Abstract:
An apparatus ( 10 ) for inflating an inflatable vehicle occupant protection device ( 12 ) comprises a container ( 14 ) defining a chamber ( 40 ). The container ( 14 ) has a portion ( 36 ) which is rupturable to form an opening in the container. A quantity of inflation fluid ( 42 ) is stored under pressure in the chamber ( 40 ). An electrically actuatable initiator ( 50 ) is connected with the container ( 14 ) for rupturing the rupturable portion ( 36 ) of the container to enable flow of inflation fluid ( 42 ) out of the inflator ( 10 ). The initiator ( 50 ) comprises an ignitable primary charge ( 90 ) and electrically energizable means ( 60 ) for igniting the primary charge. The initiator ( 50 ) further comprises a secondary charge ( 100 ) ignitable by the primary charge ( 90 ). The secondary charge ( 100 ) comprises an ignitable metal in solid form which liquefies when ignited by the primary charge ( 90 ). The primary charge ( 90 ) when ignited generates combustion products under pressure which ignite the secondary charge ( 100 ) and direct the liquefied metal into engagement with the rupturable portion ( 36 ) of the container ( 14 ) to help open the rupturable portion of the container. In a preferred embodiment, the secondary charge ( 100 ) is thermite.

Description:
BACKGROUND OF THE INVENTION 
     Technical Field 
     The present invention relates to an apparatus for inflating an inflatable vehicle occupant protection device, such as an air bag. In particular, the present invention relates to an initiator for an air bag inflator having a burst disk which is opened by the initiator. 
     Description of the Prior Art 
     A known type of inflator for inflating an air bag includes a container in which inflation fluid is stored. The container includes a rupturable burst disk. The inflator also includes an electrically actuatable initiator for, when actuated, rupturing the burst disk to release inflation fluid to flow into the air bag. The initiator typically includes a pyrotechnic material capable of generating combustion products, including hot gases, for rupturing the burst disk. Known initiators use a mixture of a metal and a metallic compound (used as the oxidizer) such as boron potassium nitrate or titanium potassium perchlorate. Ignition of such a charge primarily produces heat, hot gases, and a shock wave for rupturing the burst disk. 
     SUMMARY OF THE INVENTION 
     The present invention is an apparatus for inflating an inflatable vehicle occupant protection device. The apparatus comprises a container defining a chamber. The container has a portion which is rupturable to form an opening in the container. A quantity of inflation fluid is stored under pressure in the chamber. The apparatus also comprises an electrically actuatable initiator connected with the container for rupturing the rupturable portion of the container to enable flow of inflation fluid out of the inflator to inflate the protection device. The initiator comprises an ignitable primary charge and electrically energizable means for igniting the primary charge. The initiator further comprises a secondary charge ignitable by the primary charge. The secondary charge comprises an ignitable metal in solid form which liquefies when ignited by the primary charge. The primary charge when ignited generates combustion products under pressure which ignite the secondary charge and direct the liquefied metal into engagement with the rupturable portion of the container to help open the rupturable portion of the container. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other features of the present invention will become apparent to one skilled in the art to which the present invention relates upon consideration of the following description of the invention with reference to the accompanying drawings, wherein: 
     FIG. 1 is a longitudinal sectional view, partially broken away, of an inflator including an initiator constructed in accordance with the present invention; and 
     FIG. 2 is an enlarged view of a portion of the inflator of FIG. 1 including the initiator. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention relates to an apparatus for inflating an inflatable vehicle occupant protection device, such as an air bag. Other inflatable vehicle occupant protection devices that can be used in accordance with the invention include, for example, inflatable seat belts, inflatable knee bolsters, inflatable head liners or side curtains, and knee bolsters operated by inflatable air bags. In particular, the present invention relates to an air bag inflator having a burst disk which is opened by an initiator. As representative of a first embodiment of the present invention, FIG. 1 illustrates an inflator  10  for inflating an inflatable vehicle occupant protection device, or air bag, illustrated schematically at  12 . 
     The inflator  10  includes a container  14 . The container includes a cylindrical main body portion  16  which is centered on an axis  20  of the inflator  10 . The main body portion  16  of the container  14  includes a cylindrical, axially extending side wall  22  and a radially extending end wall  24 . The container  14  also includes an end cap or diffuser  30  welded to an open end of the main body portion  16  of the container opposite the end wall  24 . The diffuser  30  defines an opening  32  into an initiator chamber  34  in the diffuser  30 . 
     The container has a rupturable portion in the form of a burst disk  36 . The burst disk  36  is welded to the diffuser  30 . A gas outlet opening  38  extends between the initiator chamber  34  and the exterior of the diffuser  30 . 
     The container  14  defines a gas storage chamber  40 . A quantity of inflation fluid  42  in the form of a combustible mixture of gases is stored under pressure in the chamber  40 . The combustible mixture of gases includes primary gas and fuel gas. The fuel gas provides heat of combustion which heats the primary gas. This mixture of gases may have any suitable composition known in the art, but preferably has a composition in accordance with the invention set forth in U.S. Pat. No. 5,348,344. 
     The burst disk  36  blocks flow of the fluid  42  out of the chamber  40  through the opening  32 . The inflator  10  could alternatively include structure different than the burst disk  36  and opening  32 , such as a thin-walled portion of the diffuser  30  which is rupturable to provide an opening in the container. 
     The inflator  10  includes a metal retainer  44  secured in the diffuser  30 . The retainer secures an initiator  50  in position in the inflator  10 . The initiator  50  includes a base illustrated schematically at  52  (FIG.  2 ). A pair of electrical terminals  54  and  56  extend from the base  52  for engagement by an electrical connector (not shown) of the vehicle. 
     A resistive element  60  extends between an inner end portion  62  of the first terminal  54  and an inner end portion  64  of the second terminal  56 . The resistive element  60  electrically interconnects the first terminal  54  and the second terminal  56 . The resistive element  60  is operative to generate heat when an electric current is passed through the resistive element between the first and second terminals  54  and  56  of the initiator  50 . 
     The initiator  50  includes a metal can or cap  70 . The cap  70  is a single layer of metal having a cup-shaped configuration including a cylindrical, axially extending side wall  72  and a radially extending end wall  74 . The side wall  72  of the cap  70  is centered on the axis  20 . The side wall  72  of the cap  70  is sealed to the base  52  of the initiator  50  in a known manner (not shown). 
     The side wall  72  and end wall  74  of the cap  70  together define a chamber  80  in the cap. A primary charge  90  and a secondary charge  100  are disposed in the chamber  80 . The resistive element  60  is also disposed in the chamber  80 . 
     The primary charge  90  is a quantity of ignitable material. The ignitable material is preferably a known pyrotechnic material such as zirconium potassium perchlorate, titanium potassium perchlorate, or boron potassium nitrate, in powder form, which ignites when exposed to the heat generated by the resistive element  60 . The cap  70  encloses the ignitable material  90  and maintains the ignitable material in contact with the resistive element  60 . 
     The secondary charge  100  is disposed in a layer along the inside of the end wall  74  of the cap  70 . During assembly of the initiator  50 , the secondary charge  100  is placed in the empty cap  70  before the primary charge  90  is placed in the cap. The secondary charge  100  is made and assembled separately from the primary charge  90 . The secondary charge  100  may be pressed or loose powder. 
     The secondary charge  100  is thus disposed between the primary charge  90  and the end wall  74  of the cap  70 . When the initiator  50  is assembled in the inflator  10 , the end wall  74  of the cap  70  is presented toward the burst disk  36 . The secondary charge  100  is thus disposed between the primary charge  90  and the burst disk  36  when the initiator  50  is assembled in the inflator  10 . 
     The secondary charge  100  is a quantity of a finely divided active metal used as a fuel and a metal oxide used as an oxidizer (a combination often referred to as “thermite”). An “active” metal is any elemental metal which undergoes a strong exothermic reaction in reducing the metal oxide. Active metals include the alkali metals and the alkaline earth metals, among others. A preferred active metal is aluminum. Other active metals such as magnesium, titanium, or zirconium could be used. A preferred oxidizer is iron oxide. Other materials such as silicon dioxide, chromium oxide, manganese dioxide, cupric oxide or lead oxide could be used as the oxidizer. 
     The first and second electrical terminals  54  and  56  of the initiator are connected with vehicle electric circuitry indicated schematically at  110  including a power source, which is preferably a vehicle battery and/or a capacitor, and a normally open switch. The switch is part of a sensor which senses a vehicle condition indicating the occurrence of a collision. The collision-indicating condition may comprise, for example, sudden vehicle deceleration that is caused by a collision. If the collision-indicating condition is at or above a predetermined threshold level, it indicates the occurrence of a collision having at least a predetermined threshold level of severity. The threshold level of collision severity is a level at which inflation of the air bag  12  or other vehicle occupant protection device is desired to help protect an occupant of the vehicle. 
     When the sensor senses a collision-indicating condition at or above the predetermined threshold level, the switch closes and the initiator  50  is energized electrically. An electric current flows between the first and second terminals  54  and  56  of the initiator  50 , through the resistive element  60 . The resistive element  60  generates heat, which ignites the primary charge  90 . 
     The ignition of the primary charge  90  produces combustion products, including hot gases, and causes an increase in the pressure inside the cap  70 . The pressure increase inside the cap  70  causes the cap to rupture. The combustion products of the primary charge  90 , and a shock wave, travel away from the base  52  and toward the burst disk  36 . The combustion products and the shock wave strike the burst disk  36 . 
     The secondary charge  100  is ignited by the heat of the primary charge  90 . The oxidizer in the secondary charge  100  reacts with the fuel in a combustion reaction. The oxidizer is reduced by the fuel, evolving or generating a superheated elemental metal in a liquid form. When aluminum and iron oxide are used as the components of the secondary charge  100 , the aluminum reduces the iron oxide, in an exothermic reaction, to form molten iron and aluminum oxide. The metal and metal oxide are selected so that the reaction temperature of the reduction reaction between them is high, with a corresponding high calorific output, and the evolved products should have a high volatility (evaporation temperature). 
     The molten iron has a temperature typically in the range of 2000-2800° C. The molten iron is propelled by the combustion products of the primary charge into engagement with the burst disk  36 . The metal may flow, for example, in a small diameter stream of about one eighth of an inch diameter. 
     The liquid metal from the secondary charge  100  acts as a projectile and engages the burst disk  36 . The burst disk  36  ruptures when it is engaged by the stream of molten metal and by the combustion products of the primary charge. The inflation fluid  42  flows through the ruptured burst disk  36  and through the gas outlet opening  38  in the diffuser  30  to inflate the air bag  12 . 
     The metal of the secondary charge  100  remains molten, while melting the burst disk  36 , because of the high temperature of the metal (typically about 2000-2,800° C.) and the small thermal mass of the burst disk (typically about 0.5 mm thickness). The heat of the molten metal helps to weaken the burst disk  36  to enhance the projectile effect of the flowing metal. 
     After the burst disk  36  is ruptured, the molten metal ignites the fuel gas which is part of the inflation fluid  42  in the container  14 . The ignition of the fuel gas raises the pressure and temperature of the primary gas in the container  14 . No heat source other than the secondary charge  100  is needed to ignite the fuel gas. 
     In a second embodiment of the invention, an elemental metal is used as the secondary charge  100 . The secondary charge  100  is provided as a disk or similar structure, and is not mixed or compounded with any other elements or materials such as an oxidizer or other ignitable materials. The heat of combustion of the primary charge  90  causes the secondary charge  100  to liquefy. The molten metal is propelled as a projectile by the combustion products of the primary charge  90  into engagement with the burst disk  36 , and can help to rupture the burst disk. If there is adequate oxygen present, the elemental metal of the secondary charge  100  will burn rather than melt. 
     From the above description of the invention, those skilled in the art will perceive improvements, changes and modifications in the invention. For example, the inflation fluid in the container might not include a fuel gas, in which case the molten metal of the secondary charge  100  would serve only to rupture the burst disk. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims.