Abstract:
A method for testing hydrostatic pressure of an inflator housing constituting an outer shell container for various inflators, which can evaluate a hydrostatic pressure conveniently is provided. The method comprises the steps of injecting water from a small hole provided in the inflator housing until the pressure inside the inflator housing reaches a predetermined pressure of an evaluation standard, keeping the inflator housing sealed tightly, or comprises the step of increasing the pressure inside the inflator housing to the predetermined pressure while injecting water in the inflator housing, draining the injected water, and drying the interior of the inflator housing.

Description:
This application claims priority on provisional Application No. 60/359,899 filed on Feb. 28, 2002, the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for testing hydrostatic pressure of various kinds of inflators in which a pressurized medium, such as an inert gas, is charged under a high pressure, and a method for manufacturing such various kinds of inflators. 
     2. Description of Related Art 
     In an air bag apparatus mounted to an automobile or the like, an inflator which inflates an air bag by an inert gas charged under a high pressure therein or the like, is incorporated. In an inflator of this type, an inert gas such as argon, helium is charged at a high pressure into an inflator housing constituting an outer shell container. Therefore, it is required to test hydrostatic pressure on all products and confirm their safety by the law (High Pressure Gas Safety Law) to secure safety of passengers. 
     In such a test method, it is required to apply pressure of 1.5 times the internal pressure in an actual product to evaluate the safety thereof. When the charging pressure is 50 to 60 MPa, the hydrostatic pressure in the test is required to be under the high pressure of 75 to 90 MPa. Conventionally, as a method for conducting such a test, the steps of charging gas into an inflator housing and pressurizing the same, or the steps of injecting oil having a high viscosity into an inflator housing and pressurizing the same are employed. 
     However, in case of charging a gas under a high pressure, a device for charging the gas under such a high pressure is very expensive, and also, if there is a defect, such as bad welding, an inflator housing may break violently due to a gas leakage and may injure a worker. 
     Also, in case of injecting an oil having a high viscosity, although the safety at a time of the test is high, it is difficult to clean and dry the inside of the inflator housing after the oil is removed, a gas for inflating an air bag is polluted by the remaining oil, and the remaining oil may adversely affect a welded portion. 
     Further, it is preferable that the testing of hydrostatic pressure is not performed independently of inflator manufacturing processes but incorporated in the manufacturing processes. Therefore, it is also required that a smooth flow of manufacturing processes is not obstructed in view of the whole manufacturing procedure. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a method for testing hydrostatic pressure for confirming the safety of an inflator and a method for manufacturing an inflator in which the hydrostatic pressure testing method is incorporated in a manufacturing procedure. 
     The present invention provides, as a means for solving the above-described problem, a method for testing hydrostatic pressure of an inflator housing constituting an outer shell container for various inflators, comprising, injecting water from a small hole provided in the inflator housing until the pressure inside the inflator housing reaches a predetermined pressure of an evaluation standard and keeping the inflator housing sealed tightly, or comprising, increasing the pressure inside the inflator housing to the predetermined pressure while injecting water in the inflator housing. 
     Further, the present invention provides, as a means for solving the above-described problem, a method for testing hydrostatic pressure of an inflator housing constituting an outer shell container for various inflators, comprising, injecting water from a small hole provided in the inflator housing until the pressure inside the inflator housing reaches a predetermined pressure of an evaluation standard and keeping the inflator housing sealed tightly, or comprising, increasing the pressure inside the inflator housing to the predetermined pressure while injecting water in the inflator housing, and draining the injected water. 
     Further, the present invention provides, as another means for solving the above-described problem, a method for testing hydrostatic pressure of an inflator housing constituting an outer shell container for various inflators, comprising, injecting water from a small hole provided in the inflator housing until the pressure inside the inflator housing reaches a predetermined pressure of an evaluation standard, keeping the inflator housing sealed tightly, or comprising, increasing the pressure inside the inflator housing to the predetermined pressure while injecting water in the inflator housing, draining the injected water, and drying the interior of the inflator housing. 
     An application range of the method for testing hydrostatic pressure of the present invention is different between a case where an inflator housing subjected to a hydrostatic pressure evaluation is not used as a product and a case where it is used as a product. That is, in the case where the inflator is not using the inflator housing as a product, only the steps of injecting water and holding the water is sufficient. Alternatively, in view of re-use of the inflator, the step of draining water may be added to the above. However, in case of using the inflator housing as a product, it is necessary to dry the interior of the inflator housing in the end. Therefore, the drying step is further added to the above. 
     Further, the present invention provides, as yet another means for solving the above-described problem, a method for manufacturing an inflator in which a pressurized medium is charged at a high pressure, comprising, injecting water from a small hole provided in the inflator housing until the pressure inside the inflator housing reaches a predetermined pressure of an evaluation standard, keeping the inflator housing sealed tightly, or comprising, increasing the pressure inside the inflator housing to the predetermined pressure while injecting water in the inflator housing, and further comprising, draining the injected water, and drying the interior of the inflator housing. 
     The method for testing hydrostatic pressure and the method for manufacturing an inflator according to the present invention can be applied to an inflator in which a pressurized medium including nitrogen gas, oxygen gas, and an inert gas, such as argon, helium, and neon, is charged at a high pressure. Specifically, the present invention can be applied to various inflators such as an air bag inflator for a driver side, an air bag inflator for a passenger side, an air bag inflator for a side collision, an inflator for a curtain air bag, an inflator for a knee-bolster, an inflator for an inflatable seat belt, an inflator for a tubular system, and an inflator for a pretensioner. 
     When the method for testing hydrostatic pressure and the method for manufacturing an inflator, in which the test method of hydrostatic pressure is incorporated in a manufacturing process according to the present invention, are applied, the problem that occurs when gas or oil is used as the conventional method for testing hydrostatic pressure can be solved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is an explanatory view of a method for testing hydrostatic pressure of an inflator housing according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An embodiment of a method for testing hydrostatic pressure of an inflator housing of the present invention will be explained with reference to the drawing.  FIG. 1  is a sectional view in the longitudinal direction of an inflator  10  for a curtain air bag or an air bag inflator for a side collision (hereinafter, referred to as “an inflator  10 ”) in which a pressurized medium is charged under a high pressure. 
     First, the inflator  10 , which will become a finished product after the hydrostatic pressure evaluation, will be explained. An inflator housing  12  has an opening portion  14  at its one end, closed at the other end, and a pressurized medium comprising an inert gas is charged into an inner space  16  at the pressure of about 60 MPa. The inflator housing  12  has a circular cross-section in a widthwise direction, and the opening portion  14  is also circular. 
     The inflator housing  12  is nearly closed by swaging or spinning, without closing a small hole  40  which is a charging hole for charging a pressurized medium and is provided at the other end of the inflator housing. After a diffuser portion  20  is connected to the inflator housing  12 , an unillustrated seal pin having the same diameter as that of the small hole  40  is fitted into the small hole  40 . Then, the pressurized medium is charged from a gap between the small hole  40  and the seal pin, and the inflator housing  12  is welded at a portion of the seal pin to be completely closed. 
     The diffuser portion  20  is fixed at a connecting portion  18  in the opening portion  14  side of the inflator housing  12  by welding. The diffuser portion  20  has an outer shell formed by a diffuser housing  28 . The diffuser housing  28  has a plurality of gas discharging ports  22  for discharging, to an outside, the pressurized medium through the opening portion  14  upon actuation, and it is further provided with a filter  24  made of wire mesh to cover the plurality of discharge ports  22  from the inside thereof. Thereby, the pressurized medium is always discharged outside from the gas discharging ports  22  through the filter  24 . 
     The opening portion  14  of the inflator housing  12  is closed by a rupturable plate  19  mounted to the diffuser portion  20 , and before actuation, an inner space  16  of the inflator housing  12  is maintained hermetically at a high pressure. Meanwhile, the gas discharging ports  22  are not closed and in fluid-communication with the outside. 
     An igniter  26  containing an igniting charge to serve as rupturing means for rupturing the rupturable plate  19  is provided in the diffuser portion  20 . This igniter  26  is fixed inside the diffuser housing  28  and mounted to the diffuser portion  20 , and it is fixed by crimping an end portion  29  of the diffuser housing  28 . Reference numeral  30  is a conductive pin for applying electric current to the igniter  26 , reference numeral  31  is an O-ring, and reference numeral  32  represented by a broken line denotes a connector adapted to be connected to the igniter  26  after the inflator  10  is mounted on a vehicle and for supplying power upon activation. 
     In a manufacturing process of such an inflator  10 , generally, after the inflator housing  12  and the diffuser portion  20  are welded at the connecting portion  18 , testing of hydrostatic pressure is performed before mounting the igniter  26 . 
     First, water is injected into the inner space  16  through the small hole  40  provided in the inflator housing  12  until the internal pressure of the inflator housing  12  reaches a predetermined pressure which is an evaluation standard (in case of this embodiment, about 60 MPa×1.5=about 90 MPa). 
     When injecting water, a straw-shaped small-diameter pipe  50  made of stainless steel, aluminum or the like is inserted into the inflator housing  12  from the small hole  40  and then, the injection is performed through the small-diameter pipe  50 . It is preferable that water used at this time is pure. Running water, ion-exchanged water or the like can also be used. 
     Regarding the relationship between the diameter of the small hole  40  and the small-diameter pipe  50 , when the diameter of the small hole  40  is 3 mm, the small-diameter pipe having an outer diameter of 1.5 to 2 mm can be used. Also, when injecting water, it is preferable that the small-diameter pipe  50  is inserted until its tip end portion is positioned at the position indicated with “a” or so (at a position near the small hole  40 ) and then, injection is performed. 
     After water is injected until it reaches the predetermined pressure, the small-diameter pipe  50  is removed, and the inflator housing is maintained for one minute or so while hermetically closing the small hole  40  by resin or the like, and then the hydrostatic pressure of the inflator housing  12  is evaluated. In this case, whether the internal pressure has reached the predetermined pressure or not can be determined in advance from the volume of the inflator housing  12  and the injecting amount of water. 
     Also, the following method can be employed as another method. After water is injected in the above-described manner until the interior of the inflator housing  12  is filled with water, the injecting is once stopped and the small-diameter pipe  50  is pulled out. Next, after the small-diameter pipe  50 , attached at its tip end with an elastic molded body having a through hole, is pressed so that the through hole is positioned at the small hole  40 , injecting of water is restarted. And, after a pressure gauge additionally provided to the small-diameter pipe  50  confirms that the predetermined pressure is obtained, the inflator housing  12  is maintained for one minute or so. 
     Next, the injected water is drained out. At this time, it is preferable to drain the water through the small-diameter pipe  50  or the small hole  40  (a gap between the small hole  40  and the small-diameter pipe  50 ) after sealing is removed to insert the small-diameter pipe  50  from the small hole  40  again. In this case, a method includes the steps of inserting the small-diameter pipe  50  so that the tip end portion thereof reaches the position indicated with “b” or so (the position near to the rupturable plate  19 ) and draining the water from the small-diameter pipe  50  mechanically, the method further includes the steps of charging gas under pressure from the small-diameter pipe  50  and draining the water from the small hole  40 , and the steps of charging gas under pressure from the small hole  40  and draining the water from the small-diameter pipe  50  or the like can be employed. 
     Next, in preparation for the next charging step of a pressurized medium, the interior of the inflator housing  12  is dried by feeding gas therein. When gas is fed, it is preferable to feed a normal temperature gas, heated air or water vapor through the small-diameter pipe  50  to promote drying. Also, heating and drying may be conducted in an oven or the like, as necessary. 
     Thus, the hydrostatic pressure evaluation is performed during the manufacturing process of the inflator  10 , and only inflator housings which are acceptable are transferred to the next-step. Consequently, assembling of other parts such as the igniter  26  is performed to obtain the inflator  10  which is a final product. 
     EXAMPLE 
     The present invention will be explained below in detail on the basis of an example, but the present invention is not limited to this example. 
     Example 1 
     In the manufacturing process of the inflator  10  shown in  FIG. 1 , the hydrostatic pressure evaluation of the inflator housing  12  was conducted by the following method. Incidentally, the details of the inflator housing  12  shown in  FIG. 1  are as follows:
         The quality of the material for the inflator housing  12 : iron-base alloy   The thickness of the inflator housing  12 : 2 mm   The volume of the inflator housing: 100 ml   The outer diameter of the inflator housing: 30 mm   The diameter of the thin hole  40 : 3 mm   The outer diameter of the thin pipe  50 : 2 mm   The quality of material for the thin pipe  50 : stainless       

     After the inflator housing  12  and the diffuser portion  20  was connected at the connecting portion  18  by a resistance-welding (or a projection welding, a laser welding), the small-diameter pipe  50  was inserted into the small hole  40  at a manufacturing stage prior to mounting of the igniter  26  so that its tip end was positioned at the position indicated with “a”. 
     Thereafter, running water was injected from the small-diameter pipe  50 , the injecting was stopped when the inflator housing was full, and running water was injected further in a state such that the small-diameter pipe  50  attached at its tip end with a rubber-made circular elastic body (having a through hole of a hole diameter of 3 mm) was pressed to meet the through hole and the small hole  40  with each other. When the pressure gauge additionally attached to the small-diameter pipe  50  indicated about 90 MPa, the inflator housing was held for one minute while the pressure was maintained, and it was confirmed that the withstand pressure of the inflator housing  12  satisfied a level required by the Law. 
     Next, the seal was removed, the small-diameter pipe  50  was inserted until the tip end thereof reached the position indicated with “b” this time, and the running water was drained mechanically by suction. 
     Then, after air at a normal temperature (20° C.) was fed from the small-diameter pipe  50  for about two minutes at a rate of about 1000 cm 3 /minute in a state such that the tip end was maintained in the position indicated as “b”, the small-diameter pipe  50  was pulled out and the inflator housing was further left in an oven having a temperature of 100° C. for about 10 minutes. 
     Thereafter, an inert gas was charged from the small hole  40  until the predetermined pressure (60 MPa) was achieved and it was sealed. Finally, necessary parts such as the igniter  26 , the connector  32  were attached to the inflator housing so that the inflator  10  was obtained.