METHOD FOR ORIENTATION OF THE MATERIAL OF PLASTIC TUBES

Abstract: Method for orientation of the material of tubes (10) of plastic wherein the plastic tube is passed through a heated jacket tube (12). The plastic tube is held under an internal positive pressure inside the jacket tube is to be exposed to tempering in an initial stage and then to be exposed to orientation in a following wider portion of the jacket tube by the plastic tube being expanded against the tube wall under the action of the internal positive pressure in said latter portion of the jacket tube.

Description: METHOD FOR ORIENTATION OF THE MATERIAL OF PLASTIC TUBES This invention relates to a method for orientation of the material of plastic tubes, e.g. tubes of poly- ole'fins such as polyethylene or cross-linked poly¬ ethylene, PVC and ABS. It is known that plastic material can be made stronger by effecting orientation of the material. The advantage of the orientation is that the tube will be stronger and can stand a higher burst pressure, but the method of orientation of the tube to the extent it is applied on the whole, is an expensive and troublesome process. A method for cross-linking tubes of polyethylene in the manufacture of so-called PEX tubes comprises the step of passing the tube through a salt bath when the tube has left the extruder, said bath being held at a high uniform temperature. In order to avoid collapsing of the tube in the salt bath and in order to effect a precal ibration to an approximate intended diameter of the tube, the tube is held under a low internal pressure when passing through the salt bath which may have a length of about 15 m. When PEX tubes having larger dimensions (40 mm diameter or more) are to be manufac- tured the problem is encountered that the tube will be oval in the salt bath due to the fact that the tube by buoyancy in the salt bath will be pressed against the overlying ,rol lers provided to maintain the tube in the salt bath. The object of the invention is particularly to provide in direct connection with the cross-linking of polyethylene tubes, an orientation of the plastic material while the difficulties mentioned above in connection with the use of a salt bath at the same time are eliminated, and, moreover, to provide this by a OMPI method which can easily be included in the production line without causing increased production time. How¬ ever, the invention as to the broad aspects thereof can be applied to the orientation of the material of plastic tubes which are not cross-linked or which have been cross-linked in another way. For this purpose there is provided a method for orientation of the material of plastic tubes with the characteristics appearing from claim 1. In order to explain the invention in more detail reference is made to the accompanying drawing which discloses di agram atical ly an equipment for working the method of the invention for cross-linking and orientation of polyethylene. A polyethylene tube 10 is extruded from an extruder 11 and from the extruder is passed into a jacket tube 12 of steel which is kept at a uniform temperature of 130 to 300°C, preferably at about 250°C, e.g. by heating by means of external heaters 13. The polyethylene the temperature of which may be higher than the temperature maintained in the tube 12, when the polyethylene tube leaves the extruder, will be tempered in the tube 12. If the polyethylene has not said higher temperature, the polyethylene will be heated in the steel tube. Inside the steel tube 12 the polyethylene tube is held under an internal positive pressure of 0.1 - 25 bar by means of a fluid supplied, preferably an inert gas, " e.g. nitrogen. In order that the polyethylene tube will slide utfobstructedly through the steel tube a lubricant, preferably with high or relatively high viscosity (> 400 cSt at 20°C) can be supplied between the surfaces sliding against each other. In an initial stage, the plastic tube will be cross- -linked and after the distance required for the cross- -linking, the steel tube changes at a conical portion 14 into a wider portion wherein the cross-linked tube will be expanded under the action of the internal positive pressure to engage the inner side of the wider portion of the steel tube such that the plastic material will be exposed to orientation. Thus, it will be seen that the polyethylene tube immediately after the extrusion will be exposed to continuous cross-l nking and orientation while being calibrated at the same time when moving through the steel tube 12 substantially at the velocity at which the tube is being extruded. The final product is an orientated PEX tube which has improved strength properties and is calibrated and moreover presents a regular cylindrical form. The polyethylene tube supplied to the initial portion of the steel tube 12 can be cross-linked already, e.g. by radiation, the only purpose of the heating and expansion in the steel tube in that case is to effect the desired orientation of the cross-linked polyethylene material of the tube. GfΛPΪ ~-

Claims: CLAIMS '1. Method for orientation of the material of plastic tubes wherein the plastic tube (10) is passed through a heated jacket tube (12) in which the plastic tube in an initial stage is tempered in a tempering zone, and wherein the plastic tube then in a succeeding stage is exposed to orientation by the plastic tube being expanded under the action of an internal positive pressure, c h a r a c t e r i z e d in that the plastic tube (10) comprises a tube of polyethylene which is cross-linked before or during the tempering. 2. Method as claimed in claim 1, c h a r a c - t e r i z e d in that the expansion is effected in a flaring portion of the jacket tube (12) downstream of the. tempering zone. 3. Method as claimed in claim 1 or 2, c h a r a c ¬ t e r i z e d in that the jacket tube (12) is held at a temperature between 130 and 300°C, preferably at about 250°C. 4. Method as claimed in any of claims 1 to 3, c h a r a c t e r i z e d in that a lubricant is applied between the plastic tube (10) and the jacket tube (12).

Inventors: AOGREN LENNART, AOGREN, LENNART
Applicants: UPONOR AB
Application year: 1983
Main classification: B29D23
IPC codes: B29C47, B29C49, B29C55, B29K105