Rotary swaging of gas generator filters

A method for re-sizing and re-shaping wrapped multiple layered or stack-up cylindrical air bag inflator filter assemblies. This invention is specifically directed to manufacturing cylindrical air bag filter assemblies exhibiting superior dimensioned characteristics such as diameter, wall thickness, roundness, straightness, clylindricity by subjecting the as-wrapped filter assemblies to a rotary swaging operation.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to manufacturing filters for inflatable 
type modular occupant restraint systems for passenger vehicles or, as they 
are commonly known, air bag restraint systems. The filter in such systems 
functions to cool hot gases before they reach the air bag and serves to 
trap particulates and residues generated during ignition so that they do 
not enter the air bag and escape into the vehicle. More particularly, this 
invention relates to an improved method for manufacturing inflator filters 
which includes re-sizing and re-shaping said filters by subjecting the 
filters to a rotary swaging operation. 
2. Description of The Related Art 
Filter assemblies generally available in the art include alternating layers 
of screen and ceramic paper. Paxton et al, U.S. Pat. No. 4,998,751, which 
is assigned to the assignee of the present invention, disclose a filter 
assembly which comprises two wraps of nickel coated carbon or stainless 
steel 30-mesh screen, metal filters such as 80.times.700 or 
50.times.250-mesh stainless steel or 40.times.180-mesh nickel coated 
carbon steel, a single wrap of ceramic filter paper 0.080 inches thick 
followed by two wraps of 30-mesh stainless steel or nickel coated carbon 
steel. 
The filter assemblies are, typically, manufactured by wrapping the 
different filter materials circumferentially around a mandrel. Cunningham, 
U.S. Pat. No. 4,878,690, which patent is assigned to the assignee of the 
present invention, discloses a cylindrical filter assembly manufactured 
accordingly. The filter assemblies are manufactured as single units by 
hand and/or machine. The particular geometry of the filter assemblies and 
the number of wraps of filter materials are determined by the designed end 
use of the assemblies. 
There is a roundness problem associated with making cylindrical filter 
assemblies for air bag inflators, which have wrapped multiple layered or 
stack-up structural configurations. The use of such wrapped multiple 
layers or stack-ups inherently results in a screen pack assembly 
exhibiting an out-of-round, i.e., ellipsoidal, cross-section. This 
inherent lack of roundness is exacerbated as the number of layers of 
filter materials increase and is particularly acute at the overlap outside 
diameter (O.D.). 
Re-sizing cylindrical filters for use in air bag inflator systems has been 
a topic of concern in the art dating back to early driver side air bag 
applications. However, on passenger side air bag programs resizing of 
oversize filters was a dead issue from the beginning since the diameter 
requirement was only a maximum. More recently, the implementation of a 
minimum diameter requirement coupled with existing maximum requirements 
has revived interest in re-sizing. One re-sizing method comprises 
reworking oversize passenger screens by sizing them down to size through 
the use of a split tubular (clam shell) stationary die mounted to a press. 
Use of the clam shell method for re-sizing has not proved satisfactory 
because it requires operating with a minimal oversize tolerance in order 
to prevent screen damage. With a clam shell die the majority of the 
working occurs at the die separation region and this results in localized 
working. There is also a finning or winging problem associated with the 
use of a clam shell die when large diameter as-wrapped screen packs are 
worked. Thus, the use of a clam shell die places restrictions on the 
as-wrapped O.D.s of the screen pack that can be successfully re-sized. 
Further, the inherent ellipsoidal shape of the as-wrapped filter screen 
packs requires special placement of the screen pack into the clam shell 
die. The maximum diameter of the ellipsoidal shaped screen must be aligned 
with the vertical axis of the die member to effect re-sizing and 
re-shaping of the as-wrapped screen pack. 
The consistent and economical manufacture of filter assemblies for air bag 
inflator systems, which efficiently cool and clean gas from the gas 
generant, is of prime concern. The implementation of a resizing process 
for producing cylindrical filter assemblies exhibiting superior 
dimensional characteristics such as diameter, wall thickness, roundness, 
straightness, cylindricity, etc. and which exhibit lower residue levels 
has obvious advantages and benefits. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide an improved method for re-sizing 
and re-shaping wrapped multiple layered or stack-up cylindrical air bag 
inflator filters having at least one open end and internal and external 
surfaces to the required size and shape, which filters are designed for 
use in gas generators for passenger side, driver side, side impact and 
rear seat inflatable air bag occupant restraint systems. Further, an 
object of this invention is to enhance the performance of the 
manufacturing process for making air bag filters by producing filters 
exhibiting superior dimensional characteristics such as diameter wall 
thickness, roundness, straightness, cylindricality, etc. Another object of 
this invention is to provide a method of manufacturing an air bag inflator 
filter having superior characteristics which enhances its performance when 
employed in an air bag gas generator by providing lower particulate levels 
with less variation upon firing of the inflator. 
These objects have been achieved by a novel process for resizing and 
re-shaping wrapped multiple layered or stack-up cylindrical filter 
assemblies to the required size and cylindricity, which comprises 
subjecting the as-wrapped filter assemblies to a rotary swaging operation. 
The rotary swaging process of this invention can be used for re-shaping 
discrete filters or continuous filter lengths. 
While rotary swaging has been available to industry since the early 1900's 
it has generally been applied to making cylindrical parts from rods, tubes 
and wire. Rotary swaging alters the diameters or shape of such components 
by means of a large number of controlled-impact blows applied radially by 
one or more pairs of opposed dies. The dies are appropriately shaped to 
give the part the required form. Metals that are suitable for metal 
forming processes in general are also the most readily swageable. Best 
results are generally obtained with low-carbon steels and ductile 
nonferrous metals. The use of swaging to re-size and/or re-shape filtering 
systems for air bag inflators in accordance with this invention is novel 
and results in lower particulate levels when compared with filters which 
were not re-sized and/or re-shaped. High residue weights in air bag 
inflators have been attributed to an inherent lack of roundness of 
cylindrical filter units. Swaging has been discovered to improve filter 
roundness. Swaging has also been found to provide reduced residue weight 
readings compared to unswaged filter screen packs. 
Reworking of oversize filters assemblies by sizing them down to size 
through the use of a split tubular (clam shell) die mounted to a press has 
limited utility and cannot be appropriately used for re-sizing larger 
diameter filters. The clam shell die produces excess finning and 
accordingly is limited to re-sizing cylindrical filters having O.D.s about 
0.025 in. above maximum. It has been discovered that this finning problem 
is overcome through the use of rotary swaging. Further, it was discovered 
that post-swage O.D. can be maintained consistently if the initial filter 
O.D. is at least 0.035 in larger than the inside diameter (I.D.) of the 
swaging die. This permits a larger tolerance, ie O.D. range from the 
winding machine.

DESCRIPTION 
The method of re-sizing and re-shaping inflator filter assemblies according 
to this invention by employing a rotary swaging operation will now be 
described with reference to specific embodiments thereof. 
An as-wrapped cylindrical passenger side filter assembly which is subject 
to re-sizing and re-shaping by the swaging process of this invention is 
shown in FIG. 1. Filter assemblies of the type shown in FIG. 1 are 
described in co-pending application--Morton case number 2494-21-00 filed 
on even date herewith. The filter unit 10 comprises an inner 
30.times.30-mesh tube 12 and additional wraps of 18.times.18-mesh metal 
woven cloth 14, paper filter 16 and 45.times.170-mesh metal woven cloth be 
wound thereabout. Filter unit 10 is manufactured by cutting 
18.times.18-mesh metal woven cloth and 45.times.170-mesh metal woven cloth 
to length, positioning them together on a lay-up table and welding to form 
a wire cloth laminate. Paper filter 16 such a Lydall 924 filter paper, is 
then cut to length and positioned to the wire cloth laminate. The leading 
edge of the wire cloth/paper filter subassembly is welded to the exterior 
surface of the inner 30.times.30 tube 12 and the wire cloth laminate and 
paper filter are wound outward around said tube 12 in an outer wrap 
machine. The trailing edge of the radially most outward portion of said 
laminate is welded to the thus formed cylindrical filter body so that the 
wire cloth laminate and filter paper are maintained in cylindrical 
relation about said core. As shown in FIG. 1, the maximum O.D. of the 
as-wrapped screen pack occurs in the vicinity of line AA with the minimum 
O.D. displaced approximately 90.degree. therefrom. 
The 30.times.30-mesh wire cloth is cut to length and curled using a 
conventional two roll former (0.75 in. diameter steel knurled roller and a 
2 in. diameter urethane roller, 50 durometer) into cylindrical form to 
provide tube 12. The two roll former rolls the wire cloth into a capture 
tube while the cylindrical form is being produced. Two finish roll belts 
assist the wire cloth all the way into the capture tube. Once the wire 
cloth is completely in the capture tube it is stripped into a sizing tube. 
The wire cloth is sized to the same diameter as the sizing tube and then 
welded together by resistance welding to form self supporting tube 12. 
The as-wrapped cylindrical filter assembly is then re-sized and re-shaped 
by subjecting said assembly to the rotary swaging operation of this 
invention. A Fenn F-4 swaging machine, manufactured by Fenn Manufacturing, 
Newington, Conn. was utilized to carry out the swaging operation. The 
swage tooling consisted of two semi-circular, cross-section dies backed by 
hammer blocks. The die sections each had a 4.degree. relief taper. As the 
die assembly rotates, the hammer blocks strike a series of 12 rollers that 
cause the dies to be driven together. The filter assembly is formed to the 
desired size/shape by repeatedly compressing the part between the die 
halves. A spindle speed of 200 rpm was typically employed to swage the 
filter assembly. Zero clearance dies were preferred. Accordingly, shims 
were inserted as needed between the die halves and the hammer blocks to 
assure that the dies closed completely. Further, it was found that use of 
an internal mandrel during swaging improved part roundness significantly 
and is preferably used to maintain the internal diameter integrity. A feed 
rate of up to 3 inches or more per second was the typical work employed. 
The degree of resizing was found to be dependent upon the initial size of 
the filter. Maximum filter O.D. as-wrapped for the filters comprising a 
perforated support tube is targeted in the 2.145 in.-2.18 in. range. The 
target O.D. for the post swaged filters is about 2.113 in. 
By way of illustration and not limitation it is noted that for the 
passenger side filter shown in FIG. 1, the materials of which the filter 
components are made and the dimensions thereof where relevant are as 
indicated below: 
______________________________________ 
Com- 
ponents Functions Materials Dimensions 
______________________________________ 
filter Combustion screen 
carbon steel 
30 .times. 30 mesh 
cloth-12 
holds large .011" wire 
particles and slag diameter 
and provides some 
initial cooling 
coarse Support ceramic 
carbon steel 
18 .times. 18-mesh 
screen paper. Aids in .017" wire 
filter wrapping filter, diameter 
cloth-14 
provides cooling 
and supports fine 
screen. 
filter Filters particu- 
ceramic, mfd. 
paper lates. Cools gas 
by Lydall, Inc. 
Lydall flow. New Hampshire 
924-16 
fine screen 
Filters and cools 
stainless steel 
45 .times. 170- 
filter-18 
the gases mesh 
______________________________________ 
While FIG. 1 shows a filter construction comprising three wraps of the 
30.times.30-mesh screen, six wraps of the 18.times.18-mesh screen, three 
wraps of the paper filter and one wrap of the 45.times.170-mesh fine 
screen, it is to be understood that the swaging process of this invention 
is useful for re-sizing and re-shaping wrapped multiple layered or 
stack-up cylindrical filter constructions comprising different mesh 
screens, number of individual filter elements, number of wraps and/or 
additional or alternative materials in structuring the filter unit. 
While the preceding embodiment is directed to manufacturing filter units 
which do not include a perforated support tube, the swaging process of 
this invention also finds application in re-sizing and re-shaping filters 
of the type which include a perforated support tube. In this embodiment 
the leading edge of the aforesaid wire cloth/paper filter subassembly is 
welded to the exterior surface of a perforated support tube and then wound 
outward around said support tube in an outerwrap machine. The trailing 
edge of the radially most outward position of said laminate is welded to 
the thus formed cylindrical filter body so that the wire cloth laminate 
and paper filter are maintained in cylindrical relation about said 
perforated support tube. Unlike the previously described embodiment, the 
30.times.30 mesh wire cloth 12 is stripped from the capture tube into the 
perforated support tube. The wire cloth 12 is then sized to the same 
diameter as the perforated support tube and is then welded to the 
perforated support tube which has been wrapped with the 18.times.18 mesh 
wire cloth, the Lydall 924 filter paper and the 45.times.170 mesh wire 
cloth. 
The maximum filter O.D. as swaged for filters that do not employ a 
perforated support tube is targeted in the 2.100 in.-2.107 in. range. It 
was observed that post swage O.D. can be maintained consistently if the 
initial filter O.D. is at least 0.035 in. larger than the die I.D. This 
will target maximum filter O.D.s as wrapped in the 2.12 in.-2.18 in. 
range, which is a significant tolerance increase over filters including 
the perforated support tube. 
Tests of filters, which do not have a perforated support tube, swaged in a 
2.097 die (0.fwdarw.diameter) and checked for roundness using filter gauge 
0245-12 showed that the average roundness was improved from 0.0144 in to 
0.0056 with a significant improvement at the trailing end of the 
18.times.18-mesh screen. The term roundness is defined in this invention 
as: 
##EQU1## 
Firing tests conducted under substantially the same conditions showed that 
swaged tubeless filter screen packs provided lower residue levels compared 
with unswaged tubeless filter packs. The results of such tests are shown 
in Tables 3 and 4. 
TABLE 3 
______________________________________ 
Unsized 
Sample P Max.sup.1 
P40.sup.2 Delay.sup.3 
Pcomb.sup.4 
Residue.sup.5 
______________________________________ 
1 96.2 90.5 4.5 2675 7.7 
2 81.1 79.3 4.3 2623 6.25 
3 70.5 69.6 4.5 2525 2.56 
4 77.2 74.9 4.3 2831 6.04 
5 109.1 105.8 4.2 2760 15.74 
6 97.7 65.2 4.4 2446 9.8 
7 74.5 72.8 4.1 2454 6.22 
8 73.5 71.9 4.4 2633 4.06 
9 70.7 69.5 4.1 2826 2.63 
10 72.5 71 4.3 2777 3.86 
11 71.1 69.9 4.1 2819 3.91 
______________________________________ 
TABLE 4 
______________________________________ 
Swaged 
Sample 
P Max.sup.1 
P40.sup.2 
Delay.sup.3 
Pcomb.sup.4 
Residuehu 5 
______________________________________ 
12 66.9 65.8 4.1 2149 0.92 
13 67.2 65.9 4.4 2014 0.94 
14 66.2 64.9 4.5 2140 1.13 
15 66.8 65.8 6.1 2178 0.79 
16 65.6 64 4.6 2077 0.93 
17 65.7 64.2 6 2007 0.78 
18 69 67.71 4.4 2322 1.34 
19 65.8 64.7 4.1 2159 0.96 
20 64.6 63.8 3.8 2191 0.92 
21 67.2 66 4 2291 1.09 
______________________________________ 
.sup.1 ksi 
.sup.2 P at 40 msec. (psi) 
.sup.3 msec. 
.sup.4 P/combustion chamber (psi) 
.sup.5 grams 
As shown in Tables 3 and 4, the swaged screen packs had an average residue 
content of 0.980 grams whereas the unswaged screen packs had an average 
residue content of 6.2518 grams. This represents approximately an 84% 
reduction in residue content as a result of the swaging. 
The as swaged passenger side air bag inflator filter FIG. 1 produced by the 
process of this invention can be used in any of a number of known inflator 
constructions including the construction shown in FIG. 2 and illustrated 
in U.S. Pat. No. 4,296,084 to Schneiter, which patent is assigned to the 
assignee of the present invention. The inflator 52 includes generally an 
outer housing 54 into which is inserted the as swaged filter unit 36 of 
this invention. An igniter 58 containing igniter granules and the 
appropriate ignition system is then inserted into the center of the 
inflator 52. Gas generant 60 is then loaded into the inflator 52 which is 
then sealed in a conventional manner known to those in the art. After 
ignition of the gas generant, the rapidly expanding generated gases flow 
outwardly from the center of the inflator 52 through the cylindrical 
filter assembly 36 and the filter acts as a heat exchanger for cooling the 
gases. Substantially all of the solid residual matter carried by the gas 
is trapped and retained in the various wraps of filter material. 
While the invention has been specifically described in relation to swaging 
passenger side filter assemblies, it is to be understood that the present 
invention is not limited to such applications. The swaging techniques of 
the present invention can be used to re-size or re-shape cylindrical 
filter units generally having a multiple layered or stack-up structural 
configuration where such characteristics as diameter, wall thickness, 
roundness, straightness, cylindricality are desirable. The swaging 
operation of this invention is generally applicable to re-sizing and 
re-shaping any cylindrical filter assemblies which exhibit an inherent 
lack of roundness because of their multiple layered or stack-up structural 
configurations and/or method of manufacture. 
Thus, in accordance with this invention, there has been provided and 
improved method for re-sizing and re-shaping oversize out-of-round 
cylindrical filter assemblies. Additionally, there has been provided an 
improved method for eliminating the inherent lack of roundness of 
cylindrical filter assemblies having multiple layered or stack-up 
structural configurations by re-sizing and re-shaping said assemblies in a 
rotary swaging machine. 
With this description of the invention in detail, those skilled in the art 
will appreciate that modifications may be made to the invention without 
departing from the spirit thereof. Therefore, it is not intended that the 
scope of the invention be limited to the specific embodiments that have 
been illustrative and described. Rather, it is intended that the scope of 
the invention be determined by the scope of the appended claims.