Plastic and nonmetallic article shaping or treating: processes – With measuring – testing – or inspecting – Controlling rate of movement of molding material or its...
Patent
1997-01-15
1999-08-24
Heitbrink, Jill L.
Plastic and nonmetallic article shaping or treating: processes
With measuring, testing, or inspecting
Controlling rate of movement of molding material or its...
2642095, 264237, 425325, 425393, B29C 730
Patent
active
059421711
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a method for manufacturing biaxially oriented tubing from thermoplastic material according the the preamble of claim 1.
DESCRIPTION OF RELATED ART
In WO 93/19924 a method of making tubing is disclosed. The object of biaxial orientation of the plastic material of a tube, also known as biaxially stretching a tube, is to improve the properties of the tube through orientation of the molecules of the thermoplastic material in two mutually perpendicular directions; the axial direction and the hoop or circumferential direction.
In order to effect the biaxial orientation it is desired that the tube is, uniformly over the wall thickness of the tube, at the orientation temperature suitable for the thermoplastic material concerned. This has been disclosed in DE 23 57 210 and EP 0 441 142 (Petzetakis). In practice, this orientation temperature is the temperature at which the plastic material becomes form-retaining when it cools down. For PVC (polyvinylchloride) the orientation temperature lies in a range just above the glass transition temperature of PVC. PE (polyethylene) and other polyolefins exhibit no transition temperature, but an "alpha phase", which indicates the transition from a crystalline through a partially crystalline to an amorphous structure. The orientation temperature of such a plastic material lies just above the temperature range appertaining to the "alpha phase". The biaxial orientation is fixed (frozen) by cooling the tube.
To obtain the orientation temperature of the plastic material the tube which exits from the extruder at a high temperature is cooled. In practice, this cooling is achieved by passing the extruded tube through a cooling device placed downstream of the extruder, which device cools the tube externally and/or internally.
According to WO 93/19924 the extruded tube is passed through a constant temperature area before reaching the expanding section of the mandrel to obtain the desired homogeneous temperature of the tube before it is biaxially oriented. WO 93/19924 also proposes to place a pushing device in this area which has belts which grip the outside of the tube and push the tube towards the mandrel. This pushing device therefore acts as a tube speed controlling means. It is noted that most extruders allow the extrusion speed to be set at a desired value, but this possibility does not allow a control of the biaxial orientation process.
SUMMARY OF THE INVENTION
In the context of the present invention, the tube speed controlling means are separate means placed between the extruder and the expanding section of the mandrel.
The transmission of an axial force to a tube by the tube speed controlling means, without damaging the surface thereof upon which the tube speed controlling means engages (the inner and/or the outer surface of the hollow tube), will be based upon frictional forces between the tube speed controlling means and the tube.
It has been found that the axial force which can be transmitted by the tube speed controlling means to the tube with the known method is limited since the thermoplastic material of the tube has its orientation temperature as the tube speed controlling means acts upon the tube, and the tube is rather soft at that temperature. In order to exert an axial force on the tube the tube speed controlling means must apply a large radial force on the tube and due to the softness of the tube these forces would damage the tube, in particular at the locations where the tube speed controlling means grips the tube.
The object of the present invention is therefore to provide a method which allows a substantial axial force to be exerted by the tube speed controlling means on the extruded tube without the tube being damaged and without other detrimental consequences for the following biaxial orientation. It is a further object of the invention to provide a energy efficient method for the manufacturing of biaxially oriented tubing. Another object of the invention is to make it possible to manufacture biaxiall
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Eashoo Mark
Heitbrink Jill L.
Wavin B.V.
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