CROSS-REFERENCE TO RELATED APPLICATION 
     This is a division of application Ser. No. 09/829,445 filed Apr. 9, 2001 which is a division of application Ser. No. 09/479,166 filed Jan. 7, 2000 (now U.S. Pat. No. 6,213,497) which is a CIP of application Ser. No. 09/130,939 filed Aug. 7, 1998 (now U.S. Pat. No. 6,032,978) all of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This invention is related generally to inflatable vehicle protective devices and, more particularly, to an inflatable knee bolster. 
     BACKGROUND OF THE INVENTION 
     Inflatable knee bolsters have been developed to enhance vehicle occupant protection in the event of sudden vehicle deceleration. Such devices increase the duration of deceleration of occupants&#39; femurs and control upper torso kinematics during a frontal impact event. The bolster impact surface is moved from a stored position to an extended position to intercept an occupant&#39;s knees early in the impact event. This allows the knees more time to decelerate to the vehicle&#39;s final velocity. The initial positioning of the bolster impact surface in a retracted, stored position allows more styling flexibility and enhances occupant ingress and egress. 
     Typical inflatable knee bolster installations comprise an inflatable air bag sandwiched between an impact surface and a reaction surface. When the inflator is triggered, the air bag expands to move the impact surface a predetermined distance to active position. This position may be determined by tethers between the reaction and impact surfaces. These installations comprise numerous parts, bits and pieces and require careful installation. 
     It would be desirable to provide a simpler inflatable knee inflatable bolster which would be less costly to manufacture and assemble into the vehicle. To this end it is believed to be advantageous to utilize bolster configurations which utilize a relatively small number of components which may be easily assembled and which exhibit effective deployment in an operational situation. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a simpler inflatable knee bolster which would be less costly to manufacture and assemble into the vehicle. 
     This invention provides advantages and alternatives over typical prior knee bolster installations by providing an inflatable knee bolster which may be deployed from a stored state to an operational state by expansion of the bolster body itself. Such configuration reduces and more preferably eliminates any reliance on the use of an air bag within the bolster structure thereby reducing the complexity of assembly and installation and effecting a reduction in the number of active components within the operating device. 
     In one aspect this invention features an inflatable bolster for a vehicle occupant having an outer wall that is projected outwardly from a stored position to an extended position by inflation of an expansible chamber by an inflator. The expansible chamber includes a unitary hollow body having an outer wall and an inner wall interconnected by U-shaped normally folded accordion pleats. An attachment tab extends from the top of the chamber for attachment to vehicle structure, and another attachment tab extends from the bottom of the chamber for attachment to vehicle structure. Alternatively, the bottom of the outer surface could be attached directly to vehicle structure. 
     In another aspect of this invention, the bolster is an inflatable knee bolster having an outer wall that is projected outwardly to an extended position adjacent the occupant&#39;s knees by inflation of an expansible chamber by an inflator. The outer and inner walls are interconnected at their bottoms by a bottom wall and at their tops and sides by the accordion pleats which are U-shaped and extend from the bottom side corners up the sides and across the top of the body. The top tab is a frangible tab which is broken by expansion of the expansible chamber, allowing the outer wall to pilot about the bottom wall and extend. Preferably, tethers on the frangible tab limit movement of the outer wall. Also, vent holes are preferably included to provide subsequent controlled deflation of the expansible body. The hollow body is preferably a blow molding of polypropylene. 
     According to another aspect of the present invention, an inflatable knee bolster of multipiece construction is provided. The bolster is formed of separate portions forming the face and an expansible body which are joined about their peripheries. Formed within the body portion are a plurality of at least partially collapsible profile elements extending in raised and lowered profile within the cavity between the face and the expansible body. At least a portion of the profile elements undergo collapsible deformation upon pressurization of the cavity by an inflator thereby expanding the volume of the cavity and forcing the face outwardly from a stored position to an extended position. 
     According to yet other potentially preferred aspects of the invention, the face portion and body portion of the bolster of multipiece construction are formed from injection molded polymeric materials. The materials forming the face portion and the body portion are preferably of different durometers such that the face portion is more rigid than the body portion. The materials forming the face and the body portions may be either of similar or dissimilar chemical composition. 
     According to yet further potentially preferred aspects of the present invention, the face portion and body portion of the bolster of multipiece construction are joined along their peripheries by mateable projectile and aperture elements with intermediate ridges therebetween. The projectile and aperture elements are preferably fusion bonded to effect melt flow of the projectile elements within the aperture elements between the ridges. 
     In accordance with other potentially preferred aspects of the present invention the knee bolster includes mounting elements extending outwardly from the underside of the body portion facing away from the vehicle occupant for attachment to the vehicle frame. The mounting elements are preferably metal brackets which undergo flexure and subsequent plastic deformation upon impact by a vehicle occupant. 
     Advantageously, the knee bolster according to the present invention serves to provide a construction which is easily assembled and may be expanded to its operable position either with or without the use of an internally disposed air bag component. The orientation of expansion elements and the materials of construction may be selected to provide preferential expansion of designated regions of the bolster to enhance design alternatives. 
     Additional objects features and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description. While the invention is illustrated and will be described in connection with potentially preferred embodiments, it is to be understood that such illustration and description are exemplary and explanatory only and are in no way to be construed as restrictive of the invention. On the contrary, it is the intent of the applicants to cover all alternatives, modifications, and equivalents as may be included within the true spirit and scope of the invention as limited only by the full scope of allowable claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
     FIG. 1 is a perspective view of the driver&#39;s side of a vehicle passenger compartment showing the position of an inflatable knee bolster of this invention in stored position; 
     FIG. 2 is a front view of the inflatable knee bolster illustrated in FIG. 1; 
     FIG. 3 is a rear view of the inflatable knee bolster of FIG. 1; 
     FIG. 4 is a sectional view taken along line  4 — 4  of FIG. 3; 
     FIG. 5 is a sectional view taken along line  5 — 5  of FIG. 3; 
     FIG. 6 is a sectional view taken along line  6 — 6  of FIG. 3; 
     FIG. 7 is a partial side view of the passenger compartment of FIG. 1, showing the inflatable knee bolster in stored position; 
     FIG. 8 is a view similar to FIG. 7, showing the inflatable knee bolster in extended position; 
     FIG. 9 is a rear view of another embodiment of inflatable knee bolster according to this invention: 
     FIG. 10 is a sectional view taken along line  10 — 10  of FIG. 9; 
     FIG. 11 is a sectional view taken along line  11 — 11  of FIG. 9 
     FIG. 12 is an exploded perspective view of a further embodiment of an inflatable knee bolster according to this invention comprising at least two discrete panel sections; 
     FIG. 13 is an interior view of the face-forming panel of the inflatable knee bolster illustrated in FIG. 12; 
     FIG. 14 is a plan view of the underside of the body portion of the inflatable knee bolster illustrated in FIG. 12; 
     FIGS. 15A and 15B illustrate a potential arrangement for attachment of the panel elements illustrated in FIG. 12; 
     FIG. 16 is a sectional view taken along line  16 — 16  of FIG. 14; 
     FIG. 17 is a sectional view taken along line  17 — 17  of FIG. 14; 
     FIG. 18 is a sectional view taken along line  18 — 18  of FIG. 14; and 
     FIG. 19 is a view similar to FIG. 18 illustrating the knee bolster of FIG. 12 in expansion. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIG. 1, an automotive vehicle  20  has a passenger compartment  22  that includes front seats  24  for the occupants, including a driver  26 . The usual instrument panel  28  flanks the steering wheel  30 . An inflatable knee bolster  32  is mounted on the underside of instrument panel  28  adjacent the knees  34  of driver  26 . A similar arrangement is provided for the passenger. Knee bolster  34  is nearly flush with the underside of instrument panel  28 , as best seen in FIG.  8 . 
     Referring now to FIGS. 2-7, knee bolster  32  comprises a unitary hollow main body  36  formed of a blow molded polypropylene-based or other thermoplastic material. Main body  36  includes a front or outer wall  38  and a spaced inner wall  40  that are interconnected by a bottom wall  42 . At their tops and sides, outer wall  38  and inner wall  40  are interconnected by inverted U-shaped accordion pleats  44  and  46  which are normally folded, as shown. 
     A pair of transverse strengthening ribs  48  and  50  are formed in inner wail  40  and extend inwardly of body  36  into engagement with the rear side of outer wall  38 , as best seen in FIG. 5. A pair of integral lower mounting tabs  52  and  54  extend from bottom wall  42  for attachment by screws  56  to the bottom  58  of instrument panel  28  as best seen in FIG. 7. A pair of integral upper frangible mounting tabs  60  and  62 , having weakened sections  60 ′ and  62 ′, extend upwardly from body  36  for attachment to instrument panel bottom  58  by screws  56 . A tether  64  is fastened by screws  56  to both ends of upper mounting tabs  60  and  62 . 
     A cavity  66  is formed in inner wall  40  and includes an opening  68  for receiving a cylindrical inflator  70 . That is bolted at  72  to the upper wall of cavity  66 , as best shown in FIGS. 3 and 6. Inner wall  40  also includes a pair of spaced vent holes  76  and  78 , as shown in FIG.  3 . With this unitary construction, main body forms a hollow chamber  80  that is air tight except for vent holes  76  and  78 . 
     In the event of vehicle deceleration during a frontal impact event, an acceleration sensor (not illustrated) actuates inflator  70 , which pressurizes main body  36 . This forces main body  36  to expand by breaking frangible tabs  60  and  62  at  60 ′ and  62 ′ and unfolding accordion pleats  44  and  46 . This projects bolster outer wall  38  outwardly to the extended position shown in FIG. 8 adjacent occupant&#39;s knees  34 . This movement to extended position is limited by the length of tethers  64 . Thus, bolster outer wall  38  is in position to engage the occupant&#39;s knees  34  and lower leg to increase their duration of deceleration during vehicle deceleration. 
     An alternative embodiment of inflatable knee bolster, denoted  134 , is shown in FIGS. 9-11, where parts similar or identical to parts in the FIGS. 1-8 embodiment are identified by like numbers increased by  100 . Here, the main difference is the inclusion of four spaced vertical strengthening ribs  80 ,  82 ,  84  and  86  which are formed in inner wall  140 . These replace ribs  48  and  50  in the FIGS. 1-8 embodiment and provide the same strengthening function. Functioning of this embodiment is the same. 
     A further embodiment of an inflatable knee bolster  232  of multipiece construction is illustrated in FIG. 12 wherein the front wall  238  forming the face portion of the knee bolster  232  and a three dimensional expansible body portion  240  are formed as separate panels which are joined in sealed relation about their peripheries as will be described further herein. 
     The front wall  238  forming the face portion of the knee bolster preferably has a substantially smooth outer surface which may be contoured for disposition within the underside of the vehicle instrument panel. As shown in FIG. 13, the underside of the front wall  238  preferably includes a number of integrally molded male projectile elements  247  extending generally around the periphery of the underside for use in securing attachment to the panel forming the body portion  240 . In the illustrated and potentially preferred embodiment, the projectile elements  247  are of a generally hollow ellipsoidal geometry having substantially straight sides connected by curved ends and are of a height sufficient for mateable insertion and upward extension through corresponding apertures  249  disposed around the periphery of the body portion  240  (FIG.  14 ). 
     As best illustrated in FIG. 12, the front wall  238  forming the face portion of the knee bolster  232  is preferably of unitary construction having a raised perimeter edge  251  surrounding the interior surface thereby forming a tray-like structure. As illustrated, the body portion  240  preferably includes a perimeter wall  253  which is mateable in nesting relation within the perimeter edge  251  of the front wall  238  such that male projectile elements  247  extend upwardly through the corresponding apertures  249  which are disposed within a channel  257  between the perimeter wall  253  and adjacent expandable perimeter profile elements  259  which are preferably of a raised and lowered accordion configuration. This mating relationship is best illustrated in FIGS. 15A and 15B. 
     The projectile elements  247  may be held in place within the apertures  249  by joining techniques as are known to those of skill in the art including by way of example only and not limitation, sonic welding, vibration welding, laser welding, thermal melt bonding, adhesive bonding and combinations thereof. One potentially preferred method of attachment involves use of a moveable sonic welder  263  (FIG. 15B) operating a frequency of about 20 MHz. 
     According to the illustrated embodiment, rib-like ridges  265  of substantially right triangular configuration are disposed inboard of the perimeter wall  253  of the body portion  240  between the apertures  249 . Such ridges  265  may serve as weirs for the containment of melt material which flows outwardly from the projectile containing apertures during the melt bonding operation and in some configurations may provide additional support if desired. 
     Although the mateable bonding of projectile elements  247  within corresponding apertures  249  may represent one mechanism for the joinder of the discrete portions of the knee bolster  232 , it will be appreciated that a host of other mechanisms may likewise be utilized either independently or in combination which do not rely upon such geometries. Such attachment mechanisms include by way of example only, direct mechanical attachment, smooth service welding or chemical adhesion. 
     In the illustrated embodiment, the body portion  240  comprises a unitary panel which extends away from the front wall facing generally away from the vehicle occupant and towards the engine compartment. While panels of unitary molded construction may be preferred, it is to be understood that the panels forming the body portion  240  as well as the front wall  238  may likewise be formed from two or more pieces which are later joined together. 
     As previously referenced, and best illustrated in FIGS. 16-18, the underside of the body portion  240  preferably includes one or more integrally formed perimeter profile elements  259  extending in raised and lowered relief with respect to the front wall  238  inboard of and substantially adjacent to the line of attachment between the front wall  238  and the body portion  240 . As shown, the perimeter profile elements  259  are preferably arranged in a wave form accordion orientation and occupy a relatively wide band extending along the top edge of the body portion  240 . These perimeter profile elements  259  preferably taper away as they extend down the lateral sides of the body portion  240 . In addition, in the illustrated and potentially preferred embodiment, interior rib profile elements  269  extend from the back of the body portion  240  towards the front wall  238 . 
     The underside of the body portion  240  preferably includes an integral molded chamber for housing an inflator  270  as illustrated. However, as will be appreciated, the inflator  270  may likewise be housed in a location remote from the knee bolster  232  and be connected thereto by tubing or other fluid communication means. 
     According to the illustrated embodiment, raised attachment surfaces  273 ,  275  of elongated tapered profile preferably extend outwardly from the underside of the body portion  240  for attachment to brackets  277 ,  279  by retaining pins  281  (FIG.  12 ). The brackets  277 ,  279  are preferably of an inwardly angled profile and are formed of a material such as light gauge steel of about 2 mm thickness or like material to facilitate energy absorptive flexure followed by plastic deformation upon impact. As will be appreciated, such brackets  277 ,  279  may be mounted in supported relation to the vehicle frame by screws  285  or other attachment devices as may be known to those of skill in the art. 
     In the event of a collision event which does not give rise to deceleration values sufficient to activate the inflator  270 , it is believed that in some applications the brackets  277 ,  279  may nonetheless provide a degree of energy absorption through flexure and subsequent plastic deformation upon impact by an occupant. Bracket structures formed from plastically deformable material such as metal and having geometries which facilitate controlled bending under compression may be preferred for some applications. 
     Referring now to FIGS. 18 and 19, upon the occurrence of an impact event causing sufficient vehicle deceleration, the inflator  270  is actuated and releases a pressurizing medium which flows into the chamber  291  created by the enclosure between the face forming front wall  238  and the body portion  240 . Upon pressurization of the interior, both the perimeter profile elements  259  and the interior rib profile elements  269  preferably undergo at least a partial collapse thereby increasing the volume of the chamber and placing the underside of the body portion into compression as the profile elements  259 ,  269  seek a flattened profile as they are pushed away from the front wall  238 . Since the body portion  240  is held substantially rigidly in place in relation to the vehicle frame, the compressive stress is relieved by thrusting the front wall  238  outwardly to an operational position adjacent to the knees of the occupant. 
     As will be appreciated, the degree to which the front wall  238  is outwardly thrust will depend largely upon the surface area of the profile elements which undergo deformation. The preferential concentration of perimeter profile elements  259  extending along the upper edge of the knee bolster  232  will tend to provide preferential rotatable extension of the bolster at that region, while the interior rib profile elements  26  tend to distend the front wall as a whole. The orientation and concentration of such elements may thus be adjusted to facilitate characteristics of extension as may be desired for a given application. It is, of course to be understood that breakaway tethered attachment constructions including those as illustrated in FIGS. 2,  7  and  8  may also be utilized if desired. 
     In order to facilitate preferential pressurized expansion of the body portion  240 , in some applications it may be desirable that the materials forming the front wall  238  and body portion  240  are such that the face portion is more rigid than the body portion and is therefore less likely to undergo deformation. Such differential rigidity may be achieved by adjusting the thickness of the components or by using polymers of differing durometer for the front wall and body portions. Potentially preferred materials for forming both the front wall and the body portion of the knee bolster are injection molded polymers, including by way of example only, polypropylenes, polycarbonates, polyurethanes and other suitable thermoplastic materials. 
     It is believed that injection molding of the panels forming the front wall  238  and the body portion  240  may permit the achievement of extremely close tolerances which may be desirable for some applications. Other molding techniques as may be known to those of skill in the art including by way of example only, compression molding, extrusion and the like may also be utilized. 
     It is contemplated that the materials forming the front wall  238  and body portion  240  may be either of similar or dissimilar chemical composition. According to one embodiment, the front wall  238  is formed from an injection molded polycarbonate thermoplast of high rigidity and impact strength while the body portion  240  is formed from a flexible injection molded polyurethane rubber. The use of materials of such differential rigidity tends to facilitate the preferential expansion of the body portion  240 , while permitting the front wall to be substantially unaltered. Such performance characteristics may be desirable in certain applications. 
     While the present invention has been described as carried out in specific embodiments thereof, it is not intended to be limited thereby, but is intended to cover the invention broadly within the scope and spirit of the appended claims.
An inflatable knee bolster for a vehicle occupant is mounted in opposing relation to the knees of a vehicle occupant. The bolster is an expansible chamber of multipiece construction having a face portion and a separate three dimensional expansible body portion. The face portion and body portion are joined about their peripheries to define and substantially enclose the chamber. The expansible body portion is formed from injection molded polymeric material and has one or more integrally molded at least partially collapsible pleated profile elements disposed along the sides and across the top of the body portion in raised and lowered profile with respect to the face portion. Deformable mounting brackets extend outwardly from the body portion for attachment to the vehicle. The materials forming the face and body portions are of either similar or dissimilar chemical and physical character. Actuation of an inflator in fluid communication with the bolster pressurizes the expansible body causing at least a portion of the pleated profile elements to collapsibly deform such that the volume of the enclosed chamber is expanded and the face portion is thrust outwardly to an extended position.