Abstract:
This vibration welding device is a device that vibration welds an airbag holding box and an installment panel having a tear line. The vibration welding device is provided with a support jig that carries and affixes the installment panel during vibration welding. The support jig is provided with a groove that permits thermal expansion during vibration welding of the installment panel at a position differing from the tear line.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a vibration welding device and a vibration welding method for vibration welding a plurality of workpieces. 
       BACKGROUND ART 
       [0002]    Heretofore, in the case that workpieces made of resin or the like are joined together, vibration welding has been carried out in which a vibratory force is applied to the workpieces, whereby parts to be welded of the workpieces are heated and joined. In vibration welding, there is no need for adhesives, and further, after the welding process, there is no need for heat drying or the like. Thus, vibration welding is suitable for joining together interior members or components of an automobile. 
         [0003]    For example, in Japanese Laid-Open Patent Publication No. 2007-091024, a technique is disclosed in which an air bag chute is attached to an instrument panel of an automobile by vibration welding. In this case, so that the instrument panel can be broken smoothly when the airbag is deployed, a tear line, which is a breakable portion (a cutout portion, i.e., a groove-shaped thin portion), is disposed on the instrument panel. The instrument panel and the airbag chute are joined by vibration welding while sandwiching the cutout portion therebetween. 
       SUMMARY OF INVENTION 
       [0004]    Incidentally, concerning the instrument panel of an automobile, the quality of the external appearance thereof is important, and thus it is preferable to fabricate the instrument panel so that the joined portions of internal components cannot easily be distinguished. However, with an instrument panel equipped with a cutout portion, the location where the cutout portion is formed becomes deformed due to thermal effect on the instrument panel at the time of vibration welding, whereby after such vibration welding is carried out, a depression (deformation) tends to be produced along the cutout portion on a face (surface) opposite from the cutout portion. When such a deformation is produced, the appearance of the instrument panel worsens and thus the quality in external appearance of the instrument panel is deteriorated. 
         [0005]    The present invention has been devised while taking into consideration the aforementioned circumstances, and has the object of providing a vibration welding device and a vibration welding method in which, when a workpiece having a cutout portion is subjected to vibration welding, a condition of deformation of the workpiece due to thermal expansion is guided by way of a simple structure, and thus after vibration welding, the product quality of the workpiece can be enhanced. 
         [0006]    For achieving the aforementioned objects, the present invention is characterized by a vibration welding device with which a first workpiece having a cutout portion therein, and a second workpiece are stacked and vibration welded, comprising a fixing part configured to place and fix the first workpiece thereon, so that the cutout portion of the first workpiece faces toward the second workpiece during vibration welding, wherein the fixing part comprises a space configured to allow thermal expansion of the first workpiece during vibration welding, at a position overlapping with the cutout portion of the first workpiece in a fixed state of the first workpiece. 
         [0007]    According to the above features, the vibration welding device, by means of a simple structure including the space disposed at a position overlapping with the cutout portion of the first workpiece, deformation of the first workpiece after vibration welding can be suppressed significantly. More specifically, although the first workpiece in the vicinity of the cutout portion is subjected to thermal expansion by heat produced at the time of vibration welding, and easily bulges or is raised in the thickness direction due to narrowing of the cutout portion, even if the first workpiece expands and bulges, such expansion and bulging can escape into the space that exists on the opposite side from the cutout portion. Owing thereto, accompanying cooling after vibration welding, the thermally expanded portion of the first workpiece that has moved into the space is suitably returned to its original location. Thus, after vibration welding, the first workpiece exhibits superior product quality without any (or almost no noticeable) depression in the vicinity where the cutout portion is formed. 
         [0008]    In this case, the fixing part preferably fixes the first workpiece, for a deep part of the cutout portion to be located in a central portion in a widthwise direction of the space. 
         [0009]    In this manner, by fixing the first workpiece, during vibration welding, the vibration welding device is capable of allowing the thermally expanded portion on both sides in the vicinity of the cutout portion of the first workpiece to escape evenly into the space. Thus, after cooling upon completion of vibration welding, the thermally expanded portion in the vicinity of the cutout portion can return evenly, and a reduction in the occurrence of a depression can be further enhanced. 
         [0010]    Further, the fixing part may include a position adjusting mechanism configured to enable a width or position of the space to be adjusted. 
         [0011]    In this manner, by providing the position adjusting mechanism that enables a width or position of the space to be adjusted, the vibration welding device is capable of suitably adjusting the amount at which the first workpiece is displaced corresponding to the width of the space. Accordingly, after vibration welding, the condition of the first workpiece can easily be adjusted. Further, corresponding to the amount of heat, etc., at the time of vibration welding, or a change in the shape of the first or second workpiece (shape, material or thickness of the cutout portion, etc.), since the need to prepare a plurality of fixing parts is eliminated, versatility is increased. 
         [0012]    Furthermore, the second workpiece preferably comprises a plurality of welding parts, and the space preferably is disposed between the welding parts in a fixed state of the second workpiece. 
         [0013]    In this manner, when the space is disposed between the welding parts with the second workpiece fixed in position, the thermally expanded portion of the first workpiece due to the influence of heat of the welding parts generated during vibration welding can be made to move reliably into the space. 
         [0014]    The first workpiece may be an instrument panel of an automobile, and the second workpiece may be a retaining member of an airbag to be deployed into interior of the automobile at a predetermined timing, the retaining member configured to be welded to the instrument panel. The cutout portion may be a breakable portion to be broken accompanying deployment of the airbag. 
         [0015]    Consequently, in a structure in which the airbag is attached to the instrument panel of an automobile, the state of the surface of the instrument panel is such that the retaining member of the airbag cannot be discerned. 
         [0016]    Further, for achieving the objects described above, the present invention is characterized by a vibration welding method, by which a first workpiece having a cutout portion therein, and a second workpiece are stacked and vibration welded. The method comprises a fixing step of fixing the first workpiece with respect to a fixing part that is capable of placing and fixing the first workpiece so that the cutout portion thereof faces toward the second workpiece, and so that the cutout portion overlaps with a space of the fixing part, and a vibration welding step of, after the fixing step, vibration welding the first workpiece and the second workpiece by applying a vibration force to the first and second workpieces, while allowing thermal expansion of the first workpiece to escape into the space. 
         [0017]    According to the present invention, in the case that a workpiece having a cutout portion is subjected to vibration welding, by means of a simple structure, the product quality of the workpiece after vibration welding thereof can be enhanced. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0018]      FIG. 1  is a perspective view showing an instrument panel on which a vibration welding device according to an embodiment of the present invention carries out vibration welding; 
           [0019]      FIG. 2  is a cross-sectional view showing an attached state of an airbag unit and the instrument panel of  FIG. 1 ; 
           [0020]      FIG. 3  is a perspective view showing in outline the structure of the vibration welding device; 
           [0021]      FIG. 4  is a plan view showing a support jig of the vibration welding device of  FIG. 3 ; 
           [0022]      FIG. 5  is a cross-sectional view showing in enlarged form a structure of the vibration welding device that fixes a portion of the instrument panel where a tear line is disposed; 
           [0023]      FIG. 6A  is a first explanatory drawing for describing operations of the vibration welding device of  FIG. 5 ; 
           [0024]      FIG. 6B  is a second explanatory drawing, continuing from  FIG. 6A , for describing operations of the vibration welding device; and 
           [0025]      FIG. 6C  is a third explanatory drawing, continuing from  FIG. 6B , for describing operations of the vibration welding device; 
           [0026]      FIG. 7A  is a first explanatory drawing for describing operations of a conventional vibration welding device; and 
           [0027]      FIG. 7B  is a second explanatory drawing, continuing from  FIG. 7A , for describing operations of the conventional vibration welding device; 
           [0028]      FIG. 8A  is a graph showing a displacement amount of an instrument panel corresponding to the width of a groove portion at a time of vibration welding; and 
           [0029]      FIG. 8B  is a graph showing a displacement amount of a surface of the instrument panel corresponding to the width of the groove portion when cooling takes place after vibration welding; 
           [0030]      FIG. 9A  is a plan view of a support jig according to a first modification;  FIG. 9B  is a cross-sectional view taken along line IXB-IXB of  FIG. 9A ; 
           [0031]      FIG. 9C  is a plan view showing a condition in which a groove portion of the support jig according to the first modification is widened; and  FIG. 9D  is a cross-sectional view taken along line IXD-IXD of  FIG. 9C ; 
           [0032]      FIG. 10A  is a cross-sectional view of a support jig according to a second modification; and 
           [0033]      FIG. 10B  is a cross-sectional view of a support jig according to a third modification. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0034]    Hereinafter, a preferred embodiment of a vibration welding device, in relation to a vibration welding method according to the present invention, will be described in detail below with reference to the accompanying drawings. 
         [0035]    The vibration welding device according to the present invention includes a function to vibration weld predetermined interior components onto an instrument panel (dashboard) of an automobile. In particular, the vibration welding device is configured to attach a predetermined structural object (an airbag, etc.) to an inner side (outside of the vehicle compartment) of a passenger seat front portion of the instrument panel. Below, to facilitate understanding of the present invention, first, a description will be given concerning attachment of an airbag to the instrument panel. 
         [0036]    In the description of the instrument panel given below, a surface that is exposed to the vehicle compartment will be referred to as a front surface, whereas a surface that faces outside from the vehicle compartment on an opposite side from the front surface will be referred to as a rear surface. Further, in the case of describing directions of the instrument panel, the front window side will be referred to as a front side, and the passenger seat side will be referred to as a rear side corresponding to front and rear directions of the automobile (see  FIG. 2 ). 
         [0037]    As shown in  FIG. 1 , a vehicle  12  comprises a driver&#39;s seat  14  and a passenger seat  16  in the interior of a vehicle compartment  18 , and an instrument panel  20  (first workpiece: also referred to as an instrument panel  20  below) in front of the passenger seat  16 . A steering handle  22  together with meters and gauges  24  are provided on the driver&#39;s seat  14  side of the instrument panel  20 . A glove box  26 , for example, is provided on the passenger seat  16  side of the instrument panel  20 . 
         [0038]    An airbag unit  28  is provided for the purpose of protecting a vehicle occupant by expanding and deploying at the time of a collision of the vehicle  12 . In the case of the driver&#39;s seat  14 , the airbag unit  28  is disposed on a rear surface of a wall portion that constitutes a central part (shaft portion) of the steering handle  22 , and in the case of the passenger seat  16 , the airbag unit  28  is disposed on a rear surface  34  of the instrument panel  20  on a more upward side than the glove box  26 . Further, on the rear surface  34  of the instrument panel  20  on the side of the passenger seat  16 , apart from the airbag unit  28 , a duct (not shown) is provided for adjustment of air conditioning in the vehicle compartment  18 . 
         [0039]    As shown in  FIG. 2 , the airbag unit  28  on the side of the passenger seat  16  is attached so as to be capable of deploying into the vehicle compartment  18  (in front of an occupant of the passenger seat  16 ) from the rear surface  34  of the instrument panel  20 . The airbag unit  28 , for example, is equipped with an airbag device  36 , a support frame  38  that supports the airbag device  36 , and a retaining box  40  (second workpiece: retaining member) in which the support frame  38  is held. 
         [0040]    The airbag device  36  comprises an airbag  42  that is formed in the shape of a bag, and an inflator  44  that supplies air to the airbag  42 . The airbag  42  is folded and accommodated in the support frame  38  and in the interior of the retaining box  40 , at normal times when a collision or the like does not occur. At the time of a collision, the airbag  42  is constituted so as to expand outwardly based on supply of air from the inflator  44 . 
         [0041]    The support frame  38  has an open upper portion and exhibits a frame structure that accommodates the aforementioned airbag device  36  in the interior thereof. On a side portion of the upper opening of the support frame  38 , hook parts  46  are provided for hooking the support frame  38  onto the retaining box  40 . Further, in order to reliably support the airbag device  36 , the support frame  38  includes a mounting part  48  that is screwed to a vehicle body frame (not shown). 
         [0042]    The retaining box  40  indirectly supports the airbag device  36  by being fixed so that the support frame  38  hangs downwardly. The retaining box  40  comprises a retaining wall (walls)  50  that retains the support frame  38 , a flange member  52  connected to one end side of the retaining wall  50  and projecting outwardly therefrom, and door portions  54  disposed on an inner side and connected in the same manner to the one end side of the retaining wall  50 . 
         [0043]    The retaining wall  50  is formed in the shape of a rectangular tube, and includes on the inner side thereof a space  50   a  capable of accommodating the airbag device  36 . Hooking portions  56  in which the hook parts  46  are hooked are provided on a projecting end part (other end part) of the retaining wall  50 . 
         [0044]    The flange member  52  projects out at a predetermined length along the shape of the rear surface  34  side of the instrument panel  20 . Welding parts  58 , which are vibration welded on the instrument panel  20 , are disposed on an upper surface (a surface facing toward the rear surface  34  of the instrument panel  20 ) of the flange member  52 . 
         [0045]    On the other hand, the door portions  54  are formed by a pair of door plates  60 , which are divided into front and rear parts at a center portion in the forward/rearward direction. The pair of door plates  60  is separated from the retaining walls  50  on both left and right sides of the retaining box  40 , and is connected respectively through hinge members  62  to the retaining walls  50  on both front and rear sides of the retaining box  40 . The hinge members  62  are portions formed with thinner walls than the walls of the door portions  54  or the flange member  52 , and allow the door plates  60  to swing freely. Therefore, the two door plates  60  in a pair are capable of opening as double doors toward the side of a front surface  32  of the instrument panel  20 , to thereby widen or spread a gap  64  in the front and rear center portion. Similar to the flange member  52 , the welding parts  58 , which are vibration welded on the instrument panel  20 , are disposed on upper surfaces (surfaces facing toward the instrument panel  20 ) of the door portions  54 . 
         [0046]    The welding parts  58  are portions that are directly welded to the rear surface  34  of the instrument panel  20 , and are formed as protrusions  58   a  (ribs) disposed in plurality on upper surfaces of the flange member  52  and the door portions  54 . The respective protrusions  58   a  project at the same degree from the upper surfaces of the flange member  52  and the door portions  54  along the rear surface  34  of the instrument panel  20 . On upper surfaces of the flange member  52  and the door portions  54 , the respective protrusions  58   a  of the welding parts  58  may be constituted in the form of a lattice shape, a net-like shape, or a striped pattern. 
         [0047]    The door portions  54  are welded to the instrument panel  20  so as to include the gap  64  of a predetermined width between projecting ends  60   a  of the pair of door plates  60 . The welding parts  58  are disposed in proximity to the projecting ends  60   a  of the door plates  60 . At the connected portions between the retaining walls  50  and the pair of door plates  60 , the welding parts  58  are disposed in the vicinity of the hinge members  62 , however, the welding parts  58  are not disposed on the hinge members  62 . 
         [0048]    The instrument panel  20  to which the retaining box  40  is welded has a predetermined wall thickness and is formed to exhibit a complex curved surface. A peripheral edge portion of the instrument panel  20  is fixed to the vehicle body frame. As shown in  FIG. 1 , on the front surface  32  of the instrument panel  20  on the side of the passenger seat  16 , an attachment portion of the airbag unit  28  is formed with a relatively smooth surface, and is capable of allowing the airbag  42  to be deployed smoothly in front of the passenger seat. 
         [0049]    As shown in  FIG. 2 , although the rear surface  34  of the instrument panel  20  is formed as a substantially smooth surface similar to the front surface  32 , a tear line  66  (cutout portion) is provided at a predetermined position (airbag mounting portion  30 ). The tear line  66  serves as a breakable portion that prompts opening of the door portions  54 , by breaking the instrument panel  20  when the airbag  42  is deployed. As shown in  FIG. 1 , the tear line  66  is formed with a predetermined shape corresponding to the shape of the airbag unit  28  (the pair of door plates  60 ). 
         [0050]    More specifically, the tear line  66  includes a first line  66   a  facing toward the gap  64  between the projecting ends  60   a  of the pair of door plates  60 , and a pair of second lines  66   b  facing toward the pair of hinge members  62 . Further, the tear line  66  includes third lines  66   c  that connect the end portions of the first and second lines  66   a ,  66   b,  and face toward left and right detached locations between the retaining walls  50  and the door portions  54 . More specifically, the second lines  66   b  and the third lines  66   c  form an endless ring that goes around the inside vicinity of the retaining walls  50 , and the first line  66   a  is connected to the third lines  66   c  crossing the center of the ring in the lateral direction. 
         [0051]    At a time of deployment of the airbag unit  28 , the pair of door plates  60  is pushed toward the side of the front surface  32  of the instrument panel  20  due to the expansive force of the airbag  42 , and a location where the first line  66   a  of the instrument panel  20  is formed breaks. At this time, the locations where the second lines  66   b  of the instrument panel  20  are formed bend (or break) responsive to the hinge members  62 , whereas the locations where the third lines  66   c  of the instrument panel  20  are formed break along the detached locations between the retaining walls  50  and the door portions  54 . As a result, the airbag  42  flies out from the instrument panel  20  in a predetermined direction of the vehicle compartment  18 . 
         [0052]    Next, a detailed description will be made concerning vibration welding of the instrument panel  20  and the airbag unit  28  (retaining box  40 ). As already noted, the instrument panel  20  and the airbag unit  28  are joined by performing vibration welding using a vibration welding device  10 . The vibration welding device  10 , for example as shown in  FIG. 3 , is equipped with a base plate  70 , a vibration plate  72 , and a control unit  74 . 
         [0053]    The base plate  70  is a member that supports the instrument panel  20  during vibration welding, and is configured to be capable of rising and lowering with respect to the floor under an action of the control unit  74 . Multiple jigs  71  are disposed independently on an upper surface of the base plate  70 . The jigs  71  include a function to position the instrument panel  20  in a predetermined posture, and furthermore support the locations to be vibration welded from a lower side thereof. The jigs  71  are constituted from an aluminum material, for example. 
         [0054]    In the vibration welding device  10 , the instrument panel  20  is arranged on the base plate  70  such that the front surface  32  side thereof faces downward. On the rear surface  34  of the instrument panel  20 , other than the aforementioned airbag unit  28 , a non-illustrated duct is subjected to vibration welding. Therefore, on the side of the rear surface  34 , there are disposed the tear line  66  that corresponds to the airbag unit  28 , along with duct attachment parts  76  and a duct hole  78 . Vibration welding of the duct is performed in the same manner as the conventional art, and thus explanation thereof is omitted. Further, other interior components may be attached to the rear surface  34  side of the instrument panel  20 . The retaining box  40  of the airbag unit  28 , prior to vibration welding thereof, is positioned on the tear line  66  of the instrument panel  20  by a non-illustrated positioning jig (or alternatively, manually by an operator). 
         [0055]    As the jigs  71  of the base plate  70 , there may be cited, for example, duct lower side jigs  80  that are arranged at positions below the duct attachment parts  76 , and a support jig  82  (fixing part) that is provided at a position below the airbag unit  28 . 
         [0056]    On the other hand, the vibration plate  72  is connected to a vibration applying means  84  at a position above the base plate  70 . On a lower surface of the vibration plate  72 , vibration-capable jigs  73  are disposed at positions facing toward the respective jigs  71  of the base plate  70 . 
         [0057]    As the jigs  73  of the vibration plate  72 , there may be cited, for example, duct upper side jigs  86  that are arranged at positions above the duct attachment parts  76 , and a vibration-applying jig  88  (see  FIG. 5 ) that is arranged at a position above the airbag unit  28 . The support jig  82  and the vibration-applying jig  88 , by raising of the base plate  70 , are capable of sandwiching and applying a predetermined pressure to the instrument panel  20  and the retaining box  40 . The pressure can be controlled by the control unit  74 . 
         [0058]    Further, under an action of the vibration applying means  84 , the vibration plate  72  is vibrated in a lateral direction (e.g., in left and right directions of the instrument panel  20 , or in the X directions shown in  FIG. 3 ) relatively with respect to the base plate  70 . By receiving vibrations from the vibration plate  72 , the duct upper side jigs  86  and the vibration-applying jig  88  are capable of applying a predetermined vibratory force to the retaining box  40  and the instrument panel  20 . The vibration conditions (amplitude, frequency, vibration time, etc.) of the duct upper side jigs  86  and the vibration-applying jig  88  are controlled suitably by the control unit  74 . 
         [0059]    Next, the support jig  82  that supports the airbag mounting portion  30  of the instrument panel  20  will be described in detail with reference to  FIG. 4 . The support jig  82  includes a function to suitably vibration weld the airbag unit  28  to the instrument panel  20 . 
         [0060]    The support jig  82  includes a frame member  90  fixed to the base plate  70 , and a support block  92  accommodated in the frame member  90  and which directly supports the instrument panel  20 . The frame member  90  of the support jig  82  projects upwardly from the upper surface of the base plate  70 , and by surrounding the entire circumference of the support block  92 , regulates movement in the planar direction of the support block  92 . The frame member  90  is formed in a rectangular shape as viewed in plan, corresponding to the shape of the airbag unit  28 , and is fixed to the base plate  70  at each side thereof by a plurality of screws  90   a.    
         [0061]    The support block  92 , by being arranged on and fixed to the inner side of the frame member  90 , projects out on the upper surface of the base plate  70 , and supports the airbag mounting portion  30  of the instrument panel  20 . The upper surface of the support block  92  is in surface contact with the instrument panel  20 , and serves as a mounting surface  94  for supporting the instrument panel  20 . 
         [0062]    A groove portion  96  (space), which is constituted by cutting out the support block  92  downwardly at a predetermine depth, is disposed on the mounting surface  94  of the support block  92 . In a state in which the instrument panel  20  is fixed in position, the groove portion  96  is formed at a position that overlaps with the tear line  66  of the airbag mounting portion  30 , and includes a function to allow a bulge (thermally expanded portion) of the instrument panel  20 , which is generated in the vicinity of the tear line  66  during vibration welding. 
         [0063]    More specifically, the groove portion  96  includes a first groove  96   a  facing toward the first line  66   a  of the tear line  66 , second grooves  96   b  that face toward the second lines  66   b,  and third grooves  96   c  that face toward the third lines  66   c.  The first through third grooves  96   a,    96   b,    96   c  are set with the same degree of depth and width. In the vibration welding device  10 , when the instrument panel  20  and the support block  92  are vibration welded, the instrument panel  20  is fixed in position, such that a deep part (maximally cutout portion) of the tear line  66  passes through the widthwise center of the groove portion  96 . 
         [0064]    Below, with reference to the cross-sectional view of  FIG. 5 , a further detailed description will be made concerning the first line  66   a  of the instrument panel  20  and the vicinity of the first groove  96   a  of the support jig  82 . 
         [0065]    The relationship between the second and third lines  66   b,    66   c  of the instrument panel  20  and the second and third grooves  96   b,    96   c  of the support jig  82  is basically the same as the relationship between the first line  66   a  and the vicinity of the first groove  96   a,  and therefore, detailed description of such features is omitted. 
         [0066]    As viewed in cross section, the first groove  96   a  of the support jig  82  is surrounded by a bottom wall  97  and a pair of side walls  98 , and the upper surface thereof is open. The bottom wall  97  of the first groove  96   a  is formed in a flat shape, and the two side walls  98  in a pair are disposed contiguously in a perpendicular direction with respect to the bottom wall  97 . 
         [0067]    The first line  66   a  of the instrument panel  20  is arranged in a center portion in the widthwise direction of the first groove  96   a,  and is mounted on the mounting surface  94  so as to bridge the first groove  96   a.  A width α (between the pair of side walls  98 ) of the first groove  96   a  according to the present embodiment is set to be wider than the width of the tear line  66  or the gap  64  between the pair of door plates  60 . On the other hand, the depth β of the first groove  96   a,  i.e., a distance from the mounting surface  94  to the bottom wall  97 , preferably is set to a degree (also refer to  FIG. 6B ) that does not come into contact with a thermally expanded portion  20   a  of the instrument panel  20  at the time of vibration welding. 
         [0068]    The retaining box  40  is positioned by a positioning jig or the like so that the gap  64  between the pair of door plates  60  faces toward the first line  66   a,  and is stacked on the side of the rear surface  34  of the instrument panel  20 . 
         [0069]    Accordingly, the central vicinity in the widthwise direction of the first groove  96   a,  the first line  66   a  of the instrument panel  20 , and the gap  64  of the retaining box  40  are lined up uniformly in the stacking direction. Further, in a state with the retaining box  40  being fixed in position, the welding parts  58  of the door portions  54  face toward the mounting surface  94  of the support jig  82 . Thus, the mounting surface  94  of the support jig  82  is capable of suitably supporting portions (positions facing toward the welding parts  58 ) of the instrument panel  20  to which vibrations are directly transmitted. 
         [0070]    The vibration welding device  10  according to the present invention is constructed basically as described above. Below, operations and advantageous effects of the vibration welding device  10  will be described on the basis of a vibration welding method. 
         [0071]    With the vibration welding method, as shown in  FIG. 3 , at first, the instrument panel  20  is positioned, mounted, and fixed in a surface direction with respect to the base plate  70  of the vibration welding device  10  (fixing step). 
         [0072]    At this time, the instrument panel  20  is fixed with respect to the support jig  82  so that the tear line  66  of the airbag mounting portion  30  faces upwardly (an opposite surface from the mounting surface  94  of the support jig  82 ). Further, in the fixed state, as described above, the tear line  66  is arranged at a position that overlaps with the center in the widthwise direction of the groove portion  96  of the support jig  82 . 
         [0073]    Next, in the vibration welding device  10 , the retaining box  40  for the airbag unit  28  is positioned and mounted by a positioning jig with respect to the instrument panel  20 . At this time, positioning is performed such that the gap  64  between the pair of door plates  60  overlaps with the first line  66   a,  the two hinge members  62  overlap with the second lines  66   b,  and the detached locations between the retaining walls  50  and the door portions  54  overlap with the third lines  66   c.  The retaining box  40  may be positioned beforehand and fixed provisionally on the instrument panel  20 . 
         [0074]    Thereafter, the base plate  70  is raised under an action of the control unit  74 , the instrument panel  20  and the retaining box  40  are sandwiched between the vibration-applying jig  88  of the vibration plate  72  and the support jig  82  of the base plate  70 , and are pressed therebetween in the stacking direction (refer to  FIG. 6A ). Consequently, a suitable pressure is applied to the contact surfaces of the welding parts  58  of the retaining box  40  and the rear surface  34  of the instrument panel  20 . 
         [0075]    In a state in which pressure is applied, under an action of the control unit  74 , the vibration plate  72  is vibrated at a predetermined amplitude and a predetermined frequency in the X directions. As a result, a vibratory force is imparted to the retaining box  40  and the instrument panel  20  from the vibration-applying jig  88 , frictional heat is generated at the contact surfaces between the welding parts  58  of the retaining box  40  and the rear surface  34  of the instrument panel  20 , and vibration welding is carried out (vibration welding step). 
         [0076]    In the vibration welding step, welding heat is transmitted from the vicinity of the welding parts  58  to the instrument panel  20 . Therefore, as shown in  FIG. 6B , on the instrument panel  20 , in the vicinity of the tear line  66  (first line  66   a ) in close proximity to the welding parts  58 , deformation (thermal expansion) is caused due to the influence of heat. In particular, due to such vibration welding, the temperature on the side of the rear surface  34  of the instrument panel  20  increases, and a heat gradient appears in which the temperature lowers toward the side of the front surface  32  of the instrument panel  20 . Therefore, the wall portion near the rear surface  34  of the instrument panel  20  acquires greater fluidity, whereas the wall portion near the front surface  32  of the instrument panel  20  acquires lower fluidity (refer to the white arrows in  FIG. 6B ). Consequently, the thermally expanded portion  20   a  of the instrument panel  20  in the vicinity of the tear line  66  moves toward the side of the groove portion  96  (first groove  96   a ), and during vibration welding, the location where the tear line  66  of the instrument panel  20  is formed projects outwardly in the shape of a chevron in the direction (downward direction) of the bottom wall  97  of the groove portion  96 . 
         [0077]    After completion of vibration welding, the base plate  70  is lowered under an action of the control unit  74 , whereupon sandwiching between the support jig  82  and the vibration-applying jig  88  is released. In addition, the instrument panel  20  and the retaining box  40  undergo natural cooling, whereby an action takes place so that the thermally expanded portion  20   a,  which had acquired fluidity at the time of vibration welding, returns. At this time, since the temperature was higher near the rear surface  34  than near the front surface  32 , the wall portion near the rear surface  34  of the instrument panel  20  has greater fluidity, whereas the wall portion near the front surface  32  of the instrument panel  20  has lower fluidity (refer to the white arrows in  FIG. 6C ). 
         [0078]    Therefore, as shown in  FIG. 6C , the location where the tear line  66  of the instrument panel  20  is formed undergoes displacement so as to separate away from the groove portion  96  (bottom wall  97 ). The amount of displacement of the instrument panel  20  can be controlled to an appropriate amount by the shape of the groove portion  96 . The front surface  32  of the instrument panel  20 , in a state after cooling thereof (at normal temperature), can be restored exactly to its position prior to vibration welding. More specifically, after completion of vibration welding, the front surface  32  of the instrument panel  20  can be restored to a smooth surface in which a depression (deformity) thereof is almost unnoticeable. Further, in the groove portion  96  of the support jig  82 , after vibration welding, residual heat of the instrument panel  20  can suitably escape, and restoration of the instrument panel  20  can take place in a short time. 
         [0079]    Based on  FIGS. 7A and 7B , a description will be made concerning a support jig  200  of a conventional vibration welding device. A mounting surface  202  of the instrument panel  20  of the support jig  200  is constituted with a flat surface that does not have the groove portion  96  according to the present embodiment. Further, in the mounting surface  202 , there is no portion into which the thermally expanded portion  20   a  of the instrument panel  20  can escape downwardly at the time of vibration welding. Therefore, during vibration welding, due to the welding heat of the instrument panel  20 , a phenomenon occurs in which the wall portions on the side of the rear surface  34  of the instrument panel  20  where the tear line  66  is sandwiched acquire greater fluidity, whereas the wall portion on the side of the front surface  32  acquires almost no fluidity. Stated otherwise, at the time of vibration welding, the instrument panel  20  is subjected to an action such that a top side opening of the tear line  66  is narrowed. 
         [0080]    When the instrument panel  20  undergoes cooling after completion of vibration welding, although the wall portion on the side of the rear surface  34  where the tear line  66  has been narrowed upwardly has greater fluidity, due to the lower fluidity of the wall portion on the side of the front surface  32 , an action takes place such that the tear line  66  on the side of the rear surface  34  widens significantly. Consequently, the front surface  32  of the instrument panel  20  undergoes displacement by being pulled toward the side of the rear surface  34 , and is deformed such that the location where the tear line  66  is formed warps or bends backward. As a result, a depression (deformity) is generated on the front surface  32  side of the instrument panel  20 . 
         [0081]    In contrast thereto, in the vibration welding device  10  according to the present embodiment, as shown in  FIGS. 6B and 6C , by the groove portion  96  being disposed at a position that overlaps with the tear line  66 , movement of the thermally expanded portion  20   a  of the instrument panel  20  at the time of vibration welding can be permitted. Consequently, the front surface  32  side of the instrument panel  20  after completion of vibration welding can be constructed with a smooth surface without the occurrence of any depression therein. 
         [0082]    In this case, at the time of vibration welding, the amount of displacement of the thermally expanded portion  20   a  of the instrument panel  20  that enters into the groove portion  96  side is an important factor. Although the amount of displacement is influenced by the amount of heat at the time of vibration welding or the shape of the tear line  66 , in the vibration welding device  10  according to the present embodiment, on the premise that the above elements are the same, the amount of displacement is adjusted by the width α of the groove portion  96 . Below, with reference to  FIGS. 8A and 8B , the relationship between the width α of the groove portion  96  and the amount of displacement of the instrument panel  20  will be explained. 
         [0083]    The graph of  FIG. 8A  shows an amount of displacement of the instrument panel  20  corresponding to the width α of the groove portion  96  at a time of vibration welding, wherein the vertical axis represents the displacement amount of the front surface  32  of the instrument panel  20 , and the horizontal axis represents the width α of the groove portion  96 . Further, in the graphs of  FIGS. 8A and 8B , the thin solid line, the thin one-dot dashed line, the thin two-dot dashed line, the thick solid line, the thick one-dot dashed line, and the thick two-dot dashed line are shown in order from the width α of the groove portion  96  being narrowest to the width α thereof being widest. 
         [0084]    As indicated by the thin solid line of  FIG. 8A , in the case that the width α of the groove portion  96  is sufficiently narrow, almost no displacement of the front surface  32  of the instrument panel  20  takes place during vibration welding. In addition, when the width α of the groove portion  96  widens to and beyond a predetermined level, it is understood that the displacement amount of the front surface  32  of the center portion (deep part of the tear line  66 ) increases in succession. 
         [0085]    The graph of  FIG. 8B  shows the displacement amount of the front surface  32  of the instrument panel  20  corresponding to the width α of the groove portion  96  when cooling takes place after vibration welding. The respective lines correspond to different widths α of the groove portion  96  in the same manner as the lines shown in  FIG. 8A . In  FIG. 8B , the case in which the displacement amount on the vertical axis is negative indicates a situation in which the front surface  32  of the instrument panel  20  is displaced to the side of the rear surface  34  (a state in which the front surface  32  is warped or bent backward). Conversely, the case in which the displacement amount on the vertical axis is positive indicates a situation in which the front surface  32  of the instrument panel  20  projects out toward the side of the groove portion  96 . 
         [0086]    From these facts, it can be understood that if the width α of the groove portion  96  is narrow (the thin solid line, the thin one-dot dashed line), the front surface  32  of the instrument panel  20  is displace so as to be bent inwardly. Further, it can be understood that if the width α of the groove portion  96  is wide (the thick two-dot dashed line), the projecting state of the front surface  32  toward the side of the groove portion  96  remains to a slight degree. Additionally, the cases of the thin two-dot dashed line, the thick solid line, and the thick one-dot dashed line reside within a range of error in which almost no influence is imparted to the external appearance of the instrument panel  20 , and the shape thereof returns near to its original position. In particular, it can be understood that when the width α of the groove portion  96  is as shown by the thick solid line, the front surface  32  of the instrument panel  20  acquires a substantially flat shape. In other words, by adjusting the width of the groove portion  96 , the front surface  32  of the instrument panel  20  can be placed in a state that exhibits almost no deformation. 
         [0087]    As described above, with the vibration welding device  10  and the vibration welding method according to the present embodiment, by means of a simple structure including the groove portion  96  disposed at a position overlapping with the tear line  66  of the instrument panel  20 , after vibration welding, deformation of the instrument panel  20  can be suppressed significantly. More specifically, even though the location where the tear line  66  of the instrument panel  20  is formed undergoes thermal expansion due to the heat generated during vibration welding, the thermally expanded portion  20   a  is capable of escaping into the groove portion  96  on the side opposite from the tear line  66  (i.e., the front surface  32  side of the instrument panel  20 ). As a result, when cooling takes place after vibration welding, the thermally expanded portion  20   a  of the instrument panel  20  that has moved into the groove portion  96  is suitably returned to its original location. Thus, after vibration welding, the instrument panel  20  exhibits superior product quality without any (or almost no noticeable) depression or the like. 
         [0088]    In this case, by fixing the instrument panel  20  so that the support jig  82  is positioned centrally in the widthwise direction of the groove portion  96 , during vibration welding, the vibration welding device  10  is capable of allowing the thermally expanded portion  20   a  in the vicinity of the tear line  66  of the instrument panel  20  to escape evenly into the groove portion  96 . Thus, after cooling upon completion of vibration welding, the instrument panel  20  can return evenly, and the product quality of the instrument panel  20  can be further improved. Further, by fixing the retaining box  40  so that the groove portion  96  is positioned between the welding parts  58 , the thermally expanded portion  20   a  of the instrument panel  20  due to the influence of heat effect of the welding parts  58  generated during vibration welding can be made to move reliably into the groove portion  96 . 
         [0089]    The vibration welding device  10  according to the present invention is not limited to the embodiment described above, and various modifications thereto can be adopted. Below, descriptions will be given concerning some of modifications of the vibration welding device  10 . In the descriptions given below, structural features thereof having the same configuration or the same function as those of the vibration welding device  10  according to the present embodiment are designated by the same reference characters, and detailed description of such features is omitted. 
       First Modification 
       [0090]    A support jig  82 A according to a first modification, as shown in  FIGS. 9A through 9D , is equipped with a position adjusting mechanism  102  (position adjusting unit) capable of adjusting the width α and the position of a groove portion  100 . The position adjusting mechanism  102  includes a plurality of divided sub-blocks  106  into which a support block  104  is divided, and block position fixing members  108  that enable the respective sub-blocks  106  to be moved relative to the base plate  70  and to be fixed in arbitrary positions. 
         [0091]    The sub-blocks  106  include a function to individually narrow or widen first through third grooves  100   a,    100   b,    100   c  of the groove portion  100 . In this case, a support block  104   a  that surrounds the groove portion  100  close to the frame member  90  is divided into four sub-blocks  106  corresponding to the adjacent second and third grooves  100   b ,  100   c.  On the other hand, two support blocks  104   b  that are surrounded by the second and third grooves  100   b,    100   c  are each divided into six sub-blocks  106  corresponding to the first through third grooves  100   a,    100   b,    100   c.    
         [0092]    The block position fixing members  108  are constituted respectively from position adjusting elongate holes  110  disposed in the base plate  70 , and fixing screws  112 , which are screwed respectively into the sub-blocks  106 . Movements of the sub-blocks  106  are guided along the position adjusting elongate holes  110 , and by tightening the fixing screws  112 , the sub-blocks  106  are fixed at predetermined positions. 
         [0093]    In setting the width of the groove portion  100 , in the case that a deformity occurs on the front surface  32  of the instrument panel  20  due to the groove portion  100  being narrow as shown in  FIG. 9A , the fixing screws  112  of the block position fixing members  108  are loosened, and the sub-blocks  106  are moved. Accordingly, the width of the groove portion  100  is widened as shown in  FIG. 9C , and then the sub-blocks  106  are fixed by the block position fixing members  108 . At this time, by the aforementioned support block  104  being divided into the sub-blocks  106 , the groove widths of the first through third grooves  100   a,    100   b,    100   c  can all be changed evenly. 
         [0094]    Consequently, by widening the thickness of the groove portion  100 , during vibration welding, the amount of displacement of the front surface  32  side of the instrument panel  20  on which the tear line  66  is provided can be set to an appropriate amount. In this manner, by providing the position adjusting mechanism  102  that enables a width or position of the groove portion  100  to be adjusted, the vibration welding device  10  is capable of suitably adjusting the amount at which the instrument panel  20  is displaced corresponding to the width of the groove portion  100 . Accordingly, the condition of the instrument panel  20  after vibration welding can easily be adjusted. Further, corresponding to the heat at the time of vibration welding or the condition (material or thickness, etc.) of the instrument panel  20 , since the need to prepare a plurality of support jigs  82  is eliminated, versatility of the vibration welding device  10  is increased. 
       Second Modification 
       [0095]    A support jig  82 B according to a second modification, as shown in  FIG. 10A , is of a configuration in which a width α′ of a groove portion  120  is set comparatively wide, and a depth β′ of the groove portion  120  is set shallowly. The depth β′ of the groove portion  120  is of a degree that enables a bottom wall  122  to contact the front surface  32  of the instrument panel  20  at the time of vibration welding. Consequently, even though the width α′ of the groove portion  120  is formed to be wide, when vibration welding is conducted, the amount of displacement of the front surface  32  of the instrument panel  20  is regulated by the bottom wall  122 . As a result, accompanying cooling upon completion of vibration welding, the front surface  32  of the instrument panel  20  is able to return to a substantially flat shape. 
       Third Modification 
       [0096]    With a support jig  82 C according to a third modification, as shown in  FIG. 10B , the edge of a groove portion  130  is formed with rounded corners  132 . The groove portion  130  that is formed with such rounded corners  132  is capable of suppressing localized pressure on the front surface  32  of the instrument panel  20 , when sandwiched between the support jig  82 C and the vibration applying jig  88 . Therefore, even though vibration welding is carried out, it is possible to reduce the possibility for marks or the like to be formed on the front surface  32  of the instrument panel  20 . 
         [0097]    Although a preferred embodiment of the present invention has been described above, the present invention is not limited to this embodiment, and it goes without saying that various modifications can be adopted within a scope that does not depart from the essential gist of the present invention. For example, the vibration welding device  10  is not limited to being applied to vibration welding of the instrument panel  20  and the airbag unit  28 , and can suitably be applied to vibration welding of other workpieces having cutout portions such as grooves or holes, etc., therein.