Patent ID: 12246671

The reference numbers used in the figures correspond to the following:9expanded metal sheet11perforations in the expanded metal sheet13aexpanded-metal burrs—rough side13bexpanded-metal burrs—smooth side15expanded metal sheet's smooth side17expanded metal sheet's rough side21substantially cylindrical filter22substantially cylindrical inner surface of filter2123cavity of filter21defined by inner surface2224substantially cylindrical outer surface of filter2125substantially flat end sections of filter2126solid propellant101roll of metal sheet103press105punch107teeth or bits109stretcher111camera113computer controller115monitor121rollers123cutter125expanded metal strip127expanded metal strip129welder131cylinder133welder135welded mesh cylinder137female mold139mandrel141wall of perforation11(smooth side)143wall of perforation11(smooth side)145concave barrier (rough side)147concave barrier (rough side)149three-dimensional gorge in concave barrier (rough side)151sharp corner (rough side)153rounded corner (rough side)155airbag assembly157inflator159inflatable vehicle occupant protection device

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As discussed above, airbag filters have historically always been wrapped smooth side facing inward because in a commonly-used airbag configuration, the inflator's filter is in direct contact with the inflator's solid gas-generating propellant pellets (also know as tablets). The pellets are susceptible to degradation as a result of vibration because anything that is rough can scrape the surfaces of the pellets which can degrade the inflator's performance and/or make the inflator more dangerous. If the smooth side of the filter's expanded metal is wrapped facing outward and the rough side is facing inward, then the expanded metal can act exactly like a cheese grader on the solid propellant. Thus, in the past, all pyrotechnic airbag manufactures have prohibited the wrapping of filters made of expanded metal with the rough side of the expanded metal facing inward.

During a deployment of an airbag inflator, the gas that fills up the airbag is generated by a solid rocket fuel, which is, most commonly, based on guanidine nitrate. The propellant is normally highly loaded with copper and other metals, which in some cases can constitute 60% or more of the overall composition. During an airbag inflator deployment, the metals in the propellant liquefy and become entrained in the gas produced by the burning of the propellant. In this very dynamic system, this phase change from a solid to a liquid happens in a few dozen milliseconds.

The inflator filter's job is to thermodynamically diffuse and cool the hot burning gas such that the liquid copper and other metals are transformed back into the solid phase so that they can be captured in the filter, with only the cooled gas escaping. Car companies are very concerned with the amount of slag coming out of the inflator. If an inflator puts out more than 1 total gram of residues and/or airborne particulates (collectively, slag) then the inflator will be rejected by car companies for not meeting the USCAR standards established by NHTSA and other safety automotive groups to protect asthmatic occupants and others susceptible to health problems from exposure to airborne particulates.

In accordance with an aspect of the present disclosure, an expanded metal sheet, once formed, is flattened by running the sheet through a set of, for example, calendar rollers to smooth out the sheet's rough side so that pellets of the inflator's propellant will not degrade unacceptably as a result of contact with the rough side. The flattening should be enough so that from a cheese grader standpoint, the flattened smooth side and the flattened rough side of the expanded metal sheet are nearly identical. That is, the flattened rough side should grind down the inflator's pellets no faster than is normal for an un-flattened smooth side. Car companies require their inflator part suppliers to test for grinding down of an inflator's pellets by vibrational environmental testing in a laboratory. The testing is designed to replicate extended or long term vibrations in a vehicle. In an embodiment, when so tested, the flattened rough side is smooth enough so that when it forms the wall of the filter's cavity it does not degrade the inflator's pellets any faster than is normal for filters where the smooth side forms the cavity's wall. In an embodiment, after flattening both the flattened rough side and the flattened smooth side will be smooth to the touch.

Surprisingly, even though it has been flattened, by orienting the flattened rough side so that it points inward, i.e., so that it faces into the gas flow direction, more efficient cleaning is obtained than when the flattened smooth side points inward. Although not wishing to be bound by any particular theory of operation, it is believed that the flattened rough side expanded-metal burrs create slag capturing pockets at every perforation that are able to capture the slag produced by the burning propellant at a fast rate. That is, the rough fins (burrs) of the expanded metal, even though flattened, act like a little army of internal capturing pockets. Consequently, much cleaner gas filtrations can be achieved by wrapping an expanded metal filter with its flattened smooth side facing out and its flattened rough side facing in.

With reference now toFIG.3, the manufacture of an expanded metal sheet according to certain aspects of this disclosure starts with a roll of metal sheet101(for example, about nineteen inches wide, which can be cut down to between two to six inches for making filters for passenger airbags and to about 1.5 inches for driver airbags, although any width can be used depending on the equipment). For filters for airbag inflators, stainless steel, such as SS304, 309, 310, 409, 410, or 430 can be used. Carbon steel from C1006 to C1008 is often preferred for various applications. Depending on the environment in which the expanded metal is used, other metal compositions available in a sheet form can be used.

The sheet is fed first to a press103in which a punch105having a number of teeth or bits107is moved into the sheet so that the teeth perforate the sheet and then the punch is removed, just as in a stamping operation. The geometry of the bits, which are preferably identical to each other, is preferably such that a slit is formed in the sheet. Depending on the geometry of the bit, the depth of penetration of the bit will determine the length of the slit formed; the deeper the penetration, the longer the slit, and thus the more open the final structure can be after stretching. While shown with a single punch, multiple punches can be used to provide different perforation spacings, geometries, and/or depths. For airbag inflator filters, the opening is made to a size based on the airbag manufacturer's specifications for the open area of the sheet, the porosity of the sheet, or other parameter(s) required for the filter.

The sheet is advanced preferably by a servo motor (not shown) or other mechanism whereby the longitudinal advance of the sheet can be precisely controlled. The advance of the sheet is preferably in discrete steps so that the sheet is stationary when punched. Although not preferred, a roller with teeth can be used in a continuously moved sheet.

The perforated sheet produced in the press is then fed to a stretcher109in which differential rollers stretch the perforated sheet in the axial direction (that is, along the direction of travel) so that the slits are opened into diamond-shaped holes. (Of course, a hexagonal bit can be used to make hexagonal openings, or other bit geometries, can be used, but slits formed into diamonds is a common shape.)

Although slitting and stretching can be performed as separate operations, when fine patterns are to be formed, it is often preferable to produce the expanded metal sheets by performing slitting and stretching with the same teeth in the same motion. During this operation, the material hangs out over a flattened bottom blade and angled upper teeth or bits slit the sheet and then continue into the sheet. The sheet bends down and the angle formed by this bending as it relates to the teeth causes a stretching motion of the sheet. Consequently, the sheet is stretched more or less by the depth of the tooth penetrations. The amount of stretching achieved in this way is typically in the range of 20-25% and can be as much as 37%. Compared to the slit-and-stretch approach, the one step approach produces perforations (openings) that have a shape more like that of a triangle than a diamond (seeFIGS.4-5and7). As with the separate slitting and stretching approach, the one step approach forms openings by (i) forming slits in a sheet of metal and (ii) stretching the slits in the direction of the metal's longitudinal axis, but does so in one step, rather than two.

A video control system including at least one camera111, which is connected with a computer controller113running software, and an optional monitor115, examines the holes or open area, and can learn (after parameters are input to the controller) whether the perforations are within specification. The controller's software checks the opening sizes and/or shapes (geometry) to determine whether the individual openings, or open area (actual or estimated or calculated), are within specification. A second camera (not shown) can be placed between the punch and the stretcher to determine whether the initial punching is within specification. The video control system performs an optical inspection of the expanded metal sheet product and determines whether the product is within specification. To alter the process to get on, return to, or change the specification, the advance of the sheet can be altered by adjusting the servo motor (via the computer controller) to change the longitudinal spacing of the perforations. The stretcher can also be adjusted to increase or decrease the amount the perforated sheet is stretched.

Once formed, the expanded metal sheet is flattened by, for example, one or a pair121of rollers. If desired, the expanded metal sheet can be passed through multiple pairs of rollers to achieve the desired degree of flattening. Based on their knowledge of the art and this disclosure, skilled workers will readily be able to select roller configurations suitable for achieving the levels of flattening discussed herein. As discussed above, in accordance with the disclosure, the amount of flattening is selected to achieve enhanced slag capture without compromising an inflator's service life as a result of degradation of the inflator's solid propellant through the cheese grater effect. With reference again toFIG.3, a video control system camera111can be located after the flattening step and used to determine if the degree of flattening is within specifications.

FIG.4Ais a photomicrograph of the rough side17of a sheet of expanded metal produced by the one-step expanding process described above. The figure shows the rough side prior to flattening. As can be seen, perforations11in the sheet have a generally triangular shape. For this manufacturing process, the expanded-metal burrs on the rough side are largest along the intersecting short sides of the triangles. These rough side burrs have been labeled13ainFIG.4.FIG.5Ais a photomicrograph of the corresponding smooth side15of the sheet, again prior to flattening. This surface also exhibits burrs, but they are smaller and are located on the long sides of the triangles. These smooth side burrs have been labeled13binFIG.5.

The expanding process increases the thickness of the sheet by, for example, two times, the exact amount depending on the particulars of the process, the perforation pattern being formed, and the thickness of the base material. As one example, for the expanded metal ofFIGS.4-7, the base material had a thickness of 0.015 inches (15 thousandths) and the expanding process increased the sheet thickness by more than two times, i.e., after expanding and prior to flattening the sheet as shown inFIGS.4A and5Ahad a thickness of 0.033 inches (33 thousandths).

As noted above, the thickness of the expanded metal sheet prior to flattening will depend on the thickness of the base material and the amount by which the based material is expanded. The thickness after flattening will depend on the amount of flattening applied to the expanded sheet. In an embodiment, the amount of flattening needed to achieve enhanced slag capture without compromising an inflator's service life as a result of the cheese grater effect can be expressed in terms of a percentage reduction in the pre-flattened thickness t of the expanded metal sheet resulting from the flattening, e.g., in an embodiment, the thickness reduction can be in the range of 25-45% which for an expanded metal sheet having a pre-flattened thickness of 33 thousandths, corresponds to a post-flattening thickness in the range of 25 to 18 thousandths, respectively.

In certain embodiments, the thickness reduction is in the range of 30-45% or in the range of 30-40% or in the range of 30-35%. The percentage ranges referred to herein and in the claims include the endpoints of the ranges. The lower and upper endpoints can be used in other combinations such as 25-40% and 25-35%. Expanded metal exhibits a spring-back effect after flattening wherein the final thickness of the sheet ends up somewhat greater than the spacing between the rollers used for flattening. The post-flattening thicknesses to be used in calculating the percentage reduction is the final thickness after spring-back.

Sheet thicknesses both before and after flattening can, for example, be measured at multiple locations on the sheet using calipers and then averaged. Instead of using calipers, which will typically extend over multiple perforations in the sheet, a micrometer can be used at individual perforations with the measurements again being averaged for multiple locations on the sheet. Because the improvements achieved by this technology depend on a percentage reduction in sheet thickness, it normally does not matter which measuring technique is used provided the same technique is used for the pre-flattened and post-flattened measurements. However, in case of a conflict, micrometer measurements are preferred because of their higher accuracy.

FIGS.4B-4Eshow the effects of different amounts of flattening on the rough side burrs13aofFIG.4A, whileFIGS.5B-5Eshow the effects on the smooth side burrs13bofFIG.5A. Hatching has been used in these figures to make the effect of the flattening more visible. The flattened sheet thickness was 26 thousandths in the B panels, 22 thousandths in the C panels, 18 thousandths in the D panels, and 15 thousandths in the E panels. These post-flattening thicknesses correspond to reductions in the sheet's pre-flattening thickness (i.e., 33 thousandths) of 21%, 33%, 45%, and 55%, respectively.

FIGS.6A-Eshow the effects of the flattening on pairs of neighboring burrs. In this figure, the sheet's rough side is on the left in all panels. As inFIGS.4and5, the A panel shows un-flattened burrs while panels B-E show flattened burrs where the amount of flattening is the same as in the B-E panels ofFIGS.4and5, i.e., reductions in sheet thickness of 21%, 33%, 45%, and 55%, respectively. The photomicrographs ofFIG.6were prepared by cutting a cross-section from a sheet and polishing the exposed edge.

The enhanced capture of slag by the filters disclosed herein can be understood from the geometry of the neighboring burrs shown inFIG.6. As shown inFIG.6A, gases passing from the right to the left in this figure (i.e., the path through a filter where the expanded metal is wound smooth side in) are guided into a relatively smooth flow by the walls141and143of perforation11which give the perforation a funnel-like shape as seen from the smooth side. If, on the other hand, the gas were to pass through the filter from the rough side, the gas would engage concave barriers145and147(i.e., concave as seen from the incoming gas), which unlike walls141and143on the smooth side, do not present a funnel-like shape for guiding gas flow, but instead present a pocket-like geometry in which slag can become captured. In addition, the barriers include three-dimensional gorges149which further enhance their slag-capturing ability. As a result of these geometric effects, the rough side has been found to be far better than the smooth side in collecting slag.

However, the rough side is the source of the cheese grater effect and thus, as discussed above, it cannot be oriented inward without compromising the service life of the inflator. Specifically, as shown inFIG.6A(the un-flattened case), the rough side includes sharp corners151which stick out from the base surface at angles where they can engage the solid propellant and cause it to deteriorate. The challenge is then to retain the geometric features of the rough side that have been discovered to produce enhanced slag capture while at the same time minimizing the cheese grater effect. In accordance with this disclosure, this combination is achieved by controlling the amount of flattening applied to the expanded metal sheet. In particular, the amount of flattening is selected so that the reduction in sheet thickness produced by the flattening is in the range of 25-45% or one of the sub-ranges of that range discussed above.

Panels B-E ofFIG.6illustrate the effects of different amounts of flattening. Specifically, panels B (21%) and E (55%) illustrate flattening levels outside of the 25-45% range, while panels C (33%) and D (45%) illustrate levels covered by the range. Beginning with panel C (33% reduction in thickness), it illustrates burrs that have been flattened enough so that they have rounded corners153which can contact the solid propellant without a substantial degradation in inflator service life, but have not been flattened to a point where the concave barriers145and147and their associated three-dimensional gorges149are no longer able to provide enhanced slag capture. Panel E (55% reduction in thickness) illustrates the effects of over-flattening. Both the concave barriers and the three-dimensional gorges have been flattened to a point where they cannot provide enhanced slag capture. Panel D (45% reduction in thickness) is intermediate between Panels C and E and still retains enough of concave barriers and three-dimensional gorges to achieve an improvement in slag capture compared to the existing state of the art, but not as large an improvement as Panel C. Panel B illustrates the other extreme of under-flattening in which the sheet still contains sharp corners that extend out from the plane of the sheet where they can contact and degrade the inflator's propellant.

In embodiments, rough-side-in filters produced by processes in which the level of flattening has been selected to fall within the ranges disclosed herein have been found to achieve a slag retention which satisfies the 1.0 gram requirement of USCAR when tested using the USCAR procedures (see SAE Standard USCAR24-2 entitled “Uscar Inflator Technical Requirements and Validation” issued Apr. 30, 2013). For example, rough-side-in filters flattened by 33% achieved a 0.7 gram level of slag retention, i.e., 30% below the 1.0 gram requirement of the USCAR standard. For comparison, a filter having the same construction and the same level of flattening but with the rough side facing out (the conventional orientation) released 1.3 grams of slag, i.e., it failed the 1.0 gram USCAR standard. This drop in the amount of slag released from the filter from 1.3 grams to 0.7 grams represents an improvement of 46%. In embodiments, the amount of slag released by filters having a flattened rough side oriented inward is at least 10% or at least 20% or at least 30% or at least 40% less than a filter having the same construction but with the flattened rough side oriented to face outward. Significantly, although they have their rough sides facing inward, filters in which the flattening is within the ranges disclosed herein also exhibit levels of the cheese grater effect that are low enough to be commercially acceptable.

In the manufacture of a filter for a vehicle airbag inflator, one filter geometry is a cylinder having porous walls. To make such a device, and continuing withFIG.3, the flattened expanded metal sheet is cut by cutter123into individual strips125. Strips having narrower widths can be formed using additional cutters (not shown). Individual strips, if long enough, can be rolled into a filter or multiple strips, possibly having different open areas, can be placed in an overlapping relationship as shown at127inFIG.3and attached to each other via a welder129(preferably by electric welding). The individual strip or the joined composite strip is then rolled into a cylinder131and the edge of the mesh secured to the cylinder by a welder133. To produce the proper ID and OD (inner and outer diameters), the cylinder135can be placed into a female mold137optionally having a movable inner wall, and a mandrel139optionally expandable can be inserted into the central bore of the cylinder. The desired ID and OD of the final filter can be achieved by the combination of the mandrel, optionally expanding, and the mold, optionally contracting, to cold form the cylinder into the desired radial geometry and dimensions.

In an embodiment, when multiple strips of expanded metal are used to produce the filter, those strips can differ from one another in their perforation patterns including the orientations, shapes, sizes, and spacings between the perforations (e.g., the pitch between rows of perforations). In a preferred embodiment, at least one strip of variable expanded metal (VEM) having a non-constant perforation pattern is used. Such a strip can comprise all or substantially all of the filter. Commonly-assigned U.S. Pat. No. 10,717,032 discloses filters that employ variable expanded metal; the contents of this patent are incorporated herein in their entirety by reference. In addition to layers of expanded metal, the filter can include one or more layers or sections of other materials such as metal screens, ceramic fabrics, or the like.

In a typical application, the expanded metal strip is rolled upon itself to produce a structure having multiple layers, e.g., between three and twenty layers. For example, the filter can have between 10 and 15 layers. The first 360 degree wrap (first layer) can be secured with spot welds with the remaining layers being continuously wrapped around one another to reach the desired outside diameter. The outermost layer can then be secured with spot welds. Once completed, the filter can be installed within an inflator housing having a plurality of apertures which allow gases produced by the burning of the inflator's solid propellant to exit the housing and inflate the air bag which is secured about the outside of the housing.FIG.8is a schematic diagram showing the overall construction of an airbag assembly155comprising an inflator157which includes a filter21which houses a solid propellant26which when burned inflates an airbag or, in more general terms, an inflatable vehicle occupant protection device159. Details of the construction of airbag assemblies are omitted since such assemblies are well known in the art.

From the foregoing it can be seen that the technology disclosed herein can provide safer airbag inflator filters that are cleaner by combining flattening of expanded metal sheets with wrapping of the sheets to form filters that have the expanded metal sheet's flattened rough side facing in. The filters can replace existing filter designs without compromising long established performance criteria, including both cooling and ballistic performance. The filters can clean the inflator's gas at a lower cost by using less filter layers and less filter mass. The filters can also be used to obtain an output gas that is cleaner.

A variety of modifications that do not depart from the scope and spirit of the invention will be evident to persons of ordinary skill in the art from the foregoing disclosure. The following claims are intended to cover the specific embodiments set forth herein as well as modifications, variations, and equivalents of those embodiments.