FR2915053A1 - METHOD AND DEVICE FOR HEATING TUBULAR OR INDUCTIONALLY FULL PIECES. - Google Patents

Abstract

The invention relates to a device (10) for heating a tubular piece (18) or a solid part, in particular for transforming or molding a tube, comprising: a tubular body (12) made of non-magnetic material, intended to contain the part (18), - an electrically conductive inner layer (16) intended to be in contact with the part (18), - inductor means (14) surrounding the tubular body (12) for generating a magnetic field, the device (10) being such that the magnetic field generated by the inductor means induces currents in the inner layer (16), thus making it possible to locate the heating in the vicinity of the inner layer / tube to be heated interface.

Description

METHOD AND DEVICE FOR HEATING TUBULAR OR FULL PARTS

INDUCTION

  The present invention relates to induction heating of tubular or solid parts, for the purpose of performing the transformation or molding of tubular parts, consisting in particular of thermoplastic or thermosetting matrix composite materials.

  Induction heating technology is widely used in the field of molding plastic parts or composites, especially because of a higher energy transfer rate than conventional heating means, and also because of the good performance, the accuracy and repeatability it provides. The heating of tubes, in particular for the purpose of molding molded composite tubes, does not currently benefit from a satisfactory induction technology. It is known for example a device comprising an aluminum tubular body surrounded by inductors, the currents induced by the latter causing the heating of the tubular body. The disadvantage of such a device is to heat the tubular body in the mass, which involves a long heating time to take into account the time required for the diffusion of heat in the aluminum body. A long heating time is very disadvantageous, because in addition to low productivity, it induces a higher energy consumption and a proportional cooling time. A device as described above also has the disadvantage of using a material offering a low efficiency induction heating The present invention aims to provide a method and a simple and inexpensive device, allowing the efficient heating of parts tubular or solid induction, in particular for the purpose of molding tubes made of composite materials. The invention starts from the observation that the most effective molding devices are those offering a surface-type heating, that is to say allowing localized heating of the molding surface of the mold. This saves energy loss due to the heating of the mold in the mass. Thus, the invention relates to a device for heating a tubular or solid part, comprising: a metal tubular body, intended to contain the workpiece, an electrically conductive inner layer, intended to be in contact with the workpiece, induction means surrounding the tubular body to generate a magnetic field, the device being such that the magnetic field generated by the inductor means induces currents in the inner layer, thus making it possible to locate the heating in the vicinity of the interface inner layer / room to be heated . Thus, thanks to the invention, the induced currents, and hence the heating, are located in the inner layer directly in the vicinity of the tube to be heated, and not in the thickness of the tubular body. A device according to the invention therefore has the advantage of locally heating the molding zone, directly in the vicinity of the molding zone / material interface, and not in the thickness of the mold body, which represents a saving in energy. important. Such a device also has the advantage of being simple and inexpensive to manufacture. In one embodiment, the inner layer is disposed on the inner surface of the tubular body. In one embodiment, the magnetic field generates induced currents at the interface between the inner layer and the tubular body. In one embodiment, the inner layer is disposed on the outer surface of an inner cylindrical core, the core being disposed coaxially within the body.

  In one embodiment, the device comprises a second inner layer disposed on the inner surface of the tubular body. In one embodiment, the tubular body comprises two elements, movable relative to each other, allowing the opening of the body.

  In one embodiment, the two elements are electrically isolated when the body is closed. In one embodiment, the inner layer comprises a magnetic compound, preferably of relative magnetic permeability and high electrical resistivity.

  In one embodiment, the tubular body comprises a non-magnetic compound, preferably of high electrical resistivity. In one embodiment, the tubular body comprises a magnetic compound, preferably of relative magnetic permeability and high electrical resistivity.

  In one embodiment, the tubular body comprises, on its outer surface facing the inductor means, a layer of a non-magnetic material, preferably of high electrical conductivity. In one embodiment, the device comprises internal pressure means for pressing the tube to be heated against the tubular body. The invention also relates to a method for manufacturing parts made of composite materials using a device as defined above. Other features and advantages of the invention will become apparent with the description given below, the latter being carried out for descriptive and non-limiting purposes with reference to the following figures in which: FIG. 1 represents a device conforming to FIG. invention, seen in section in a plane perpendicular to the axis of symmetry of the device, - Figure 2 shows the device of Figure 1, in a half sectional view in a radial plane of the device, - Figure 3 shows Particular embodiments of the device of Figure 2, - Figure 4 shows an alternative embodiment of the device of Figure 1, - Figure 5 shows another alternative embodiment of the device according to the invention. FIG. 1 represents a device 10 according to the invention, in the example intended for molding cylindrical parts made of composite material. The device, or mold 10, comprises a tubular body 12 surrounded by inductor means 14. The body 12 is made of a non-magnetic material, in the example a stainless steel. The inner surface of the body 12 is covered with an inner layer 16 of a magnetic material, for example a nickel-based alloy, or an alloy steel with elements such as nickel, chromium or titanium. This inner layer 16 constitutes a molding zone and a heating zone, being intended to be in contact with the workpiece to be heated and / or molded. When supplying the inductor means 14 with an alternating current Il of frequency F, the device is shaped in such a way that the generated magnetic field induces currents 12 in the internal magnetic layer 16. FIG. 2, which represents a half-view in section along a radial plane of the mold 10, this time adapted to the heating of a tubular piece, or tube 18, the piece of thickness e3 to be heated being disposed inside the body 12. The tube 18 is for example in a thermoplastic or thermosetting matrix composite material. Near the outer surface of the body 12 are arranged the turns 141, 142, 143, 144, 145 inductor means 14, which are traversed by internal cooling channels 15 for the circulation of a cooling fluid. To induce currents in the inner layer 16, the magnetic field generated by the inductor means must be non-zero at this layer of thickness e2. In other words, the magnetic field created by the turns of inductors, which envelops the body 12, must pass through the body 12 of thickness e. The depth of penetration of the magnetic field is defined by a quantity called skin thickness.

  The skin thickness 8 in the body 12 may be approximatively determined by the following formula: ## EQU1 ## where p is the resistivity of the non-magnetic material constituting the body 12, the relative magnetic permeability of the material and the frequency of the inducing currents. For a nonmagnetic material, we take: r = 1, and the formula becomes: 8 = 50. (p / F) 1'2. Thus, it can be seen that the skin thickness is proportional to the resistivity of the non-magnetic material constituting the body 12. The choice of a non-magnetic material with a high resistivity thus makes it possible to obtain the penetration of the desired magnetic field. In this respect, the choice of a stainless steel represents a good compromise between resistivity and mechanical strength, the body 12 having to withstand significant efforts in the case of a device dedicated to molding. However, the body 12 may be made of any non-magnetic material having a high resistivity, such as a manganese-based alloy, with alloying elements such as nickel and copper.

  Once the resistivity of the body 12 is known, the frequency F of the inductor currents II is chosen so as to obtain a skin thickness 8 which is greater than the thickness e ,. Preferably, the highest frequency will be chosen to fulfill this condition. Indeed, the Joule power induced by induction heating is proportional to, / T 'so the higher the frequency, the greater the energy injected will be large. Depending on the applications and materials constituting the device, it will be in a frequency range from 100 Hertz to a few kilohertz, which allows to obtain a skin thickness of several tens of millimeters. Thus, the magnetic field passes through the body 12 and reaches the inner layer 16, generating within it induced currents 12, also called eddy currents. The inner layer 16 is then heated by the Joule effect under the action of these induced currents 12, thus making it possible to heat and bring the tube 18 to the desired temperature in a very short time. It has thus been realized the object of the invention, which is to locate induction heating directly in the vicinity of the mold / material interface. Indeed, the induced currents are localized in the inner layer 16, whose thickness e2, less than one millimeter, is very small compared to the thickness of the mold 10, which makes it possible to obtain a surface heating. On the other hand, the body 12 being made of a non-magnetic material, it is very little heated by induction. One of the advantages of the invention is that it allows a surface heating while enjoying the advantages of a metal structure for the body 12 of the device. Indeed, the use of a metal to make the body 12 allows to obtain the mechanical resistance (to the effort and the fatigue) and the thermal properties (low expansion, good thermal conduction for cooling, etc.) that can be expected from a mold body. The use of a material transparent to the field such as ceramics would not have benefited from these qualities. In a preferred variant, the depth of penetration of the magnetic field is such that induced currents are generated in the inner layer 16, at the interface inner layer 16 / tubular body 12. Thus, the surface of the inner layer 16 in contact with the tubular part to be heated is not traversed by any induced current. In other words, the skin thickness 8 is greater than e, but strictly less than (e, + e2). The interface inner layer 16 / body 12 is therefore heated directly by induction, the heat produced then propagating towards the inner layer interface 16 / part 18 by conduction. This variant has the advantage of performing a surface heating according to the invention, while allowing the heating of electrically conductive parts (for example carbon fibers) since no current is induced at the interface between the inner layer 16 and the room to be heated. Fixing the inner layer 16 of magnetic material on the mold body can be carried out in various ways, for example by fixing a sheet or by deposition of material, for example a plasma or electrolytic deposit. The magnetic material used for this inner layer 16 is a magnetic compound that can have a Curie temperature, as well as a higher electrical resistivity than copper, such as, for example, nickel-based steel, chromium alloys and / or or titanium. A significant electrical resistivity of this inner layer 16 is an advantage because it allows more efficient induction heating. However, it should be noted that the magnetic permeability of the material constituting this layer also influences the efficiency of induction heating (see formula cited above). In a variant, the mold 10 comprises internal pressure means 20 (see FIG. 2), disposed inside the tube 18, and which make it possible to apply the external surface of the tube 18 against the body 12 during the operation. molding. These pressure means 20 are for example an expandable device of the balloon type (metal or silicone), or a cylindrical metal piece, tubular or solid, intended to expand with the increase in temperature, this expansion being sufficient to come to plaster the tube 18 against the inner surface 16. In another example, the shape memory properties of certain alloys can be used in a similar manner, so that a part made of such a material has a different shape depending on the temperature, and more particularly between the ambient temperature and the molding temperature. Such a piece may be for example a thin plate wound on itself. FIG. 3 represents a variant of the device, in which a metal core 22 is provided, on which the inner layer 16 is disposed. In this configuration, the device is such that the magnetic field generated by the inductors passes through the body 12 and the tube 18. to reach the core 22. In other words, the penetration depth of the magnetic field in the mold 10 is greater than e ,. Indeed, if the skin thickness is greater than e ,, the magnetic field directly reaches the inner layer 16, because it passes through the workpiece 18, the latter being transparent to the field because electrically non-conductive. Thus, as for the previous variants, the magnetic field induces eddy currents in the inner layer 16, which is disposed on the outer surface of the core 22. In this embodiment, the heating of the tube 18 is therefore effected by the intermediate of its internal surface. In the example, the core 22 comprises a magnetic material similar to that which composes the inner layer 16, these two elements then forming a single piece in one piece, which simplifies the manufacture of the core 22. In a variant not shown, the device 10 comprises two internal magnetic layers, the first being disposed on the inner surface of the body 12, as shown in Figure 2, the second being disposed on the outer surface of a core, as shown in Figure 3. In this configuration, the inductive means are implemented in such a way that the depth of penetration of the magnetic field is greater than (e, + e2), which allows the magnetic field to pass through the tubular body 12, the first inner layer, the part to be heated ( transparent to the field), to reach the second inner layer and thus generate in the latter induced currents. This configuration makes it possible to obtain double heating of the tubular part simultaneously on its internal and external surfaces. In practice, the tubular body 12 is opening, so as to allow the ejection of the finished part. In such a case, one can realize the tubular body 12 in two elements 121, 122 (Figure 4 or 5), forming half tubes movable relative to each other. The body 12 is further provided with cooling channels, arranged in the thickness of the body 12, and direction parallel to the axis of symmetry of the latter. These channels make it possible to circulate a cooling fluid to cool the part after transformation. In a variant shown in FIG. 4, the tubular body comprises, on one of its diametrical planes, an electrical cutoff separating said body into two electrically insulated elements 121 and 122, for example by means of a layer 123 of an insulating material. . In this configuration, the electrical insulation between the two elements 121 and 122 acts as an air gap in which the magnetic field generated by the inductors circulates. Thus, the magnetic field envelops each of the elements 121 and 122, thereby generating induced currents 13 and 14 on the inner and outer surfaces of these two elements. The advantage of such a configuration is that it makes it possible to overcome the influence of the depth of penetration of the magnetic field in the elements 121 and 122. In fact, whatever the thickness of skin in these elements the induced currents circulate on the inner surface of these two elements. In the case where the mold 10 is provided with a magnetic inner layer 16 located in the tubular body 12, it ensures that it is traversed by the induced currents. In the case where the mold 10 is provided with an inner core comprising the inner layer 16, it is subjected directly to the action of the magnetic field circulating in the mold 10, and, in addition, the currents flowing on the inner surface of the elements 121 and 122 also induce currents on the surface of the core 22, in the inner layer 16. As previously described, the advantage of the configuration of FIG. 4 is that it makes it possible to overcome the electromagnetic skin thickness, and therefore to have a greater freedom of choice as regards the frequency of the currents which feed the inductor means 14.

  However, when all the parameters are set, it is known that increasing the frequency of the electromagnetic field will improve the efficiency of the heating. This will further decrease the skin thickness. Therefore, it becomes advantageous to provide a body 12 made of magnetic material, for example a material similar to that which composes the layer 16, and to equip each half-body with an outer layer 124, 125 of non-magnetic material, for example copper. The nonmagnetic layers 124 and 125 are, like the elements 121 and 122 electrically isolated, in the example by means of the same insulating layer 123. Such a device will ideally be implemented with high frequencies, for example between 10 and 100 kHz . Indeed, the frequency of the electromagnetic field will be chosen so that the skin thickness is less than the thickness of the outer non-magnetic layer, which will be greater than one millimeter. Thus, the electromagnetic field does not penetrate to the outer surface of the body 12, because the nonmagnetic outer layer 124, 125 constitutes an electromagnetic shielding layer. In addition, the latter being non-magnetic and low electrical resistivity, it is very little heated by induction. On the other hand, as described above, as currents 13, 14 flow on the internal surface of the body 12, the latter being made of magnetic material, it will be very reactive to induction heating and thus heat a lot, while generating at its In this way, a double heating of the tubular piece 18 is achieved simultaneously on its inner and outer surfaces. To avoid any energy loss in the tubular body, there is also provided a non-magnetic shielding layer between each element 121, 122 and the insulating layer 123. In a variant shown in Figure 5, there is also provided a body of magnetic material in two parts. electrically insulated 121 and 122 and provided with a non-magnetic shielding layer 124 and 125, but without inner core. The operating principle is identical to the case of FIG. 4, with the difference that the inner magnetic layer 16 is on the inner surface of the elements 121 and 122. Thus, the layer 16 is traversed directly by the induced currents 13 and 14, and is therefore heated locally. In practice, the inner layer 16 is merged with the inner surface of the tubular body, since it is made of the same magnetic material. To avoid any loss of energy, the layers 124 and 125 also separate the elements 121 and 122 from the insulating layer 123, as shown in FIG. 5. Depending on the case, these two shielding layers may be extended to the level of the inner layer 16. Such a device can particularly be applied to heating and molding of solid parts. The device according to the invention is particularly suitable for molding tubes or cylinders made of composite materials, for example thermoplastic or thermosetting matrix. It makes it possible to considerably reduce the cycle times necessary for the transformation of a part. Indeed, the energy of the inductors is injected directly into the inner layer 16. The very thin thickness of the latter makes it possible to quickly bring its surface into contact with the part to be heated / transformed to the desired temperature, since no heating not the device 10 in the mass. To heat the room to a given temperature, therefore less energy is injected, and in a shorter time, than in a conventional device. As a result, the cooling time required is also reduced because there is less thermal energy to dissipate, which provides additional gain on the cycle time. Finally, the least energy required represents a source of economy, due to the lower capacity required for the induction generator. Conventionally, mechanical means (not shown) for ejecting the manufactured part are also provided. In order to simplify the positioning and removal of the parts, opening inductors are also provided, made for example in two integral parts respectively of each half-body 121, 122 of the body 12, these parts being in electrical contact when the body 12 is closed. The manufacturing method is implemented in the following manner: positioning of the material (s) of the tubular part inside the tubular body, heating of the molding zone, and pressurization by means of internal pressure means during a given time, implementation of the cooling of the mold bodies, in order to cool the workpiece, ejection / removal of the workpiece.

Claims (13)

  1. Device (10) for heating a tubular piece (18) or solid, comprising: - a tubular body (12) for containing metal part (18), - an inner layer (16) electrically conductive, to be in contact with the piece (18), - inductor means (14) surrounding the tubular body (12) for generating a magnetic field, the device (10) being such that the magnetic field generated by the inductor means induces currents directly in the inner layer (16), thus making it possible to locate the heating in the vicinity of the interface inner layer / tube to be heated.   2. Device according to claim 1, wherein the inner layer (16) is disposed on the inner surface of the tubular body (12).   3. Device according to claim 2, wherein the magnetic field generates induced currents at the interface between the inner layer (16) and the tubular body (12).   4. Device according to claim 1, wherein the inner layer (16) is disposed on the outer surface of an inner cylindrical core (22), the core being disposed coaxially inside the body (12).   5. Device according to claim 4, comprising a second inner layer disposed on the inner surface of the tubular body (12).   6. Device according to one of claims 1 to 5, wherein the tubular body (12) comprises two elements (121, 122), movable relative to each other, allowing the opening of the body (12). .   7. Device according to claim 6, wherein the two elements (121, 122) are electrically insulated when the body (12) is closed.   8. Device according to one of claims 1 to 7, wherein the inner layer (16) comprises a magnetic compound, preferably of relative magnetic permeability and high electrical resistivity.   9. Device according to one of claims 1 to 8, wherein the tubular body (12) comprises a non-magnetic compound, preferably of high electrical resistivity.   10. Device according to one of claims 1 to 8, wherein the tubular body (12) comprises a magnetic compound, preferably of relative magnetic permeability and high electrical resistivity.   11. Device according to claim 10, wherein the tubular body comprises, on its outer surface facing the inductor means, a layer (124, 125) of a non-magnetic material, preferably of high electrical conductivity.   12. Device according to one of claims 1 to 11, comprising internal pressure means, for pressing the tube to be heated against the tubular body.   13. A method of manufacturing tubes, using a device according to one of claims 1 to 12.