Airbag cover and apparatus for producing an invisible tear seam therein

A cover for an air bag unit is disclosed having a patterned tear seam therein defined by an integrally bonded, thermoplastic filler strip as the cover material and comprising a material having properties different than that of the cover material. Apparatus for forming the cover includes a thin shell mold having an inner surface that is heated to melt thermoplastic material cast thereon. A powder box adapted to contain thermoplastic material therein for distribution onto the thin shell mold carries a gasket having a shape corresponding to the tear seam shape. The powder box and gasket are adapted to be connected to the thin shell mold for forming a sealed footprint on the inner surface during distribution of the cast material against heated surface portions of the thin shell mold to form a thermoplastic cover with an opening therein that have the tear seam pattern. The opening is later filled with the lesser strength thermoplastic material to form an integrally bonded thermoplastic filler strip.

BACKGROUND OF THE INVENTION 
1. Technical Field 
This invention relates to covers for air bag units located within the 
interior of a motor vehicle and in particular relates to covers having a 
tear seam therein that is configured to define an uninterrupted outer 
surface on the cover. Further, the invention relates to apparatus and a 
method for forming hidden tear seams in such covers for air bag units. 
2. Description of the Related Art 
Various proposals for covering air bag units have evolved. One primary 
objective is to provide a cover that has a weakened section therein that 
will separate upon air bag deployment to provide an opening in the cover 
through which the air bag can escape following vehicle impact that is 
sensed by a controller. A gas generator is ignited by the controller to 
produce gas for inflating the air bag as it is deployed. 
Air bags are mounted in canisters that can be mounted in the steering wheel 
or in an instrument panel or crash pad on the driver's side of the 
vehicle. If the air bag is mounted on the passenger side, typically the 
canister is stowed behind an instrument panel either in a top mount 
position or in front mount position. Such mounting arrangements require 
that the cover material of the steering wheel hub or the skin or cover of 
the instrument panel be specially configured during manufacture to produce 
a weakened section to define a seam therein that will readily open during 
bag deployment. 
Such manufacturing methods and resultant structure must consider various 
functional and aesthetic requirements. The seam must be configured to 
separate such the air bag can be deployed immediately after vehicle 
impact. However, the cover must have a seam that is configured such that 
the air bag unit is protected against either accidental or intentional 
tampering and additionally the seam should be configured to meet long term 
weathering and interior design appearance requirements. As to design 
appearance, there is a desire in some cases to form the surface of the 
cover as a smooth uninterrupted surface so that there is no indication 
that there is a air bag unit disposed beneath the surface. This interior 
design objective is desired both from a human psychology standpoint and 
from a pure esthetics standpoint. In the case of the psychological 
considerations, the smooth surface effectively meets the adage of out of 
sight out of mind such that a passenger is not reminded of the possibility 
of deployment of an air bag unit. The interior design consideration 
enables smooth flowing surfaces to be incorporated in the cover material 
of an instrument panel on the passenger side thereof. 
U.S. Pat. No. 3,640,546, issued Feb. 1, 1972 to D.S. Brawn, includes a 
cover for a air bag unit having two doors that are covered by an outer 
layer of material that is weakened along a joint line by reducing the 
thickness of the outer layer. Such thin sectioned seam lines can cause a 
resultant depression over time that will essentially outline the outlet 
from the canister for the air bag component of the air bag unit. The 
arrangement does not solve either the psychological awareness problem or 
the problem of retaining a smooth, flowing surface appearance. 
U.S. Pat. No. 4,246,213, issued Jan. 20, 1981 to Y. Takamatsu et al., 
discloses a method for casting a V-shaped section at the tear seam. While 
the outer surface of the cover material is smooth the V-shaped 
configuration also reduces the thickness of the outer layer in an 
undesirable manner. 
U.S. Pat. No. 5,082,310 discloses a cover for an air bag unit. The cover 
includes two doors that are hinged for pivotal movement during air bag 
deployment. The doors are covered by a layer of foam and the layer of foam 
is covered by an outer skin having a V-shape groove that is cut to weaken 
the skin. 
One problem common to designs that cut or score a tear seam into an outer 
skin is that plastic materials especially, at cold temperatures, are prone 
to becoming brittle such that the cut or score line can induce cracking 
along the seam. At opposite temperature extremes typical skin cover 
material such as thermoplastic olefins or thermoplastic polyvinyl chloride 
become relatively soft and deformable such that they will stretch and 
remain unbroken during initial bag deployment. Another problem that can 
arise with cut or scored tear seams is that they become observable (read 
through outer surface) following extended periods of exposure to heat and 
sunlight. 
Another specific disadvantage of the cover shown in the '310 patent and 
another cover shown in U.S. Pat. No. 4,886,630 is that they require that 
the outer skin be separately formed on a heated mold and then removed from 
the mold and cut before being co-molded to a foam layer in a subsequent 
molding operation. 
The problems arising from separately forming a skin and then cutting the 
grooves therein are obviated by the present method that employs casting 
techniques disclosed in U.S. Pat. No. 4,623,503, issued Nov. 18, 1986 to 
E. Anetis et al., U.S. Pat. No. 4,562,025, issued Dec. 31, 1985 to John D. 
Gray and U.S. Pat. No. 4,925,151 issued May 15, 1990 to John D. Gray all 
of which are commonly assigned to the assignee of the present invention 
and each of which is hereby incorporated by reference. 
SUMMARY OF THE INVENTION 
The present invention is directed to a cover assembly for an air bag unit. 
The cover assembly has a skin or a shell with a tear seam therein having a 
desired configuration such as the C, H, U, and X patterns commonly used to 
form tear patterns in the skin that will open during air bag deployment to 
form an opening through which the air bag will pass as it inflates to 
protect an occupant of the vehicle. In particular the skin or shell is 
formed of a first thermoplastic material having a predetermined thickness 
and a cast opening therein that has a configuration corresponding to a 
desired tear seam configuration and wherein the cast opening is filled 
with a second thermoplastic material to form an integrally bonded filler 
strip in the skin or shell but having tensile and elongation properties 
less than that of the first thermoplastic material that forms the skin or 
cover material such that the tear seam will not be elongated as much as 
the skin or shell at higher temperatures but will have sufficient 
compliance at lower temperatures to prevent cracking at the tear seam. 
This invention also contemplates the use of filler strip materials having 
other different properties, e.g. lesser shear strength, flexural strength, 
burst strength, etc. 
Apparatus for forming the skin or shell for the cover assembly includes a 
heatable mold. A powder box carries a seal gasket that is shaped to 
correspond to the shape of the desired tear seam configuration. The powder 
box and seal gasket thereon is adapted to be connected to the heatable 
mold and to contain thermoplastic material such as thermoplastic olefins 
or thermoplastic material such as polyvinyl chloride (PVC) such as the 
pigmented particles or powder set-forth in U.S. Pat. No. 4,784,911 also 
commonly assigned to the assignee of the present invention and also 
incorporated herein by reference. The gasket pattern is arranged to 
impress against the surface of the heatable mold during the casting 
process and to leave a sealed footprint thereon that corresponds to the 
shape of the desired tear seam configuration. 
When the heatable mold is connected to the powder box, the gasket seals the 
mold surface such that following casting and curing of the skin or shell 
an opening is formed therein around the perimeter of the gasket having the 
same pattern as the tear seam. In order to produce the cover of the 
present invention the opening is then filled with thermoplastic material 
having different tensile and elongation properties than that of the 
originally cast material so as to form a filler strip of material that 
will separate on air bag deployment to form an opening through which the 
air bag can pass immediately upon vehicle impact. Additionally, the filler 
strip blends with the remainder of the skin or shell such that the cover 
assembly for the air bag unit with the cover assembly having a smooth 
outer surface that will hide an underlying air bag unit. 
Accordingly, an object of the present invention is to provide an easily 
assembled low cost outer skin or shell for use in a cover assembly for an 
air bag unit deployable into the passenger compartment of a motor vehicle 
wherein the outer skin has a preformed opening therein of the same 
thickness as the remainder of the outer skin but conforming to a desired 
tear seam configuration established by an end user and wherein the opening 
is filled to form an integrally bonded filler strip that combines with the 
outer skin to define a smooth, uninterrupted outer surface extending 
across both the preformed opening and the remainder of the outer skin. 
A feature of the present invention is to provide such a low cost outer skin 
having a filler strip of a thermoplastic material that fills the preformed 
opening and wherein the filler strip is integrally molded with the 
remainder of the outer skin. 
Another feature of the present invention is to provide a filler strip of 
thermoplastic material that fills the preformed opening having tensile and 
elongation properties different than such properties of the material 
forming the surrounding material of the outer skin. 
Still another feature of the present invention is to provide such a low 
cost outer skin having a cast thermoplastic outer skin with a preformed 
opening therein conforming to a desired tear seam configuration and a 
filler strip of a second thermoplastic material that fills the preformed 
opening to form a smooth uninterrupted outer surface on the outer skin. 
Another object of the present invention is to provide an improved apparatus 
for manufacturing the low cost outer skin of the present invention 
including a heatable mold member; a powder box and a gasket connected to 
the powder box that has a configuration that corresponds to the shape of a 
desired seam configuration for a cover assembly that hides an underlying 
air bag unit and wherein the gasket further is mounted to engage a surface 
portion of the interior of the heatable mold member to form a sealed 
footprint on the surface portion around which a thermoplastic material is 
cast to form an outer skin with a preformed opening therein corresponding 
to the shape of a desired seam configuration. 
Still another object of the invention is to provide a method for forming a 
weakened tear seam in the cover for an air bag unit comprising the steps 
of sealing along a footprint corresponding to a tear seam, forming a 
preformed opening corresponding to a tear seam pattern in a first 
thermoplastic material, and filling the preformed opening with a second 
thermoplastic material having different tensile and elongation properties 
than those of the first material. 
These and other objects, features and advantages of the present invention 
and a more complete understanding thereof will be apparent to those 
skilled in the art from the succeeding detailed description of the 
invention and the accompanying drawings thereof.

DESCRIPTION OF PREFERRED EMBODIMENT 
In accordance with a preferred embodiment of the invention in FIG. 7 an 
apparatus 10 is illustrated for manufacturing the outer skin and air bag 
deployment cover assembly of the present invention. The apparatus 10 
includes a powder box 12 having a peripheral portion 14 adapted to be 
connected to the peripheral portion 16 of a shell tool 18 by suitable 
clamps that join the peripheral portions in a known manner. 
The powder box 12 has a tear seam pattern 20 connected thereto that 
includes two cross legs 22, 24 and a center portion 26 that form an 
H-shaped pattern. Each of the legs 22, 24 and the center portion extend 
beyond the depth of the powder box and outwardly of the peripheral portion 
14 of the powder box such that they will penetrate into the interior of 
the shell tool 18 when the powder box 12 is clamped to the shell tool 18. 
Each of the legs 22, 24 and center portion 26 have a gasket mounting 
surface 28 thereon on which is affixed a gasket 30. 
The gasket 30 is shown as an H-shaped configuration and is conformable to a 
flat surface region 32 on the inner surface 34 of the shell tool 18 such 
that it will bridge the distance between the mounting surface 28 and the 
flat surface region 32 and form a sealed foot print on the flat surface 
region 32. The gasket 30 can, of course, have any shape desired so long as 
it corresponds to the tear seam pattern. 
The process for manufacturing the outer skin and cover assembly of the 
present invention includes initially locating the shell tool 18 in an 
inverted position as shown in FIG. 6 such that the inner surface 34 faces 
downwardly. The shell tool 18 is mated to the powder box 12 that contains 
a powdered thermoplastic material. When the peripheral portions 14, 16 are 
connected the shell tool 18 and powder box 12 form a sealed mold chamber 
36 defined by the inner surface 34 and the interior walls of the powder 
box 12. The thermoplastic material can be a dry resin powder material with 
suitable coloring and plasticizer content, such as described in U.S. Pat. 
No. 4,923,657 issued May 8, 1990 to J.C. Gembinski et al. Shell tool is 
heated to a sufficient temperature to cause approximately one millimeter 
of dry powder to attach and fuse. Mold apparatus 10 is then rotated as a 
unit 180 degrees to dump the dry powder against the surface 34. Since the 
inner surface is heated, a layer of the dry powder forms across the 
surface 34 and about the outer peripheral edge 30a of the gasket 30 and 
fuses to form a skin 40 of plastic with an H-shaped gap or opening 40a 
therein at the foot print of the gasket 30. The remaining powder is dumped 
back into the powder box by rotating the mold apparatus 10 as a unit back 
to the initial position shown in FIG. 6. The powder box 12 and shell tool 
18 are then disconnected and the shell tool 18 is heated more to cure the 
thermoplastic layer. Thereafter a second type of thermoplastic material is 
deposited within the H-shaped gap or opening 40a while the skin 40 is in 
the shell tool 10 (FIG. 8). The shell tool 10 is heated to suitably cure 
both the originally deposited material that makes up the skin 40 and the 
second deposited material that makes up a filler strip 40b that fills the 
gap 40a. Finally, the shell tool 10 is cooled and the skin 40 and 
H-configured filler strip formed integrally therewith are removed as a 
unit from the inner surface 34. 
Automated production apparatuses for performing such steps are well known 
to those skilled in the art including U.S. Pat. No. 4,623,503, issued Nov. 
18, 1986 to E. Anetis et al., U.S. Pat. No. 4,562,025, issued Dec. 31, 
1985 to John D. Gray and U.S. Pat. No. 4,925,151 issued May 15, 1990 to 
John D. Gray. The manner of heating the shell tool 18 will depend upon a 
particular operation but it is contemplated that heating by directing a 
heated air stream on the outer surface 18a of the shell tool 18 will work. 
The gasket 30 cross-section can be a foam compressible material such as a 
temperature resistant foam or it can be a hollow gasket either of the 
inflatable or non-inflatable type such as is shown in U.S. Pat. No. 
4,925,151 issued May 15, 1990 to John D. Gray. 
FIGS. 1-5 show an automobile interior trim structure that includes a cover 
assembly 52 for hiding an air bag unit 54 shown in outline broken line 
form in FIG. 2. In FIG. 1 an outer skin 56 is shown having a H-shaped 
filler strip 58 therein of an integrally molded thermoplastic material 
having a lesser tensile strength than that of surrounding thermoplastic 
material of the outer skin 56. The cover assembly 52 has a smooth 
uninterrupted outer surface 60 that extends across the filler strip 58. 
Since the filler strip 58 is formed from a different type material than 
the remainder of the outer skin 56, in order to provide an identical 
surface appearance a thin paint coating 62 can be provided on the outer 
surface 60. Alternatively, the filler strip material and skin material can 
be blended to provide an identical appearance match. In the illustrated 
embodiment the cover assembly 52 includes a pair of doors 64, 66 formed as 
part of an insert 68 that forms an enclosure for an air bag canister 70 
having a deployable air bag 72 therein. The air bag 72 is inflated by a 
gas generator 74 upon ignition in response to a signal from a controller 
76 including collision sensing device 76a as is well known in the art. The 
configuration of the air bag unit and the arrangement of the components 
thereof will depend upon a particular application and form no part of the 
present invention. Furthermore, the cover assembly 52 can be formed to 
have a single door or other configuration that will spread open to bear 
against the outer skin 56 so as to cause separation thereof at the filler 
strip 58 that defines a lesser strength region in the outer skin 56 that 
will immediately separate upon air bag deployment. In the illustrated 
embodiment, the filler strip 58 forms an H-shaped tear seam within the 
outer skin 56. The doors 64, 66 will pivot about hinges 64a, 66a as shown 
in FIG. 3 to press through a foam layer 78 located between the insert 68 
and the outer skin 56. The doors 64, 66 will act on the lesser strength 
region defined by the filler strip 58 to produce an immediate separation 
of the outer skin 56 to form an opening 80 as shown in FIG. 5 such that 
the air bag 72 will be free to deploy into the passenger compartment 
immediately following a collision that will ignite the gas generator 74 
and inflate the air bag 72 for deployment through the opening 80 formed by 
the separation of the H-shaped tear seam. 
In accordance with the invention the H-shaped tear seam has a surface 
smoothness like that of the surrounding regions of the outer skin such 
that the cover assembly 52 will appear as a continuous uninterrupted 
surface overlying the outlet from the canister that houses the air bag. It 
will of course be understood that the filler strip 58 that forms the 
lesser strength material region within the outer skin 56 can be shaped in 
other ways including C, U or X shapes as illustrated in co-pending U.S. 
patent application Ser. No. 680,837 filed Apr. 5, 1991 and commonly 
assigned with the present application. In one embodiment, the material of 
lesser strength is a spray deposited highly glass filled or carbonate or 
other filler filled thermoplastic PVC or polyolefin material and the 
surrounding material of the outer skin 56 is a thermoplastic material 
formed from PVC. The lesser strength material that forms the filler strip 
58 has a lower tensile and elongation property in the operational 
temperature range than that of the material comprising the remainder of 
the outer skin that is integrally bonded to and in surrounding 
relationship to the periphery of the filler strip 58. Typically, the 
operational temperature range in an instrument panel can range from 
-30.degree. C. to 104.degree. C. 
The outer skin 56 provides a tear seam at the filler strip 58 that has the 
necessary thickness to hide any tendency to read through the location of 
the doors or the outlet from the canister. Nevertheless, the filler strip 
58 defines a tear seam that will provide an weak link at which the outer 
skin 56 will be easily separated. 
In some cases, the need for a hidden seam line is less critical. If so, the 
method will include providing a nickel alloy mold shell 82 having a 
styling line protrusion 84 therein. As shown in FIG. 9, the sealing gasket 
86 is positioned over the protrusion 84 to form a sealed footprint 
corresponding to a desired tear seam pattern. The first thermoplastic 
material is cast as layer 90. The gasket 86 is removed and a preformed 
opening 88 in layer 90 is then filled by casting, spraying or otherwise 
depositing a second layer of thermoplastic material of desired, different 
properties than of the first material. A resultant recessed styling filler 
strip 92 is formed on the layer 90 as shown in FIG. 10. 
It will be appreciated from the foregoing description that the present 
invention not only includes an outer skin configuration that will assure 
cover assembly opening immediately upon vehicle collision but additionally 
a outer skin configuration that will hide an underlying air bag unit over 
long periods of time. Furthermore, the arrangement will protect an 
underlying air bag unit against either accidental or intentional tampering 
since there is no structure or door that can be pried open or 
inadvertently depressed into the structure of the air bag unit. It will be 
further appreciated by those skilled in the art that the foregoing 
description is for a preferred exemplary embodiment of the invention and 
that the invention is not limited to the specific embodiments or material 
selections shown or discussed. For instance, in the broader aspect of the 
outer skin, the seam forming gap can be filled with a suitable bonding 
material with desired tensile strength properties so long as the gasket 
formed gap therein is filled to form a smooth outer surface and a 
thickness of material that will prevent read through of underlying air bag 
unit components. For instance, in the broader aspect of the apparatus the 
gasket can be carried by a separate carriage apart from a powder box such 
that the initial skin can be formed around a gasket footprint by spray 
deposit of the thermoplastic material rather than rotational coating of 
the thermoplastic powder. For instance, in the broader aspect of the 
method, the initial coating with preformed opening can be formed by an 
injection molding or a spray step and the filling of the preformed opening 
can be accomplished by a rotational deposition of thermoplastic material 
of lesser strength and lesser elongation under operating temperature 
conditions. In addition to thermoplastic PVC spray coatings, the method of 
the invention contemplates the use of spray coating urethane materials to 
either form the preformed opening in the skin or shell or to form the 
filler strip. 
Various changes and modifications will be apparent to those skilled in the 
art and all such variations and modifications are intended to come within 
the spirit and scope of the appended claims.