United States Patent [19]
`Breed et al.
`
`|||||||||||||||||||||||||||||||||||
`US005653464A
`[11] Patent Number:
`5,653,464
`[45]. Date of Patent:
`Aug. 5, 1997
`
`[54]
`
`AIRBAG SYSTEM WITH SELF SHAPING
`AIRBAG
`
`[75]
`
`Inventors: David S. Breed, Boonton Township,
`Morris County; William Thomas
`Sanders, Rockaway Township, Morris
`County, both of N.J.
`
`
`
`1/1995 Glance ............................... 290/743.14
`5,382,051
`5,390,950 2/1995 Barnes et al. .
`280/728 B
`5,505,485 4/1996 Breed ...................................... 280/729
`5,538,280 7/1996 Gray et al. ........................... 280/743.1
`FOREIGN PATENT DOCUMENTS
`OC
`0478897 4/1992 European Pat. Off. ............ 280/743.1
`2162134 6/1990 Japan .................................. 280/743.1
`-
`-
`-
`-
`Primary Examiner—Christopher P. Ellis
`[57]
`ABSTRACT
`-
`-
`-
`-
`Appl. No.: 539,676
`An inflatable occupant restraint device in an occupant pro
`tection system of a vehicle having at least two pieces of
`Filed:
`Oct. 5, 1995
`6
`substantially flat inelastic plastic film having peripheral
`
`Int. Cl* … * 21/16 edges attached together only at their peripheral edges to
`U.S. Cl. ---------------------------------------- 280/743.1; 428/35.5
`form 3. substantially sealed airbag, all inflator coupled to the
`Field of Search .............................. 280/743.1, 728.1;
`airbag and to the vehicle for inflating the airbag by providing
`428/137, 224, 35.2, 35.5
`gas to the airbag through at least one portin one of the pieces
`e
`of elastic film, and an initiator for initiating the inflator in
`References Cited
`response to a crash of the vehicle. Upon inflation of the
`U.S. PATENT DOCUMENTS
`airbag, the at least two pieces of inelastic plastic film deform
`-
`substantially without stretching, in particular to create a
`§ º: i. ------------------------------ fº, shape for the airbag where the thickness is substantially
`5248,275 9/1993 McGrath et al. ...
`... 446/224 * º and/or to provide the airbag
`5,279,873
`1/1994 Oike ...............
`... 428/35.4
`p
`pe.
`5,295,892 3/1994 Felton ........
`446/224
`5,336,538 8/1994 Kitamura ............................... 428/35.2
`
`Assignee: Automotive Technologies
`International Inc., Denville, N.J.
`
`[73]
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`[21]
`[22]
`[51]
`[52]
`[58]
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`[56]
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`Oil &l 3}. .......
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`AIRBAG SYSTEM WITH SELF SHAPING
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`desirable to combine the properties of a film airbag which
`automatically attains the proper driver airbag shape with a
`fabric. In such cases, interaction with the driver needs to be
`minimized.
`Airbags made of plastic film are disclosed in the copend
`ing patent application Ser. No. 08/247,763 referenced above
`and incorporated herein by reference. Many films have the
`property that they are quite inelastic under typical stresses
`associated with an airbag deployment. If an airbag is made
`from flat circular sections of such films and inflated, instead
`of forming a spherical shape, it automatically forms the flat
`ellipsoidal shape required for driver airbags. This unex
`pected result vastly simplifies the manufacturing process for
`driver airbags since tethers are not required. Furthermore,
`since the airbag can be made by heat sealing two flat circular
`sections together without the need for tethers, the entire
`airbag can be made without sewing reducing labor and
`production costs. In fact, the removal of the requirement for
`tethers permits the airbag to be made by a blow molding or
`other similar process. Indeed, this greatly reduces the cost of
`manufacturing driver airbags.
`In addition to the above referenced patent application,
`film material for use in making airbags is described in U.S.
`Pat. No. 4,963,412 to Kokeguchi. The film airbag material
`described in the Kokeguchi patent is considerably different
`in concept from that disclosed in the above referenced patent
`application Ser. No. 08/247,763 or the instant invention. The
`prime feature of the Kokeguchi patent is that the edge tear
`resistance of the airbag film material can be increased
`through the use of holes in the plastic films. Adding holes,
`however, reduces the tensile strength of the material by
`factor of two or more due to the stress concentration effects
`of the hole. It also reduces the amount of available material
`to resist the stress. As such, it is noteworthy that the
`Kokeguchi airbag is only slightly thinner than the conven
`tional fabric airbag (320 micrometers vs. the conventional
`400 micrometers) and is likely to be as heavy or perhaps
`heavier than the conventional airbag. Also, Kokeguchi does
`not disclose any particular shapes of film airbags. As will be
`discussed below in detail, the airbags constructed in accor
`dance with the present teachings attain particular shapes
`based on the use of the inelastic properties of particular film
`materials.
`The neoprene or silicone coating on conventional driver
`airbags, as mentioned above, serves to trap hot particles
`which are emitted from the inflator, such as a conventional
`sodium azide inflator. A film airbag will be vulnerable to
`such particles and as a result will not work effectively with
`sodium azide inflators. Fortunately, new inflators are being
`developed which do not produce hot particles but instead
`produce gases which are substantially cooler than gases
`produced by sodium azide inflators.
`In the conventional airbag, the propellant which is used to
`inflate the airbag also is used to force open a hole in the
`vehicle trim, called the deployment door, permitting the
`airbag to deploy. Since the mass of a film airbag is substan
`tially less than the conventional fabric airbag, much less
`energy is required to deploy the airbag in time. However,
`substantial pressure is still required to open the deployment
`door. Also, if the pressure now used to open the deployment
`door is used with film airbags, the rate of deployment, once
`the door has been opened, will be substantially faster than
`conventional airbags. This rapid deployment puts excessive
`stresses on the film airbag and increases the chance that the
`occupant will be injured thereby. For most implementations
`of the film airbag, an alternate less energetic method of
`opening the deployment door is necessary.
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`This application is related to U.S. patent application Ser.
`No. 08/247,763 filed May 23, 1994 to D. S. Breed titled
`Plastic Film Airbag now U.S. Pat. No. 5,505,485.
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`This invention relates to an airbag made at least partially
`from two panels, e.g., front and back panels, of relatively
`inelastic plastic film in such a manner that it automatically
`attains a desired shape without the use of tethers which
`connect the front and back panels.
`2. Description of the Prior Art
`A conventional driver side airbag (also referred to herein
`as a driver airbag) is made from pieces of either Nylon or
`polyester fabric which are joined together by sewing. The
`airbag is coated on the inside with neoprene or silicone for
`the purposes of (i) capturing hot particles emitted by the
`inflator and preventing holes from being burned in the
`fabric, and (ii) sealing the airbag to minimize the leakage of
`an inflating gas through the fabric. These airbags are con
`ventionally made by first cutting two approximately circular
`25
`sections of a material having a coating on only one side
`which will form a front panel and a back panel, and sewing
`them together with the coated side facing out. The back
`panel contains a hole for attachment to an inflator. Fabric
`straps, called tethers, are then sewn to the front panel.
`Afterwards, the airbag is turned inside out by pulling the
`fabric assembly through the inflator attachment hole placing
`the coated side on the inside. Assembly is completed by
`sewing the tethers to the back panel adjacent the inflator
`attachment hole.
`If a conventional driver airbag is inflated without the use
`of tethers, the airbag will take an approximately spherical
`shape. Such an airbag would protrude significantly into the
`passenger compartment from the steering wheel and, in most
`cases, impact and injure the driver. To prevent this possible
`injury, the tethers are attached to the front and rear panels of
`the airbag to restrict the displacement of the front panel
`relative to the back panel. The result of the addition of such
`tethers is an airbag which has the shape of a flat ellipsoid
`with a ratio of the thickness of the airbag to its diameter of
`approximately 6. In the conventional airbag, the tethers are
`needed since the threads which make up the airbag fabric are
`capable of moving slightly relative to each other. The airbag
`is elastic for stresses which are not aligned with the warp or
`woof of the fabric. As a result, the fabric distorts to form an
`approximate sphere.
`Moreover, the above-mentioned method of manufacturing
`an airbag involves a great deal of sewing and thus is highly
`labor intensive and, as a result, a large percentage of all
`driver airbags are manufactured in low labor cost countries
`such as Mexico.
`.
`Many people are now being injured and some killed by
`interaction with the deploying airbag. One of the key advan
`tages of the film airbag described in the above referenced
`patent application is that, because of its much lower mass,
`the injury caused by this interaction is substantially reduced.
`In accordance with the teachings of that patent application,
`the driver airbag system can be designed to permit some
`significant interaction with the driver. In other words, the
`film airbag can be safely designed to intrude substantially
`further into the passenger compartment without fear of
`injuring the driver. Nevertheless, in some cases it may be
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`One such system is disclosed in Barnes et al. U.S. Pat. No.
`5,390.950 titled “Method and arrangement for forming an
`air bag deployment opening in an auto interior trim piece”.
`This patent describes a method of forming an air bag
`deployment opening in an interior trim piece having an vinyl
`skin overlying a rigid substrate so as to be invisible prior to
`operation of the air bag system and comprises an energy
`generating linear cutting element arranged in a door pattern
`beneath the skin acting to degrade or cut the skin when
`activated.
`The goal of the Barnes et al. patent is to create an invisible
`seam when the deployment door is located in a visible
`interior trim panel. This permits greater freedom for the
`vehicle interior designer to create the particular aesthetic
`effect that he or she desires. The invisible seam of the Barnes
`et al. patent is thus created for aesthetic purposes with no
`thought toward any advantages it might have to reduce
`occupant injury or advantages for use with a film airbag, or
`to reduce injuries at all for that matter. One unexpected
`result of applying the teachings of this patent therefore, is
`that the pressure required to open the deployment door is
`substantially reduced. When used in conjunction with a film
`airbag, this result is important since the inflator can be
`designed to provide only sufficient energy to deploy and
`inflate the very light film airbag thereby significantly reduc
`ing the size of the inflator. The additional energy required to
`open a conventional deployment door above that required to
`open a door constructed in accordance with the teachings of
`the Barnes et al. patent is not required within the inflator.
`Furthermore, since a film is more vulnerable to being injured
`by ragged edges on the deployment door than a conventional
`fabric airbag, the device of the Barnes et al. patent can be
`used to pyrotechnically cut open the deployment door per
`mitting it to be easily displaced from the path of the
`deploying airbag, minimizing the force of the airbag against
`the door and thus minimizing the damage to the film airbag
`from the deployment door. Since Barnes et al. did not
`contemplate a film airbag, advantages of its use with the
`pyrotechnically opening deployment door could not have
`been contemplated.
`The discussion of the self-shaping airbag thus far has been
`limited to film airbags. An alternate approach is to make an
`airbag from a combination of fabric and film. The fabric
`provides the tear resistance and conventional airbag appear
`ance. The film forces the airbag to acquire the flatellipsoidal
`shape desired for driver airbags without the use of tethers
`and permits the airbag to be assembled without sewing using
`heat and/or adhesive sealing techniques. Such a hybrid
`airbag is made from fabric and film which have been
`laminated together prior to the cutting operation. Naturally,
`the combination of a film and net, as described in the above
`referenced patent application, is equally applicable for the
`airbag described here and both will be referred to herein as
`hybrid airbags.
`A finite element analysis of conventional driver side
`airbags shows that the distribution of stresses is highly
`unequal. Substantial improvements in conventional airbag
`designs can be made by redesigning the fabric panels so that
`the stresses are more equalized. Today, conventional airbags
`are designed based on the strength required to support the
`maximum stress regardless of where that stress occurs. The
`entire airbag must then be made of the same thickness
`material as that chosen to withstand maximum stress con
`dition. Naturally, this is wasteful of material and attempts
`have been made to redesign the airbag to more closely
`equalize the stress distribution and thus permit a reduction in
`fabric strength and thus thickness and weight. However, this
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`optimization process when used with conventional fabric
`airbags can lead to more complicated assembly and sewing
`operations. Thus, there is a tradeoff between manufacturing
`cost and airbag optimization.
`With the film airbag manufactured using blow molding
`techniques, for example, much greater freedom is permitted
`to optimize the airbag vis-à-vis equalization of the stress.
`First, other than tooling cost, the manufacturing cost of an
`optimized airbag is no greater than for a non-optimized
`airbag. Furthermore, the thickness of the film can be varied
`from one part of the airbag to another to permit the airbag
`to be thicker where the stresses are greater and thinner where
`the stresses are less. A further advantage of blow molding is
`that the film can be made of a single constituent material.
`When the airbag is fabricated from sheet material, the
`outside layer of the material needs to be heat sealable such
`as polyethylene or else a special adhesive layer is required
`where the sealing occurs.
`One example of an inflatable film product which illus
`trates the technology of this invention is the common
`balloon made from metalized “Mylar” plastic film found in
`many stores. Frequently these balloons are filled with
`helium. They are made by heat sealing two flatpieces offilm
`together as described in U.S. Pat. Nos. 5,188,558, 5.248,275,
`5,279,873, and 5,295,892. The shape of these balloons,
`which is circular in one plane and elliptical in the other two
`planes, is very nearly the shape which is desired for a driver
`side airbag. This shape is created when the pressure within
`the balloon is sufficiently low such that the stresses induced
`into the film are much smaller than the stresses needed to
`significantly stretch the film. The film used is relatively rigid
`and has difficulty adjusting to form a spherical shape. In
`contrast, the same airbag made from woven material more
`easily assumes an approximate spherical shape requiring the
`use of tethers to create the shape which comes naturally with
`the Mylar balloons.
`One problem with film balloons is that when a hole is
`punctured in the balloon it fails catastrophically. One solu
`tion to this problem is to use the combination of a film and
`net as described in the above referenced patent application.
`Such materials have been perfected for use as sail material
`for lightweight high performance sails for sailboats. One
`example is marketed under the trade name Bainbridge
`Sailcloth SL SeriesTM, and in particular SL 500-PTM, 1.5
`mill. This material in a laminate of a film and a net. Such
`materials are designed to permit heat sealing thereby elimi
`nating threads and the stress concentrations associated there
`with. Heat sealing also simplifies the manufacturing process
`for making sails. Another preferable solution is to make the
`airbags from a film material which naturally resists tears,
`that is, one which is chemically formulated to arrest a tear
`which begins from a hole, for example.
`Applications for the self shaping airbag described herein
`include all airbags within the vehicle which would otherwise
`required tethers or complicated manufacturing from several
`separate panels. Most of these applications are more difficult
`to solve or unsolvable using conventional sewing technol
`ogy. The invention described herein solves the above prob
`lems by using the inelastic properties of film, otherwise
`stated as their high modulus of elasticity, plus innovative
`designs based on analysis including mathematical modeling
`plus experimentation. In this manner, the problems dis
`cussed above, as well as many others, are alleviated or
`solved by the self shaping airbags described in the para
`graphs below.
`OBJECTS AND SUMMARY OF THE
`INVENTION
`A principal object of this invention is to form an airbag
`from flat sheets of film or composite material, or by blow
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`molding or a similar process in order to create an airbag for
`use as a driver side airbag which is substantially elliptical in
`two orthogonal planes and is circular in a third orthogonal
`plane. If a composite material composed of film and a net.
`or film and a fabric, is used to form a hybrid design, the
`relatively inelastic properties of the film are used to create
`the desired flat elliptical shape while the net or fabric is used
`to provide other desirable features including tear resistance.
`Other objects and advantages of this invention include:
`1. To provide an airbag which can be made without the
`use of sewing.
`2. To provide an airbag which is considerably lighter and
`smaller when folded in the inoperative condition than
`conventional fabric airbags.
`3. To provide a driver airbag which does not require the
`use of tethers.
`
`4. To provide an airbag for use on the front passenger side
`of the vehicle which can be easily manufactured from
`a minimum number of parts without the use of sewing.
`5. To provide a substantially conventional driver fabric
`airbag which can be manufactured without the use of
`tethers.
`
`6. To provide an airbag which can the manufacturing
`using a low cost blow molding or similar technology.
`7. To provide an airbag which has been optimized to
`substantially equalize the stresses in the material.
`8. To provide an airbag where the material thiclmess is
`varied to reduce the stress in the high stress areas of the
`airbag.
`9. To provide an airbag where optimization procedures
`have been used to substantially eliminate folds and
`wrinkles in the surface of the inflated airbag.
`10. To provide an airbag comprising film where the
`thickness to diameter ratio is less than 7 without the use
`of tethers and, in some cases, less than 6.
`11. To provide a very low cost airbag.
`12. To provide a method of manufacturing an airbag
`permitting any desired shape airbag to the manufac-
`tured from two panels.
`13. To provide an airbag module utilizing the combination
`of an airbag made substantially of film and a pyrotech-
`nically opening deployment door.
`14. To provide an airbag module which is substantially
`less injurious to out-of position occupants during air-
`bag deployment.
`Other objects and advantages of the present invention will
`become apparent from the following description of the
`preferred embodiments taken in conjunction with the
`accompanying drawings.
`Briefly though, in a most basic embodiment, the inflatable
`occupant restraint device in an occupant protection system
`of a vehicle in accordance with the invention comprises at
`least two pieces of substantially flat inelastic plastic film
`having peripheral edges, a first one of the pieces of inelastic
`plastic film having at least one port; first attachment means
`for attaching the pieces of inelastic plastic film together only
`at the peripheral edges to form a substantially sealed airbag;
`inflating means for inflating the airbag by directing gas into
`an interior of the airbag through the at least one port;
`coupling means for coupling the airbag to the inflating
`means; initiation means for initiating the inflation means in
`response to a crash of the vehicle to thereby cause deploy-
`ment of the airbag; and second attachment means for attach-
`ing the inflating means to the vehicle. In this manner, upon
`
`inflation of the airbag by the inflating means, the pieces of
`inelastic plastic film deforms substantially without stretch-
`ing. Also, the attaching of the pieces of plastic film together
`only at the peripheral edges precludes the use of tethers
`which are conventionally used to connect inner regions of
`the pieces of airbags together to provide the desired ellipsoid
`shape.
`With respect to specific parts of the vehicle to which the
`airbag may be attached, suitable parts include the steering
`wheel of the vehicle, the headliner of the vehicle and an
`instrument panel of the vehicle. The airbag when deployed
`from the headliner preferably extends substantially across a
`major portion of the front seat of the vehicle if not the entire
`front seat.
`
`The airbag may also include means for preventing the
`propagation of a tear, such as a laminated fabric or net,
`which are connected to each of the pieces of inelastic plastic
`film, or within the formulation of the plastic film material
`itself.
`
`In another embodiment, the piece of inelastic plastic film
`having the port also includes at least one variable outlet vent
`comprising pressure responsive means for controlling open-
`ing thereof and thus flow of gas therethrough in response to
`pressure within the airbag. In this manner, upon inflation of
`the airbag, the variable vent is substantially closed until the
`airbag is impacted by the occupant at which time the
`variable vent opens by an amount determined by the pres-
`sure in the airbag thereby controlling the deceleration of the
`occupant.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The following drawings are illustrative of embodiments
`of the invention and are not meant to limit the scope of the
`invention as encompassed by the claims.
`FIG. 1A is a partial cutaway perspective view of a
`conventional prior art driver side airbag showing the tethers
`which create the flat ellipsoidal shape of the airbag.
`FIG. 1B is a partial cutaway perspective view of the prior
`art driver side airbag of FIG. 1A with the tethers removed.
`FIG. 2 is a partial cutaway perspective View of a driver
`side airbag made from plastic film.
`FIG. 3A is a partial cutaway perspective view of a driver
`side airbag made from plastic film and a fabric to produce a
`hybrid airbag.
`FIG. 3B is a partial cutaway perspective view of a driver
`side airbag made from plastic film and a net to produce a
`hybrid airbag.
`FIG. 4A is a partial cutaway perspective View of a driver
`side airbag made from plastic film using a blow molding
`process.
`FIG. 4B is a partial cutaway perspective view of a driver
`side airbag made from plastic film using a blow molding
`process so that the airbag design has been partially opti-
`mized using finite element airbag model where the wrinkles
`have been eliminated and where the stresses within the film
`are more uniform.
`
`FIG. 4C is a cutaway view of a driver side airbag made
`from plastic film showing a method of decreasing the ratio
`of thiclmess to effective diameter.
`
`FIG. 4D is a view of a driver side airbag of FIG. 4C as
`viewed along line 4D—4D.
`FIG. 5 is a partial cutaway perspective view of a passen-
`ger side airbag made from plastic film.
`FIG. 6 is a partial cutaway perspective view of an airbag
`made from plastic film providing protection for all of the
`occupants of the front seat of the vehicle.
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`7
`FIG. 7 is a partial cutaway perspective view of a driver
`side airbag made from plastic film having a variable vent in
`the seam of the airbag
`FIG. 7A is a cross-sectional view of the driver side airbag
`having a variable vent in the seam of the airbag taken along
`the line 7A.—7A of FIG. 7.
`FIGS. 8 and 9 are reproduced FIGS. 7 and 8 from U.S.
`Pat. No. 5,390,950 to Barnes et al. showing a prior art
`method and arrangement for forming an airbag deployment
`opening in an automobile interior trim piece.
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`Referring to the accompanying drawings wherein like
`reference numerals refer to the same or similar elements, a
`partial cutaway perspective view of a conventional driver
`side airbag showing the tethers which create aflatellipsoidal
`shape is illustrated in FIG. 1A. The airbag is constructed
`from two flat disks of material 110 and 112. The material is
`typically manufacturing from nylon fabric and coated on one
`side with a coating 114 of either neoprene or silicone, both
`of which are elastic rubbery films. The two pieces of fabric
`are initially laid on top of each other so that the circumfer
`ences match with the coated side facing out. A seam 120 is
`then created by sewing peripheral edges of the disks
`together. In some cases, a reinforcing piece of material is
`added between the edges in order to further strengthen the
`seam. Typically two, or sometimes more, rows of stitches
`are used to spread the stresses generated during inflation
`over a significant area of the seam 120.
`A reinforcing flat ring or annulus 160 of material is
`typically sewn to what will become the front section of the
`airbag and the tethers 140 are sewn to the opposite side using
`stitches 162. With the airbag in this condition, the assembly
`is pulled through an inflator attachment hole 150 in what will
`become the rear side or panel of the airbag and then turned
`inside out so that the coated side of the airbag 114 is now on
`the inside as shown in FIG. 1A. Tethers 140 are now sewn
`to a reinforcing ring 151 surrounding hole 150 which serves
`to attach the airbag to the inflator, not shown. Holes 152 are
`placed in reinforcing ring 151 for this purpose. A vent hole
`130 is also provided in one of the panels, i.e., the rear panel
`as shown, to complete the airbag. This hole 130 permits the
`gas to escape when the occupant loads the airbag during a
`crash, thereby absorbing the kinetic energy of the occupant.
`Port or hole will be used herein to mean an opening in the
`airbag through which gas can flow either in or out depending
`on the design and vent will mean a hole through which gas
`can flow out of the airbag.
`When this airbag is inflated, folds and wrinkles 122
`appear in the circumference of the airbag due to the unequal
`stress distribution in the fabric caused by the attempt to form
`a three dimensional object having curved surfaces from two
`dimensional flat sheets.
`What has been described with reference to FIG. 1A is
`prior art and is presented here to illustrate the complexity of
`manufacturing a driver airbag and the effect of the tethers. At
`every location where threads and sewing is used, additional
`pieces of material are usually attached so as to minimize the
`stress concentrations in the material at those locations. The
`manufacture of airbags is labor intensive which is the reason
`that many are made in low labor cost countries.
`Furthermore, the fabric itself is quite expensive as is the
`process by which neoprene or silicone is used to coat the
`material.
`It is obvious when observing an inflated airbag that it has
`not been optimized from a stress point of view. As a result,
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`the material thickness required is that which will resist the
`maximum stresses in the airbag which typically occur at
`only one or a few locations. Nevertheless, since it is not
`convenient to vary the thickness of the material, the same
`thickness is used throughout. Observations of an inflated
`airbag also show that there are typically many wrinkles
`which occur, primarily at the circumference. These wrinkles
`could be eliminated through a different pattern of material;
`however this complicates the sewing process.
`FIG. 1B, where the same reference numbers are used to
`represent the same parts here and throughout this disclosure,
`illustrates what the driver airbag would approximately look
`like if the tethers were removed. This figure is a partial
`cutaway perspective view of the driver side airbag of FIG.
`1A with the tethers removed. Experiments have shown that
`the airbag attains an approximate spherical shape. This, it
`can be shown, results from the fact that the threads can move
`relative to each other giving the bag some elasticity. If the
`tethers were not present, the occupant would usually interact
`with the airbag during deployment. Since this interaction
`results in injury to the occupant, the bag must be restrained
`from obtaining such a spherical shape (hence the required
`use of tethers).
`In contrast to the fabric airbag illustrated in FIG. 1B, an
`equivalent airbag made from plastic film is illustrated in
`FIG. 2 which is a partial cutaway perspective view of a
`driver side airbag 200 made from film. This airbag is
`constructed from two flat disks offilm material 210 and 212
`which are sealed together by heat welding or adhesive to
`form seam 220. A hole 250 is provided as before for
`attachment to an inflator. This hole 250 is reinforced with a
`ring of plastic material 260 and holes 252 are provided in
`this ring 260 as before for attachment to the inflator. A vent
`hole 230 is also provided and it has a reinforcing plastic disk
`231. Since this airbag is formed from flat plastic sheets 210
`and 212, an unequal stress distribution occurs causing the
`customary wrinkles and folds 222.
`Several different plastic materials are used to make plastic
`films for balloons as discussed in U.S. Pat. Nos. 5,188,558,
`5.248,275, 5.279,873, and 5,295,892, which are included
`herein by reference. These films are sufficiently inelastic that
`when two flat disks of film are joined together at their
`circumferences and then inflated, they automatically attain a
`flat ellipsoidal shape. This is the same principle used herein
`to make a film airbag, although the particular film materials
`chosen are different since the material for an airbag has the
`additional requirement that it cannot fail during deployment
`when punctured.
`When the distinction is made herein between an “inelas
`tic” film airbag and an elastic conventional airbag, this
`difference in properties is manifested