The present invention refers to an improved group and a method for depositing an extruded thermoplastic coating on welded pipes in rotation and forward movement . At present arranging a layer of extruded thermoplastic coating on the outer surface of welded pipes, intended for various uses, is considered a normal procedure in view of the protection that such an extruded thermoplastic coating gives to the aforementioned welded pipes against corrosive agents and against possible collisions that can occur during transportation and application. Of course, the process for depositing the coating is influenced by different factors such as the type and size of the pipe to be treated.

Furthermore, such protective coating of pipes is regulated by standards, such as standard DIN 30670, which prescribes a minimum coating thickness required based on the diameter of the pipe to be treated. In general, the process for applying a coating in a standard installation can be summarised with the following points : - preparation of the surface of the pipes,- heating of the pipes; application of the first epoxy layer,- application of the second layer of thermoplastic resin (adhesive) ; - application of the third layer of thermoplastic resin (polyolefin) ; cooling; brushing of the heads; checking the quality and continuity of the coating; and

Packaging/storage of the coated pipe. As stated earlier, standard DIN 30670 requires that there be a minimum coating thickness based on the diameter of the pipe to be treated.

During the various process steps shown above for depositing the coating of pipes with longitudinal and/or helical welding, the thermoplastic resin coating (polyolefin) , immediately after application, undergoes an actual squashing at the weld.

Such a phenomenon of squashing, and thus reduction in thickness of the coating, occurs due to the weight of the pipe that weighs down heavily on the "step" due to the thickening formed by the welding material. In this condition the great pressure generated reduces the thickness of coating not yet set. In order to avoid this drawback, and to respect the aforementioned standard DIN 30670, the usual techniques foresee applying a coating layer of thermoplastic resin (polyolefin) with increased thickness over the entire circumference of the pipe, so as to be able to make up for the reduction of the layer in the welded area and thus be able to meet the standard.

It is clear how the solution proposed by the prior art is extremely disadvantageous in view of the excessive coating material deposited on the pipes not at the welding lines, where there is no squashing, with a consequent increase in costs.

Indeed, due to the above it is necessary to use a large amount of material also in areas where not strictly needed.

The purpose of the present invention is to make a device capable of solving the aforementioned drawbacks of the prior art in an extremely simple, cost-effective and particularly functional manner.

Another purpose is to make an improved group and a method for depositing an extruded thermoplastic coating on welded pipes in rotation and forward movement capable of administering a controlled amount of material on each area of the pipe.

Yet another purpose is to be able to have an improved group and a method for depositing an extruded thermoplastic coating on welded pipes in rotation and forward movement capable of providing a surplus of extruded product only at the welding seam, so as to have a correct distribution of the material that is still fluid during squashing and, after cooling, to respect the minimum values foreseen by the standard in force . These purposes according to the present invention are accomplished by making an improved group and a method for depositing an extruded thermoplastic coating on welded pipes in rotation and forward movement as outlined in claims 1 and 9, respectively. Further characteristics of the invention are outlined by the dependent claims .

The characteristics and advantages of an improved group and of a method for depositing an extruded thermoplastic coating on welded pipes in rotation and forward movement according to the present invention shall become clearer from the following description, given as an example and not for limiting purposes, referring to the attached schematic drawings, in which: figure 1 is an elevation view partially in section of an extruder of an extruded thermoplastic coating equipped with an improved group for depositing the aforementioned extruded thermoplastic coating on welded pipes in rotation and forward movement according to the present invention; figure 2 shows an enlarged detail of the extruder of figure 1; figure 3 is a section view of a first embodiment of the improved group for depositing the aforementioned extruded thermoplastic coating on welded pipes in rotation and forward movement according to the present invention in a use step; figure 4 is a section view of an improved group of figure 3 in a different use step; figure 5 is a section view of a welded pipe coated with thermoplastic material extruded through an improved group for depositing the aforementioned extruded thermoplastic coating on welded pipes in rotation and forward movement according to the present invention; figure 6 shows an enlarged detail of a welded pipe of figure 5; and figure 7 is an elevation view partially in section of another embodiment of an improved group for depositing the aforementioned thermoplastic coating.

With reference to the figures, an improved group for depositing an extruded thermoplastic coating on welded pipes in rotation and forward movement according to the present invention is shown with 10.

Such a group 10 for depositing an extruded thermoplastic coating on welded pipes 20 in rotation and forward movement, as can be seen in figure 1, is of the type able to be associated with an extrusion unit 100. As shown for example in figures 2 and 7, the deposit group 10 comprises a main pipe fitting 11 provided with a first end 12 for connecting to the aforementioned extruder 100, from which the extruded thermoplastic coating is fed, and with a second end 13 associated with an extruder head 15 that is positioned facing the outer surface of the welded pipes in rotation and forward movement for depositing extruded thermoplastic coating, coming from the extruder and passing in the main pipe fitting 11, on the pipes themselves. In particular, according to the invention the deposit group 10 comprises means for cyclic insertion of the extruded thermoplastic coating in the main pipe fitting 11 between the first end 12 and the extruder head 15. According to a first embodiment shown in figures 1-4, according to the invention it is foreseen for the deposit group 10 to also comprise means for removing part of the extruded thermoplastic coating passing in the main pipe fitting 11 and for the aforementioned means for cyclic insertion in the main pipe fitting 11 to be for cyclic reinsertion into the main pipe fitting 11 of the same part of the extruded thermoplastic coating removed previously.

According to such an embodiment shown for example in figure 2, such means of selective removal and cyclic reinsertion of part of the extruded thermoplastic coating passing in the main pipe fitting Ii comprise a secondary pipe fitting 14, which is provided with a first end 16 that intercepts the main pipe fitting 11 upstream of the extruder head 15 and a second end 17 associated with a piston device 18.

In particular, such a piston 18 is cyclically moveable between a first position of maximum excursion and a second position of minimum excursion inside the secondary pipe fitting 14.

This piston 18, as shown in figure 3, during the stroke from the position of maximum excursion to that of minimum excursion, by creating a "syringe-type" decompression effect, extracts part of the extruded thermoplastic coating passing in the main pipe fitting 11 conveying it to the secondary pipe fitting 14. In particular, as schematically indicated in figure 5, such a stroke from the position of maximum excursion to that of minimum excursion of the piston 18 takes place for a time period equal to that in which the welding line is not located at the extruder head 15. The angle of rotation of the pipe 20 in which the piston 18 carries out the aforementioned depression action is indicated in figure 5 with α.

Throughout such an angular sector α of rotating forward movement of the pipe 20 the piston 18 thus creates a depression in the secondary pipe fitting 14 in which part of the extruded thermoplastic coating proceeds to collect passing in the main pipe fitting 11. Since the stroke of the piston 18 in the angular sector α takes place at a constant speed removing a constant part of extruded thermoplastic coating passing in the main pipe fitting 11 per unit time, it follows that, with a constant flow rate introduced in the group 10 by the extruder 100, in the aforementioned angular sector α the extruded thermoplastic coating deposited has a constant thickness "S", indicated in figure 6, which corresponds to the minimum thickness foreseen by law for the diameter of the pipe 20. In the remaining angular sector β of the pipe 20, shown enlarged in figure 6 or rather in the area at the welding line 22, the piston 18 as shown in figure 4, inverts its own motion from the position of minimum excursion to that of maximum excursion pushing the part of extruded thermoplastic coating previously conveyed in the secondary pipe fitting 14 in the main pipe fitting 11.

Consequently, with a constant flow rate introduced into the group 10 by the extruder 100, in the angular sector β of the pipe 20 at the welding line 22 the extruded thermoplastic coating deposited has a thickness "S'" that is greater than that "S" of the angular sector a. As shown in figure 6, advantageously, the minimum coating thickness "S" foreseen by law is thus ensured in the entire circumference of the pipe 20 and, moreover, at the welding line 22 such a coating has a thickness "S'" such as to ensure a correct distribution of material that is still fluid during squashing and, after cooling, to respect the minimum values foreseen by the standards in force.

Preferably, the piston device 18 comprises an oil- dynamic cylinder 21, connected to an oil-dynamic control unit, and a relative stem 21' . According to another embodiment, shown in figure 7, the means for cyclic insertion of the extruded thermoplastic coating in the main pipe fitting 11 between the first end 12 for connecting with the extruder 100 and the extruder head 15 comprise a secondary pipe fitting 14 provided with a first end 16 that intercepts the main pipe fitting 11 upstream of the extruder head 15 and a second end 17 associated with a device 101 for melting and injecting extruded thermoplastic coating.

By the terms used above of "device 101 for melting and injecting extruded thermoplastic coating" we mean a secondary extruder as such but also another device capable of carrying out the melting and the controlled injection of extruded thermoplastic coating in the secondary pipe fitting 14.

According to such an embodiment, therefore, the extruded thermoplastic coating that is introduced cyclically in the main pipe fitting 11 between the first end 12 for connecting with the extruder 100 and the extruder head 15 does not represent part of extruded thermoplastic coating removed previously from the main pipe fitting 11 itself, but is cyclically introduced in the secondary pipe fitting 14 by the device for melting and injecting 101.

In order to ensure the aforementioned cyclic nature it is foreseen for there to be means 17' for controlling the extruded thermoplastic coating passing from the device 101 for melting and injecting to the secondary pipe fitting 14 such as a collection tank 17' associated with the second end 17 of secondary pipe fitting 14 cyclically openable in connection with the secondary pipe fitting 14. Preferably, it is possible to foresee thrust means of the extruded thermoplastic coating associated with the collection tank 17' and operating in the secondary pipe fitting 14 suitable for quickly conveying said coating, as fast as necessary, from the device 101 for melting and injecting to the main pipe fitting 11. For example, a small controlled-movement injection press can be foreseen as thrust means .

The operation of this embodiment is totally similar to that described earlier.

With reference to figure 5, the period in which the collection tank 17' does not inject thermoplastic coating in the secondary pipe fitting 14 corresponds to the period in which the welding line is not located at the extruder head 15.

The angle of rotation of the pipe 20 in which the secondary pipe fitting 14 does not introduce thermoplastic coating in the main pipe fitting is indicated in figure 5 with α.

Throughout such an angular sector a. of rotating forward movement of the pipe 20 thermoplastic coating collects in the collection tank 17' not passing through the main pipe fitting 11.

With a constant flow rate introduced in the group 10 by the extruder 100, in the aforementioned angular sector α the extruded thermoplastic coating deposited has a constant thickness "S", indicated in figure 6, which corresponds to the minimum thickness foreseen by law for the diameter of the pipe 20.

In the remaining angular sector β of the pipe 20, shown enlarged in figure 6 or rather in the area at the welding line 22 the thermoplastic coating collected in the collection tank 17' passes through the secondary pipe fitting 14 and then through the main pipe fitting 11 increasing the flow rate of thermoplastic coating that is deposited on the pipe 20.

Consequently, with a constant flow rate introduced in the group 10 by the extruder 100, in the angular sector β of the pipe 20 at the welding line 22 the extruded thermoplastic coating deposited has a thickness "S'" that is greater than that "S" of the angular sector a. As shown in figure 6, also through the embodiment of figure 7 advantageously the minimum coating thickness "S" foreseen by law is ensured in the entire circumference of the pipe 20 and, moreover, at the welding line 22 such a coating has a thickness "S'" such as to ensure a correct distribution of the material that is still fluid during squashing and, after cooling, to respect the minimum values foreseen by the standard in force.

In order to ensure the synchrony of the cyclic movements of the means for introducing the coating in the main pipe fitting 11, the deposit group 10 can comprise optical sensors capable of identifying the welding line 22 during the rotation of the welded pipes

20.

In particular, such sensors are connected to the control unit that is able to control the aforementioned means for cyclic insertion of the extruded thermoplastic coating in the main pipe fitting 11.

According to an embodiment such optical sensors for identifying the welding line 22 of the welded pipes 20 can comprise adjustable-position laser sensors. Moreover, in order to ensure correct operation of the group 10 in all conditions it is possible to foresee for there to be an electrical panel for monitoring the means for cyclic insertion of part of the extruded thermoplastic coating in the main pipe fitting 11, means for regulating the speed and flow rate of the extruded thermoplastic coating passing in the main pipe fitting 11, as well as means for regulating the speed and flow rate of the extruded thermoplastic coating passing in the secondary pipe fitting 14. It is absolutely easy to understand how the device object of the invention operates. Indeed, the group 10 for depositing a extruded thermoplastic coating on welded pipes 20 as described earlier performs the following process steps: a) feeding the extruded thermoplastic coating from an extruder 100 to the main pipe fitting 11, with a constant flow rate; b) introducing extruded thermoplastic coating in the main pipe fitting 11 through the secondary pipe fitting 14, such a step being carried out with continuity when the sensors detect that the welding line is close to the extruder head 15 during the rotation and forward movement of the welded pipes 20.

In the case of the embodiments of figures 1-4, the method described above also comprises the step of removing part of the extruded thermoplastic coating passing in the main pipe fitting 11, with constant flow rate, and conveying it in the secondary pipe fitting 14 arranged between the first end 12 for connecting to the extruder 100 and the extruder head 15.

Such a step is carried out with continuity when the sensors detect that the welding line is far away from the extruder head 15 during the rotation and forward movement of the welded pipes 20 and it precedes the step of introducing the extruded thermoplastic coating removed previously from the secondary pipe fitting 14 in the main pipe fitting 11.

Of course, it is possible to foresee the preliminary step of adjusting the speed and flow rate of the extruded thermoplastic coating passing in the main pipe fitting 11 and/or in the secondary pipe fitting 14 as needed.

It has thus been seen that an improved group and a method for depositing an extruded thermoplastic coating on welded pipes in rotation and forward movement according to the present invention achieves the purposes outlined previously.

Indeed, the improved group and the method for depositing an extruded thermoplastic coating on welded pipes in rotation and forward movement of the present invention is able to administer a controlled amount of material on every area of the pipe as well as to provide a surplus of extruded product only at the welding seam, so as to have a correct distribution of material that is still fluid during squashing and, after cooling, to respect the minimum values foreseen by the standard in force.

The improved group and the method for depositing an extruded thermoplastic coating on welded pipes in rotation and forward movement of the present invention thus conceived can undergo numerous modifications and variants, all of which are covered by the same inventive concept; moreover, all of the details can be replaced by technically equivalent elements . In practice, the materials used, as well as their sizes, can be whatever according to the technical requirements .