structures have a rounded form in cross-section.

10. Device as claimed in claims 1-9, comprising a compression gap which follows the conical gap of the extruder head and which is bounded by a compression inner wall and a compression outer wall, wherein the compression inner wall and/or the compression outer wall is provided with separately controllable heating/cooling means distributed over the periphery and received in peripheral segments.

11. Device as claimed in claim 10, wherein an end gap of the extruder head which is bounded by an end gap inner wall and an end gap outer wall is provided with separately controllable heating/cooling means distributed over the periphery in the end gap inner wall and/or end gap outer wall and received in peripheral segments .

12. Device as claimed in claim 10 or 11, wherein the heating/cooling means are arranged in the compression outer wall and/or end gap outer wall.

13. Device as claimed in claims 10-12, wherein the number of peripheral segments amounts to 2-12, preferably 4-10, and more preferably 8.

14. Device as claimed in claims 10-13, wherein the peripheral segments at the position of the compression gap are arranged offset over the periphery relative to the segments at the position of the end gap.

15. Device as claimed in claims 1-14, wherein an outlet gap of the extruder is provided with means for avoiding burr growth.

16. Device as claimed in claim 15, wherein the means for avoiding burr growth comprise a constriction arranged in the outer wall of the outlet gap.

17. Device as claimed in claim 15 or 16, wherein the means for avoiding burr growth comprise an inner wall extended axially relative to the outer wall.

18. Extruder head as claimed in claims 1-17.