Abstract:
An airbag for a motor vehicle includes an airbag skin having two side walls which enclose a gas chamber. The airbag skin is at least partially rolled to a shaped package in the resting state of the curtain airbag, with the shaped package having a non-circular cross-section. In order to provide a cost-effective way to produce such an airbag and to ensure that the airbag shows a deployment behaviour of a rolled package, the non-circular cross-section is permanent as long as the pressure inside the gas chamber does not exceed the pressure of the environment and is obtainable by plastic conversion of a rolled package with a circular cross-section.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The invention relates to an airbag according to the preamble of claim  1  and to a method for its manufacture according to claim  13 . 
         [0002]    So-called curtain airbags are widely used in the automotive industry. They serve for the protection mainly of the head of a vehicle occupant in case of a side impact or a roll-over accident. 
         [0003]    A curtain airbag comprises an airbag skin with two parallel side walls which enclose a gas chamber. The side walls usually have a rather large surface and the airbag skin is rolled and/or folded to a package whose diameter is small compared to its length. The upper edge of the airbag skin is directly or indirectly mounted to the inner structure of the vehicle and the curtain airbag is covered by the ceiling. Here, a general problem is that the space available between the inner structure and the ceiling is limited. 
       PRIOR ART 
       [0004]    Two basic concept of forming the airbag skin to a package are known in the prior art: 
         [0005]    In the first concept, the airbag skin is folded with a so-called Z-folding like an accordion. This kind of forming the package has the advantage that one is relatively free in defining the cross section of the package. Especially it is possible to fold the airbag skin such that the package has a basically rectangular shape with a longer side and a shorter side, which often fits better into the space available between the inner structure of the vehicle (mostly the roof rail) and the ceiling. A curtain airbag with such a folding is for example known from DE 10 2004 052 466 A1. 
         [0006]    In a second concept, the airbag skin is rolled to a package. This has the advantage that the deployment behaviour is generally superior in relation the deployment behaviour of a folded curtain airbag. Generally it is preferred that the package is rolled to an “outboard” roll which means that the airbag skin unrolls towards the outside during deployment leading to a gentle behaviour towards the occupant. An especially good deployment behaviour is achieved when the upper edge of the airbag skin points downwards in the mounted state. Such a configuration is for example shown in WO 2005/049390 A1 (so-called p-roll). 
         [0007]    A variation of this type of packaging is for example known from WO 2002/085674 A1. Here, the main section of the airbag skin is rolled to a package and a further section extending between the main section and the upper edge of the airbag skin shows one fold. The cross-section of the total package including the further section is mainly defined by the shape of the rolled package. 
         [0008]    The drawback of a curtain airbag whose airbag skin is completely or to a large part rolled to a package is that the package has usually a circular cross-section which does often not use the available space in an ideal way. 
         [0009]    The generic patent document WO 2004/094198 A1 describes a method for packing a curtain airbag. Here, the airbag skin of the curtain airbag is first rolled to a package and the package is then deformed by means of one or more deformation elements pushing into the rolled package. By this deformation of the rolled package it is possible to generate cross-sections of the package that differ from a circle, but two drawbacks need to be accepted. First, additional measures must be taken in order to keep the new formed package (the deformed rolled package) in its new shape. Without such measures—usually a cover is wrapped around the package—the package would return into its original state, namely a rolled package with a basically circular cross-section. This applying of a cover makes of course additional efforts—and thus costs—necessary, which is not desired. Second, the package obtained by such a deformation is no roll anymore in the following geometrical sense: The geometrical characteristic of a roll is that if one follows the roll—for example from the lower edge to the upper edge of the airbag skin—the direction of curvature is monotonous: One has either a left curvature or a right curvature. This is no more the case in a package as described in the WO 2004/094198 A1: Here, one has sections with left and sections with right curvature and thus in some cases the perfect deployment behaviour of a “real” roll may not be achieved. 
       SUMMARY OF THE INVENTION 
       [0010]    Starting form this the invention sets itself the problem to improve a generic airbag and a generic method for manufacture of such an airbag in such a way that the curtain airbag is easy and thus cost effective to manufacture and shows the deployment behaviour of a rolled package. 
         [0011]    This problem is solved by an airbag according to claim  1  and a method for the manufacture of such an airbag according to claim  13 . 
         [0012]    According to the invention at least a part the airbag skin is a rolled package with a non-circular cross-section; this rolled package is referred to as “shaped package”. The non-circular cross-section is permanent as long as the airbag is not deployed, that means as long as the pressure in the gas chamber does not exceed the pressure of the environment. This permanent non-circular cross-section can be obtained by a plastic, especially a thermoplastic, deformation of a rolled package, especially of a rolled package with a circular cross-section. 
         [0013]    The production of the airbag according to the invention is quite easy: The airbag skin is first rolled to a package like a “traditional” rolled airbag. The entire airbag is then inserted into a press with at least two plates. The rolled package is pressed between these two plates and kept there for a certain amount of time. Depending on the material of the airbag skin and/or a coating of the airbag skin, heat is applied to the pressed package, preferably by heating at least one of the plates. Due to memory effects of the airbag skin and/or its coating the rolled package does not return to its original circular cross-section but remains basically in the shape applied by the plates. In the simplest and most common case, the package is pressed between two plates being parallel to each other. As a result, the shaped package has an oval shape with a longer side and a shorter side (mathematically more correct one should say: a maximum diameter and a minimum diameter). The mentioned memory effects result in most cases from a softening or “near melting” of airbag skin material and/or coating material during the pressing and a “re-solidification” after the pressing of the package. 
         [0014]    Since the usually used Polyamide (Nylon, PA) for the plastic fabric of the airbag skin has a rather high melting point, it is in many applications to be preferred that the airbag skin is coated, preferably with a coating made of thermoplastic components. 
         [0015]    The invention will now be described in detail by means of examples in view of the accompanying drawings. The drawings show: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1 : a bird eye&#39;s view to a curtain airbag; 
           [0017]      FIG. 2 : the sectional view along the cutting plane A-A in  FIG. 1 ; 
           [0018]      FIG. 3 : what is shown in  FIG. 3  and a cylinder; 
           [0019]      FIG. 4 : the curtain airbag of  FIG. 3  after the airbag skin has been wrapped around the cylinder; 
           [0020]      FIG. 5 : the curtain airbag of  FIG. 4  after the cylinder has been removed; 
           [0021]      FIG. 6 : the curtain airbag of  FIG. 5  located between the plates of a press; 
           [0022]      FIG. 7 : what is shown in  FIG. 6  after the press has been activated; 
           [0023]      FIG. 8 : the curtain airbag after it has been released from the press, and a stiff mounting element; 
           [0024]      FIG. 9 : The curtain airbag of  FIG. 8  with attached stiff mounting element installed to a roof rail of a vehicle; 
           [0025]      FIG. 10 : a second embodiment of the invention in a view according to  FIG. 9 ; and 
           [0026]      FIG. 11 : a third embodiment of the invention in a view according to  FIG. 9 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0027]      FIGS. 1 to 8  show schematically the last steps in the production of a curtain airbag according to a preferred embodiment of the invention: 
         [0028]      FIG. 1  is a bird eye&#39;s view onto an unfolded curtain airbag  10 .  FIG. 2  is a sects view along plane A-A in  FIG. 1 . The curtain airbag  10  comprises an-airbag skin  12  with two side walls  14  and  16 . The side walls  14  and  16  and thus the airbag skin  12  enclose a gas chamber G which has in the embodiment shown two sub-chambers G 1 , G 2 . The airbag skin has four edges, namely an upper edge  12   a , a lower edge  12   b , a right edge  12   c  and a left edge  12   d . The two side walls are connected to each other by means of an edge seam  18 , but it needs to be emphasized that other kinds of connections between the side walls are also possible. For example the airbag skin can be made in a one-piece-woven technique. 
         [0029]    Mounting lugs  30 ,  30 ′,  30 ″ are attached to the upper edge; in the embodiment shown by means of the edge seam  18 . A gas feed element extends into the gas chamber through a section of the upper edge. This gas feed element  20  whose upper  22  section serves to accommodate a gas generator (not shown) is only drawn very schematically. It can be built like the gas feed element which is disclosed in WO 2006/117121 A1. Other possibilities of feeding the gas into the gas chamber can of course also be used. 
         [0030]    Preferably the airbag skin is made of a plastic fabric. A suitable and preferred material for this plastic fabric is polyamide (PA, nylon). It is further preferred that the airbag skin is coated, especially with a coating made of thermoplastic components. 
         [0031]    The next step after the assembly of the airbag as such is the rolling of the airbag skin  12 . Since it is desired that the rolled package of the airbag skin  10  is hollow, such that the rolled package  25  has the shape of a tube, the rolling can be done by wrapping the airbag skin around a cylinder  60 , as is shown  FIGS. 3 and 4 . After the rolling of the package is completed, the cylinder is removed, such that the rolled package  25  of the airbag skin remains in a state as shown in  FIG. 5 , namely in form of a tube. 
         [0032]    In the next step the curtain airbag  10  is inserted between two plates  52 ,  54  of a press  50 . In this step it can be necessary to keep the rolled package  25  in shape by arranging some elastic bands around its outer surface (not shown). The two plates are heatable and are heated to preferably 80° C. to 110° C. The maximum temperature is limited by the material of the airbag skin and/or the coating of the airbag skin (if any). With the materials described above, the preferred temperature is 110° C. 
         [0033]    As shown in  FIG. 7 , the press is actuated by lowering the upper plate  54  towards the lower plate  52 . The force is preferably between 5000 and 10000 N per meter of the rolled airbag skin. A good value for the pressure for the materials and the temperature given above is 7500 N/m. The curtain airbag remains in the press for some time, for example for 5 minutes. In the press, the airbag skin of the rolled package is flattened such that it shows layers that are basically parallel to each other and sharp bending lines along which the airbag skin is bent around 180°. Especially in the areas of the bends, the airbag skin and/or the coating of the airbag skin is thermoplastically deformed. Further it is possible that by the heating and the pressing a certain gluing effect between adjacent layers of the airbag skin in the shaped package occurs. 
         [0034]    When the press is opened, the rolled package expands to some extend but does not return to its original shape with a circular cross-section due to thermoplastic deformation. This package being obtained form the original rolled package with circular cross-section by the described plastic conversion (or plastic deformation) is referred to as shaped package  27  ( FIG. 8 ). The shape of the shaped package  27  is permanent as long as no sufficient forces are applied, thus in the resting state of the airbag. Of course the shaped package looses its shape when the gas generator is triggered, the pressure inside the gas chambers rises and the airbag deploys. 
         [0035]    The cross-section of the shaped package  27  is oval after the plastic deformation of the rolled package  25  with circular cross-section. The areas of the bends show the strongest curvature. It is to be noted here that the amount of the curvature of the airbag skin building the shaped package  27  is not constant (as is a characteristic of an oval) but that the direction of the curvature does not change within the shaped package  27  (as is also a characteristic of an oval). In mathematical terms: The curvature of the airbag skin in the shaped package  27  is monotonous. 
         [0036]    Even though mathematically not exact one can say that the shaped package has a longer side with a length a and a shorter side with a length b. Mathematically correct “a” denotes the maximum diameter and “b” denotes the minimum diameter. Preferably a is between 20 mm and 50 mm and b is between 10 mm and 25 mm. Further it is preferred that a is at least 1,5×b. 
         [0037]    As one can see from  FIG. 8 , the upper edge  12   a  of the airbag skin extends along the longer side (taking the above given definition). As one can also see from  FIG. 8 , the mounting lugs (shown is mounting lug  30 ) extend basically parallel to the longer side and end on the upper edge  12   a , so that the mounting lug  30  and the shaped package  27  form a “b” in the shown cross-section. A stiff mounting element (shown in  FIG. 8  is the stiff mounting element  32 ) can be attached to each mounting lug  30 ,  30 ′,  30 ″. For example it is possible to glue this stiff mounting element  32 —which can especially being made of sheet metal or of plastic—or to lay it inside the mounting lug  30  if this mounting lug  30  is double-walled. With these stiff mounting elements  32  it is achieved that the curtain airbag can be mounted in such a way at the roof rail  65  of a vehicle, that the longer side of the shaped package extends basically in an up-down-direction which is usually preferred ( FIG. 9 ). 
         [0038]      FIG. 10  shows a second embodiment in a sectional view according to claim  8 . The difference to the first embodiment is that the orientation of the mounting lugs  30  and the stiff mounting elements  32  is opposite: The mounting lug  30  and the shaped package form a “p”. 
         [0039]      FIG. 11  shows a third embodiment in a sectional view according to claim  8 . Here, the airbag skin has two sections: A first section being rolled to a rolled package and deformed to a shaped package with an oval cross-section as described above, and a second section extending between the first section and the upper edge showing one fold  19 . This fold can be applied before or after the plastic deformation of the package. 
         [0040]    In the first two embodiments usually no wrapper or similar is needed in order to keep the shaped package in its shape. The third embodiment will usually need one or more wrappers in order to keep its shape. 
         [0041]    In the embodiments described the shaped packages have a basically oval cross section and are obtained by plastic deformation of rolled packages with usually circular cross-sections. This shape is usually preferred since it can be made with a simple press with two plates. But it is also possible to generate a shaped package with a more complex cross-section, for example a triangular cross-section. The advantage of such a shape can be that the curtain airbag does even better fit into the space available between the roof rail and the ceiling. A drawback is that a more complicated press, namely one with at least three plates is necessary. 
         [0042]    Although the invention has been described at the example of curtain airbags, which is likely to be the most important application, it needs to be emphasised that an airbag according to the invention can also be another kind of airbag, especially a side airbag or a knee airbag. 
       LIST OF REFERENCE NUMBERS 
       [0000]    
       
           10  curtain airbag 
           12  airbag skin 
           12   a  upper edge 
           12   b  lower edge 
           12   c  right edge 
           12   d  left edge 
           14  first side wall 
           16  second side wall 
           18  edge seam 
           19  fold 
           20  gas feed element 
           22  upper section of the gas feed element 
           25  rolled package 
           27  shaped package 
           30 , 30 ′, 30 ″ mounting lug 
           32  stiff mounting element 
           50  press 
           52  lower plate 
           54  upper plate 
           60  cylinder 
           65  roof rail 
         G gas chamber 
         G 1 ,G 2  sub-chamber