[0001] The present invention relates to a joining tool for heating an, at least in some portions, inductively heatable joining member, wherein the joining tool has a joining head having a holding device for holding the joining member, wherein the joining tool has an inductive heating device having an electric coil which is arranged such that a joining member held in the holding device can be inductively heated.

[0002] The invention further relates to a method for inductively heating a joining member held in a holding device, comprising the steps: feeding the joining member into the holding device, and inductively heating the joining member by means of an inductive heating device having an electric coil.

[0003] Joining tools of the above-described type are usable, in particular, in the field of automotive engineering. In vehicle construction and, in particular, in body making, there is a need to join joining members in the form of fastening elements to components such as sheets, body sheets and/or vehicle frames. Fastening elements of this type here frequently serve as anchors for the fastening of further items, such as, for instance, electrical leads or fluid lines, interior trim parts, etc. The fastening of further items is realized, for instance, by means of plastic clips, which are clipped onto a joining element of this type in the form of a fastening element.

[0004] It is generally known to weld such joining members onto components or to join them in some other way. Joining by welding is also known as stud welding. It is further known to weld joining members thermoplastically to components which are made, for instance, of plastic.

[0005] Whilst the heating of metallic joining members for the purpose of welding is generally realized by an electric current (electric arc), for the gluing of joining members of this type and/or for the thermoplastic welding of joining members, it is also known to heat these joining members. The heating serves, for instance, to melt and/or activate an adhesive and/or to melt a thermoplastic joining portion of a joining member of this type. [0006] The heating is here realized, for instance, inductively. For this purpose, the joining member can be made of an inductively heatable metal material, to which an adhesive layer is applied. In the case of joining members which are thermoplastically welded, inductively heatable metal particles can be embedded, for instance, into a thermoplastic joining portion and ensure that the thermoplastic joining portion is melted.

[0007] In inductive heating, apart from the (desired) heat input in the joining member, a heat input also takes place in the inductive heating device, in particular in the electric coil.

[0008] It is therefore necessary to cool the coil, or at least to transport away the heat of the coil, in order thus to enable a longer working life of the inductive heating device, or to achieve a faster cadence of the individual joining operations.

[0009] It is known to achieve a cooling or a heat transport by the coil being formed of a metal tube, preferably of a copper tube, wherein the metal tube is flushed with a cooling fluid, preferably water.

[0010] This cooling is complex in design terms, since a hermetically closed cooling system, in which the cooling fluid is fed into the copper tube by means of a pump, is required. In addition, a heat exchanger or the like is necessary for the cooling of the cooling fluid. As a result, an automated production plant comprising such a joining tool or such a cooling apparatus is prone to faults and also expensive. Moreover, as a result of the active pumping of the cooling fluid and active cooling of the cooling fluid in the cooling circuit, the energy consumption is significantly increased.

[0011 ] A joining method in which a fastening element has an adhesion surface, to which a thermally meltable or curable adhesive is applied, wherein the adhesive is heatable, for instance, by an inductive heating device, is known from Document DE 10 2009 042 467 A1 .

[0012] In the light of the above, the object of the invention is to define an improved joining tool for heating an, at least in some portions, inductively heatable joining member, and an improved method for inductively heating a joining member held in a holding device. [0013] In the case of the joining tool stated in the introduction, this object is achieved by virtue of the fact that the inductive heating device has a heat exchanger tube.

[0014] The object is further achieved by a method for inductively heating a joining member held in a holding device, in particular by means of a joining tool according to the invention, comprising the steps: feeding a joining member into the holding device, and inductively heating the joining member by means of an inductive heating device having an electric coil, wherein heat is led away from the electric coil by means of a heat exchanger tube.

[0015] A heat exchanger tube is a heat exchanger, which, using heat of vaporization of a medium, enables a high heat flow density. The transport of a working medium inside the heat exchanger tube is generally realized passively, that is to say without any accessories, such as, for example, a circulating pump.

[0016] A heat exchanger tube basically contains a hermetically encapsulated volume, which in the present case is preferably formed by a tubular body. Into this tubular body is preferably filled a working medium, such as, for example, water or ammonia, which fills the volume inside the tubular body to some extent in liquid, to a greater extent in vaporous state.

[0017] The above statements and further supplementary information on the technology of heat exchanger tubes derive from the appropriate entry under

www.wikipedia.org/wiki/Heat_pipe.

[0018] Through the use of a heat exchanger tube, heat can consequently be rapidly transported away from the coil. A supplementary water cooling of the coil is preferably unnecessary. In addition, this cooling concept preferably contains no pumps or liquid heat exchangers. The working medium is fully accommodated in the closed volume of the heat exchanger tube, so that no problems are to be expected in terms of leak- tightness or similar.

[0019] In a heat exchanger tube, when a heat input takes place, for instance in the region of the coil, the working medium or fluid inside the heat exchanger tube vaporizes and thereby extracts heat from a warmer region of the heat exchanger tube. The vapour then flows to a region with lower temperature, where it condenses and gives off heat. The condensed fluid then flows due to gravity (two-phase thermosiphon) or due to capillary action (heat pipe) back to the warm region of the heat exchanger tube.

[0020] The thermal conductivity of a heat exchanger tube usually increases the higher is the temperature difference between a warm region of the heat exchanger tube and a cold region of the heat exchanger tube. Moreover, the heat transport is more efficient or faster over short distances than over long distances.

[0021] By virtue of the above-described principle, a rapid heat transport is achieved due to a high thermal conductivity.

[0022] The object is thus fully achieved.

[0023] It is of particular advantage if the coil itself is formed of a heat exchanger tube, such that a tubular body of the heat exchanger tube conducts an electric current flowing through the coil.

[0024] In this embodiment, the tubular body is preferably formed of a material having good electrical conductivity, for instance a metal, in particular copper. For the formation of the coil, in particular a commercially available heat exchanger tube can here be used.

[0025] In this embodiment, overall savings on coil material can be made, which simplifies the design of the inductive heating device.

[0026] It is of particular advantage if on the joining head is arranged a heat sink, wherein the heat sink is assigned to the heat exchanger tube.

[0027] As a result, the heat sink can absorb heat transported away from the heat exchanger tube and release it to the environment, whereby a higher temperature difference between a warm region of the heat exchanger tube (in the region of the coil) and a colder region of the heat exchanger tube (in the region of the heat sink) can be achieved. This leads to a faster heat transport and thus to a better cooling of the coil. The heat sink is preferably made of a metal material and, in particular, is electrically conducting. [0028] According to a particularly preferred embodiment, a tubular body of the heat exchanger tube is here connected to the heat sink by electrically insulating connecting elements.

[0029] Via the electrically insulating connecting elements, the heat exchanger tube can be thermally coupled with the heat sink. The electrically insulating connecting elements, which can jointly form a one-piece connecting element arrangement, are preferably configured such that they have good thermal conductivity.

[0030] One example of a material from which insulating connecting elements of this type can be produced is a ceramic material or a silicone material.

[0031] Through the use of connecting elements of this type, an electrical short circuit between the tubular body of the heat exchanger tube and the heat sink can be prevented.

[0032] In addition, it is advantageous overall if the heat exchanger tube has at least one winding forming the electric coil, as well as electrical connecting portions extending from the winding.

[0033] The electrical connecting portions preferably extend from the winding to a cooling arrangement, which can be configured, in particular, as a heat sink. The cooling arrangement is here configured, in particular, on the joining head. The coil is preferably provided in a region which surrounds a portion of the joining member that protrudes from the holding device. This region is generally distanced from the housing of the joining head. The connecting portions here preferably bridge the distance between the coil and the joining head on which the cooling arrangement is fixed.

[0034] It is here of particular advantage if the winding of the coil defines a coil plane, wherein the electrical connecting portions form with the coil plane an angle within a range from 30° to 90°, in particular within a range from 35° to 80°, preferably maximally 60°.

[0035] The cooling arrangement which is connected to the connecting portions of the heat exchanger tube can contain a heat sink and/or a fan. [0036] The heat sink can be configured with cooling fins. The heat sink can be part of a housing of the joining head.

[0037] In addition, it is advantageous if the electrical connecting portions of the coil, in a region between the coil and the joining head, are of straight configuration.

[0038] As a result, the distance between the coil at which the heat to be transported away is generated and the joining head on which the cooling arrangement is disposed can be kept short.

[0039] For the thermal conductivity of a heat exchanger tube is frequently adversely affected by bends and kinks.

[0040] It is further preferred if the coil has a maximum of two windings, and preferably precisely one winding.

[0041] The number of windings of a coil determines, inter alia, the inductivity of the coil.

[0042] As a result of a comparatively low inductivity, a comparatively small capacitor can also be used in an electrical induction circuit containing the coil and such a capacitor. Consequently, an electric current which is used for the heating can be comparatively large.

[0043] It is of particular advantage if in a tubular body of the heat exchanger tube are accommodated means for supporting the capillary action.

[0044] As a result, an orientation dependence of the thermal conductivity of the heat exchanger tube can be prevented, whereby the joining tool, for instance, can also be operated upside down with constant cooling of the coil, i.e. also at constant cadence.

[0045] Means for supporting the capillary action can be, for instance, a wire mesh inserted in the heat exchanger tube, or a fibre composite or the like. The flowing of fluid is thereby promoted, in much the same way as with a wick of a candle. [0046] It is further preferred if on the joining head is arranged a Peltier element, wherein the Peltier element is assigned to the heat exchanger tube in order to absorb heat transported away from the coil by means of the heat exchanger tube.

[0047] As a result of a Peltier element, a greater temperature difference between a warm region of the heat exchanger tube and a cold region of the heat exchanger tube can be achieved, which, as mentioned above, additionally improves the heat transport.

[0048] With a Peltier element it is possible to generate temperatures below the ambient temperature, so that, if need be, an additional cooling of the coil can be achieved. As a result, the working frequency of the joining tool, for instance, can be increased, since less time is needed between the work cycles for cooling of the coil.

[0049] In a preferred embodiment, the joining tool has a robot having a robot arm, wherein the joining head is disposed on the robot arm of the robot. As a result, an efficient and automated joining tool can be realized, as is usual, for instance, in a production for the automotive industry.

[0050] When metals are inductively heated with the aid of an inductor containing a coil and a capacitor, a very strong magnetic field is generated. A very high electric current can here flow through the coil, whereby the coil is heated rapidly. In order to dissipate this heat, the inductive heating device contains in the present case a heat exchanger tube, by means of which the generated heat can be dissipated in an optimized manner.

[0051] Of course, the above-stated features, and the features which are yet to be set out below, can be used not only in the combination respectively specified, but also in other combinations or in isolation, without departing from the scope of the present invention.

[0052] Illustrative embodiments of the invention are represented in the drawings and are explained in greater detail in the following description, wherein:

Fig. 1 shows a schematic representation of a first embodiment of the joining tool according to the invention; Fig. 2 shows a schematic top view of a heat exchanger tube of a heating device of the joining tool of Fig. 1 ;

Fig. 3 shows a schematic sectional view along a line Ill-Ill of Fig. 2;

Fig. 4 shows a schematic representation of a heat exchanger tube with connection to a heat sink and an electrical induction circuit; and

Fig. 5 shows a further embodiment of a joining tool according to the invention.

[0053] In Fig. 1 , a joining tool is represented schematically and is denoted generally by 10. The joining tool 10 has a joining head 12. The joining head 12 can be a manually operated joining head, yet in the present case is fixed to a robot arm 16 of a robot 14 of the joining tool 10 and is consequently freely movable in three dimensions by means of the robot 14.

[0054] The joining tool 10 serves to join joining members 18, having a shank 20 and a flange 22, onto workpieces 24. More precisely, the joining tool 10 serves to respectively join a joining member 18 onto the workpiece 24 by the use of inductive heating energy. The joining member 18 can be, for instance, a plastics joining member, in whose flange 22 are integrated metal particles which can be inductively heated. Alternatively, the joining member 18 can contain a shank 20 and a flange 22 made of a metallic, inductively heatable material, wherein an inductively activatable adhesive (which for reasons of clarity is not represented in Fig. 1 ) is applied to a joining surface, facing the workpiece 24, of the flange 22.

[0055] The joining head 12 contains a holding device 28, which is configured to respectively hold a joining member 18, such that the latter is oriented along a joining axis 30 along which the joining member 18 is to be joined onto the workpiece 24.

[0056] The joining tool 10 can have a joining member feed device 32, by means of which joining members 18 are fed in an automated manner to the joining head 12.

[0057] In addition, to the joining tool 10 is assigned a power supply device 34, which is configured to supply power to an inductive heating device 36 of the joining tool 10. [0058] The inductive heating device 36 contains an electrical induction circuit 38, which typically contains a capacitor and a coil. The electrical induction circuit 38 can be disposed, in particular, on the joining head 12. A coil 40 of the electrical induction circuit 38 is arranged concentrically to the joining axis 30 around the joining member 18, which is held in the holding device 28.

[0059] In the present case, the coil 40 has an individual winding, which extends over at least 300°, in particular over at least 330°.

[0060] The coil 40 can be arranged around the shank 20. As a result, either the joining member 18 as a whole, and thus an adhesive attached to the flange 22, is heated, or else a metal arrangement is inductively heated, which latter is integrated in the joining member in the region of the flange 22 insofar as the joining member is made of a thermoplastic material or, at least in the region of the flange 22, has a thermoplastic joining portion.

[0061] As is shown also in Fig. 2, the coil 40 extends around the joining member 18 and is connected at its ends to electrical connecting portions 42 extending from the coil 40 in the direction of the joining head 12, in particular in the direction of the electrical induction circuit 38.

[0062] The electrical connecting portions 42 can here preferably be of rectilinear or straight configuration and are preferably oriented at an angle 44 relative to a coil plane running perpendicular to the joining axis 30, wherein the angle 44 lies preferably within a range between 30° and 90°, in particular within the range from 45° to 80°.

[0063] In Fig. 2, ends 46a, 46b of the electrical connecting portions 42a, 42b are also shown, wherein the ends 46a, 46b of these connecting portions 42a, 42b are electrically connected to the electrical induction circuit 38, though this is not represented in detail in Figs. 1 and 2.

[0064] On the joining head 12 is fixed a heat sink 48. To the heat sink 48 can be assigned a fan 49.

[0065] In the present case, the inductive heating device 36 contains a heat exchanger tube 50, by means of which thermal energy which is generated in the region of the coil 40 during an inductive heating operation can be transported away in the direction of the joining head 12, in particular in the direction of the joining body 48.

[0066] More precisely, in the present case the coil 40 itself, as well as the thereto connected connecting portions 42, are formed by the heat exchanger tube 50. As is represented in Fig. 3, the heat exchanger tube 50 contains a tubular body 52 made of an electrically conducting material, such as, for instance, copper, as well as a working medium 54 disposed in the tubular body 52. The heat exchanger tube 50 is closed off at the ends 46a, 46b and consequently forms a closed, sealed volume for the working medium 54. The working medium 54 is present in the volume of the tubular body 52 partly in a liquid phase 56, partly in a gaseous phase 58, as is indicated schematically in Fig. 3.

[0067] Within the tubular body 52 can be arranged capillary supporting means 60, which can be formed, for instance, by a mesh or the like. In this case, the heat exchanger tube 50 can be configured, for instance, as a heat pipe.

[0068] If, in a joining operation by means of the joining tool 10, the electrical induction circuit 38 is supplied with power in order to conduct an electric current i through the coil 40 (as is shown schematically in Fig. 2), at least a portion of the joining member 18 is inductively heated. In addition, the tubular body 52 of the heat exchanger tube 50 is in this case heated in the region of the coil 40.

[0069] Consequently, the working medium 54 vaporizes in the region of the coil 40 and the vapour is transported in the direction of a cooler portion of the heat exchanger tube 50, namely into the connecting portions 42a, 42b and up to the ends 46a, 46b,

[0070] For, in the region of the ends 46a, 46b of the connecting portions 42a, 42b, the heat sink 48 is assigned to the heat exchanger tube 50, so that the tubular body 52 of the heat exchanger tube 50 is cooler in this region. As a result of this temperature difference and as a result of this cooler portion of the heat exchanger tube, the working medium in this region condenses and flows back in the direction of the coil 40.

[0071] Fig. 4 shows a schematic binding of the heat exchanger tube 50 to a heat sink 48. [0072] The tubular body 52 of the heat exchanger tube 50 is electrically conducting and the heat sink 48 is preferably likewise made of a metallic material. Consequently, electrically insulating connecting elements 62a, 62b are provided, by which the connecting portions 42a, 42b are thermally coupled with the heat sink 48. The connecting elements 62 can be made, for instance, of ceramic material, silicone or the like.

[0073] On the heat sink 48, in particular on the side situated opposite to the connecting elements 62, cooling fins 64 can be configured.

[0074] In addition, the ends 46a, 46b at which the heat exchanger tube 50 is closed off, are electrically contacted with electrical connecting leads 66a, 66b, which are connected to the electrical induction circuit 38.

[0075] Fig. 5 shows a further embodiment of a joining tool 10', which, in terms of structure and working method, generally corresponds to the joining tool 10 of Fig. 1. Same elements are therefore denoted by same reference symbols. The differences are substantially illustrated below.

[0076] Thus, in the joining tool 10' of Fig. 5, a Peltier element 70 is assigned to the electrical induction circuit 38.

[0077] A Peltier element is an electrically operated heat pump. Through the application of an electric current, the Peltier element 70 can generate a temperature difference between its opposing surfaces. The surface facing the heat exchanger tube 50 here constitutes the colder surface of the Peltier element 70, and the side lying opposite this side constitutes the warmer surface of the Peltier element 70.

[0078] In the embodiment shown in Fig. 5, the Peltier element 70 can generate on its colder surface a temperature which lies below the ambient temperature, and can thus improve the heat transport in the heat exchanger tube and, ultimately, the cooling of the coil 12.

[0079] It is also conceivable to switch on the Peltier element 70 only when required, in order to save energy. Furthermore, it is conceivable to arrange the Peltier element 70 between the heat exchanger tube 50 and a heat sink 48 in order thus to increase the efficiency of the Peltier element 70.