Gas generator for a safety device

The invention relates to a gas generator for a safety device, in particular for a vehicle occupant restraint system, comprising a housing of insulating material, a combustion chamber formed in the housing containing solid propellant for the production of hot gas, and with an igniter inserted into the housing and projecting into the combustion chamber. The gas generator is wherein the housing is constructed so as to be conductive in the region of the igniter, and the solid propellant and, optionally, the combustion chamber, is arranged in a conductive container, the conductive region of the housing and the conductive container being in conducting connection with each other.

The invention relates to a gas generator for a safety device, in particular 
for a passive vehicle occupant restraint system. 
BACKGROUND OF THE INVENTION 
Usually, such a gas generator contains a housing with a combustion chamber 
formed therein, into which chamber a pyrotechnic solid propellant is 
filled. To activate the solid propellant, an igniter is provided which is 
inserted into an opening of the housing and projects into the combustion 
chamber. In the wall of the housing delimiting the combustion chamber 
there are several outflow openings for the gases which are released,on 
combustion of the propellant. These gases are then supplied to the safety 
device, in particular to an inflatable gas bag or to a knee protector 
which is able to be activated pneumatically. 
In recent times, it has been proposed to construct the gas generator 
housing from plastic, in particular from fibrereinforced plastic, to save 
weight. A proposal for the construction of gas generators with a plastic 
housing has been described for example in the thesis written by Frau Karen 
Reichart entitled "Airbag-Gasgeneratorgehause aus faserverstarktem 
Kunststoff", University of Rostock, Faculty of Mechanical Engineering and 
Marine Technology, Engineering Institute, published in September 1995. In 
this thesis, reference is already made to the fact that gas generator 
housings formed from fibre-reinforced plastic require particular measures 
to prevent an electrostatic charging. In this context, it is proposed in 
the said thesis to provide the gas generator housing either with a steel 
casing or else to metallize it galvanically. These measures can indeed 
reduce the high surface resistance of the plastic housing. However, they 
require additional components and/or production steps in the manufacture 
of the gas generator and are therefore not able to be realized at a 
favourable cost. 
From the DE-C2-38 24 162 it is known, in addition, to pack the solid 
propellant in an airtight manner by means of an aluminium foil, for 
protection against moisture, a detection of any leaks being made possible 
by evacuating the aluminium foil. 
There is therefore still a need for gas generators which are able to be 
produced at a favourable cost with a plastic housing, the pyrotechnic 
components of which are protected from ignition by electrostatic charge 
and its conductive discharge. 
SUMMARY OF THE INVENTION 
The invention provides a gas generator for a safety device, in particular a 
gas generator for a vehicle occupant restraint system, which comprises a 
housing of insulating material, a combustion chamber formed in the housing 
with solid propellant for the production of hot gas, and an igniter 
inserted into the housing and projecting into the combustion chamber. The 
gas generator according to the invention is wherein the housing is 
constructed so as to be conductive in the region of the igniter, and the 
solid propellant and also, if necessary, the combustion chamber, is 
arranged in a conductive container, the conductive region of the housing 
and the conductive container being in conducting connection with each 
other. 
A gas generator of a non-metallic, non-conductive material is realized by 
the invention, the pyrotechnic and, if necessary, electronic components of 
which are effectively protected from an ignition or destruction by 
electrostatic charge and its conductive discharge. In addition, 
electromagnetic radiation is reliably shielded and a protection of the 
pyrotechnic components is achieved in the case of a possible mechanical 
destruction of the housing. 
In a particularly preferred embodiment of the invention, the gas generator 
housing is sprayed with a conductive lacquer in the region of the igniter. 
Advantageously, the gas generator housing can also be provided with a 
conductive insert piece in the region of the igniter. Finally, it is 
proposed as a further embodiment according to the invention, to construct 
the gas generator housing from a conductive plastic in the region of the 
igniter. Such plastics are known to those skilled in the art and can be 
selected according to mechanical requirements to be set for the gas 
generator housing. 
In all embodiments the conductive container containing the propellant 
and/or the combustion chamber is preferably in direct, conducting 
connection with the conductive housing parts in the region of the igniter. 
Through this, a potential difference is prevented from being able to build 
up between the housing surface and the propellant, which difference flows 
off via the igniter and hence brings about an ignition of the propellant.

DESCRIPTION OF PREFERRED EMBODIMENTS 
The gas generator 10 illustrated in FIG. 1 has a housing composed of a 
upper housing part 12 and a lower housing part 14. The two housing parts 
12, 14 are connected firmly with each other by known types of connection, 
such as for example welding or glueing. Both housing parts consist of an 
insulating material, preferably of a plastic reinforced with glass fibres 
and/or carbon fibres, the fibre content of which preferably amounts to 
between 30 and 50 wt.-%. Particularly preferably, the plastic is a 
glass-fibre-reinforced polyamide. 
Inside the gas generator housing 12, 14 a combustion chamber 16 is formed, 
with a solid propellant 18 contained therein for the production of hot 
gas. The solid propellant is preferably present in tablet form, however it 
can also have a different form known in the art. An igniter 20 is inserted 
into the lower housing part 14, which igniter 20 projects into the 
combustion chamber 16. 
In the embodiment illustrated here, both the solid propellant 18 and also, 
the combustion chamber housing 22 defining the combustion chamber 16 are 
surrounded by a container 24 of conductive material, preferably aluminium. 
The container 24 is preferably of an air- and moisture-tight aluminium 
foil, which encloses the propellant and the combustion chamber housing and 
is at least partially evacuated. However, it is also conceivable to only 
enclose the propellant in a conductive casing of aluminium. Several 
outflow openings 26 are provided in the wall of the housing 12, 14 
distributed over the periphery of the housing, which outflow openings 26 
are in flow connection with the interior volume of an inflatable gas bag. 
In the embodiment illustrated in FIG. 1, the lower housing part, or at 
least a region of the lower housing part 14 adjoining the igniter 20, is 
sprayed with a conductive lacquer 28. The conductive aluminium foil 24 is 
in direct contact with the conductive lacquer 28 which is also sprayed on 
the inner face of the lower housing part 14, and is therefore in 
conducting connection with the outer face of the lower housing part 14. 
In a further embodiment shown in FIG. 1, in the production of the lower 
housing part 14 a conducting insert piece 29, constructed here 
approximately in a U-shape, is arranged in the region of the igniter 20 
and is molded around with the plastic material of the lower housing part 
14. Also in this embodiment a conducting connection is produced between 
the aluminium foil lying against the inner face of the lower housing part 
14 in the region of the igniter 20, and the outer face of the lower 
housing part 14. 
FIG. 2, finally, shows a further embodiment of the gas generator according 
to the invention, the basic construction of which coincides with the gas 
generator shown in FIG. 1. The same reference numbers designate the same 
features in this respect. In the embodiment shown here, the lower housing 
part, at least, however, a region of the lower housing part 14 adjoining 
the igniter, is formed from a conductive plastic 30. The aluminium 
container 24 surrounding the propellant 18 and the combustion chamber 16 
is on the inner face of the lower housing part 14 in direct, conducting 
connection with the conducting plastic 30. Therefore, the occurrence of 
potential differences between the propellant and the gas generator housing 
and also a voltage breakthrough via the igniter can be reliably prevented. 
FIG. 3 shows a diagrammatic sketch of the gas generator according to the 
invention, incoporated in a safety device. The gas generator 10 is secured 
to the steel trough 34 of the airbag module by means of a clamping device 
32. The clamping device 32 holds at the same time the airbag 36 of the 
safety device which is positioned over the gas generator 10. If an 
accident is detected by a sensor (not shown here), a current impulse 
originating from the sensor activates the igniter 20. The latter ignites 
the solid propellant 18 in the combustion chamber 16 for the production of 
hot gas, which is supplied via the outflow openings 26 to the interior 
volume of the airbag 36. 
In the case of an electrostatic charging of the gas generator housing 12, 
14, which is made from a non-conducting plastic, the danger exists that 
the charge built up on the housing surface either comes through onto the 
propellant 18 or flows off via the igniter 20. In both cases, an undesired 
deflagration of propellant would be brought about. The lower housing part, 
which according to the invention is constructed so as to be conductive at 
least in the region of the igniter 20 and which is in conducting 
connection with a container (not shown here) encasing the propellant 18 
and/or the combustion chamber 16, ensures, however, an conductive 
discharge of the charge built up on the gas generator housing via the 
steel trough 34 of the airbag module which is to be regarded as earthing. 
Therefore, the danger of a voltage break-through and hence an undesired 
deflagration of propellant can be effectively avoided.