Abstract:
A method for producing a trim panel for an airbag support assembly comprising the steps of forming a spray aromatic urethane skin and forming a tear seam in the skin by scoring the skin with an ultrasonic knife.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates generally to motor vehicle passenger restraint systems and more particularly to a seamless trim panel for a motor vehicle airbag support assembly and a method of making the same.  
           [0002]    The incorporation of airbags into motor vehicles has resulted in many design challenges for vehicle designers. It is desirable to incorporate an airbag into a motor vehicle trim panel, such as the instrument panel in the front passenger compartment of the vehicle. When incorporating an airbag into the trim panel, it is necessary to provide an airbag cover that will reliably and safely tear during a collision. To this end, a tear seam has generally been provided in the trim panel cover to insure that the airbag safely deploys.  
           [0003]    Vehicle designers prefer to have a continuous surface on the front face of the trim panel whenever possible, so the designers would prefer to avoid having a tear seam in the trim panel that is visible to the vehicle passengers. Thus, recently there has been an effort to achieve a “seamless” design, wherein there is no visible indication on the front face of the trim panel that an airbag is located behind the trim panel. To achieve this design, tear seams provided in the trim panel are only provided on the under face, and not on the front face, of the trim panel.  
           [0004]    A major design challenge with such “seamless” designs has been to provide a cover for the trim panel that has a tear seam that is not visible on its front face. A conventional cover has a relatively soft skin with a tear seam on its under face. The presence of the tear seam is often visible in the form of a protrusion on a front face of the soft skin due to its soft, pliable nature. A harder skin may be employed, however, providing a tear seam in a harder skin has proven to be difficult because harder skins interfere with the deployment of the airbag.  
           [0005]    It is desirable to create a cover for the trim panel that has a skin that is sufficiently hard to prevent a visible sign of a tear seam on its front surface while not interfering with the deployment of the airbag. Preferably, such a cover would have a greater tensile strength, a greater tear strength, and greater elongation than covers currently used. Such a cover also would be easier and more cost effective to produce.  
         SUMMARY OF THE INVENTION  
         [0006]    In a disclosed embodiment of this invention, an airbag support assembly includes a trim panel cover that includes a spray aromatic urethane skin having a tear seam in its under face, wherein the tear seam is formed with an ultrasonic knife. The invention is also directed to a method for producing a trim panel for an airbag support assembly comprising the steps of forming a spray aromatic skin and scoring the skin with an ultrasonic knife.  
           [0007]    These and other features and advantages of this invention will become more apparent to those skilled in the art from the following detailed description of the presently preferred embodiment. The drawings that accompany the detailed description can be described as follows. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a front view of an instrument panel having an airbag assembly designed according to the present invention.  
         [0009]    [0009]FIG. 2 is a cross-sectional view along line  2 - 2  of FIG. 1.  
         [0010]    [0010]FIG. 3 is a perspective view of the skin on a fixture and a mechanical arm supporting an ultrasonic knife for scoring the skin to make a tear seam therein.  
         [0011]    [0011]FIG. 4 is an enlarged cross-sectional view of a skin with a tear seam therein.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0012]    An embodiment of a vehicle instrument panel according to the present invention is shown generally at  20  in FIG. 1. The vehicle instrument panel  20  has a skin or shell  24 . Shown in phantom line is an airbag support assembly  28  located behind the skin  24 . Although the airbag support assembly  28  in the illustrated embodiment is located behind the skin  24  of the instrument panel  20 , it can be located behind any other trim panel that is positioned in the passenger compartment of the vehicle.  
         [0013]    As illustrated in FIG. 2, the instrument panel  20  preferably includes a substrate or support  22 , which may be in the form of a structural plastic material, such as acrylonitrile-butadiene styrene (ABS)/polycarbonate, thermoplastic poly-olefin (TPO), polypropylene, polyphylene oxide (PPO)/high-impact polystyrene (HIPS), or styrene malaeic anhydrid (SMA). The substrate  22  has an opening formed therein through which an airbag may be deployed. The opening is closed by a door flap  32 . The door flap  32  is preferably in the form of plastic, aluminum, or other suitable lightweight material. An outer surface of the substrate  22  and the door flap  32  are preferably bonded to a layer of a suitable energy absorbing elastomeric foam, such as urethane foam, that forms a cushion  50  in an underlying relationship to the skin  24 . The skin  24  forms an outer decorative surface for the instrument panel  20 .  
         [0014]    The door flap  32  has a plurality of outer edges and the opening has a plurality of sides. A flexible hinge member  36  connects one of the outer edges of the door flap  32  to one of the sides of the opening. The hinge member  36  is contiguous along substantially the entire length of one of the outer edges of the door flap  32 . As should be appreciated by one of ordinary skill in the art, the hinge member  36  can be in the form of multiple hinges spaced along an outer edge of the door flap  32 . Alternatively, the hinge member  36  can be in the form of a single hinge that extends along an outer edge of the door flap  32 . The hinge member  36  may be any suitable shape or configuration.  
         [0015]    In the illustrated embodiment, the outer edges of the door flap  32  are spaced inwardly from the sides of the opening, although such is not necessary. This spacing creates a gap  35  between the sides of the opening and the outer edges of the door flap  32 . The gap  35  or the edge of the door closely overlies the tear seam  38  on an under surface of the skin  24 .  
         [0016]    The airbag support assembly  28  in the illustrated embodiment is supported beneath the door flap  32  by a welded joint between an outer rim  30  of the airbag support assembly  28  and an under face of the substrate  22 . A support structure  52  extends rearward from the outer rim  30 . The support structure  52  includes a pair of L-shaped brackets  54 . An airbag module  56  includes a pair of channels  62 . The channels  62  are shaped to slidably mate with a portion of the L-shaped brackets  54 .  
         [0017]    The airbag module  56  contains the airbag  60  and an airbag inflator  58  for inflating the airbag  60 . The airbag module  56 , the airbag  60 , and the airbag inflator  58  are all located behind the door flap  32 . The airbag module  56 , the airbag  60 , and the airbag inflator  58  are shown schematically for illustration purposes since such modules are well known in the art and their construction forms no part of this invention. It should be appreciated by one of ordinary skill in the art that the shape of the airbag module  56 , the airbag  60 , and the airbag inflator  58  may vary from that shown.  
         [0018]    During a collision, when the airbag  60  deploys, it will initially be forced against an underside of the door flap  32 . The airbag  60  then ruptures the skin  24  along the tear seam  38  so that the portion of the skin  24  within the outlines of the tear seam  38  separates from the rest of the skin  24  and moves with the door flap  32 . The flexible hinge member  36  permits the door flap  32  to rotate outwardly (in the direction of arrow A when viewing FIG. 2) so that the airbag  60  can be released into the passenger compartment. During and after deployment of the airbag  60 , the weld joint maintains contact between the under face of the substrate  22  and the outer rim  30  of the airbag support assembly  28 . In the illustrated embodiment, the outer rim  30  surrounds the door flap  32  and the tear seam  38 . Thus, the outer rim  30  is supported around the entire opening in the substrate  22 , around the tear seam  38 , and around the door flap  32 .  
         [0019]    In accordance with the present invention, the skin  24  is a spray aromatic urethane skin. The skin  24  is formed from two-components, which, when mixed together, produce a chemical reaction. The mixture is forced through a tube (not shown) and further through a nozzle (not shown) at the end of the tube to produce various spray patterns. The mixture is deposited on a heated tool where it solidifies after a brief period of time (e.g.,  45  seconds) to form the skin  24 . The skin  24  according to the present invention has a greater tensile strength, a greater tear strength, and a greater elongation than skins currently in use. The skin  24  according to the invention is also harder and thus does not permit visible signs of the tear seam  38  to appear on the front face thereof. Moreover, the skin  24  is easier and more cost effective to produce.  
         [0020]    As illustrated in FIG. 3, the skin  24  is placed on a fixture  64  with the front face of the skin  24  tightly against the fixture  64 . This can be accomplished in any suitable manner. A mechanical or robotic arm  66  is located next to the fixture  64 . The mechanical arm  66  supports an ultrasonic knife  68 . Mechanical arms and ultrasonic knives are well known in the art and their construction forms no part of this invention. The ultrasonic knife  68  is guided by the mechanical arm  66  along the skin  24  to score the skin  24  to form the tear seam  38 . This weakens the skin  24  in a localized area.  
         [0021]    The ultrasonic knife  68  preferably modulates or vibrates at about  20 , 000  cycles per second to about  100 , 000  cycles per second. The ultrasonic knife  68  makes a very clean V-shaped tear seam, as illustrated at  38  in FIG. 4. The tear seam  38  may be a continuous tear seam, or a discontinuous tear seam. The mechanical arm  66  precisely controls the distance between the surface of the fixture  64  and the tip of the blade of the ultrasonic knife  68  so that the material of the skin  24  remaining between the tear seam  38  and the front face of the skin  24  remains substantially constant. This is important because the thickness of the skin  24  may vary. Consequently, the depth of the tear seam  38  varies according to variations in the depth of the skin  24 . In a preferred embodiment of the invention, the skin  24  is about  0 . 8  mm to about  1 . 3  mm and the material remaining between the tear seam  38  and the front face of the skin  24  is in a range of about 0.1 mm and about 0.9 mm. In such case, the skin  24  is sufficiently hard and thick to conceal the tear seam  38  on the front face of the skin  24  but the tear seam  38  sufficiently weakens the skin  24  to allow the airbag  60  to be deployed without interference.  
         [0022]    The foregoing description is exemplary rather than limiting in nature. Variations and modifications to the disclosed embodiment may become apparent to those skilled in the art and do come within the scope of this invention. Accordingly, the scope of legal protection afforded this invention can only be determined by studying the following claims.  
         [0023]    In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.