Biaxial stretching of plastic tubes

Details Biaxial stretching of plastic tubes Biaxial stretching of plastic tubes

1998-03-13

2001-10-02

Silbaugh, Jan H. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Direct application of fluid pressure differential to...

Production of continuous or running length

C264S150000, C264S209400, C264S209500, C264S290200, C264SDIG007, C425S326100, C425S387100, C425S393000

Reexamination Certificate

active

06296804

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to process and apparatus for the manufacture of oriented plastics tubes, and in particular to the manufacture of tubes which have been stretched (drawn) in both the circumferential and axial directions to orient the material in the direction of stretching and enhance its mechanical properties.
International Patent Application No. WO 90/02644 describes one process for the manufacture of thermoplastics tubes for example for unplasticised polyvinyl chloride (uPVC) which have a degree of orientation in the circumferential direction that improves properties such as resistance to hoop stresses, and renders the tubes particularly suitable for transmission of water under pressure. The process described in that patent application comprises:
(i) extruding a tube of plastics material;
(ii) temperature conditioning the extruded tube to bring it to a temperature suitable for expansion;
(iii) diametrically expanding the tube by application of an internal pressure to the tube that is limited at its downstream end by a plug that is inflatable or otherwise expandable to maintain pressure within the expansion zone; and
(iv) cooling the expanded tube to set the tube in its diametrically expanded configuration.
To pull the tube through this process line a first haul-off tractor is provided before the temperature conditioning zone and another haul-off tractor is provided downstream of the expansion and cooling zones. Axial draw may be introduced into the product by running the downstream tractor at a higher haul-off speed than the first.
Other processes have been proposed in which the diametrical expansion is achieved by drawing the tube over a solid mandrel. The present invention is applicable also to such processes.
SUMMARY OF THE INVENTION
In a first form, the invention provides a continuous process for producing oriented plastic tube comprising the steps of extrusion, temperature conditioning, diametrical expansion and cooling, characterised in the step of inducing axial draw of the tube between first and second haul-off means located before said expansion step.
A second form of the invention provides a process line for continuous production of oriented plastic tube, comprising an extruder for extruding a tube, temperature conditioning means for bringing the tube to a temperature suitable for expansion, expansion means for causing diametrical expansion of the extruded tube and cooling means for setting the tube in its diametrically expanded configuration, characterised in that first and second haul-off means are located upstream of the expansion means for inducing axial draw of the tube prior to said diametrical expansion.
The term “haul-off means” as used herein is to be understood as referring to means which grip the tube and regulates the axial speed of the tube.
Preferably, the axial draw is induced prior to at least the completion of the temperature conditioning step. In one preferred form, the axial draw is induced concurrently with at least part of the temperature conditioning step. This may be achieved by locating said first haul-off means upstream (relative to the direction of tube travel) of a temperature conditioning zone and locating said second haul-off means between the temperature conditioning and expansion zones.


REFERENCES:
patent: 3074108 (1963-01-01), Wiley et al.
patent: 4153667 (1979-05-01), Brady et al.
patent: 4157235 (1979-06-01), Lagabe et al.
patent: 4443399 (1984-04-01), Takashige et al.
patent: 4828770 (1989-05-01), Fabian et al.
patent: 4886634 (1989-12-01), Strutzel et al.
patent: 5096634 (1992-03-01), Tsadares et al.
patent: 5665297 (1997-09-01), Ward et al.

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