CA2025995C

CA2025995C - Process for producing a large calibre plastic pipe and an extrusion tool for the process

Info

Publication number: CA2025995C
Application number: CA002025995A
Authority: CA
Inventors: Byron Dikis; John Charaktinos
Original assignee: Individual
Current assignee: Individual
Priority date: 1989-09-22
Filing date: 1990-09-21
Publication date: 2001-01-16
Anticipated expiration: 2010-09-21
Also published as: NO904089D0; EP0418760A2; ZA907575B; FI904665A0; AU6300390A; CA2025995A1; DE3931614C2; JPH03205137A; ES2080773T3; AU633362B2; RU1823818C; PT95370B; EP0418760B1; EP0418760A3; GR3019367T3; NO904089L; PT95370A; ATE131770T1; DE3931614A1

Abstract

A process for production of large calibre plastic tube, in which a hollow thermoplastic profile of rectangular cross-section is extruded and is coiled on a drum helically with contiguous turns with adequate deformability, and in which during the coiling operation the turns are welded together at the contact surfaces by a butt-welded seam under axial pressure and heat. The hollow plastic profile is moulded in an extrusion tool to form a multi-cell profile having outer profile walls and cell-forming webs and at the same time and/or thereafter, by internal cooling of the outer profile walls and the cell-forming webs, is sized and given a design strength which is stable up to a predetermined critical pressure. The calibrated mufti-cell profile is again heated at the contact surfaces to the welding temperature of the thermoplastic material. The butt-welded seam between the turns is welded with an axial pressure below the critical pressure of the design strength. Extrusion tools for performing the process are also described.

Description

~~)~j~~~~) PROCESS FOR PRODUCING A LARGE CALIBRE PLASTIC PIPE
AND AN EXTRUSION TUOL FOR THE PROCESS
The invention relates to a process for the production of a large calibre plastic pipe, in which a hollow thermoplastic profile having a rectangular cross-section is extruded and coiled helically on a drum with contiguous turns with adequate deformability, and in which during the coiling operation the turns are welded together at the contact surfaces by a butt-welded seam under application of axial pressure and heat. Essentially, any desired thermoplastic suitable for the manufacture of tubes can be used for the plastic from which the hollow plastic profile is made. More particularly, the plastic may belong to the group of polyolefins such as polyethylene PE, polypropylene PP, etc.. The plastic may have integrated fibre reinforcement and be provided with finely divided fillers, more particularly mineral fillers.
In DE-AS 23 08 418, a hollow plastic profile of a single-cell profile of square or rectangular external cross-section is shown. The hollow plastic profile is extruded with a substantially round internal cross-section. A coolant fluid is introduced into the hollow plastic profile and passed through at least one turn during the extrusion. The turns are welded in contiguous relationship to the contact walls of the hollow plastic profile at appropriate temperature and under suitable axial pressure.
These known process steps are based on the idea that flanges with overlapping weld seams or a positive connection are not necessary in the manufacture of tubes, and particularly large calibre pipes, made from hollow plastic profiles. The known steps make use of the fact that a hollow plastic profile can be coiled with contiguous turns at deformation temperature without difficulty. Axial pressures required to weld the adjacent contact walls are then produced preferably kinematically. At the same time, a support core produced in the hollow plastic profile by the introduction of coolant fluid acts as an effective abutment. This allows adequate compression and upsetting in the axial direction for a proper ~~1~~~~~
welding of the surfaces of the adjacent contact walls of the helically coiled hollow plastic profiles. In the known steps, it is possible so to adjust conditions that the axial pressure produces permanent deformation and changes the cross-section of the hollow plastic profile on welding. Whether one or other occurs, i.e. the upsetting or compression, depends on the axial pressure and the coolant temperature. The finished pipe is drawn off from the coiling mandrel or drum. The known steps have proved satisfactory but result in plastic pipes of relatively high weight for a predetermined design stress. They can be manufactured only with hollow plastic profiles whose rectangular cross-sectional shape deviates only little from being square.
In contradistinction thereto, the object of the invention is to provide a process whereby it is possible to manufacture plastic tubes of the construction described with a reduced weight but which are nevertheless capable of taking the predetermined stresses without difficulty.
As here described, a hollow plastic profile having an elongate rectangular overall cross-section is moulded in an extrusion tool to form a multi-cell profile having outer profile walls arid cell-forming webs and at the same time and/or thereafter, by internal cooling of the outer profile walls and of the cell-Forming webs, is sized and given a design strength which is stable up to a predetermined critical pressure, the calibrated multi-cell profile set to the design strength is again heated at the contact surfaces to the welding temperature of the thermoplastic material, and the seam between the turns is butt-welded with an axial pressure below the critical pressure of the design strength. It has been found possible to obtain a considerable weight reduction without impairing the stress characteristics provided the hollow plastic profile used has an elongate rectangular cross-section and, comprises a plurality of rectangular cells separated from one another by cell-forming webs. The outer wall of the profile and the cell-forming webs may be constructed with relatively thin wall thickness, which allows considerable weight reduction. A hollow plastic profile of this kind, however, cannot be coiled into a pipe in the known manner described and be welded torn-wise by the application of axial pressure, Unless other steps were taken, it would bulge outwards at the _ 2 deformation temperature required and under the influence of the axial pressure required for welding. This can be prevented by the internal cooling and the resultant stabilization and setting of the design strength.
Further, an elongate mufti-cell hollow plastic profile of this kind cannot be used without sizing, because the outer walls of the profile would collapse in the cell area. This would be visually disturbing and also impair the moment of inertia of cross-section of the hollow plastic profile and hence the annular rigidity and stability of the complete pipe. It is known to size a hollow plastic profile, including a mufti-cell hollow profile, by vacuum sizing. Vacuum sizing cannot be incorporated without difficulty into the prior art method described in the introduction. We have discovered that adequate internal cooling of the outer walls and of the cell-forming webs causes the mufti-cell hollow plastic profile to be simultaneously externally sized and be given a design strength (in the sense of conventional engineering) such that it can take the axial pressure required to make a butt-welded seam between the adjacent turns without the risk of bulging. This is possible if the axial pressure does not have to be too high. This can be achieved by heating the contact surfaces. The butt-welded seam between the turns can always be welded at an axial pressure below-the critical pressure of the design strength. For the rectangular cross-section, its length may be I to 5 or even more times greater than its height or thickness of the rectangular overall cross-section of the hollow plastic profile. The hollow plastic profile may have a number of cells.
Generally, the internal cooling is produced by cooled components of the extrusion tool and/or cooling components connected to them. The sizing and the design strength are set by the temperature gradient during cooling and by the cooling time. In a preferred embodiment, the internal cooling is effected by a cooled liquid of predetermined cooling temperature.
Surprisingly, very accurate external sizing of the hollow plastic profile is achieved by the internal cooling if the described procedure is carried out and the design strength is set to an adequate value. Nevertheless, it is possible additionally to improve the external sizing either by external cooling and/or use of sizing tools.
_ 3 _ In accordance with a first aspect of the invention there is provided a process for the production of large calibre plastic pipe wherein a hollow thermoplastic profile of rectangular cross section is extruded and coiled helically on a drum in contiguous turns in contact with one another which are butt-welded at such contacting surfaces under application of axial pressure and heat, comprising the steps of:
molding the hollow plastic profile in an extrusion tool to form a mufti-cellular profile having outer profile walls and cell forming webs, sizing said hollow plastic profile and imparting a given design strength stable to a predetermined critical pressure by internally cooling the outer profile walls and the cell forming webs, repeating the contacting surfaces to welding temperature coiling said hollow plastic profile and butt-welding the seam between said contiguous turns at axial pressure which is below the predetermined critical pressure.
In accordance with a second aspect of the invention there is provided an extrusion tool for the production of large calibre plastic pipe wherein a hollow thermoplastic profile of rectangular cross section is extruded to form a mufti-cellular profile having outer profile walls and cell forming webs, comprising, an outer tool and an inner tool, said outer tool initially shaping an external surface of said hollow thermoplastic profile, said inner tool having coolable cell forming mandrels for forming internal surfaces of said profile walls and said cell forming webs.
In accordance with a third aspect of the invention there is provided an extrusion tool for the production of large calibre plastic pipe wherein a hollow thermoplastic profile of rectangular cross section is extruded to form a mufti-cellular profile having outer profile walls and cell forming webs, comprising:
an outer tool for forming external surfaces of said hollow plastic profile, and an inner tool, the inner tool having a plurality of cell-forming mandrels, cooling components for each individual mandrel being connected thereto and having spaced peripheral ribs in the form of rings or helices, a return tube for a coolant within each mandrel and the respective cooling component feeding the coolant through an annular chamber between the return tube and an outer wall of the cooling component, a coolant supply tube surrounding the return tube in the mandrel and connected to the annular chamber at one end of the cooling component, the coolant emerging adjacent the ribs through at least one bore -4a-adjacent the other end of the cooling component (14), the mandrel (13) having an evacuation bore (20) for the coolant at least in the region of the mandrel end adjacent the cooling component, said evacuation bore leading to an annular extraction chamber (21) which surrounds the coolant supply tube (15), part of the coolant in the cooling component (14) flowing off in the extrusion direction of extrusion of the hollow profile into the adjacent cell (6) of said hollow profile and being discharged through the return tube (17).
Embodiments of the invention will now be described with reference to the accompanying drawings wherein;
Fig. 1 is a diagrammatic side elevation of an installation arranged for a process embodying the invention.
Fig. 2 is a section in the direction II-II shown in Fig. 1 to a much enlarged scale in comparison with Fig. 1.
Fig. 3 is the detail III of the article shown in Fig. 2 to a still larger scale.
Fig. 4 is a section in the direction IV-IV shown in Fig. 1, in part, to an enlarged scale compared with Fig. 1.
Fig. 5 is a section to the same scale as Fig. 2 in the direction V-V
shown in Fig. 1.
Fig. 6 is a section in the direction VI-VI shown in Fig. 5.
Fig. 7 is a corresponding section to Fig. 2 through another embodiment of the apparatus of Fig. 1, and Fig. 8 is a section in the direction VIII-VIII of Fig. 7.

1~~~~ ~~s.
.;
Fig. 1 shows the tool head 1 of a plastic extruder to which a cooling extension 2 is connected. A rotating drum 3, which may also be termed a coiling mandrel, is seen on the right. In detail, this installation may be otherwise constructed so as largely to correspond to the known steps already described in the introduction (DE-PS 23 OS G18). A comparison of Figs. 1 and 4 will show that a thermoplastic hollow profile 4 having a rectangular cross-section is extruded and coiled on the drum 3 with contiguous turns with adequate deformability. During the coiling operation the turns are welded together at the contact surfaces by a butt-welded seam 5 using axial pressure and heat.
It will be apparent particularly from Fig. 2 that the hollow plastic profile 4 is provided with an elongate rectangular cross-section and is constructed as a multi-cell profile. Four cells 6 are provided in the exemplified embodiment. The hollow plastic profile 4 has outer profile walls 7 and cell-forming webs 8. It is moulded in the tool head 1 in an appropriate extrusion tool and is sized during that operation and/or thereafter by internal cooling of the outer profile walls 7 and of the cell-forming webs 8. The resulting sizing is an external sizing although it is effected by internal cooling. At the same time, however, the hollow plastic profile 4 is, as a result of this internal cooling, given a design strength which is stable up to a predetermined critical pressure, and this applies even at deformation temperature. The double hatching in Fig. 3 shows that the internal cooling statically results in effect in a shell formation and ultimately a box girder structure is obtained which has the said design strength. The sized multi-cell hollow plastic profile 4 adjusted to the design strength has again been heated to the welding temperature of the thermoplastic at the contact surfaces 9, e.g. by irradiation of the corresponding contact surfaces 9 with infrared rays as indicated by wavy arrows 30 in Fig. 2. The butt-weld seam 5 between the turns is welded with an axial pressure below the critical pressure of the design strength, as indicated diagrammatically by the arrows 31 in Fig. 4.
The axial pressure is generally about 10 to 15$ less than is equivalent to the critical pressure of the design strength set. The internal cooling is effected by a cooled fluid coolant, e.g. water or oil.

a'~~~~' Figs. 5 and 6 illustrate details of an extrusion tool 10 specially arranged for performing the new method and disposed in the tool head 1. The extrusion tool 10 comprises an outer tool 11 and an inner tool 12, which has hollow cell-forming mandrels 13 and/or cooling components 14 connected thereto both constructed so as to be coolable. To this end, a cooling tube extends the entire length of the cell-forming mandrels 13 and the connected cooling components 14, but in such manner that the cooling fluid supplied through each cooling tube 15 can emerge from its component 14. The 10 coolant is deflected at the closing end of the cell-forming mandrels 13 or cooling components 14 and flows 'back around the exterior of the cooling tube 15. Tube 15 can be of thermally insulated construction. The cooling fluid flow path may also be reversed. Fig. 5 shows part of the outer tool 11.
This too can be cooled. - The steps and apparatus described are particularly 15 suitable for the production of large calibre plastic pipes of the construction described from polyvinyl chloride.
The construction of another extrusion tool 10 for performing the process embodying the invention will be apparent from Figs. 7 and 8. This extrusion tool 10 also consists of an outer tool 11 and an inner tool 12, the latter comprising cell-forming mandrels. A comparison of Figs. 1 and 8 will show that in the embodiment according to Figs. 7 and 8 the cell-forming mandrels 13 and the adjoining cooling components 14 also extend into a zone which is no longer surrounded by the outer tool 11. The cooling components 14 adapted to the individual mandrels are connected to the mandrels 13. The cooling components comprise spaced peripheral ribs 16 in the form of rings or heliees. A return tube 17 for a coolant is disposed in the centre of the mandrels 13 and of the cooling components 14. As already described the coolant may be any cooled liquid, and particularly water or oil. The coolant can be fed via an annular chamber 18 between the return tube 17 and the cooling component outer wall. Connected to the annular chamber 18 is a coolant supply tube 15 which surrounds the return tube 17 in the mandrel 13. At the end of the cooling components 14 the coolant passes via bores 19 into the zone of the ribs 16 already mentioned. The mandrels 13 have exit bores 20 for the coolant at least in the area of the mandrel ends. The 7 _ ~., ..r ~:
bores 20 lead into an annular evacuation chamber 21 which surrounds the associated coolant supply tube 15. The design is also such that a sub-flow of the coolant flows off at the cooling components 14 in the extrusion direction into the cells 6 as shown by arrows in Fig. 8, both in the outflow direction and towards the bores 20. The coolant can be discharged via the associated return tube 17.
While in the embodiment shown in Figs. 2 to 6 the cells 6 of the hollow plastic profiles 4 have a rectangular cross-section on the inside, it will be seen from Fig. 7 that the cells 6 of the hollow plastic profiles 4 in this ease have an internal round cross-section. This embodiment of the extrusion tool is particularly suitable for the manufacture of large calibre plastic pipes from polyethylene. The hollow plastic profiles 4 are first extruded in the conventional manner and undergo sizing mainly in the area outside the outer tool 11 on the mandrels 13. Sizing continues at the cooling components 14 which, however, are used primarily for cooling. The coolant flowing away through the annular evacuation chambers 21 is already heated in comparison with the coolant leaving the cooling components 14, and this assists the sizing operation.
_ g

Claims

1. Process for the production of large calibre plastic pipe wherein a hollow thermoplastic profile of rectangular cross section is extruded and coiled helically on a drum in contiguous turns in contact with one another which are butt-welded at such contacting surfaces under application of axial pressure and heat, comprising the steps of:
molding the hollow plastic profile in an extrusion tool to form a multi-cellular profile having outer profile walls and cell forming webs, sizing said hollow plastic profile and imparting a given design strength stable to a predetermined critical pressure by internally cooling the outer profile walls and the cell forming webs, repeating the contacting surfaces to welding temperature, coiling said hollow plastic profile and butt-welding the seam between said contiguous turns at axial pressure which is below the predetermined critical pressure.

2. A method as defined in claim 1, wherein the internal cooling step is effected by cooled components selected from components of the extrusion tool and components connected to such tool.

3. A method as defined in claim 1 or 2, said sizing and design strength being effected by setting temperature gradient and cooling time of material comprising said outer profile walls and said cell-forming webs during said internal cooling.

4. A method as defined in claim 1 or 2, said internal cooling step being effected by a cooled liquid of predetermined temperature.

5. An extrusion tool for the production of large calibre plastic pipe wherein a hollow thermoplastic profile of rectangular cross section is extruded to form a multi-cellular profile having outer profile walls and cell forming webs, comprising:
an outer tool and an inner tool, said outer tool initially shaping an external surface of said hollow thermoplastic profile, said inner tool having coolable cell forming mandrels for forming internal surfaces of said profile walls and said cell forming webs.

6. An extrusion tool as defined in claim 5, said outer tool also being coolable.

7. An extrusion tool for the production of large calibre plastic pipe wherein a hollow thermoplastic profile of rectangular cross section is extruded to form a multi-cellular profile having outer profile walls and cell forming webs, comprising:
an outer tool for forming external surfaces of said hollow plastic profile, and an inner tool, the inner tool having a plurality of cell-forming mandrels, cooling components (14) for each individual mandrel (13) being connected thereto and having spaced peripheral ribs (16) in the form of rings or helices, a return tube (17) for a coolant within each mandrel (13) and the respective cooling component (14) feeding the coolant through an annular chamber between the return tube (17) and an outer wall of the cooling component, a coolant supply tube (15) surrounding the return tube (17) in the mandrel (13) and connected to the annular chamber at one end of the cooling component, the coolant emerging adjacent the ribs (16) through at least one bore (19) adjacent the other end of the cooling component (14), the mandrel (13) having an evacuation bore (20) for the coolant at least in the region of the mandrel end adjacent the cooling component, said evacuation bore leading to an annular extraction chamber (21) which surrounds the coolant supply tube (15), part of the coolant in the cooling component (14) flowing off in the extrusion direction of extrusion of the hollow profile into the adjacent cell (6) of said hollow profile and being discharged through the return tube (17).

8. An extrusion tool according to claim 7, characterized in that the respective mandrels (13), the cooling components (14), the return tubes (17) and the coolant supply tubes (15) are of circular cross section.