Method for manufacturing a plastic tube provided with a wound reinforcement, in which one or more reinforcement layers are wound around an extruded inner tube made of thermoplastic material and an outer casing is extruded around the layers, in such a way that the reinforcement is joined to both the inner tube and the outer casing over at least a part of the tube length. Such methods are known for the manufacture of various types of reinforced tubes; the reinforced tubes are intended for holding a medium under pressure.

Applicants' International Patent Application PCT/NL/91/00120 relates to a type of tube in which the reinforcement is welded firmly only at some points to the inner tube and the outer casing and lies loose between those points, which ensures easy winding up of the tube. EU-O 111 169 relates to a type in which the reinforcement is joined to the inner tube and the outer casing. In that case use is made of a metal reinforcement which is wound around the inner tube in such a heated state that it is welded firmly directly it is laid, while the bond between the reinforcement and the outer casing is effected by means of a cold-setting or thermosetting adhesive.

If one does not wish to use metal reinforcement layers or adhesive, but if one still wishes to fuse the reinforcement material directly to the inner tube and the outer casing, heating must be applied immediately after the extrusion of the outer casing. For this purpose, infrared heating or heating in a hot-air oven can be selected. In both cases the plastic material is heated through and through, and this gives rise to inevitable deformation of the tube structure, thus to a lower quality.

The object of the invention is to overcome this problem and propose a method for producing a welded joint between the reinforcement material and the plastic by means of which an end product of high quality is obtained. Another object of the invention is to achieve this in a

simple way, using as little energy as possible.

To this end, the method according to the invention is characterized as described in claim 1.

In this way only the reinforcement and the plastic immediately surrounding it are brought to welding temperature, while little temperature increase occurs at places further away from the reinforcement, due to the poor heat conduction of plastics. This means that a good welding effect is obtained, while the shape of the tube is still retained.

For industrial use, for purposes such as the heating involved here, internationally different frequency values are reserved. These lie partly in the radiowave band (RF below 300 MHz) , and partly in the microwave band (MW above 300 MHz) . With regard to the fixing of those frequencies, we would refer to, for example, IEC CISPR publication 11 of 1975 "Limits and methods of measurement of radio interference characteristics of industrial, scientific and medical (ISM) radio-frequency equipment (excluding surgical diathermy apparatus)". As ISM frequencies - frequencies fixed for use in industrial, scientific and medical applications - 13, 27 and 41 MHz have been fixed in the radiowave band. In addition, other frequencies are permitted in some countries, with or without the addition of requirements relating to possible radio interference.

For the application involved here, the values 27 and 41 MHz are preferred, because in their case a heating requiring a relatively low field strength occurs, while no account need be taken of the penetration depth of the radiation, and the cost is lower than that of equipment for MW heating.

It is possible to design or place the RF electrodes in such a way that the electromagnetic field is at right angles to the plane of the reinforcement. However, it has been found that the heating is better, i.e. uses less electromagnetic power, if the field is generated in the plane of the reinforcement. This is achieved in the preferred embodiment, which is characterized in that the

combined tube is passed through at least one pair of circular RF electrodes surrounding the tube.

The heat development in the reinforcement material increases in proportion to the dielectric loss factor. Dielectric loss factors can vary very widely for various materials, even by a factor of 10 ⁸ . In order to obtain a heating time which is efficient in practice, and which is also adapted to the rate of travel of the tube and the working of the extruder, the reinforcement material embedded in plastic preferably has a dielectric loss factor of at least 0.2.

A material which is suitable for use in practice and has such a dielectric loss factor is aramid. Ara id material has a number of excellent properties making it suitable for reinforcement; the dielectric loss factor for PE with 30 vol.-% aramid is 0.3.

On the other hand, the cost of aramid is relatively high. The cost per kilogram of glass fibres is only about one tenth of that of aramid, while such glass fibres are in many cases undoubtedly acceptable as reinforcement material for the type of reinforced tube involved here. Glass by itself has an inadequate dielectric loss factor, but the idea of the invention can still be realized if glass-fibre reinforcement with a metallization layer is used. In connection with the abovementioned experience concerning the most advantageous direction of the electric field relative to the plane of the reinforcement, the idea of the invention also leads to a new method for manufacturing flat, internally reinforced sheets of thermoplastic material. If such sheets are to be manufac¬ tured using electric heating, this is not possible other than by irradiating the flat semi-finished product at right angles to the surface. However, if one proceeds in the manner indicated in the characterizing part of claim 8, it is possible to use the advantage of placing the electrodes around the tube, with the result that the electric field is created in the plane of the reinforcement.

An additional advantage of initial manufacture in tubular form is that the inner tube and the outer casing

between which the reinforcement lies provide a build-up of pressure in the region of the reinforcement through thermal expansion of the reinforcement and the plastic in the boundary region. A certain welding pressure consequently arises by itself, this welding pressure being necessary for achieving good welding quality, and being less easy to achieve when a reinforcement between two flat sheets can expand freely in a direction at right angles to the plane of the sheet. In order to prevent that, an additional roller or press after the HF heating would be necessary, and that is avoided by starting with the manufacture of a tube.

The invention will be explained below with reference to the appended drawing. Fig. 1 shows diagrammatically an arrangement for creating an electric field around a tube of the type discussed here;

Fig. 2 illustrates in the same way, in this case in part-sectional view of the tube, the method for manufac- turing reinforced sheets, while

Fig. 3 explains further in isometric projection how the tube is laid out after being sawn through.

In Fig. 1 the inner tube extruded in a preliminary stage is indicated by 1, around which inner tube one or more reinforcement layers 2 are wound in a subsequent preliminary stage. In a manner known per se, each rein¬ forcement layer is made of fibres which are surrounded by a thermoplastic material in strip form. The illustration given here is considered to relate to the stage at which the product leaves the extrusion head 3, in which the outer tube 4 has now also been extruded around the combination of inner tube and reinforcement.

At some distance from the extruder 3 - which distance can be greater in reality than is indicated in the drawing, in order to permit some cooling of the tube - pairs of electrodes 5,, 5 ₂ are provided. These electrodes are circular and are placed in such a way relative to the extruder head that the tube moves through them. This creates an electric field between said two electrodes,

which electric field in its effect also reaches the reinforcement layer 2, with the lines of flux lying substantially in the plane of the reinforcement. This produces a more advantageous heating of the reinforcement than if the lines of flux are at right angles to the plane of the reinforcement.

Figs. 2 and 3 show how the same method of manu¬ facture can be used for finally manufacturing reinforced flat sheeting. A saw 6, preferably a circular saw, is set up at a fixed point, which saw makes a single longitudinal cut through the full thickness of the heated tube 1, 2, 4. Thereafter, the cut tube can be laid out flat, for example using a pair of conveyor rollers 7,, 7 ₂ , the first of which will lay out flat the edges, indicated by 8, of the tube. The end result is that continuous flat sheeting is produced behind the rollers 7,, 7 ₂ , with a width which is substantially equal to the tube periphery.