Abstract:
A method for covering a vehicle interior part with leather, particularly a vehicle steering wheel, includes the following steps: A leather piece being cut to size and having a front side, a rear side, and edges is provided with a thermoplastic layer on the rear side. The leather piece is fastened on the vehicle interior part by a high frequency welding process in a region of the thermoplastic layer.

Description:
TECHNICAL FIELD 
     The invention relates to a method for covering a vehicle interior part with leather, in particular a vehicle steering wheel, and a corresponding vehicle interior part. 
     BACKGROUND OF THE INVENTION 
     Vehicle interior parts such as vehicle steering wheels, handles of handbrake levers or gearshift knobs are covered with leather in higher quality fitting variants. A piece of leather which is cut to size is wrapped around the vehicle interior part, and the opposed ends, which abut each other after wrapping, are sewn together. Until now, this sewing cannot be carried out by machine and must therefore still take place by laborious manual work. Seamstresses require 50 to 70 minutes for one vehicle steering wheel. 
     It is an object of the invention to indicate a more favourably priced method for covering a vehicle interior part with leather, in particular a leather steering wheel. In addition, a favourably priced vehicle interior part is also to be provided. 
     BRIEF SUMMARY OF THE INVENTION 
     A method for covering a vehicle interior part with leather, particularly a vehicle steering wheel, includes the following steps: A leather piece being cut to size and having a front side, a rear side, and edges is provided with a thermoplastic layer on the rear side. The leather piece is fastened on the vehicle interior part by a high frequency welding process in a region of the thermoplastic layer. 
     The leather itself is not, by nature, able to be welded, so that it first has to be provided with a thermoplastic layer on the rear side. Of course, the connection between this layer and the leather piece must be permanent, in order to prevent the leather piece from detaching. A weld connection between two thermoplastic parts is very reliable nowadays, so that there is also no risk of detachment. 
     According to the preferred embodiment, the leather piece is only welded onto the vehicle interior part in the region of the edges. The leather piece is preferably stretched taut over the vehicle interior part, so that no formation of bubbles occurs. 
     Before the welding, the leather piece is of course to be held on the vehicle interior part by means of clips or other fixing means, so that the position during welding is able to be clearly predetermined. 
     The vehicle interior part is to have the leather piece wrapped around it, so that opposed edges of the leather piece lie in abutment against each other, with the leather piece then being welded on both edges. 
     In this connection, it is entirely possible to provide a groove in the vehicle interior part in the region of the adjacent edges, into which groove the edges are inserted before they are welded. This embodiment prevents the edges of the leather from being visible and from becoming rough in the course of time. 
     The thermoplastic layer is preferably only applied onto the leather piece in the region of the edges. Between the thermoplastic layers in the region of the edges the leather piece has no coating on its rear side. 
     The application of the thermoplastic layer can preferably take place by means of laminating. 
     The invention allows the welding process and the relative movement of the vehicle interior part and the welding apparatus to be carried out fully automatically, for example by means of a multiple-axle, freely programmable device. The expensive manual work can thereby be dispensed with entirely. Here, of course, either the vehicle interior part or the welding apparatus can be moved, or both. In a steering wheel, at least one direction of movement is able to be realized relatively simply, namely the peripheral direction. Here, the steering wheel only has to be held in the hub and rotated. 
     On the vehicle interior part, a surface is present which permits high frequency welding. This can be achieved for example in that a second thermoplastic layer is likewise applied onto the vehicle interior part in the region of the thermoplastic layer of the leather. 
     This application of the thermoplastic layer can be carried out only partially, if necessary, in order to save costs. 
     In addition, the hardness of the thermoplastic layer is in part undesired because of the feel, so that the thermoplastic layer only occupies a small portion of the outer surface of the vehicle interior part. 
     Alternatively, the vehicle interior part itself can of course consist at least partially of a thermoplastic material. 
     The feel is also very important in vehicle interior parts. There are therefore several vehicle interior parts which consist of a hard inner layer and a soft, e.g. foamed, outer layer. The leather piece could not be welded onto this foamed outer layer. One embodiment therefore proposes allowing the thermoplastic inner material which is enclosed by a non-thermoplastic outer material, to extend in the weld seam region towards the outer side. In a leather steering wheel, this region would be the abutment region of the leather edges. 
     Furthermore, the invention also provides a vehicle interior part, in particular a vehicle steering wheel, which is provided with a leather covering which is fastened to the vehicle interior part by means of high frequency welding. On the rear side, the leather covering has a thermoplastic layer which is welded on. 
     In a radial cross section of the vehicle steering wheel, the leather covering extends around the entire circumference of a steering wheel rim. 
     The thermoplastic layer is preferably an adhesive film which is glued to the edge of the leather part. 
     Preferably the thermoplastic layer is not a layer of foam. 
     One embodiment makes provision that a narrow, shared thermoplastic strip is provided for the mounting of the leather on the vehicle interior part, onto which the leather piece is welded and to which non-thermoplastic material of the vehicle interior part is laterally adjacent. 
     Natural leather can, of course, be processed in particular by the invention. However, if synthetic leathers which are not able to be welded, in particular synthetic suede, are to be used for the leather covering, these can also be provided with a thermoplastic layer and used according to the invention. 
     Preferably the weld seam is realized in a way that its extension is visible from the exterior and provides depressions in the leather piece to mimic a handsewn seam. A high frequency welding apparatus or a welding tool presses against the leather under pressure which leaves behind a permanent depression in the leather piece. 
     The invention can also be used on vehicle interior parts which have a large area, such as leather-covered instrument panels or parts thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a top view onto a vehicle steering wheel according to the invention, which has been covered with leather by the method according to the invention, 
         FIG. 2  shows a cross-sectional view along the line II-II in  FIG. 1  through the steering wheel rim, 
         FIG. 2   a  shows an enlarged part of the cross-section view of the  FIG. 2  in the welding region, 
         FIG. 3  shows a sectional view according to  FIG. 2  through the steering wheel rim according to a second embodiment, and 
         FIG. 4  shows a sectional view according to  FIG. 2  through the steering wheel rim according to a third embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In  FIG. 1  a vehicle interior part  11  is illustrated in the form of a vehicle steering wheel which has an encircling steering wheel rim  10 , a hub  12 , and spokes  14  connecting hub  12  and steering wheel rim  10 . 
     The cross-section through the steering wheel rim, shown in  FIG. 2 , provides further information regarding the structure of the steering wheel itself. A so-called steering wheel skeleton  16  which consists of a metal alloy is provided in the interior of the steering wheel. This steering wheel skeleton  16  is enclosed by an outer layer  18 . This outer layer  18  is preferably a foam surround. 
     As an alternative to this, however, there are also solutions in which prefabricated shells are applied onto the steering wheel skeleton  16  or a combination of shells and foam surround is provided. The steering wheel skeleton  16  and the outer layer  18  form the previously mentioned vehicle interior part  11  which is covered with leather. 
     A leather piece  20  which has been cut to size surrounds the previously described vehicle interior part  11  and is fastened to it. The leather piece  20  has two opposed edges  22  which abut each other after wrapping around the steering wheel rim and, if applicable, also the spokes. The abutment is given reference number  24 . 
     In the radial cross section of the steering wheel the leather piece  20  extends around the total circumference of the steering wheel rim  10  (see  FIGS. 2 to 4 ). 
     A thermoplastic layer  26  is fastened, in particular by laminating, on the rear side, i.e. the inner side of the leather piece  20  in the region of the edges  22 . Between the thermoplastic layers  26  the leather piece has no coating on its rear side. 
     One option for the application of the thermoplastic layer consists in sticking an adhesive film of thermoplastic material onto the leather piece  20 . 
     The vehicle interior part  11  also has its own thermoplastic layer  28 , fastened to the vehicle interior part  11  before the covering with leather, in the region of the thermoplastic layer  26 . 
     This thermoplastic layer  28  can, for example, likewise be an adhesive strip or a corresponding piece which has been embedded into the outer layer  18  during the surrounding of the skeleton  16  with foam. The thermoplastic layer  26  is not a layer of foam. 
     The leather piece  20  is fastened to the vehicle interior part  11  in the region of the edges  22  by means of high frequency welding. In so doing, the thermoplastic layers  26  and  28  become connected with each other. The corresponding weld lines or weld points are given reference number  30  and are illustrated on a slightly enlarged scale in  FIG. 2   a.    
     Depressions  42  in the leather piece  20  in the region of the weld seam mimic a handsewn seam which is visible from the exterior. 
     The leather piece  20  itself is not able to be welded; its material would not permit a high frequency welding process. 
     In the embodiment according to  FIG. 3 , the skeleton  16  is firstly enclosed by a thermoplastic inner material  32  which is again enclosed by a non-thermoplastic outer material  34 . This outer material  34  can, for example, likewise be a foam surround. In order eliminate the separate thermoplastic layer  28 , the thermoplastic inner material  32  is formed to reach the outer side  36  of the vehicle interior part  11 . Here, a corresponding bulge  38  is envisaged, which the thermoplastic layer  26  can be welded onto. 
     In addition, the inner material  32  is also distinctly harder than the outer material  34 . 
     The leather piece here likewise again has the thermoplastic layer  26 , by means of which the leather piece  20  is fastened to the vehicle interior part  11 , more precisely to the bead  38 . 
     The embodiment according to  FIG. 4  corresponds substantially to that of  FIG. 3 . However, in the region of the bead  38  an encircling groove  40  is provided on the outer side, into which the edges  22  are inserted, so that a pre-positioning takes place. In the region of the edges  22 , the thermoplastic layers  26  are of course again applied on the rear side onto the leather piece  20 , so that a high frequency welding can take place from the exterior. 
     The manufacture of the vehicle interior part  11  according to the invention is explained below. 
     Firstly the leather piece  20  is cut to size and is then provided on the rear side with the thermoplastic layer  26  on the edges  22 . 
     Preferably, a thermoplastic layer  26  is only provided in the region of the edges  22 . In this embodiment the leather piece  20  has no coating on its rear side between the thermoplastic layers  26  in the region of the edges  22 . 
     The leather piece  20  is then stretched around the interior part of the vehicle and is held securely in position by means of clips or other fastening means. 
     The interior part of the vehicle, which is thus covered with leather, is inserted into a device which is provided with a high frequency welding apparatus which is indicated by reference number  50  in  FIG. 1 . The leather piece  50  is then fastened to the vehicle interior part  11  by means of the high frequency welding apparatus  50 . 
     A linear weld seam or punctiform weld seams can be formed. A zigzag-shaped weld seam pattern would also be conceivable. 
     To mimic a handsewn seam, the weld seam is realized in a way that its extension is visible from the exterior. The imitated weld seam, visible from the exterior, is formed in that the high frequency welding apparatus  50  or a welding tool presses against the leather under pressure and a rise in temperature, which leaves behind a permanent depression  42  (see  FIG. 2   a ). It is of course possible that the weld seam is only in part visible from the exterior. 
     The interior part of the vehicle is turned around the hub  12  between welding processes. The entire welding process takes place fully automatically. 
     The processed leather piece  20  is preferably a natural leather, but also synthetic leathers are also conceivable which, however, per se would not make any high frequency welding possible.