Patent Publication Number: US-2023138018-A1

Title: Vehicle seat and method for folding side airbag

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2021-179712 filed on Nov. 2, 2021, incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a vehicle seat including a side airbag and a method for folding the side airbag. 
     2. Description of Related Art 
     WO2011/006560 discloses a side airbag in which a thorax section that inflates and deploys from a lateral region of a backrest of a vehicle seat and a head section that inflates and deploys from an upper region of the backrest are composed of separate bags and connected to each other via a tubular filling section. In this side airbag, the head section inflates and deploys between a shoulder belt and an occupant&#39;s head. Therefore, the head can be restrained early in the event of side collision. c 
     SUMMARY 
     In the technology described in WO2011/006560, the head section is housed along the upper end of a seat frame. When the head section is housed in a roll form, the head section may overlap the skeleton of a headrest, thereby affecting deployment performance. The above related art has room for improvement in this respect. 
     The present disclosure provides a vehicle seat including a side airbag and a method for folding the side airbag that can improve the deployment performance of a head chamber constituting the side airbag. 
     A first embodiment of the present disclosure relates to a vehicle seat. The vehicle seat includes a seat back including an upper frame, an inflator configured to supply gas, and a side airbag. The side airbag includes a head chamber housed along the upper frame of the seat back and configured to inflate and deploy between a shoulder belt and a head in response to supply of gas from the inflator in an event of side collision. The head chamber is housed while being folded in a seat up-and-down direction to reduce a vertical width and then rolled from a front end in a seat fore-and-aft direction. 
     In the first embodiment, the side airbag inflates and deploys in response to the supply of gas from the inflator in the event of side collision. The side airbag includes the head chamber housed along the upper frame of the seat back. The head chamber inflates and deploys between the shoulder belt and the head of an occupant to restrain the head at an early stage. The head chamber is housed while being folded in the seat up-and-down direction to reduce the vertical width and then rolled from the front end in the seat fore-and-aft direction. Therefore, the head chamber can be housed along the upper frame at a position that does not overlap the skeleton of a headrest. As a result, the inflation and deployment of the head chamber toward a front side of the seat are not impaired by the headrest, and the deployment performance of the head chamber is improved. In the first embodiment, the description “the head chamber is housed along the upper frame” may be a broad concept including not only a case where the entire head chamber is housed along the upper frame, but also a case where a part of the head chamber is housed along the upper frame. The description “in the event of side collision” may include not only a case where the occurrence of the side collision of the vehicle is actually detected, but also a case where the occurrence of the side collision of the vehicle is predicted. 
     In the first embodiment, the head chamber may be housed while being folded in the seat up-and-down direction multiple times into a bellows form and then rolled from the front end in the seat fore-and-aft direction. 
     In the structure described above, the head chamber may be folded in the seat up-and-down direction multiple times into the bellows form and then rolled from the front end in the seat fore-and-aft direction. Therefore, the rolled portion is released and the bellows portion is released at the same time during the inflation and deployment of the head chamber. When the bellows portion is released, the head chamber deploys toward an upper side of the seat over the shoulder belt. As a result, the inflation and deployment of the head chamber toward the front side of the seat are not impaired by the shoulder belt, and the deployment performance of the head chamber is improved. 
     In the first embodiment, the head chamber may be housed while being rolled from the front end in the seat fore-and-aft direction after an upper end in the seat up-and-down direction is folded downwardly toward an occupant and a lower end obtained by folding the upper end is folded upwardly toward the occupant. 
     In the structure described above, the head chamber may be rolled from the front end in the seat fore-and-aft direction after the upper end in the seat up-and-down direction is folded downwardly toward the occupant and the lower end obtained by folding the upper end is folded upwardly toward the occupant. By folding the upper end of the head chamber twice toward the occupant into the bellows form, the rolled portion is released and, at the same time, the bellows portion is released toward the upper side and an inner side of the seat during the inflation and deployment of the head chamber. As a result, when the head chamber deploys toward the upper side of the seat over the shoulder belt, a deployment space can satisfactorily be secured between the shoulder belt and the headrest, thereby facilitating the deployment of the head chamber toward the front side of the seat. 
     In the first embodiment, the head chamber may be housed while being folded in the seat up-and-down direction to reduce the vertical width, rolled from the front end in the seat fore-and-aft direction, and inwardly rolled into a roll form at least at a rear end that is an end of winding with a distal end of rolling located on an inner side of the seat. 
     In the structure described above, the head chamber may be folded in the seat up-and-down direction to reduce the vertical width, rolled from the front end in the seat fore-and-aft direction, and inwardly rolled at least at the rear end that is the end of winding. At an initial stage of the inflation and deployment of the head chamber, the inwardly rolled portion is released toward the inner side of the seat between the shoulder belt and the headrest. As a result, the inflation and deployment of the head chamber toward the front side of the seat are not impaired by the headrest and the shoulder belt, and the deployment performance is improved. 
     In the first embodiment, the seat back may include a side frame, and the side airbag may include a torso chamber housed along a region from a side portion to a shoulder of the side frame of the seat back and configured to inflate and deploy on a side of a torso in the event of side collision. The head chamber and the torso chamber may be provided in a single bag, and may be housed while being inwardly rolled from the front end in the seat fore-and-aft direction. 
     In the structure described above, the side airbag may include the torso chamber housed along the region from the side portion to the shoulder of the side frame of the seat back. In the event of side collision of the vehicle, the torso chamber inflates and deploys on the side of the torso of the occupant to restrain the torso at an early stage. In the side airbag, the head chamber and the torso chamber may be provided in a single bag. Therefore, the side airbag may be housed along the upper frame via the shoulder from the side portion of the side frame. The head chamber and the torso chamber may be housed while being inwardly rolled from the front end in the seat fore-and-aft direction. Therefore, the side airbag can easily be folded by inwardly rolling the entire side airbag including the head chamber and the torso chamber. Further, the deployment performance of the head chamber can be improved. 
     A second embodiment of the present disclosure relates to a method for folding a side airbag including a head chamber to be housed along an upper frame of a seat back. The method for folding the side airbag includes folding the head chamber in a seat up-and-down direction to reduce a vertical width and rolling the head chamber from a front end in a seat fore-and-aft direction. 
     According to the second embodiment, the inflation and deployment of the head chamber toward the front side of the seat are not impaired by the headrest, and the deployment performance of the head chamber is improved. 
     As described above, according to the first embodiment and the second embodiment of the present disclosure, it is possible to improve the deployment performance of the head chamber constituting the side airbag. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the disclosure will be described below with reference to the accompanying drawings, in which like signs denote like elements, and wherein: 
         FIG.  1    is a front view showing a vehicle seat according to an embodiment in a state in which a side airbag inflates and deploys; 
         FIG.  2    is a partially cutout front view showing the vehicle seat according to the embodiment in a state in which the side airbag is housed; 
         FIG.  3    is an enlarged side view showing a state in which the side airbag according to the embodiment inflates and deploys; 
         FIG.  4 A  is a side view of a head chamber for description of a method for folding the side airbag according to the embodiment, showing a step of downwardly folding the upper end of the head chamber in a seat up-and-down direction toward an occupant; 
         FIG.  4 B  is a side view of the head chamber for the description of the method for folding the side airbag according to the embodiment, showing a step of upwardly folding the folded lower end toward the occupant; 
         FIG.  4 C  is a side view of the head chamber for the description of the method for folding the side airbag according to the embodiment, showing the head chamber in a state in which the steps in  FIGS.  4 A and  4 B  are completed; 
         FIG.  4 D  is an enlarged plan of the head chamber for the description of the method for folding the side airbag according to the embodiment, showing a step of inwardly rolling the head chamber and a torso chamber from the front ends in a seat fore-and-aft direction; 
         FIG.  4 E  is an enlarged plan of the head chamber for the description of the method for folding the side airbag according to the embodiment, showing a state in which the step of folding the side airbag is completed; 
         FIG.  5    is a side view of the side airbag according to the embodiment, showing a state before the folded side airbag is subjected to a thickness reduction process; 
         FIG.  6    is a schematic diagram for describing the thickness reduction process by heat press according to the embodiment; 
         FIG.  7 A  is a plan of the vehicle seat for description of behavior of the side airbag according to the embodiment during inflation and deployment, showing behavior at an initial stage of the inflation and deployment; 
         FIG.  7 B  is a plan of the vehicle seat for the description of the behavior of the side airbag according to the embodiment during the inflation and deployment, showing behavior at a latter stage of the inflation and deployment; 
         FIG.  8 A  is a partial rear view of the vehicle seat for the description of the behavior of the side airbag according to the embodiment during the inflation and deployment, showing the behavior at the initial stage of the inflation and deployment; 
         FIG.  8 B  is a partial rear view of the vehicle seat for the description of the behavior of the side airbag according to the embodiment during the inflation and deployment, showing behavior at a middle stage of the inflation and deployment; 
         FIG.  8 C  is a partial rear view of the vehicle seat for the description of the behavior of the side airbag according to the embodiment during the inflation and deployment, showing the behavior at the latter stage of the inflation and deployment; 
         FIG.  9    is a schematic diagram corresponding to  FIG.  6    for description of a thickness reduction process by heat press according to a modification of the embodiment; 
         FIG.  10 A  is a schematic diagram for describing a thickness reduction process according to a modification of the embodiment, relating to a modification in which a part of an outer peripheral portion of the head chamber is covered with a surface material; 
         FIG.  10 B  is a schematic diagram for describing a thickness reduction process according to a modification of the embodiment, relating to a modification in which a part of the outer peripheral portion of the head chamber is covered with a surface material; 
         FIG.  10 C  is a schematic diagram for describing a thickness reduction process according to a modification of the embodiment, relating to a modification in which a part of the outer peripheral portion of the head chamber is covered with a surface material; 
         FIG.  10 D  is a schematic diagram for describing a thickness reduction process according to a modification of the embodiment, relating to a modification in which a part of the outer peripheral portion of the head chamber is covered with a surface material; 
         FIG.  11    is a partially enlarged front view showing a state in which the head chamber subjected to any one of the thickness reduction processes shown in  FIGS.  10 A to  10 D  is housed; 
         FIG.  12    is an enlarged sectional view of a cross section cut along a line XII-XII in  FIG.  5   , showing the state before the folded side airbag according to the embodiment is subjected to the thickness reduction process; and 
         FIG.  13    is an enlarged sectional view of a cross section cut along a line XIII-XIII in  FIG.  5   , showing the state before the folded side airbag according to the embodiment is subjected to the thickness reduction process. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, a vehicle seat  14  including a side airbag device  10  according to an embodiment will be described with reference to  FIGS.  1  to  8 C . Arrows FR, UP, LH, and RH in the drawings indicate a seat front side, a seat upper side, a seat left side, and a seat right side of the vehicle seat  14  including the side airbag device  10 , respectively. When description is made using directions of “front” and “rear”, “right” and “left”, and “up” and “down”, the terms “front” and “rear” indicate a front and a rear in a seat fore-and-aft direction, the terms “right” and “left” indicate a right and a left in a seat width direction, and the terms “up” and “down” indicate a top and a bottom in a seat up-and-down direction unless otherwise specified. 
     A dummy P for a collision test is seated on the vehicle seat  14  shown in  FIGS.  1 ,  7 A, and  7 B  instead of an actual occupant. Examples of the dummy P include AM50 (50th percentile of an American adult male) of the World Side Impact Dummy (World SID). In the following description, the dummy will be referred to as “occupant P”. 
     Vehicle Seat 
     As shown in  FIGS.  1  and  2   , the vehicle seat  14  including the side airbag device  10  includes a seat cushion  16 , a seat back  18 , and a headrest  20 . For example, the vehicle seat  14  is arranged in a driver&#39;s seat on a right side of a vehicle, and the front of the seat and the front of the vehicle coincide with each other. The right and left in the seat width direction and the right and left in a vehicle width direction coincide with each other. 
     The seat cushion  16  extends in the seat fore-and-aft direction and the seat width direction, and can support buttocks and thighs of the occupant P. The seat back  18  is turnably coupled to the rear end of the seat cushion  16  and extends in the seat up-and-down direction. The seat back  18  can support the back of the occupant P. The headrest  20  is provided at the upper end of the seat back  18  and can support a head H of the occupant P. 
     Seat Belt Device 
     The occupant P is restrained to the vehicle seat  14  by a seat belt device  24 . The seat belt device  24  includes a webbing  26  that restrains the upper body and the waist of the occupant. 
     The webbing  26  has a long band shape, and includes a shoulder belt  26 A that restrains the upper body of the occupant P to the seat back  18  and a lap belt  26 B that restrains the waist of the occupant when the webbing  26  is attached to the occupant P. The shoulder belt  26 A extends diagonally from the shoulder on the right side of the occupant P to the waist on the left side. The lower end of the shoulder belt  26 A is inserted through a tongue plate  28 . 
     The tongue plate  28  is detachable from a buckle (reference symbol is omitted) provided on the left side of the seat. The occupant P is restrained by the webbing  26  by attaching the tongue plate  28  to the buckle. 
     The one end of the webbing  26  that is looped back through the tongue plate  28  extends to the right side of the seat, and a portion extending in the seat width direction serves as the lap belt  26 B. Therefore, the left end of the lap belt  26 B is connected to the lower end of the shoulder belt  26 A. The right end of the lap belt  26 B is fixed to a belt anchor (not shown) provided on a floor panel. 
     The upper end of the shoulder belt  26 A is looped around a shoulder anchor (not shown) provided on a vehicle body. The end of the webbing  26  is reeled by a retractor (not shown). 
     Side Airbag Device 
     As shown in  FIGS.  2  and  3   , the side airbag device  10  includes a side airbag  12 , an inflator  42 , and a sock  44 . 
     The side airbag  12  is housed inside the seat back  18 , and is fixed to a seat back frame  30  constituting the skeleton of the seat back  18 . As partially shown in  FIG.  2   , the seat back frame  30  has a rectangular frame shape when viewed in the seat fore-and-aft direction, and its right and left side portions are side frames  32  extending in the seat up-and-down direction. The upper end of the seat back frame  30  is an upper frame  34  extending in the seat width direction to bridge the shoulders (upper ends) of the right and left side frames  32 . The side airbag  12  is housed along the upper frame  34  via the shoulder from the side portion of the side frame  32  on the right side of the seat while being folded into an elongated rod shape. 
     The side airbag  12  inflates by being supplied with gas generated by the inflator  42 . The side airbag  12  inflates and deploys between the body of the occupant P and the side of a vehicle cabin by rupturing an outer skin of the seat back  18  in the seat width direction with an inflation pressure. 
     The side airbag  12  is a single bag, and includes a head chamber  12 A that is an upper part on the upper side of the seat, and a torso chamber  12 B that is a lower part on the lower side of the seat. 
     The head chamber  12 A inflates and deploys between the shoulder belt  26 A and the head H of the occupant P in response to the supply of gas from the inflator  42  to cover the side of the head H. The torso chamber  12 B inflates and deploys on the side of a torso T of the occupant P (portion extending from the shoulder to the waist via the thorax) in response to the supply of gas from the inflator  42  to cover the side of the torso T. The detailed structure of the side airbag  12  will be described later. 
     The inflator  42  is a cylinder-type gas generator having a substantially cylindrical shape, and its axial direction is along the side frame  32  of the seat back  18 . A gas injector  42 A is provided at the lower end of the inflator  42 . When a side collision of the vehicle is detected or predicted, the gas injector  42 A generates gas and supplies the gas to the side airbag  12 . 
     As shown in  FIG.  3   , a part of the inflator  42  is arranged inside the side airbag  12 . The inflator  42  has stud bolts  48  extending inward in the seat width direction from an outer surface located inside the side airbag  12 , and is fixed to the side frame  32  together with a base cloth  50  of the side airbag  12  by inserting the stud bolts  48  through the side frame  32  and tightening the stud bolts  48  with nuts. 
     The sock  44  made of a cloth material is further provided inside the side airbag  12 . The sock  44  is referred to also as “duct”, “inner tube”, “baffle cloth”, or “diffuser”. 
     The sock  44  has a substantially cylindrical shape with upper and lower open ends, and extends from the head chamber  12 A to the torso chamber  12 B in the seat up-and-down direction. A bag-shaped inflator insertion portion  46  is integrally provided at the rear end of the sock  44 . The gas injector  42 A of the inflator  42  and the inside of the sock  44  communicate with each other via the inflator insertion portion  46 . Therefore, the gas supplied from the inflator  42  flows to the head chamber  12 A and the torso chamber  12 B through the sock  44 . 
     Side Airbag 
     The detailed structure of the side airbag  12  will be described below. The side airbag  12  is formed into a bag shape by arranging two base cloths  50  made of, for example, nylon-based or polyester-based cloth materials in the seat width direction and sewing their outer peripheral portions. In  FIG.  3   , the sewn portion of the outer peripheral portions of the base cloths  50  is represented by reference symbol “ 51 ”. 
     An upper portion of the side airbag  12  is the head chamber  12 A that covers the side of the head H of the occupant P during inflation and deployment. The head chamber  12 A has a substantial C-shape with its break oriented to the lower side of the seat when viewed in the seat width direction. A band-shaped support belt  52  is sewn at the rear end of the head chamber  12 A. One end of the support belt  52  is attached to a stay  36  (see  FIG.  2   ) constituting the skeleton of the headrest  20 , thereby stabilizing the behavior of the head chamber  12 A during the inflation and deployment. 
     A lower portion of the side airbag  12  is the torso chamber  12 B that covers the side of the torso T of the occupant during the inflation and deployment. The torso chamber  12 B has a shape of a drop bulging downward when viewed in the seat width direction, and extends in the up-and-down direction along the seat back  18 . 
     Method for Folding Side Airbag 
     As described above, the side airbag  12  having the structure described above is housed along the skeleton of the seat back  18  while being folded into the elongated rod shape. Hereinafter, a method for folding the side airbag  12  will be described with reference to  FIGS.  4 A to  4 E . In  FIGS.  4 A to  4 E , the sewn portion  51  of the side airbag  12  is omitted. 
     The method for folding the side airbag  12  according to the present embodiment is roughly divided into a first step for folding the head chamber  12 A in the seat up-and-down direction, and a second step for rolling the side airbag  12  from its front end in the seat fore-and-aft direction after completion of the first step. 
     The first step will be described with reference to  FIGS.  4 A to  4 C . In  FIGS.  4 A and  4 B , a folding line of the head chamber  12 A in each step is represented by a long dashed double-short dashed line. 
     In the first step shown in  FIG.  4 A , the upper end of the head chamber  12 A in the seat up-and-down direction is first folded downwardly toward the occupant P (toward the inner side of the seat). Through this step, the portion previously located at the upper end of the head chamber  12 A before the folding is currently located at the lower end by the folding. 
     As shown in  FIG.  4 B , the folded lower end is then folded upwardly toward the occupant P. 
     Through the steps described above, the head chamber  12 A is folded in the seat up-and-down direction multiple times (twice in the present embodiment) into a bellows state. Therefore, the vertical width of the folded head chamber  12 A is reduced as shown in  FIG.  4 C . Thus, the first step is completed. 
     In the second step shown in  FIGS.  4 D and  4 E , the side airbag  12  is rolled by winding the base cloth  50  around an elongated rod-shaped winding shaft  40 . 
     In the second step shown in  FIG.  4 D , the winding shaft  40  is set in a posture in which its axial direction is substantially the seat up-and-down direction, and the base cloth  50  of the side airbag  12  is inwardly rolled from the front end to the rear end of the side airbag  12  in the seat fore-and-aft direction. In this step, the entire side airbag  12  including the head chamber  12 A and the torso chamber  12 B is inwardly rolled. 
     The inward rolling is a folding method in which the side airbag  12  is wound (rolled) into a roll form with the distal end of the rolling located on the inner side of the seat. 
     Through the step described above, the rear end of the head chamber  12 A in the seat fore-and-aft direction that is the end of winding is inwardly rolled as shown in  FIG.  4 E . Thus, the second step is completed. 
     The side airbag  12  folded through the first step and the second step has an elongated rod shape. In the state in which the side airbag  12  is housed in the seat back  18  as shown in  FIG.  2   , the head chamber  12 A is housed along the upper frame  34  of the seat back frame  30 , and the torso chamber  12 B is housed along a region from the side portion to the shoulder of the side frame  32 . 
     Since the vertical width of the head chamber  12 A is set small through the first step described above, the distal end (upper end) is arranged on an outer side of the seat with respect to the stay  36  of the headrest  20 . That is, the head chamber  12 A is housed at a position that does not overlap the skeleton of the headrest  20 . 
     Thickness-Reduced Portion and Thickness Reduction Process 
       FIGS.  5 ,  12 , and  13    are a side view and enlarged sectional views of the side airbag  12  folded through the first step and the second step. As shown in those figures, a thickness of the side airbag folded in the roll form when housed, that is, a fold thickness corresponds to a roll diameter (diameter) φ when the side airbag  12  is viewed in the seat up-and-down direction (winding axis direction). 
     The roll diameter φ of the side airbag  12  increases as the number of layers of the base cloth  50  in a radial direction increases. Therefore, the roll diameter φ increases as the number of folds of the base cloth  50  increases along with an increase in the size and capacity of the side airbag  12 . Particularly in the head chamber  12 A of the present embodiment, the base cloth  50  is folded in the seat up-and-down direction prior to the rolling. Therefore, the roll diameter φ in a predetermined region is larger than that in the other region. 
     For example,  FIG.  12    shows a cross section of the lower end of the head chamber  12 A cut along a line XII-XII in  FIG.  5   . In one example of the present embodiment, the lower end of the folded head chamber  12 A is a first region  12 A 1  having a relatively small number of layers of the base cloth  50  in the radial direction. A roll diameter φ 1  of the first region  12 A 1  is substantially equal to the roll diameter of the torso chamber  12 B. 
       FIG.  13    shows a cross section of the upper end of the head chamber  12 A cut along a line XIII-XIII in  FIG.  5   . As shown in  FIG.  13   , in one example of the present embodiment, the upper end of the folded head chamber  12 A is a second region  12 A 2  in which the number of layers of the base cloth  50  in the radial direction is larger than that in the first region  12 A 1 . Therefore, a roll diameter φ 2  of the second region  12 A 2  is larger than the roll diameter φ 1  of the first region  12 A 1 . 
     In a state in which the head chamber  12 A is housed along the upper frame  34  of the seat back frame  30 , the roll diameter φ of the head chamber  12 A is a fold thickness in a seat height direction. In a region having a large roll diameter, such as the second region  12 A 2 , it is necessary to secure a mounting space in consideration of the fold thickness in the seat height direction. 
     Since the headrest  20  is provided above the upper frame  34 , there is a great design restriction on the mounting space in the seat height direction (seat up-and-down direction). In the region having a large roll diameter φ, such as the second region  12 A 2  of the head chamber  12 A, it is desirable to take measures to save the mounting space at the time of housing. 
     In the present embodiment, in the region having a large fold thickness of the head chamber  12 A, such as the second region  12 A 2 , a thickness-reduced portion  60  is provided in at least a part of the outer peripheral portion of the head chamber  12 A. The thickness-reduced portion  60  is subjected to a thickness reduction process for reducing the fold thickness of the head chamber  12 A by pressurization. 
       FIG.  6    is a schematic diagram showing an example of the thickness reduction process. This schematic diagram shows the head chamber  12 A viewed in the winding axis direction. As shown in  FIG.  6   , the thickness reduction process of the present embodiment is heat press using a die  62 . In the process by the heat press, the folded head chamber  12 A is placed to face a recess  64  formed inside the die  62 , and the outer peripheral portion of the head chamber  12 A is pressurized and heated in the die  62 . Thus, the roll diameter φ of the head chamber  12 A can be reduced. In  FIG.  6   , the outline of the head chamber  12 A before the thickness reduction process is represented by long dashed double-short dashed lines. 
     The recess  64  of the present embodiment has a substantially circular outer peripheral shape, and the entire outer peripheral portion can be pressurized and heated uniformly. 
     The heat press having the structure described above provides a certain effect even when the side airbag  12  is made of the base cloth  50 . In some embodiments, the base cloth  50  may be a coated base cloth. When the base cloth  50  is the coated base cloth, a part of the surface of a coating material such as a silicone resin or a nylon resin applied to the base cloth  50  is melted and cured again by heating. Therefore, the shape reduced by pressurization can be kept satisfactorily as compared with the case where the base cloth  50  is a non-coated base cloth. 
       FIG.  2    shows the head chamber  12 A housed in the seat back  18  through the thickness reduction process. In  FIG.  2   , the second region  12 A 2  including the thickness-reduced portion  60  is shown by a region enclosed by a long dashed double-short dashed line. As shown in  FIG.  2   , the fold thickness of the second region  12 A 2  is reduced by the thickness-reduced portion  60 , and the fold thickness of the region from the proximal end (first region  12 A 1 ) to the distal end (second region  12 A 2 ) of the head chamber  12 A housed along the upper frame  34  is set to a substantially uniform fold thickness. In this way, it is possible to save the mounting space of the head chamber  12 A. 
     Deployment Behavior of Head Chamber 
     Hereinafter, behavior of the head chamber  12 A during inflation and deployment will be described in detail. First, the behavior of the head chamber  12 A will be described with reference to plans of  FIGS.  7 A and  7 B . 
     When the gas from the inflator  42  is supplied to the side airbag  12 , the head chamber  12 A inflates and deploys toward the front side of the seat by rupturing the outer skin at the upper end of the seat back  18 . Since the head chamber  12 A is arranged along the upper frame  34  at the position that does not overlap the skeleton of the headrest  20 , the head chamber  12 A can deploy toward the front side of the seat while suppressing interference with the skeleton of the headrest  20 . 
     As shown in  FIG.  7 A , the inwardly rolled portion at the rear end of the head chamber  12 A is released at an initial stage of the inflation and deployment. The inwardly rolled portion of the head chamber  12 A is released toward the inner side of the seat between the shoulder belt  26 A and the headrest  20 . Therefore, a deployment space is secured between the shoulder belt  26 A and the headrest  20 . As a result, the head chamber  12 A can deploy toward the front side of the seat over the shoulder belt  26 A as shown in  FIG.  7 B . 
     Next, the behavior of the head chamber  12 A will be described with reference to rear views of the vehicle seat  14  in  FIGS.  8 A to  8 C . During the inflation and deployment of the head chamber  12 A shown in  FIGS.  8 A to  8 C , the rolled portion is released, and the bellows portion formed by folding the head chamber  12 A in the seat up-and-down direction is released at the same time. When the bellows portion is released, the upper end of the head chamber  12 A deploys toward the upper side of the seat to extend easily over the shoulder belt  26 A. 
     In the present embodiment, the head chamber  12 A is folded in the seat up-and-down direction toward the occupant P. Therefore, the bellows portion is released toward the upper side and the inner side of the seat. Thus, the deployment space of the head chamber  12 A can satisfactorily be secured between the shoulder belt  26 A and the headrest  20 . 
     Even if the head chamber  12 A is housed with its upper end folded once toward the lower side of the seat, it is possible to obtain a certain effect that the head chamber  12 A quickly deploys toward the upper side of the seat. When the head chamber  12 A is folded twice as in the present embodiment, the swing of the head chamber  12 A in the seat width direction can be reduced during the deployment. Thus, the deployment performance toward the upper side of the seat can be increased. 
     Actions and Effects 
     In the vehicle seat  14  of the present embodiment described above, the side airbag  12  inflates and deploys in response to the supply of gas from the inflator  42  in the event of side collision. The side airbag  12  includes the head chamber  12 A housed along the upper frame  34  of the seat back  18 . The head chamber  12 A inflates and deploys between the shoulder belt  26 A and the head H of the occupant P to restrain the head H at an early stage. 
     The head chamber  12 A is housed while being folded in the seat up-and-down direction to reduce the vertical width and then rolled from the front end in the seat fore-and-aft direction. Therefore, the head chamber  12 A can be housed along the upper frame  34  at the position that does not overlap the skeleton of the headrest  20 . As a result, the inflation and deployment of the head chamber  12 A toward the front side of the seat are not impaired by the headrest  20 , and the deployment performance of the head chamber  12 A is improved. 
     In the present embodiment, the head chamber  12 A is folded in the seat up-and-down direction multiple times into a bellows form. Specifically, the upper end of the head chamber  12 A in the seat up-and-down direction is folded downwardly toward the occupant P, the folded lower end is folded upwardly toward the occupant P, and then the head chamber  12 A is rolled from the front end in the seat fore-and-aft direction. At the initial stage of the inflation and deployment of the head chamber  12 A, the rolled portion is released and the bellows portion is released toward the upper side and the inner side of the seat at the same time. Therefore, the head chamber  12 A deploys toward the upper side of the seat over the shoulder belt  26 A, and the deployment space is secured between the shoulder belt  26 A and the headrest  20 . As a result, the inflation and deployment of the head chamber  12 A toward the front side of the seat are not impaired by the shoulder belt  26 A, and the deployment performance of the head chamber  12 A is improved. 
     In the present embodiment, the side airbag  12  is housed while being inwardly rolled from the front end in the seat fore-and-aft direction. Therefore, the rear end of the head chamber  12 A that is the end of winding is inwardly rolled. At the initial stage of the inflation and deployment of the head chamber, the inwardly rolled portion is released toward the inner side of the seat between the shoulder belt and the headrest. As a result, the inflation and deployment of the head chamber  12 A toward the front side of the seat are not impaired by the headrest  20  and the shoulder belt  26 A. Therefore, the deployment performance of the head chamber  12 A can be increased. 
     In the present embodiment, the side airbag  12  is a single bag, and the head chamber  12 A and the torso chamber  12 B are housed while being inwardly rolled from the front end in the seat fore-and-aft direction. Therefore, the side airbag  12  can easily be folded by inwardly rolling the entire side airbag  12  including the head chamber  12 A and the torso chamber  12 B. 
     In the present embodiment, the head chamber  12 A includes the thickness-reduced portion  60  in at least a part of the outer peripheral portion. Since the thickness-reduced portion  60  is subjected to the thickness reduction process for reducing the fold thickness by pressurization, the folded shape can be downsized by compressing at least the part of the outer peripheral portion. Therefore, it is possible to save the mounting space by providing the thickness-reduced portion  60  in the portion having a large fold thickness relative to the mounting space permitted depending on the seat design. As a result, it is easy to secure a mounting space adapted to the seat design, thereby responding to an increase in size and capacity of the head chamber  12 A. 
     The thickness-reduced portion  60  of the present embodiment is formed by heat-pressing the outer peripheral portion of the folded head chamber. The thickness-reduced portion formed in this way does not require an additional member other than the side airbag. Therefore, the manufacturing process is not complicated and the manufacturing can be facilitated. 
     In the present embodiment, the side airbag  12  is housed in the seat back  18  while being rolled from the front end in the seat fore-and-aft direction. Therefore, there is little variation in the base cloth at the outer peripheral portion, and the shape can be kept easily. Thus, the thickness reduction process can easily be performed after the head chamber  12 A is folded. 
     Supplementary Description 
     While the vehicle seat  14  according to the embodiment of the present disclosure has been described above, the present disclosure may be implemented in various modes within the scope of the claims. 
     Modification 1 of Thickness Reduction Process 
     For example, in the heat press used in the embodiment described above, the fold thickness is reduced by uniformly pressurizing and heating the entire outer peripheral portion of the head chamber  12 A, but the heat press is not limited to this case. For example, as in Modification 1 shown in  FIG.  9   , the die  62  to be used for the heat press may have a recess  72  having a substantially elliptical outer peripheral shape to form a thickness-reduced portion  70  so that the shape of the head chamber  12 A after the process is flat in the seat height direction. As a result, the fold thickness in the seat height direction can further be reduced by the thickness-reduced portion  70  when the head chamber  12 A is housed along the upper frame  34 . 
     As shown in  FIGS.  10 A to  10 D , the thickness reduction process may be performed with a surface material arranged on the outer peripheral portion of the folded head chamber  12 A. 
     Modification 2 of Thickness Reduction Process 
     For example, as in Modification 2 shown in  FIG.  10 A , the outer peripheral portion of the head chamber  12 A may be heat-pressed in the die  62  while being covered with a hot-melt surface material  82 . Examples of the surface material  82  include a hot-melt felt (nonwoven fabric) made of a resin material having heat meltability. In a thickness-reduced portion  80  formed in this way, the reduced shape of the head chamber  12 A is kept by the surface material  82  melted on the surface of the outer peripheral portion. Therefore, the side airbag  12  is manageable, and the attachment to the seat back  18  can be facilitated. 
     Modification 3 of Thickness Reduction Process 
     For example, as in Modification 3 shown in  FIG.  10 B , the thickness reduction process may be a method using pressurization based on a shrinkage force of a surface material  92 . The thickness reduction process of Modification 3 is performed by covering the outer peripheral portion of the folded head chamber  12 A with a heat-shrinkable film having a heat-shrinking property and then heating the surface material  92  from the outside. The surface material  92  pressurizes the outer peripheral portion of the head chamber  12 A with a shrinkage force generated by the heating, thereby forming a thickness-reduced portion  90  in which the fold thickness of the head chamber  12 A is reduced. In Modification 3, the surface material  92  is the film, but is not limited to the film, and may be a heat-shrinkable tube. 
     Modification 4 of Thickness Reduction Process 
     As in Modification 4 shown in  FIG.  10 C , a surface material  102  to be used for the thickness reduction process may be a sheet-shaped member having a shrinking property with an elastic force. For example, the surface material  102  is a sheet-shaped member made of an elastic rubber material, and is wound around the outer peripheral portion of the head chamber  12 A while being stretched by elastic deformation. The head chamber  12 A is pressurized by an elastic restoration force of the surface material  102 , thereby forming a thickness-reduced portion  100  in which the fold thickness is reduced. The surface material  102  according to Modification 4 is not limited to the sheet-shaped member, but may be a tubular member. 
     In the thickness-reduced portions  90  and  100  formed by the thickness reduction processes of Modifications 3 and 4, the outer peripheral portions are covered with the surface materials  92  and  102  to keep the shapes after the shrinkage. Therefore, the side airbag is manageable, and the attachment to the seat back can be facilitated. 
     Since the surface materials  92  and  102  have the heat-shrinking property or the shrinking property based on the elastic force, a dedicated die or press machine, which is used in the case of performing the thickness reduction process by the heat press, is not required. As a result, the scale of manufacturing equipment can be reduced. 
     Modification 5 of Thickness Reduction Process 
     As in a thickness reduction process of Modification 5 shown in  FIG.  10 D , a surface material  112  need not have a shrinkage force. In this case, it is possible to perform a thickness reduction process for reducing the fold thickness by, for example, pressurizing the outer peripheral portion of the head chamber  12 A in such a manner that the surface material  112  is wound around the outer peripheral portion under tension. A thickness-reduced portion  110  can be formed in this way by using a wrapping material to be used as a packaging material for an airbag. Thus, the material cost can be reduced. 
     As shown in  FIG.  11   , each of the surface materials  82 ,  92 ,  102 , and  112  of the thickness-reduced portions  80 ,  90 ,  100 , and  110  according to Modifications 2 to 5 has a rupturable portion  120  facing the front side of the seat in a state in which the head chamber  12 A is housed. Examples of the rupturable portion  120  include an array of a plurality of thin grooves formed on the surface of each of the surface materials  82 ,  92 ,  102 , and  112  or a plurality of through holes formed through each of the surface materials. During the inflation and deployment of the side airbag  12 , each of the surface materials  82 ,  92 ,  102 , and  112  is quickly ruptured along the rupturable portion  120  by a gas pressure applied from the head chamber  12 A. As a result, the inflation and deployment of the head chamber  12 A toward the front side of the seat are not impaired by each of the surface materials  82 ,  92 ,  102 , and  112 , and the deployment performance can be improved. 
     In the embodiment described above, the side airbag  12  inflates and deploys from the outer lateral side in the vehicle width direction with respect to the body of the occupant P, but the side airbag  12  is not limited to this case. The side airbag  12  may inflate and deploy from an inner lateral side in the vehicle width direction (center side of the vehicle) with respect to the body of the occupant P. 
     In the embodiment described above, the side airbag device  10  is mounted on the front seat of the vehicle, but the present disclosure is not limited to this case. The side airbag device  10  may be mounted on a rear seat of the vehicle.