NL9301364A - Method for manufacturing reinforced plastic pipe and sheet. - Google Patents

Description

Short designation: Method for manufacturing reinforced plastic pipe and sheet.

The invention relates to a method for connecting parts of different material by means of the supply of heat, wherein at least one material is thermoplastic plastic.

Heat can be supplied in various ways, for example by infrared radiation or by placing it in a heated environment, in particular an oven. In this type of heating, the parts are generally heated as a whole, which can lead to deformation. Induction heating also exists. In order for heat to develop by induction of frequencies in the kHz region of Foucault currents, conductive material must be present, generally metal. Insofar as metal is not - accidentally - one of the parts to be joined, it is usual to arrange metal in or between the materials to be joined especially for that purpose. It can be powder, or a solid or perforated metal sheet. Examples of this technique are found in JP 62.148243 and EU-B1-26 191. As a rule, that metal remains between the joined parts, which may be undesirable under certain circumstances. The inductive heating is not suitable for this. Inductive heating may also not be applicable for other reasons.

The object of the invention is to provide a solution to this problem.

To that end, the method according to the invention is characterized in that the thermoplastic plastic material chosen is that which is almost indifferent to high-frequency electric fields, that the second material is a non-metal with a relatively high dielectric loss factor, that the Laying parts are subjected to treatment with an electric field with a frequency in the radio wave (RF) or microwave (MG) range for a time sufficient to allow, as a result of the dielectric heating that occurs, the second material, to heat the thermoplastic plastic in its areas adjacent to the second material to the welding temperature, so that the two parts are welded together.

The invention is particularly applicable for the manufacture of a plastic tube provided with a wound reinforcement, wherein one or more layers of reinforcement are wound around an extruded inner tube of thermoplastic plastic and an outer jacket is extruded around it in such a way that the reinforcement over at least a part of the pipe length is connected to both the inner pipe and the outer jacket.

Such methods are known for the manufacture of different types of reinforced pipes; the reinforced pipes are intended to contain a medium under pressure.

Applicant International Patent Application PCT / NL / 91/00120 relates to a house type in which the reinforcement is welded only locally to the inner tube and the outer jacket and is loose between those locations, thereby obtaining good windability of the tube.

EU-0 111 169 relates to a type in which the reinforcement is connected to the inner tube and the outer jacket. In that case, a metal reinforcement is used which is wound around the inner tube in such a way that it is welded directly upon laying, while the connection between the reinforcement and the outer jacket is made with a cold or heat-setting adhesive.

If one does not want to use metal reinforcement layers or glue, but wants to melt the reinforcement material directly on the inner tube and the outer jacket, a heating must be applied after extruding the outer jacket. Infrared heating or a convection oven can be selected for this. In both cases, the plastic material is heated through and through, and in practice, as already stated, this inevitably gives rise to deformation of the pipe construction, thus to a decrease in quality.

The object of the invention is to overcome this problem and to propose a method for establishing a welded connection between the reinforcement material and the plastic with which a high-quality end product is obtained. Furthermore, the invention aims at achieving this object in a simple manner, using as little energy as possible.

To this end, the method according to the invention has the feature described in claim 2.

In this way, only the reinforcement and the immediately surrounding plastic are brought to welding temperature, while due to the poor heat conduction of plastics in places further away from the reinforcement, little temperature rise occurs. As a result, a good welding effect takes place, while the shape of the tube is retained.

Various industrial values are reserved internationally for industrial use, for purposes such as the heating involved. These lie partly in the region of radio waves (RF below 300 MHz), partly in the region of the microwaves (MG above 300 MHz). Regarding the determination of those frequencies, see 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 defined for use in industrial, scientific and medical applications - in the area of radio waves, 13, 27 and 41 MHz are defined. In addition, other frequencies are permitted in some countries, with or without the addition of requirements related to possible radio interference.

For the application considered here, the values 27 and 41 MHz are preferred, because a heating occurs which requires a relatively low field strength, while the penetration depth of the radiation and the cost price is less favorable than that of equipment. for MG heating.

It is possible to design the RF electrodes in such a way. to place the electromagnetic field perpendicular to the plane of the reinforcement. However, it has been found that heating takes place better, i.e. with less consumption of electromagnetic power, when the field is generated in the plane of the reinforcement. This is achieved in the preferred embodiment characterized in that the composite 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 widely for different materials, even by a factor of 108 differences. In order to obtain a practically efficient heating time, which is moreover adapted to the running speed of the tube to the operation of the extruder, the reinforcement material embedded in plastic preferably has a dielectric loss factor of at least 0.2.

A material which is useful in practice with such a dielectric loss factor is aramid. Aramid material has some excellent properties to act as reinforcement; the dielectric loss factor for PE with 30% by volume aramid is 0.3.

The cost of aramid, on the other hand, is relatively high. The cost per kilogram of glass fibers is only about one-tenth that of aramid, while in many cases those glass fibers are certainly acceptable as reinforcement material for the type of reinforced pipe eligible here. Glass itself has an insufficient dielectric loss factor. However, the inventive idea can then still be realized when use is made of glass fiber reinforcement with a metallization layer.

In connection with the above-mentioned experience with regard to the most favorable direction of the electric field relative to the plane of the reinforcement, the inventive idea also leads to a new method for manufacturing flat, internally reinforced plates of thermoplastic plastic material. If one wishes to manufacture such plates using electric heating, it is only possible that the flat semi-product is irradiated perpendicular to the plane. However, when proceeding in the manner indicated in the feature of claim 8, one can take advantage of the placement of the electrodes about the tube, thereby generating the electric field in the plane of the reinforcement.

An additional advantage of initial tubular fabrication is that the inner tube and outer jacket between which the reinforcement is located provide pressure build-up in the area of the reinforcement by thermal expansion of the reinforcement and plastic in the boundary area. This automatically creates a certain welding pressure, which is necessary to achieve a good welding quality, and which is less easy to achieve when a reinforcement between two flat plates can expand freely in a direction perpendicular to the plane of the plate. To counteract that, an additional roller or press after the HF heating would be necessary, and this is avoided by starting with the manufacture of a tube.

The invention will be explained below with reference to the appended drawing.

Fig. 1 schematically shows an arrangement for generating an electric field about a tube of the type discussed here;

Fig. 2 similarly, in this case partly in view and partly in section through the tube, indicates the method for the production of reinforced plates, wherein

Fig. 3 illustrates in isometric projection how the pipe is explained after sawing.

In Fig. 1, 1 is the inner tube extruded at an earlier stage and why, in a subsequent earlier stage, one or more reinforcement layers 2 are wound. In a manner known per se, each reinforcement layer consists of fibers surrounded by a thermoplastic plastic in hand form. The present representation is thought 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.

Pairs of electrodes 51 # 52 are placed some distance from the extruder 3 - which may in reality be greater than that shown in the diagram to allow some cooling of the tube. These are circular and they are positioned relative to the extruder head so that the tube travels through it. This creates an electric field between those two electrodes, which in operation also reaches the reinforcement layer 2, with the field lines lying substantially in the plane of the reinforcement. This provides a more favorable heating of the reinforcement than when the field lines are perpendicular to the plane of the reinforcement.

Fig. 2 and 3 show how the same manufacturing method can be used for the final fabrication of reinforced flat sheet. In a fixed position, a saw 6, preferably a circular saw, is arranged which makes a single longitudinal cut through the entire thickness of the tube 1, 2, 4 produced. Then the cut tube can be laid out flat, for example using a pair of conveyor rollers 7lr 72, the former of which will flatly explain the sides of the tube indicated by 8. The end result is that vlakkeλ, 72 continuous flat sheet is produced behind the rollers, with a width substantially equal to the pipe circumference.

Claims (10)

Method for connecting parts of different material by the application of heat, wherein at least one material is thermoplastic plastic, characterized in that the thermoplastic plastic material is chosen which is almost indifferent for high-frequency electric fields that the second material is a non-metal with a relatively high dielectric loss factor, that the abutting parts are subjected to an electric field treatment with a frequency in the radio wave (RF) or microwave (MG) - area for a time sufficient to heat the thermoplastic plastic in its areas adjacent to the second material, due to the dielectric heating of the second material, so that the two parts are welded together. A method according to claim 1 for producing a coiled plastic tube, in which one or more layers of reinforcement are wound around an extruded inner tube of thermoplastic plastic and an outer jacket is extruded around it, in such a way that the reinforcement over at least a part of the pipe length is connected both to the inner tube and to the outer jacket, characterized in that the reinforcement material is chosen with a relatively high dielectric loss factor, that the reinforcement layers are wound on the inner tube at ambient temperature, that the outer jacket is extruded in direct contact with the outermost reinforcement layer, but not connected to it, and that the whole is subjected to treatment with the RF or MG electric field. Method according to claim 2, characterized in that the composite tube is passed through at least one pair of circular RF electrodes surrounding the tube. Method according to any one of claims 1-3, characterized in that the reinforcement material has a dielectric loss factor of at least 0.2. Method according to any one of claims 1-4, characterized in that the reinforcement material is an aramid material. A method according to any one of claims 1-4, characterized in that the reinforcement material is formed by metallized glass fibers. A method according to any one of claims 1-6, characterized in that the frequency of the electric field is in the RF region, with a preference for 27 and 41 MHz. A method for manufacturing flat, internally reinforced plates of thermoplastic plastic material, characterized in that a tubular product is manufactured in the manner according to any one of claims 2 to 7, wherein after the treatment step with the RF field The obtained tube is cut longitudinally in one place and then laid out flat. Reinforced pipe obtained by the method according to any one of claims 2-7. Reinforced plastic sheet obtained by the method according to claim 8.