Abstract:
The present method of forming a groove in an inner surface of a member for covering an airbag includes the steps of heating a blade by use of an induction-heating device and pushing the blade against the inner surface of the member. The induction-heating device is preferably a coil for induction heating. In addition, the present method of forming the groove preferably includes the steps of placing the member on a work bench having the coil, supplying alternating current to the coil, bringing the blade close to the coil, pushing the blade against the inner surface of the member, stopping current supply to the coil, and pulling the blade away from the inner surface of the member.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims priority from Japanese Patent Application No. 2007-217423 filed Aug. 23, 2007. The entire content of this priority application is incorporated herein by reference. 
   TECHNICAL FIELD 
   The present invention relates to a method of forming a groove in an inner surface of a member for covering an airbag. 
   BACKGROUND OF THE INVENTION 
   A known airbag device includes an airbag and an airbag case that accommodates the airbag therein. The airbag device is mounted in the front passenger seat side of a vehicle in some cases. In these cases, the airbag device is mounted behind an instrument panel. 
   The instrument panel is provided with an opening for allowing the airbag to inflate therethrough into the vehicle interior. The opening is, in normal conditions, covered with an airbag lid. In a crash of the vehicle, the airbag inflates, pushes the airbag lid from behind, and thereby opens the opening. 
   The airbag device has an airbag lid manufactured integrally with the instrument panel these days, whereby the airbag lid is invisible from the outer surface side of the instrument panel. The instrument panel with this type of airbag device has a groove in the inner surface. The groove has a depth to the extent not to reach the outer surface. The groove surrounds an area, and the area defines the airbag lid. Note that the groove is referred to as “a tear line” or the like in some cases. 
   A known method of forming the groove is described in Japanese unexamined patent application publication No. 2004-82865. In the method, a heated blade is pushed against the inner surface of the instrument panel. With the method, the groove having the depth to the extent not to reach the outer surface of the instrument panel is formed in the inner surface of the instrument panel. 
   In another known method, laser light is emitted onto the inner surface of the instrument panel. Also with this method, the groove having the depth to the extent not to reach the outer surface of the instrument panel is formed in the inner surface of the instrument panel. 
   The known methods of forming the groove have problems as below. 
   With the method of forming the groove by use of the heated blade, the blade edge is heated with a heater or the like installed at the base of the blade. Therefore, heat conduction to the blade edge is not uniform, which causes unevenness in temperature at the blade edge. This results in a problem that it is difficult to form the groove having a uniform depth and width in the inner surface of the instrument panel. 
   Also with the known method of forming the groove, when the blade edge is heated, the blade edge is difficult to be cooled. This causes molten resin to be partially raised by the blade edge when the blade edge is pushed against the inner surface of the instrument panel and then pulled up. This results in the problem that it is difficult to form the groove having the uniform depth and width in the inner surface of the instrument panel. 
   As for the method of forming the groove by use of the laser light, since the laser light is equal to or more than 1 mm in width, the formed groove is unexpectedly wider. This causes a problem that the groove formed in the inner surface of the instrument panel can be seen through the outer surface of the instrument panel. That is, there is the problem that the groove formed in the inner surface of the instrument panel is unexpectedly visible from the outer surface side. 
   SUMMARY OF THE INVENTION 
   The present invention was completed based on the above situation, and its purpose is to form a groove having a uniform width and depth in an inner surface of a member for covering an airbag such as an instrument panel. 
   A first aspect of the present invention is a method of forming a groove in an inner surface of a member. The member can cover an airbag. The method includes the steps of heating a blade by use of an induction-heating device, and pushing the blade against the inner surface of the member. 
   In accordance with the first aspect of the present invention, because the blade is heated by the use of the induction-heating device, the groove having a uniform width and depth can be formed in the inner surface of the member for covering the airbag. 
   Also in accordance with the first aspect of the present invention, only a blade edge can be heated by the use of the induction-heating device. Therefore, the blade edge can be heated and cooled within a short time period. The groove having the uniform and accurate width and depth thus can be formed in the inner surface of the member for covering the airbag. 
   A second aspect of the present invention is the method according to the first aspect of the present invention, wherein the induction-heating device includes a coil for induction heating. The method further includes the steps of placing the member on a work bench, the work bench including the coil, supplying alternating current to the coil, bringing the blade close to the coil, pushing the blade against the inner surface of the member, stopping the current supplied to the coil, and pulling the blade away from the inner surface of the member. 
   In accordance with the second aspect of the present invention, the blade can be pulled away from the inner surface of the member after current supply to the coil for induction heating is stopped. Therefore, the groove having the uniform width and depth can be formed in the inner surface of the member for covering the airbag. 
   A third aspect of the present invention is the method according to the first or second aspect of the present invention, wherein the member includes a skin of a vehicle interior material. 
   In accordance with the third aspect of the present invention, the groove having the uniform width and depth can be formed in the inner surface of the skin of the vehicle interior material. 
   A fourth aspect of the present invention is the method according to any one of the first to third aspects of the present invention, wherein the skin is genuine leather. 
   In accordance with the fourth aspect of the present invention, the groove having the uniform width and depth can be formed in the inner surface of the skin made of genuine leather. 
   A fifth aspect of the present invention is the method according to the third aspect of the present invention, wherein the vehicle interior material includes an instrument panel. 
   In accordance with this aspect of the present invention, the groove having the uniform width and depth can be formed in the inner surface of the instrument panel for covering the airbag. 
   A sixth aspect of the present invention is the method according to any one of the first to fifth aspects of the present invention, wherein the blade includes a thin blade. 
   In accordance with the sixth aspect of the present invention, the narrower groove can be formed. Therefore, the groove is difficult to be seen from the outer surface side. 
   In accordance with the present invention, the groove having the uniform width and depth can be formed in the inner surface of the member, such as the instrument panel, for covering the airbag. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of an instrument panel; 
       FIG. 2  is a sectional view of the instrument panel taken along line A-A of  FIG. 1 ; 
       FIG. 3  shows illustrative configurations of a groove forming apparatus for forming a groove in an inner surface of a skin of the instrument panel; 
       FIG. 4  is a perspective view of a blade having a continuous blade edge; 
       FIG. 5  is a perspective view of the instrument panel having substantially H-shaped groove formed in the inner surface of thereof; and 
       FIG. 6  is a perspective view of a blade having a notched blade edge. 
   

   DETAILED DESCRIPTION 
   An embodiment in accordance with the present invention will be now described with reference to drawings. 
     FIG. 1  is a perspective view of an instrument panel  10 .  FIG. 2  is a sectional view of the instrument panel shown in  FIG. 1  taken substantially along line A-A. 
   As shown in  FIGS. 1 and 2 , the instrument panel  10  that is a kind of vehicle interior materials includes a skin  12 , a cushion  14 , and a base  16 . The skin  12 , the cushion  14 , and the base  16  are laminated in that order from an outer surface side to an inner surface side. Note that the skin  12  corresponds to “a member for covering an airbag” of the present invention. 
   The skin  12  may be made of, for example, genuine leather, synthetic leather, thermoplastic polyolefin resin (TPO resin), flexible polyvinyl chloride resin (PVC resin), thermoplastic polyurethane resin (TPU resin), or the like. 
   The cushion  14  may be made of, for example, resin foam such as polyurethane foam, polyolefin foam. 
   The base  16  may be made of, for example, polypropylene (PP) resin alone, or a mixture of PP resin with reinforcing filler such as talc, mica, glass, or else, modified polyphenylene oxide (PPO) resin, acrylonitrile butadiene styrene (ABS) resin, or the like may be used. 
   As shown in  FIGS. 1 and 2 , an airbag device  18  is mounted behind the instrument panel  10  in the front passenger seat side. The airbag device  18  includes an airbag and an airbag case. The airbag is accommodated in the airbag case. In a crash of the vehicle, an inflator is activated and thereby instantly inflates the airbag. The airbag device  18  may be any known airbag device. 
   A groove  22  is formed in the inner surface of the instrument panel  10  in the front passenger seat side. The groove  22  substantially has an inverted U shape. The groove  22  is referred to as a tear line in some cases. The groove  22  has a depth in the inner surface of the instrument panel to the extent not to reach the outer surface. 
   An area surrounded by the groove  22  defines an airbag lid  20 . The airbag lid  20  is integral with the instrument panel  10 . The airbag lid  20  is almost invisible from the outer surface side of the instrument panel  10 . 
   When the airbag device  18  is actuated in the crash of the vehicle, the airbag inflates. Then, inflation pressure of the airbag fractures the instrument panel  10  along the groove  22 , while pressing the airbag lid  20  from the inner surface side toward the vehicle interior side. As a result, an opening is formed at a place where the airbag lid  20  existed. The airbag inflates through the opening toward the vehicle interior. 
     FIG. 3  illustratively shows a groove forming apparatus  40  for forming the groove  22  in the inner surface of the instrument panel  10 . 
   As shown in  FIG. 3 , the groove forming apparatus  40  includes a work bench  24 , a blade  26 , a coil  28  for induction heating, and a power source  30 . The work bench  24  is arranged to support the skin  12 . The blade  26  is made of metal such as stainless steel. The coil  28  is arranged to heat the blade  26 . The power source  30  is arranged to supply alternating current to the coil  28 . 
     FIG. 4  is a perspective view of the blade  26 . As shown in  FIG. 4 , the blade  26  used with the groove forming apparatus  40  of this embodiment has a continuous blade edge. The blade  26  is constituted by a thin blade having the blade edge equal to or less than 0.1 mm in thickness. 
   A method of forming the groove  22  in the inner surface of the skin  12  by use of the groove forming apparatus  40  will be now described. 
   First, the skin  12  is placed with the inner surface up on the work bench  24 . 
   Next, alternating current (e.g. alternating current of 20 kHz, 2 kW) is supplied to the coil  28  and thereby lines of magnetic force are generated, while the blade  26  is brought close to the coil  28 . Since eddy current is thus generated in the surfaces of the blade  26 , joule heat is generated and the blade  26  is heated. Note that the coil  28  corresponds to “an induction-heating device” of the present invention. 
   Next, the heated blade  26  is pushed against the inner surface of the skin  12 . The groove  22  is thus formed in the inner surface of the skin  12 . 
   Next, alternating current supply to the coil  28  is stopped. The temperature at the blade edge of the blade  26  thus rapidly decreases, because the heat at the blade edge of the blade  26  transfers to the other portion, i.e. because the heat at the blade edge of the blade  26  dissipates into a base portion of the blade  26  or the surrounding atmosphere. 
   By the rapid decrease in temperature at the blade edge of the blade  26 , the temperature in molten resin existing at the portion abutting against the blade edge of the blade  26  rapidly decreases. Thus, the molten resin is instantly solidified, and thereby the groove  22  having a desired width and depth can be formed. 
   After then, the blade  26  is pulled away from the inner surface of the skin  12 . Thus, the groove  22  having the depth to the extent not to reach the outer surface is formed in the inner surface of the skin  12 . 
   With the method of forming the groove in accordance with this embodiment, the following effects are obtained: 
   The blade edge of the blade  26  can be evenly heated by the use of the heat-induction device having the coil  28  for induction heating. Therefore, the groove  22  having a uniform width and depth can be formed in the inner surface of the instrument panel. 
   With the method of this embodiment for forming the groove, the groove  22  can be formed in the inner surface of the skin  12  also after the skin  12 , the cushion  14 , and the base  16  are laminated. 
   For example, a groove or a slit are formed in advance in the base  16  and the cushion  14 . Next, the skin  12 , the cushion  14 , and the base  16  are laminated in that order. Next, the heated blade  26  is pushed against the inner surface of the skin  12 , and thereby the groove  22  is formed. Thus, the groove  22  having the uniform width and depth can be formed in the inner surface of the instrument panel  10  constituted by the skin  12 , the cushion  14 , and the base  16 . 
   With the method of forming the groove in accordance with this embodiment, only the blade edge of the blade  26  can be heated by the use of the induction-heating device. Therefore, the blade edge of the blade  26  can be heated within a short time period, and likewise, the blade edge of the blade  26  can be cooled within a short time period. 
   Therefore, the groove  22  having the uniform width and depth can be formed in the inner surface of the skin  12 . 
   With the method of forming the groove in accordance with this embodiment, the blade  26  is constituted by a thin blade equal to or less than, for example, 0.1 mm in thickness at the blade edge thereof. Therefore, the significantly narrower groove can be formed in the inner surface of the skin  12 . Therefore, the airbag lid  20  that is almost invisible from the outer surface side of the instrument panel  10  can be realized. 
   Other Embodiments 
   The present invention is not limited to the embodiments described above with reference to the drawings, the following embodiments are also included within the scope of the present invention. Further various variations other than the following embodiments are also possible within the scope of the invention. 
   (1) In the above embodiment, the groove  22  illustratively has an substantial inverted U shape, however, the invention is not limited to this. For example, a groove  22   a  may have a substantial H shape as shown in  FIG. 5 . Even in this case, the inflation pressure of the airbag can fracture the instrument panel  10  along the groove  22   a . Likewise, in the crash of the vehicle, the airbag can be allowed to inflate into the vehicle interior. 
   (2) In the above embodiment, the blade  26  illustratively has a continuous blade edge, however, the invention is not limited to this. For example, a blade  26   a  having a notched blade edge may be used as shown in  FIG. 6 . In a case where such a blade  26   a  is used, a dashed-lined groove  22   b  can be formed in the inner surface of the skin  12 . 
   (3) In the above embodiment, a material of the blade  26  is illustratively stainless steel, however, the present invention is not limited to this. It is only necessary for the material of the blade  26  to be any electrical conducting material which is available for electromagnetic induction heating. The material of the blade  26  may be any electrical conducting material such as, for example, titanium, spring steel, or the like. 
   (4) In the above embodiment, the member for covering the airbag is illustratively the skin  12  of the instrument panel  10 , however, the present invention is not limited to this. The present invention is applicable also for any vehicle interior material other than the instrument panel  10 . The present invention is applicable also for any vehicle interior material such as, for example, a steering pad, a door trim, a pillar garnish, or the like. 
   (5) In the above embodiment, the airbag device  18  is mounted illustratively in the front passenger seat side, however, the present invention is not limited to this. The present invention is applicable also for an airbag to be mounted behind a steering pad, a side airbag, a curtain airbag, or the like. 
   (6) In the above embodiment, the blade  26  is pushed against illustratively only the skin  12  to form the groove, however, the present invention is not limited to this. The blade  26  may be pushed against the skin  12  and cushion  14  at once after these members are laminated. Or else, the blade  26  may be pushed against the skin  12 , cushion  14 , and base  16  at once after these members are laminated.