Abstract:
In a hood airbag device ( 10 ) for use in a vehicle, an airbag module ( 14 ) is installed in a hood ( 12 ) of the vehicle from below an inner hood panel ( 12 ) and a door ( 18 ) for covering an opening ( 22 A) in an outer panel ( 24 ) is mounted thereto independently of the airbag module ( 14 ). Accordingly, it is possible to prevent the outer hood panel ( 12 ) from being damaged when the airbag module ( 14 ) is installed, and also readily performed a fine adjustment of a dimensional deviation when the door ( 18 ) is mounted, so that the installability of the airbag module ( 14 ) and the door ( 18 ) can be significantly enhanced.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a hood airbag device that deploys an airbag over a hood of a vehicle. 
         [0003]    2. Background of the Invention 
         [0004]    Japanese Patent Application Publication No. JP-A-2005-271645 describes an airbag module that is accommodated in the rear portion of the hood of a vehicle and a door, assembled in advance, to the airbag module that openably closes an opening extending in a width direction of the vehicle at the rear portion of the hood. 
         [0005]    Typically, the airbag module of the hood airbag device has an elongated shape and a heavyweight nature. As in the above-cited reference, when the airbag device is structurally designed such that the airbag module with the door is to be installed into the hood by inserting the same through the opening of the hood from above and, the opening is formed in a small size, for example, in consideration of an appearance, i.e., design of the hood, the airbag module suffers from deteriorated installability and is likely to damage the ornamental surface of an external hood panel in the installation process. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention provides a hood airbag device that significantly improves the installability of an airbag module and a door when they are mounted into a hood of a vehicle. 
         [0007]    In accordance with a first aspect of the present invention, there is provided a hood airbag device for use in a vehicle, including: an airbag module having an inflator and an airbag adapted to be deployed by a gas supplied from the inflator, the airbag module is provided between an outer hood panel and an inner hood panel of the hood through the inner hood panel, the airbag module is installed on a reinforcement and the inner hood panel, the reinforcement connecting the inner hood panel with the outer hood panel and reinforcing the hood; and a door for covering an opening formed in the outer hood panel that is aligned to correspond to the airbag module. The door is held in place on the peripheral portion around the opening in such a manner that, when the airbag is inflated the force of the inflating airbag unfolds the door, which opens the opening. 
         [0008]    In such a hood airbag device, the door that closes the opening of the outer hood panel is mounted on the outer hood panel from above and the airbag module is installed into the hood through the inner hood panel. This drastically improves the installability of the airbag module and the door when they are fitted into the hood. 
         [0009]    Further, it is preferred that the door has a hinge for supporting the door when the door is unfolded. The hinge may be less rigid than the door so that the hinge is plastically deformed with ease in response to the unfolding of the door. The hinge may be provided with a deformable portion, as a spare extensible part for securing a deformation stroke thereof, and may be connected to the reinforcement by means of a connector member such that the hinge is relatively displaceable within a predetermined range with respect to the reinforcement. 
         [0010]    In the hood airbag device described above, the hinge of the door is connected to a reinforcement by means of the connector member in a floating condition such that it can be relatively displaced with respect to the reinforcement. This makes it possible to accommodate the dimensional deviation between the door and the outer hood panel upon their assembling process. Thus, there is no need to take into account the dimensional deviation when assembling the airbag module and when installing the airbag module into the hood. This helps to further enhance the mountability of the door when it is mounted to the outer hood panel. 
         [0011]    Furthermore, in the hood airbag device described above, because the hinge is less rigid than the door, the hinge is plastically deformed to support the door in the opened state as the door is pushed upwardly by the force of the inflating airbag. The deformable portion as a spare extensible part is extended in response to the unfolding of the door, thereby allowing the door to be reversed onto the outer hood panel about the hinge. This makes it possible, when the airbag is inflated, to unfold and open the door while the door is supported by the hinge. 
         [0012]    In accordance with a second aspect of the present invention, there is provided a hood airbag device for use in a vehicle, including: an airbag module having an inflator and an airbag adapted to be deployed by a gas supplied from the inflator, the airbag module is provided between an outer hood panel and an inner hood panel of the hood through the inner hood panel, the airbag module installed in the inner hood panel; and a door, pre-assembled to the airbag module, for closing an opening formed in the outer hood panel that is aligned to correspond to the airbag module, wherein the door is adapted to unfold and open by the force of the inflating airbag when the airbag is deployed and has a sealing surface formed in such a manner that a sealing member is disposed between the sealing surface and a peripheral edge of the opening. 
         [0013]    In the hood airbag device of the second aspect, the airbag module having the door pre-assembled thereto is inserted and installed in the hood through the inner hood panel. This eliminates the possibility that the external surface of the outer hood panel is inadvertently damaged during the installation process, which makes it possible to maintain the design integrity of the hood and hence to improve the external appearance quality of the vehicle. 
         [0014]    In the hood airbag device of the second aspect, the door has a hinge with a smaller rigidity than the door that supports the door when the door is unfolded. Because the hinge is less rigid than the door, it is plastically deformed with ease in response to the unfolding of the door. In addition, the hinge may be provided with a deformable portion as a spare extensible part that secures a deformation stroke thereof. The hinge is connected to the airbag module. 
         [0015]    In the hood airbag device described above, the hinge, which is less rigid than the door, is plastically deformed to support the door in an opened state as the door is pushed upward by the force of the inflating airbag. The deformable portion as the spare extensible part is extended in response to the unfolding of the door, thereby allowing the door to be reversed onto the outer hood panel about the hinge. This makes it possible, when the airbag is inflated, to unfold and open the door while the door is supported by the hinge. 
         [0016]    Preferably, the hood airbag device of the second aspect further includes a reinforcement that connects the inner hood panel with the outer hood panel and that reinforces the hood, the airbag module is installed on the reinforcement and the inner hood panel. 
         [0017]    In the hood airbag device described above, the inner hood panel and the outer hood panel are interconnected by the reinforcement and the airbag module is installed on the reinforcement and the inner hood panel. This makes the portion of the hood to which the airbag module is installed more rigid. 
         [0018]    In the hood airbag device of the first aspect and the second aspect, preferably, the inner hood panel and the reinforcement have an insertion aperture through which the airbag module is inserted and installed in place, the insertion aperture having a greater size in the longitudinal direction of the vehicle than the width of the opening of the outer hood panel. 
         [0019]    In the hood airbag device described above, the airbag module insertion aperture formed in the inner hood panel and the reinforcement has a width in the longitudinal direction of the vehicle greater than that of the opening of the outer hood panel. Not only does this improve the installability of the airbag module but also it allows the airbag module to have an increased front-rear dimension and a relatively low height. 
         [0020]    Moreover, in the hood airbag device described above, the width of the opening of the outer hood panel in the longitudinal direction of the vehicle can be reduced to maintain design integrity, while making the height of the airbag module small. This enhances the installability of the airbag module into the vehicle. 
         [0021]    In the hood airbag device of the second aspect, preferably, the inner hood panel has an insertion aperture through which the airbag module is inserted and installed in place, the insertion aperture having a greater size in the longitudinal direction of the vehicle than the width of the opening of the outer hood panel. 
         [0022]    In the hood airbag device described above, the airbag module insertion aperture formed in the inner hood panel and the reinforcement has a width in the longitudinal direction of the vehicle that is greater than that of the opening of the outer hood panel. Not only does this improve the installability of the airbag module but also it allows the airbag module to have an increased front-rear dimension and a relatively small height. 
         [0023]    Additionally, in the hood airbag device as described above, the width of the opening of the outer hood panel in the longitudinal direction of the vehicle can be reduced to maintain design integrity, while making the height of the airbag module small. This enhances the installability of the airbag module into the vehicle. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The above and other objects and features of the present invention will become apparent from the following description of preferred embodiments, given in conjunction with the accompanying drawings, in which: 
           [0025]      FIG. 1  is a cross-sectional view of a hood airbag device in accordance with a first embodiment of the present invention when the airbag is not deployed, which is taken along a line passing through a hinge in a longitudinal direction of the vehicle; 
           [0026]      FIG. 2  is a cross-sectional view of the hood airbag device of the first embodiment when the airbag is not deployed, which is taken along a line passing through a clip in the longitudinal direction of the vehicle; 
           [0027]      FIG. 3  is an exploded cross-sectional view depicting the hood airbag device of the first embodiment, which is taken along the line passing through the hinge in the longitudinal direction of the vehicle; 
           [0028]      FIG. 4  is a cross-sectional view of a hood airbag device in accordance with a second embodiment of the present invention when the airbag is not deployed, which is taken along a line passing through a hinge in a longitudinal direction of the vehicle; and 
           [0029]      FIG. 5  is an exploded cross-sectional view depicting the hood airbag device of the second embodiment, which is taken along the line passing through the hinge in the longitudinal direction of the vehicle. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    Hereinafter, there will be described preferred embodiments of the present invention with reference to the accompanying drawings. 
         [0031]    Referring to  FIG. 1 , a hood airbag device  10  in accordance with a first preferred embodiment of the present invention is provided in, e.g., a front hood  12  of a vehicle and includes an airbag module  14  and a door  18 . 
         [0032]    As can be seen in  FIGS. 1 through 3 , the airbag module  14  has an airbag  16  that can be deployed by the gas supplied from an inflator  26 . The airbag module  14  is inserted between an outer hood panel  22  and an inner hood panel  24  of the hood  12  from below the inner hood panel  24 . The airbag module  14  is attached to a reinforcement  40  and the inner hood panel  24 . The reinforcement  40  connects the inner hood panel  24  with the outer hood panel  22  and reinforces the hood  12 . The airbag  16  and the inflator  26  are all accommodated within a module case  28 . The airbag  16  is may be inflated by gas supplied from the inflator  26  to deploy the airbag outside the hood  12  by expanding through an opening  22 A of the outer hood panel  22 . 
         [0033]    The module case  28  is a box-like member that opens toward the opening  22 A of the outer hood panel  22  and extends in a width direction of the vehicle. More specifically, the module case  28  is formed into a box-like shape by, e.g., welding a front-side wall member  30 , a rear-side wall member  32  and a bottom member  34  together. The bottom member  34  is provided with flange portions  34 F respectively extending in the forward direction of the vehicle from the front-side wall member  30  and in the rearward direction of the vehicle from the rear-side wall member  32 . The flange portions  34 F of the module case  28  in the airbag module  14  are assembled and fastened to the inner hood panel  24  together with the reinforcement  40  by means of bolts  36  and nuts  38 . 
         [0034]    Furthermore, the bottom member  34  is provided with a recessed portion which recessed downwardly, and the lower part of the inflator ( 26 ) is accommodated in the recessed portion. The recessed portion protrudes below the inner hood panel ( 24 ). 
         [0035]    The reinforcement  40  is of an inverted hat shape in cross-section and extends in the vehicle width direction between the outer hood panel  22  and the inner hood panel  24 . Furthermore, the reinforcement  40  is directly coupled to the inner hood panel  24  and supports the outer hood panel  22  through, e.g., a mastic  42 . 
         [0036]    The inflator  26  accommodated within the module case  28  is a gas source for supplying an inflating gas to the airbag  16  when an impact object (not shown) collides against the frontal area of the vehicle. The inflator  26  is fixedly secured by means of, e.g., a mounting bracket  44 , to the bottom member  34  of the module case  28  in vicinity of the front-side wall member  30 . 
         [0037]    As shown in  FIG. 3 , an insertion aperture  70  through which the airbag module  14  can be installed in place is formed in the inner hood panel  24  and the reinforcement  40 , the insertion aperture  70  is larger than the opening  22 A of the outer hood panel  22  in the longitudinal direction of the vehicle. 
         [0038]    The door  18  covers or closes the opening  22 A formed in the outer hood panel  22  that is aligned to correspond to the airbag module  14  and is held in place on the peripheral portion  22 B around the opening  22 A of the outer hood panel  22  in such a manner that it can be unfolded by the force of the inflating airbag when it is deployed, thereby opening the opening  22 A. The door  18  includes a doorframe  46  formed of, e.g., an aluminum plate or a steel plate, and a synthetic resin cover  48  fixed on the doorframe  46 . As illustrated in  FIG. 2 , the door  18  is securely engaged with the peripheral portion  22 B around the opening  22 A of the outer hood panel  22  by means of a plurality of clips  62  that can be elastically deformed so that it can be detached from the outer hood panel  22 . The door  18  is held by the clips  62  at a position where it closes the opening  22 A. A sealing member  50  is interposed between the cover  48  and the peripheral portion  22 B around the opening  22 A of the outer hood panel  22 . The clips  62  are affixed to, e.g., the doorframe  46 , and are made to be elastically deformed to thereby be detached from the peripheral portion  22 B at the moment when the force of the inflating airbag  16  is exerted on the door  18 . 
         [0039]    Referring again to  FIGS. 1 and 3 , a hinge  21  is connected to a stepped bolt  52 , which is an exemplary connector member to the inner hood panel  24 , in such a manner that it can be relatively displaced within a predetermined range with respect to the inner hood panel  24 . The hinge  21  is adapted to support the door  18  when the latter is opened. The hinge  21  may be provided in plural numbers at the rear edge of the doorframe  46  of the door  18 , for example, along the length of the door  18 . The hinge  21  has a tip end portion  21 B connected to the stepped bolt  52 , the tip end portion  21 B being reinforced by a reinforcing member  58 . Furthermore, each of the tip end portion  21 B and the reinforcing member  58  has a through-hole  60  through which a span  52 A of the stepped bolt  52  is extended with a gap left therebetween. 
         [0040]    The stepped bolt  52  is fastened by means of a nut  56  to a mounting bracket  54 , which is fixedly secured to the reinforcement  40 . The span  52 A of the stepped bolt  52  allows the hinge  21  to be relatively displaced within the predetermined range in an up-down direction and the longitudinal direction of the vehicle. 
         [0041]    The through-hole  60  provided in the tip end portion  21 B of the hinge  21  has an elongated shape extending, e.g., in a direction perpendicular to an axis of the stepped bolt  52 , namely, in a substantially vertical direction of the vehicle, so as to allow the hinge  21  to make relative displacement at least in the substantially up and down direction of the vehicle. By designing the through-hole  60  to have an increased length, the gap between the tip end portion  21 B and the span  52 A can be increased, thereby making it possible to extend the range in which the hinge  21  is relatively displaced in the upward direction of the motor vehicle with respect to the stepped bolt  52 . Accordingly, if the shape of the through-hole  60  is properly set, it becomes possible to allow the clips  62  to be detached away from the outer hood panel  22  within the range in which the hinge  21  is relatively displaced in the up and down direction of the vehicle with respect to the stepped bolt  52 . The shape of the through-hole  60  is not limited to the above, but may, for example, have an increased size in the vehicle width direction to permit relative displacement of the hinge  21  in the vehicle width direction. 
         [0042]    Moreover, the hinge  21  may be integrally formed with, e.g., the rear end of the doorframe  46  of the door  18 , and is less rigid than the doorframe  46  of the door  18  so that the hinge  21  can be plastically deformed with ease in response to the unfolding of the door  18 . In addition, the hinge  21  has a deformable portion  21 A as a spare extensible part for securing a deformation stroke thereof. The deformable portion  21 A is made extensible by bending the low rigidity portion of the hinge  21  and extends as the door  18  is unfolded. It should be appreciated that the bent shape and the extension amount of the deformable portion  21 A are not restricted to the ones illustrated in the drawings. 
         [0043]    In the following, description will be given to an operation of the hood airbag device of this embodiment configured as above. First, there will be described a process of mounting the door  18  on the hood  12 . Referring to  FIG. 3 , the door  18  is fabricated by fixing the cover  48  to the doorframe  46  and fixedly securing the reinforcing member  58  to the tip end portion  21 B of the hinge  21 . The door  18  thus fabricated is fitted into the outer hood panel  22  from thereabove and then the clips  62  are engaged with the peripheral portion  22 B around the opening  22 A (see  FIG. 2 ). Subsequently, as shown in  FIGS. 1 and 3 , the stepped bolt  52  is penetrated through the through-hole  60  of the tip end portion  21 B of the hinge  21  and fastened into the nut  56  affixed to the mounting bracket  54 , thereby attaching the door  18  to the inner hood panel  24 . The door  18  is lightweight and therefore can be mounted on the hood  12  with ease. 
         [0044]    At this time, the hinge  21  of the door  18  is connected to the stepped bolt  52  in a floating condition and can be relatively displaced within a predetermined range, namely, within the gap between the span  52 A and the through-hole  60  and within the length of the span  52 A. This makes it possible to finely adjust the position of the door  18 . The hinge  21  can make relative displacement with respect to the stepped bolt  52  to thereby accommodate the dimensional deviation between the opening  22 A and the door  18  in assembling the door  18  to the hood  12 . 
         [0045]    Since the door  18  is not a sub-assembly of the airbag module  14 , there is no need to take into account the dimensional deviation between the door  18  and the airbag module  14 . This makes it possible to keep a good design of the opening  22 A of the outer hood panel  22  and that of the door  18  provided on the opening  22 A, thus improving the external appearance of the vehicle. Since the door  18  is separate from the airbag module  14 , it is possible to reduce the vertical movement of the door  18  due to the vertical vibration of the vehicle as compared with a case where the door  18  is combined with the airbag module  14 . This makes it possible to suppress the floating movement of the door  18  as a countermeasure for the vertical vibration. 
         [0046]    Next, the process of installing the airbag module  14  into the hood  12  will be described. The airbag module  14  in the form of a sub-assembly is fabricated by fitting the inflator  26  and the airbag  16  into the module case  28 . The airbag module  14  is inserted into the hood  12  through the airbag module insertion aperture  70  provided in the inner hood panel  24  such that the flange portions  34 F are brought into contact with the inner hood panel  24 . Then, the bolts  36  are made to penetrate through the inner hood panel  24  and the reinforcement  40  and fastened into the nuts  38 , thus completing the installation process of the airbag module  14 . 
         [0047]    As clearly shown in  FIG. 3 , the insertion aperture  70  has a width in the vehicle the longitudinal direction greater than that of the opening  22 A of the outer hood panel  22 . Not only does this improve the installability of the airbag module  14  but also it allows the airbag module  14  to have an increased front-rear dimension and a relatively low height. Thus, the width of the opening  22 A of the outer hood panel  22  in the longitudinal direction of the vehicle may be reduced for the purpose of design, while making the height of the airbag module  14  small, thereby enhancing the installability of the airbag module  14  into the vehicle. For instance, even when the space between the outer hood panel  22  and the inner hood panel  24  is limited due to their positional relationships with components (not shown) in an engine compartment provided below the hood  12 , it is possible to install the airbag module  14  into the limited space. In case of the airbag module  14  having an increased dimension in the longitudinal direction of the vehicle, it is preferred that the top portion of the module case  28  has a size equal to or smaller than the width of the opening  22 A of the outer hood panel  22  in preparation for the inflation of the airbag  16 . 
         [0048]    As described above, in the hood airbag device of this embodiment, the door  18  and the airbag module  14  are separately installed into the hood  12 , thereby drastically improving the installability of the airbag module  14  and the door  18  into the hood  12 . 
         [0049]    When the airbag  16  is not deployed, as illustrated in  FIG. 2 , the door  18  is held in place by the clips  62  at a position where the door  18  closes the opening  22 A of the outer hood panel  22 , so that the opening  22 A remains stably covered by the door  18 . Thus, it is possible to maintain the design integrity of the opening  22 A and that of the door  18  when the airbag  16  is not deployed. 
         [0050]    Next, the operation of the hood airbag device  10  when an impact object collides against the frontal area of the vehicle will be described. In this case, in  FIG. 1 , the inflator  26  of the hood airbag device  10  is actuated to supply a large quantity of gas into the airbag  16  accommodated within the module case  28  in a collapsed state. In response, the airbag  16  begins to be inflated and the door  18  is unfolded by the force of the inflating airbag  16 . The airbag  16  is deployed outside the hood  12  and makes contact with the impact object to absorb the shock. 
         [0051]    Specifically, as can be seen in  FIG. 2 , the inflating force of the airbag  16  is exerted on the door  18  while being inflated. The door  18  is pushed upward by the force of the inflating airbag  16 , and the clips  62  are elastically deformed and detached from the outer hood panel  22 , so that the door  18  can be unfolded outwards. At this time, the hinge  21  may be displaced relative to the stepped bolt  52  until the lower end of the through-hole  60  reaches the stepped bolt  52 . At the position where the lower end of the through-hole  60  rests against the stepped bolt  52 , the tip end portion  21 B of the hinge  21  is held by the stepped bolt  52 . The tip end portion  21 B of the hinge  21  is reinforced by the reinforcing member  58  and the stepped bolt  52  is affixed to the reinforcement  40 , so that the holding condition of the tip end portion  21 B is stably sustained by the stepped bolt  52 . 
         [0052]    By properly setting the limit of relative displacement of the hinge  21 , the clips  62  can be detached away from the outer hood panel  22  while the hinge  21  moves within the relative displacement range. Thus, the presence of the hinge  21  does not hinder the detachment of the clips  62 , which makes sure that the clips  62  are smoothly separated from the outer hood panel  22 . 
         [0053]    As the door  18  is pushed upwardly by the force of the inflating airbag  16 , the hinge  21 , which is less rigid than the door  18 , is plastically deformed so that the door  18  is stably unfolded. Furthermore, the deformation-stroke-securing deformable portion  21 A as the spare extensible part is extended in response to the unfolding of the door  18 . This allows the door  18  to be reversed onto the outer hood panel  22  about the hinge  21 , after the door  18  has been pushed upwards and all of the clips  62  have separated from the outer hood panel  22 . Further, the clips  62  can be detached in a reliable manner, so that the door  18  can be unfolded smoothly (the unfolded state thereof is not shown in the drawings). Once the door  18  is unfolded, and the opening  22 A opened, the airbag  16  can be deployed outside the hood  12  through the opening  22 A. 
         [0054]    Although the stepped bolt  52  is taken as one example of connector members in the aforementioned embodiment, the connector members are not limited to the stepped bolt  52  but any appropriate connector member may be used that allows the hinge  21  to be connected the connector member in a floating condition. Likewise, the door  18  is not restricted to the aforementioned one formed by fixing the cover  48  to the doorframe  46 . For example, the doorframe  46  itself may cover up the opening  22 A of the outer hood panel  22  without using the cover  48 . 
         [0055]    Referring to  FIG. 4 , a hood airbag device  20  in accordance with a second preferred embodiment is provided on, e.g., the front hood  12  of the vehicle and includes an airbag module  74  and a door  78 . 
         [0056]    The airbag module  74  has an airbag  16  that can be deployed by the gas supplied from an inflator  26 . The airbag module  74  is inserted between an outer hood panel  22  and an inner hood panel  24  of the hood  12  from below the inner hood panel  24  to be fixed thereto. The airbag  16  and the inflator  26  are all accommodated within a module case  76 . The airbag  16  is configured such that it can be inflated by the gas supplied from the inflator  26  and then deployed outside the hood  12  through an opening  22 A of the outer hood panel  22 . 
         [0057]    The module case  76  is a box-like member that opens toward the opening  22 A of the outer hood panel  22  and is extended in a width direction of the vehicle. More specifically, the module case  76  includes a bottom member  84  formed into a generally box-like shape and a wall member  80  fixed to the front side of the bottom member  84  by, e.g., welding. Fixedly secured to a top edge  80 A of the wall member  80  is a clip  82  for holding a frontal edge  78 A of the door  78 . A mounting bracket  86  is affixed to the bottom member  84 . The mounting bracket  86  extends beyond the bottom member  84  in a front direction as well as in a rear direction of the vehicle. To be more specific, the mounting bracket  86  extends in the longitudinal direction of the vehicle longer than the longitudinal width of an insertion aperture  70 . The airbag module  74  is attached to the inner hood panel  24  through the mounting bracket  86  by using bolts  36  and nuts  38 . 
         [0058]    The inflator  26  includes a mounting bracket  44  having a male threaded portion  88  extended through the bottom member  84  and the wall member  80 . By tightening a nut  90  onto the male threaded portion  88 , the inflator  26  is fixed to the module case  76  and the bottom member  84  and the wall member  80  are also fastened together. 
         [0059]    As illustrated in  FIG. 5 , the door  78  is pre-assembled to the airbag module  74  and closes an opening  22 A formed in the outer hood panel  22  that is aligned to correspond to the airbag module  74 . The door  78  is configured such that, when the airbag  16  is inflated, it can be unfolded and opened by the force of the inflating airbag  16  to open the opening  22 A. The door  78  has a sealing surface  78 B, and a sealing member  92  is disposed between the sealing surface  78 B and the peripheral edge  22 B of the opening  22 A. Specifically, the door  78  is provided at its rear end with a hinge  96 , which is fixed to a rear wall  84 E of the bottom member  84  by, e.g., a rivet  94 . The frontal edge  78 A of the door  78  is held in place by the clip  82  provided at the top edge  80 A of the wall member  80 . The sealing member  92  may be, for example, a rubber seal for improving water-tightness between the door  92  and the outer hood panel  22 . 
         [0060]    The hinge  96  is less rigid than the door  78  so that it can be plastically deformed with ease in response to the unfolding of the door  78 . In addition, the hinge  96  has a deformable portion  96 A as a spare extensible part for securing a deformation stroke thereof. The deformable portion  96 A is made extensible by bending the low rigidity portion of the hinge  96  and is made to extend as the door  78  is unfolded. It should be appreciated that the bent shape and the extension amount of the deformable portion  96 A are not limited to the ones illustrated in the drawings. 
         [0061]    As depicted in  FIG. 5 , although the sealing member  92  is provided along the peripheral edge  22 B of the opening  22 A of the outer hood panel  22 , prior to the airbag module  74  is installed in the hood  12 , the present invention is not limited thereto. Instead, the sealing member  92  may be provided on the airbag module  74 , i.e., on the sealing surface  78 B of the door  78 . 
         [0062]    In this embodiment, no reinforcement is employed for the reason that the outer hood panel  22  and the inner hood panel  24  are connected to, and reinforce, each other through the airbag module  74  assembled thereto. However, the present invention is not limited to the above, and a reinforcement may be added thereto as in the first embodiment shown in  FIGS. 1 to 3 . In such a case, the mounting bracket  86  may be fastened to the inner hood panel  24  together with the reinforcement by means of bolts  36  and nuts  38 . 
         [0063]    Because the other parts or components are the same as those of the first embodiment, they are denoted by like reference numerals and descriptions thereon will be omitted. 
         [0064]    In the following, the operation of the hood airbag device  20  of this embodiment configured as above will be described. The process of installing the airbag module  74  into the hood  12  will be described with reference to  FIG. 5 . The airbag module  74  in the form of a sub-assembly is fabricated by fitting the inflator  26  and the airbag  16  into the module case  76 . The airbag module  74  is inserted into the hood  12  through the airbag module insertion aperture  70  provided in the inner hood panel  24  such that the mounting bracket  86  is brought into contact with the inner hood panel  24 . Then, the bolts  36  are made to penetrate through the inner hood panel  24  and fastened into the nuts  38 , whereby the door  78  closes the opening  22 A and the sealing surface  78 B makes a contact with the sealing member  92 , thus completing the installation process of the airbag module  74 . 
         [0065]    As set forth above, in the hood airbag device  20  of this embodiment, the airbag module  74  having the door  78  pre-assembled thereto is inserted and installed into the hood  12  from below the inner hood panel  24 , so that it is possible to prevent the external surface of the outer hood panel  22  from being damaged during the installation process, thereby keeping a good design integrity of the hood  12  and improving the external appearance of the vehicle. Furthermore, because the sealing member  92  is disposed between the peripheral edge  22 B of the opening  22 A and the sealing surface  78 B of the door  78 , the door  78  is not damaged when the airbag module  74  is inserted into the hood  12  from below the inner hood panel  24 . 
         [0066]    Next, the operation of the hood airbag device of this embodiment when an impact object collides against the frontal area of the vehicle will be described. Referring to  FIG. 4 , in this case, the inflator  26  of the hood airbag device  20  is actuated to supply a large quantity of gas into the airbag  16  accommodated within the module case  76  in a collapsed state, so that the airbag  16  begins to be inflated and the door  78  is unfolded by the force of the inflating airbag  16 . Then, the airbag  16  is deployed outside the hood  12  and makes contact with the impact object to absorb the shock. 
         [0067]    More specifically, as the airbag  16  is inflated and the force of the inflating airbag  16  pushes the door  78  upward, the frontal edge  78 A of the door  78  is plastically deformed and separated from the clip  82 , unfolding the door  78 . As the door  78  unfolds, the hinge  96 , which is less rigid than the door  78 , is plastically deformed to stably keep the unfolding of the door  78 . Furthermore, the deformation-stroke-securing deformable portion  96 A as the spare extensible part is extended in response to the unfolding of the door  78 . This allows the door  78  to be reversed onto the outer hood panel  22  about the hinge  96 , after the frontal edge  78 A of the door  78  has been separated from the clip  82 . Accordingly, it is possible to unfold the door  78  smoothly (the unfolded state of the door is not shown in the drawings). Once the door  78  is unfolded to thereby open the opening  22 A, the airbag  16  can be expanded and deployed outside the hood  12  through the opening  22 A. 
         [0068]    While the invention has been shown and described with respect to the preferred embodiments, it will be understood by those skilled in the art that various changes and modification may be made without departing from the scope of the invention as defined in the following claims.