Note: Descriptions are shown in the official language in which they were submitted.
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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
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PAT 16075-1
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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
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PAT 16075-1
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.
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PAT 16075-1
CA 02025995 2000-03-02
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,
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CA 02025995 2000-03-02
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
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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.
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PAT 16075-1
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.;
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.
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PAT 16075-1
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
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PAT 16075-1
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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.
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PAT 16075-1
