Patent Publication Number: US-2003222063-A1

Title: Method of processing a laser scored instrument panel with an invisible seam airbag opening

Description:
FIELD OF THE INVENTION  
       [0001] This invention relates to a process for forming an automotive interior trim component and particularly for such a component which has been subjected to a laser scoring process.  
       BACKGROUND OF THE INVENTION  
       [0002] Multi-layer instrument panels for automobiles that incorporate an invisible seam airbag opening often utilize laser scoring to weaken the airbag opening to allow for deployment. During manufacturing, the laser cuts a continuous or perforated scoring pattern in the substrate through the underside (“B” surface) of the panel. In a trilaminate panel, the beam generally cuts through the substrate layer, foam layer, and then partially into the top skin layer which forms the visible (“A” side) of the panel. This system produces a weakened opening for a deployable restraint system in vehicles which is intended to be invisible on the “A” surface. One drawback of present production parts is that during heat ageing (as is seen in motor vehicles during natural life cycles or through validation heat cycling tests), the laser score line becomes visible on the top “A” surface as an indented groove or “witness mark” along the score line contour. Conventional wisdom indicates this is caused by the destruction of the underlying foam layer (via the laser) and the inability of the void to support the top skin layer during heating.  
       BRIEF SUMMARY OF THE INVENTION  
       [0003] This invention utilizes a pre-heat ageing method step to prevent the appearance of the witness mark on the “A” surface after laser scoring. By heat ageing the panel briefly before laser scoring, the appearance of the line on the “A” surface after laser scoring is reduced or completely eliminated.  
       [0004] Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which the present invention relates from the subsequent description of the preferred embodiment and the appended claims, taken in conjunction with the accompanying drawings. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0005]FIG. 1 is a cross-sectional view of an example trilaminate instrument panel material having a laser scored groove.  
     [0006]FIG. 2 is a process description flowchart in accordance with a prior art process.  
     [0007]FIG. 3 is a process description flowchart of a method in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0008]FIG. 1 illustrates a cross-sectional view of an example motor vehicle instrument panel  10  formed of a trilaminate configuration. The instrument panel  10  includes a top skin layer  12 , foam layer  14 , and substrate layer  16 . An additional interlayer  17  may also be provided between layers  14  and  16 . This composite structure forms what is known in the industry as an “A” surface  18  which is exposed toward the interior of the associated motor vehicle, and the backside or “B” surface  20 . Various processes for the forming of instrument panel  10  may be provided. For example, instrument panel  10  can be a laminate injection molded composite in which layers  12 ,  14  and  17  are melt bonded on a substrate  16  made of a hard or rigid material. During the laminate injection molded composite process, the foam layer  14  undergoes a compression set, due to the heat and pressure from injecting the material forming substrate layer  16  behind the trilaminate (layers  12 ,  14  and  17 ). This same compression set of the foam layer  14  is also created during the manufacturing of other trilaminate and bilaminate manufacturing process as well as other thermal forming processes. This compression set is released during heat ageing of instrument panel  10  as the foam layer  14  expands back toward its original thickness. Such heat aging occurs during normal exposure of instrument panel  10  to thermal cycles during its normal use in a motor vehicle, or during thermal cycling testing of the instrument panel. This tendency of relief of compression set causes the previously noted witness lines to appear on the instrument “A” surface  18 .  
     [0009]FIG. 1 further illustrates a laser score groove  22 . Laser scoring involves the use of a controlled laser light beam which is directed against the “B” surface  20  and destroys the substrate and foam layer  14  to create an opening for deployment of an associated airbag module (not shown). Laser score groove  22  weakens instrument panel  10  in a precise manner such that skin layer  12  will tear along score line  26  aligned with score groove  22 . This allows interior trim components, including instrument panels, to be formed which have airbag openings which are invisible to the motor vehicle occupant (viewing “A” surface  18 ). However, as mentioned previously, heat ageing of instrument panel  10  in use causes a release of the compression set of foam layer  14 .  
     [0010] The interactions between foam layer  14  and skin layer  12  differ in the area where the foam layer  14  supports skin layer  12 , and where it is removed in the area of score groove  22 . Laser score groove  22  imposes an impediment to the release of the compression set and the expansion of the foam of foam layer  14 . Score groove  22  is formed by burning caused by the laser light which makes the inside lining of score groove  22  hard and stiff. A U-shaped channel  28  (inverted as illustrated) of melted foam  24  is developed. Melted foam layer  24  acts like a solid plastic member having structural and compression characteristics differing from that of the remaining foam forming foam layer  14 . This “tube” formed by channel  28  also becomes welded to the underside of skin layer  12  which mechanically anchors skin layer  12  to the underlying substrate layer  16 . When the foam of foam layer  14  begins to release its compression set and expand during subsequent heating, the “tube” of channel  28  cannot expand and therefore holds the skin layer  12  in place. This “tube” becomes an anchor for the skin layer  12  at score line  26 . The surrounding foam of foam layer  14  expands, releasing its compression set while the score line  26  above score groove  22  does not. This creates a depressed groove along the laser score line  26 . The groove is not, as previously believed, caused by the foam  14  compressing along the score line  26  and sinking under the weight of the skin, but rather by the expansion of the foam of foam layer  14  around the score line  26 , while the skin layer  12  material along line  26  itself remains stationary. It is this new understanding of the mechanism of failure that gives rise to the method of this invention to correct the problem.  
     [0011] The appearance of such witness lines may also occur in irregular and unpredictable manner. Not only is the appearance of any witness line undesirable where the object is to produce an invisible airbag seam, but further such witness lines may be irregular in appearance around the perimeter of the airbag opening.  
     [0012] Laser scoring to form score groove  22  may be accomplished by numerous processes. One such process is described by U.S. Pat. No. 5,744,776 which is hereby incorporated by reference. Laser scoring processes typically use a CO 2  gas laser which creates a beam focused to a point. The beam may be advanced continuously to create score groove  22  which forms an invisible seam airbag deployment opening. Alternatively, the scoring can be a series of closely spaced holes forming a score groove  22  through closely spaced perforations. In either instance, however, the disadvantage of the prior art noted previously appears.  
     [0013] Now with reference to FIG. 2, a flowchart for processing instrument panel  10  in accordance with the prior art method is illustrated. Step  30  designates the beginning of the process. The part is removed from an injection mold at step  32 . This step could also include any other type of processing in which a final instrument panel  10  (or other type of panel) is produced. Thereafter, instrument panel  10  is stored for further processing at step  34 . Next, the instrument panel  10  is loaded into a laser scoring fixture at step  36 . Parts handling equipment next moves instrument panel  10  to a laser scoring station at step  38 . Laser scoring through any one of a number of potential process as described previously occurs at step  40 , yielding a scored instrument panel  10  at step  42 .  
     [0014] Now with reference to FIG. 3, a process in accordance with the present invention is illustrated. In this invention the instrument panel  10  is preheated to release the foam compression set prior to the laser scoring step. This is illustrated in the additional steps  44  through  50  shown in FIG. 3. Those steps which are common between FIGS. 2 and 3 are identified by like reference numbers.  
     [0015] After the instrument panel  10  is removed from mold at step  32  it is transferred to a dimensional holding fixture at step  44 . This holding fixture is designed to maintain the dimensional integrity of instrument panel  10  during further processing steps will now be described. Once fastened the holding fixture at step  44  instrument panel  10  is moved into an oven or another station where it is heated at step  46 . As stated previously, this heating is designed to release internal compression set forces within foam layer  14  acting on the instrument panel  10  structure. Thereafter, the part is cooled to room temperature at step  48  and is finally removed from the holding fixture at step  50 . Next, the part enters the process stream beginning at step  34  as previously described and at step  40  is laser scored.  
     [0016] When the instrument panel  10  has finally been scored at step  40 , the foam layer  14  has already expanded to its maximum thickness. Further heat aged testing does not produce a previously mentioned “groove” or witness line because foam  14  no longer expands.  
     [0017] As an example of implementation of the process of this invention, a Visteon Laminate Injection Molded (VLIM) instrument panel  10 , such as that seen on the Mazda Tribute (J14) vehicle is used in this example. In this case, the instrument panel design includes a seamless airbag opening on the passenger side. The trilaminate skin is a thermoplastic olefin (TPO) skin layer  12 , an olefinic foam middle layer  14 , with a polypropylene based interlayer  17 . The substrate layer  16  is a talc filled polypropylene such as ATX 832 produced by ATC, Inc., which is commercially available. The instrument panel  10  is placed in a heating oven at 230° F. for 30 minutes, prior to laser scoring. The part is then allowed to cool to room temperature before laser scoring the airbag door opening. The part is then used in the usual fashion for production.  
     [0018] Various alternatives can be implemented in accordance with the present invention. For example, various other heating times have been experimented with. It was found that 15 minute exposure at 250° F. in step  46  also produced the desired result for the previous mentioned material system comprising instrument panel  10 . Alternate methods of heating may also be implemented, for example, a portable heating element to locally heat the airbag opening area of instrument panel  10  or a heating bank used directly after molding at step  32  while the parts are on a fixture as may be required. In any event, the concept of this invention involves essentially the full release of compression set of the foam layer  14  prior to laser scoring. Experiments utilizing the process of this invention reveal no discernable witness lines following heat ageing cycling tests which are required by many original equipment manufacturers (OEM) customers.  
     [0019] In addition to instrument panels, there are numerous other interior trim components of motor vehicles which conceal airbag modules. For example, side impact airbags and side curtain airbag systems may be mounted to motor vehicle seats, door trim panels, headliners or various pillar structures of the motor vehicle interior compartment. For many of these applications, it is desirable to conceal the deployable restraint system and such concealment may be provided by laser scoring. Accordingly, this invention may also be implemented in connection with interior trim components in addition to instrument panels. The invention is believed best adapted for such applications where compression set forces act on the surface forming the panel&#39;s “A” surface.  
     [0020] While the above description constitutes the preferred embodiment of the present invention, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope and fair meaning of the accompanying claims.