REINFORCED THERMOPLASTIC TUBE

Description

- DORSTON INCORPORATION LIMITED a TROPIC ISLE BUILDING “THERMOPLASTIC REINFORCED TUBE DESCRIPTION The present invention relates to a method for producing thermoplastic pipes reinforced with a metal wire armor by extrusion forming. This method is particularly suitable for the manufacture of thermoplastic pipes used in the construction of pipelines primarily used to convey corrosive fluids at relatively high pressures in industrial sectors such as the chemical, petroleum, and electric power industries. A method for manufacturing thermoplastic pipes reinforced with a wire armour by extrusion forming is known (USSR 657997), according to which a transverse helical reinforcement is wound onto tensioned and translated longitudinal reinforcing strips spaced evenly along the circumference of a fixed welding electrode, said reinforcement being welded, while being wound, to the longitudinal reinforcing strips with the aid of a roller welding device, said roller welding device rotating about the axis of the transverse helical reinforcement in synchronism with its winding, said welded wire armour being introduced into a forming cavity into which thermoplastic material is simultaneously fed during extrusion.According to this method, the transverse helical reinforcement is wrapped with a single reinforcing coil and welded with a single electrode on the roller welding device. This method is only advantageous up to a certain diameter of the thermoplastic tube, exceeding which leads to a reduction in process productivity or a decrease in the strength of the tubes for the reasons described below. With the geometric parameters set for a thermoplastic pipe, including the mesh size of the reinforcing wire mesh, the maximum linear productivity of the continuous process, corresponding to the wire mesh travel speed, is ultimately determined by the maximum speed allowed by the resistance welding employed, expressed in terms of weld spots performed per unit of time. As the pipe diameter increases, for the same wire mesh size and travel speed, the required welding speed increases proportionally, a condition that may become unacceptable due to the deterioration in weld quality.In the method described above, the welding speed can practically be reduced to a tolerable value only by reducing the travel speed of the wire reinforcement, which reduces the linear productivity of the process, or by increasing the mesh size of the wire reinforcement, which in this case causes a decrease in the strength of the tube. Therefore, the current state of the art does not allow the production of such larger-diameter thermoplastic tubes with equal or higher quality and productivity characteristics. An object of the present invention is to improve this method of continuous production of thermoplastic tubes reinforced with a metal wire armature, so as to allow the production of tubes of larger diameter together with an increase in quality characteristics and productivity. This problem is solved, according to the invention, in that in addition to the known method described above, the winding of the transverse helical reinforcement is carried out with at least two reinforcement helices wound simultaneously and unidirectionally with a symmetrical central reciprocal winding phase shift, and in that the welding is carried out with a plurality of roller electrodes distributed along the circumference of the roller welding device, the number of roller electrodes being in accordance with that of the reinforcement helices. The characteristics of the method according to the present invention described above have the advantage, compared to those of the prior art, consisting in the fact that the production of thermoplastic pipes of larger diameter no longer causes a reduction in linear productivity of the process and / or a decrease in the strength of the pipe, since the resistance welding speed employed is no longer a limiting factor. This is achieved through a relatively greater complexity of the device implementing this method. The implementation of the invention will be described in more detail below with reference to the schematic drawings, in which: Fig. 1 is an example of the device embodying the invention, shown in cross-section; and Fig. 2 is a left side view of the device according to Fig. 1. The thermoplastic tube 1 reinforced with a metal wire armor comprises a cylindrical wall of thermoplastic material in the central part of which is a metal wire armor formed by longitudinal reinforcing strips 2 welded to a transverse helical reinforcement 3. According to the embodiment of the invention, the transverse helical reinforcement 3 consists of two unidirectional reinforcing helices 3a, 3b wound with a central mutual phase shift with respect to each other of 360° / 2 = 180°. The number of reinforcing helices of the helical reinforcement 3 can be equal to three, five, or more, mainly depending on the diameter of the tube. Therefore, the mutual phase shift angle of winding will be 360° / x, where x is the number of reinforcing helices. The production method of the thermoplastic tube 1 reinforced with a metal wire armature is based on the continuous extrusion forming of thermoplastic material such as, for example, polyethylene. The method consists in the transverse helical reinforcement 3 being wound onto tensioned and translated reinforcing strips 2, spaced equally apart along the circumference of the fixed welding electrode 4. This reinforcement is then welded, while wound, to the longitudinal reinforcing strips 2 with the aid of the roller welding device 5, which is rotated around the axis of the transverse helical reinforcement 3 in synchronization with the winding of the latter. After this, the welded metal wire armature 2, 3 is introduced into the forming cavity 6, into which the thermoplastic material 7 to be extruded is fed. According to the invention, the transverse helical reinforcement 3 is wound as shown in Fig. 2 with at least two reinforcing helices 3a, 3b. In this case, the number of reinforcing helices of the transverse helical reinforcement 3 can be increased, as the pipe diameter increases, up to three, four, or more. All reinforcing helices of the transverse helical reinforcement 3 are wound simultaneously in one direction with a symmetrical central reciprocal winding phase shift equal to an angle of 360° / x, where x is the number of reinforcing helices. Welding is performed, as shown in Fig. 2, with two roller electrodes 8a, 8b, distributed along the circumference of the roller welding device 5, the number of roller electrodes corresponding to that of the reinforcing helices. High welding quality is achieved by applying current to each roller electrode individually, however it is possible to connect said electrodes in series. The device for applying the present invention, shown in the drawings, comprises the extrusion head 9 connected to a conventional extruder (not shown), said head carrying the fixed welding electrode 4 and the winding mandrel 10. The frame 11 contains the roller welding device, mounted for rotation and provided with a conventional drive device (not shown). The roller welding device 5 is provided with diametrically opposed and spring-loaded roller electrodes 8a, 8b (not shown), deflection rollers 12a, 12b for winding the reinforcing coils 3a, 3b and freely rotatable bobbins 13 for the wire of the reinforcing coils 3a, 3b. The roller welding device 5 also comprises the brush current collector 14 for supplying current to the roller electrodes 8a, 8b, said collector, in the illustrated example, being possibly of double structure. The invention can be put into practice, with the aid of the device described above, in the manner described below. The longitudinal reinforcing strips 2 are unwound from external reels (not shown) and are moved together with the already formed thermoplastic tube through the fixed welding electrode 4, with the aid of a conventional traction device (not shown). As the roller welding device 5 rotates, the deflection rollers 12a, 12b, rotating with it, wind the reinforcing spirals 3a, 3b, unwound from the freely rotating reels 13, onto the longitudinal reinforcing strips 2 supported by the fixed welding electrode 4. The reinforcing spirals 3a, 3b in the process of being wound are welded to the longitudinal reinforcing strips 2 with the aid of the roller electrodes 8a, 8b rotating with the device 5.The welded wire mesh is instantly fed into the forming cavity 6, into which thermoplastic material is simultaneously fed to be extruded along the circular channel of the extrusion head 9, said material pressurizing the forming cavity 6 and covering the wire mesh 2, 3 on both sides. The thermoplastic tube 1 is cooled and then cut into separate pieces as it exits the forming cavity 6 without interruption in the direction of arrow A. The following table shows examples with numerical data characterizing the invention compared to the state of the art. Description Tube according to the Tube according to the prior art the invention diam. 150 mm diam. 240 - 320 mm Thermoplastic material high density polyethylene for low pressure Pipe outer diameter mm 150 240 320 Pipe thickness, mm 12 13 14 Wire material low carbon steel Wire diameter, mm 2.5 2.5 3.0 Number of longitudinal reinforcement strips, pieces 42 70 90 Number of reinforcement helices, pieces 1 2 3 Winding pitch of one helix, mm 10 20 33 Free size of reinforcement meshes, mm x mm 7.6x7.5 7.6x7.5 7.6x8.0 Welding current, kA 6 6 7 Welding pulse duration, ms 100 100 120 Welding pulse frequency, Hz 7 7 5.5 Linear productivity, m / min 0.10 0.12 0.12

Claims

CLAIMS 1. A thermoplastic tube with a mesh reinforcement consisting of a longitudinal reinforcement in the form of uniformly spaced circumferential strips, onto which a transverse helical reinforcement is wound and welded, the improvement being that the transverse helical reinforcement is formed by at least two reinforcing helices wound in the same direction with equal spacing and with a winding phase shift of 360° / x, where x is the number of reinforcing helices.

2. The method of producing thermoplastic tubes reinforced with a wire armour by extrusion forming, according to which a transverse helical reinforcement is wound on tensioned and translated longitudinal reinforcing strips, spaced uniformly along the circumference of the fixed welding electrode, said reinforcement being welded, while being wound, to the longitudinal reinforcing strips with the aid of a roller welding device, rotating about the axis of the transverse helical reinforcement in synchronization with the winding of the latter, after which the welded wire armour is introduced into the forming cavity into which thermoplastic material is simultaneously fed during extrusion, characterised in that the winding of the transverse helical reinforcement is achieved with at least two reinforcing helices wound simultaneously with a symmetrical central reciprocal phase shift,and by the fact that the welding is carried out with a plurality of roller electrodes distributed along the circumference of the roller welding device, the number of said electrodes corresponding to that of the reinforcing helices.

3. A method according to claim 2, for manufacturing mesh-reinforced thermoplastic pipes by extrusion forming, wherein a transverse helical reinforcement is wound onto tensioned and gradually translated strips of longitudinal reinforcement, spaced uniformly around the fixed welding electrode, and the transverse helical reinforcement is welded to the strips of longitudinal reinforcement while the helical reinforcement is wound with the aid of a continuous welding mechanism which is rotated.The welded mesh reinforcement is fed into the forming cavity into which thermoplastic material to be extruded is simultaneously fed, the improvement being that the transverse helical reinforcement is wound into at least two reinforcing helices with a centrally symmetrical winding offset, and that the welding is performed with the aid of a plurality of circumferentially arranged roller electrodes, the number of which corresponds to the number of reinforcing helices.

4. A method according to claim 2 or 3, the improvement being that the welding current is sent to each electrode through an independent welding circuit. FLORENCE 1 MAY 1994 Or, N. 189 Ordino Consulenti