Airbag inflator filter using expanded metal with the rough surface facing the inner diameter of the filter

Abstract

A piece of flattened expanded metal (9) is wrapped around an airbag inflator filter (21) with the flattened rough side (17) of the piece facing the inside diameter (ID) of the filter and the flattened smooth side (15) facing the outside diameter (OD). This orientation allows the filter (21) to capture more slag than a filter made with a standard wrapping method in which the smooth side faces inward. The expanded metal can be of various types including, for example, variably expanded metal (VEM) or standard expanded metal (SEM).

Description

[Technical Field]

[0001] FIELD OF THE DISCLOSURE This disclosure relates to vehicle safety, and more particularly to filters for airbag inflators, where the filters are made at least in part from expanded metal. [Background technology]

[0002] A. Expanded metal Expanded metal has found a variety of uses, from firefighting mats to airbag inflator filters. Expanded metal can be made in a variety of ways. For example, expanded metal can be made by taking a metal sheet, punching holes in the sheet to create multiple slits, and then pulling the sheet perpendicular to the direction of the slits to stretch the slits and create openings in the sheet. Another common method for manufacturing expanded metal is by piercing and cold-forming the openings, often referred to as "diamonds" due to their final shape. The final length of the sheet, along with the associated holes, is longer than the original, thus enlarging the openings formed.

[0003] Thus, although details vary depending on the specific process, expanded metal sheets are typically produced by forming perforations in the sheet using rows of perforator teeth or bits. The side of the sheet facing the perforators (the smooth side) has depressions around the perforations, and the back side of the sheet (the rough side) has corresponding raised portions, or burrs, around the perforations. In accordance with the present disclosure, it has been discovered that burrs also form on the smooth side of the sheet. These burrs are substantially smaller than the burrs formed on the rough side. In particular, the burrs on the rough side are large enough to exhibit a "cheese grater" effect when contacted with the solid propellant of an inflator (see below), while the burrs on the smooth side are too small to exhibit this effect at commercially unacceptable levels. To distinguish between the two types of burrs, the rough side burrs are referred to herein as "rough-surface expanded metal burrs" and the smooth side burrs are referred to herein as "smooth-surface expanded metal burrs."

[0004] As used herein, the side of an expanded metal sheet that does not contain a roughened expanded metal burr is referred to as the "smooth side" of the sheet, and the side that contains a roughened expanded metal burr is referred to as the "rough side" of the sheet. This terminology is illustrated in Figure 1, where 9 is the expanded metal sheet, 11 is the perforations in the expanded metal sheet, 13a is the roughened expanded metal burr, 13b is the smooth expanded metal burr, 15 is the smooth side of the expanded metal sheet, and 17 is the roughened side of the expanded metal sheet. As discussed and quantified below, in certain embodiments, the flattened rough side of the expanded metal of the filters of the present disclosure is at least somewhat rougher than the flattened smooth side.

[0005] B. Filter for airbag inflator Airbag inflator filters must meet many stringent standards. These filters are responsible for capturing the extensive debris (slugs) generated when the solid propellant in an airbag rapidly burns. These slugs can damage the airbag and, if released from the airbag, can injure vehicle occupants in a deployed vehicle. Furthermore, these slugs are often chemically toxic to humans. To address these concerns, airbag inflator manufacturers have established strict standards for the amount of slugs that can be released from an airbag inflator upon deployment. In the United States, for all types of airbag assemblies, the maximum amount of total particles (total slugs) that can reach the airbag cushion upon deployment is 1,000 milligrams.

[0006] For an airbag inflator to meet this standard, its filter must be highly effective in its filtering function. However, it must also allow the gases generated by the combustion of the solid propellant to quickly reach and inflate the airbag; that is, the filter must not create excessive backpressure. Furthermore, the filter must meet the conflicting criteria of providing effective filtering at low backpressure in the face of the high forces generated by the rapid combustion of the solid propellant. In addition to these criteria, the filter also functions as a diffuser to even out the flow of inflation gases entering the airbag and as a heat sink to reduce the temperature of the gases so that they do not harm the airbag or the person protected by the airbag.

[0007] In addition to these considerations, cost is always an issue for mass-produced products, especially those used in the automotive sector. Expanded metal offers an advantage over other filter materials because the metal expansion process reduces the amount of raw material (sheet metal) contained in each filter, thus lowering the cost of the filter. Lower metal content also reduces the weight of the filter, which is desirable from the perspective of improving fuel economy and, therefore, reducing vehicle emissions.

[0008] As shown in FIG. 2, a typical airbag filter configuration is a tube 21 having an inner surface 22, an outer surface 24, and substantially flat ends 25 extending between the inner surface 22 and the outer surface 24, each of which defines a central bore or cavity 23. In an assembled inflator, the filter bore typically contains some, and often all, of the inflator's solid propellant 26. The solid propellant is in the form of compressed pellets of a pyrotechnic composition. The pellets are packed within the filter bore and are able to move (vibrate) within the bore, at least to some extent, as a result of forces experienced by the airbag inflator during vehicle use. While the pellets are very strong, they are susceptible to at least some physical degradation, for example, from contact with rough surfaces. This physical degradation, in turn, leads to changes in the performance of the airbag inflator.

[0009] Once installed in a vehicle, an airbag inflator has a service life of at least 20 years. Therefore, the inflator and its contents (including the solid propellant) are subjected to long-term vibration. To avoid significant degradation of the solid propellant due to this vibration, airbag filters using expanded metal must have the rough side of the expanded metal facing outward, with only the smooth side in contact with the solid propellant. It has been feared that the rough side acts as a "cheese grater" and degrades the solid propellant over time. To prevent any confusion about the orientation of the rough and smooth sides, the smooth side of the expanded metal is colored, allowing easy visual confirmation that the smooth side is on the inside and forms the inner surface of the filter in the completed filter. The prior art documents relevant to the invention of this application are as follows (including documents cited in the international phase after the international filing date and documents cited when the application entered the national phase in other countries). (Prior art document) (Patent document) (Patent Document 1) U.S. Patent No. 7,823,919 (Patent Document 2) U.S. Patent No. 9,700,825 (Patent Document 3) U.S. Patent Application Publication No. 2010 / 0146922 (Patent Document 4) U.S. Patent Application Publication No. 2004 / 0244345 (Patent Document 5) U.S. Patent Application Publication No. 2013 / 0291756 (Patent Document 6) U.S. Patent Application Publication No. 2020 / 0316512 (Patent Document 7) JP 2016-055669 A Summary of the Invention

[0010] As described more fully below, it has been surprisingly discovered that, contrary to conventional wisdom, a roughened surface of an expanded metal can form the interior surface of an airbag filter without causing unacceptable levels of degradation of the inflator's solid propellant, provided the expanded metal is planarized to a sufficient degree to reduce the roughness. Even more surprisingly, it has been discovered that, when the planarized roughened surface faces inward, the filter substantially favors capturing the slag produced by combustion of the solid propellant, as long as the expanded metal is not overly planarized. As described more fully below, it has been discovered that a reduction in the thickness of the expanded metal in the range of 25-45% can achieve this desirable combination of enhanced slag capture without compromising the inflator's service life.

[0011] In one aspect, a method of manufacturing a filter for an airbag inflator is provided, the method comprising: (1) providing a sheet of expanded metal having a smooth surface and a rough surface; (2) flattening the sheet of expanded metal, the flattening reducing the thickness of the sheet; (3) forming a piece of flattened expanded metal from a sheet of said flattened expanded metal; (4) forming the filter by a process including the step of winding the flattened piece of expanded metal about a shaft; and (a) the filter has a cavity for receiving a solid propellant, the cavity having a surface that can contact the solid propellant in an assembled airbag inflator; (b) the flattened piece of expanded metal has a flattened rough surface; (c) the flattened piece of expanded metal is rolled so that at least a portion of the surface of the cavity has a portion of the flattened roughened surface;

[0012] In one aspect, a filter for an airbag inflator is provided, the filter comprising a piece of expanded metal rolled up about an axis to form a plurality of rolled layers, the filter having a cavity for receiving a solid propellant, the cavity having a surface contactable with the solid propellant in an assembled airbag inflator; (a) the piece of expanded metal has a planarized smooth side and a planarized rough side; (b) the piece of expanded metal is rolled up so that (1) the flattened roughened surface faces the inside of the cavity and (2) at least a portion of the surface of the cavity has a portion of the flattened roughened surface.

[0013] In one aspect, an apparatus is provided for assisting in protecting an occupant of a vehicle, the apparatus comprising: (1) an inflatable vehicle occupant restraint device; (2) an inflator operable to supply inflation fluid for inflating the inflatable vehicle occupant protection device, the inflator comprising: (A) Solid propellants that are susceptible to degradation due to contact with rough surfaces (B) a filter having a flattened piece of expanded metal rolled up about an axis to form a plurality of roll layers, the filter having an inner surface and an outer surface; (1) the flattened piece of expanded metal has a flattened smooth side and a flattened rough side; (2) the flattened piece of expanded metal is rolled up so that the flattened smooth side of the piece is on the outside and the flattened rough side is on the inside; (3) at least a portion of the inner surface of the filter has the planarized rough surface; (4) At least a portion of the solid propellant and an inner surface of the filter are in contact within the assembled inflator, the contact occurring prior to activation of the inflator.

[0014] Additional features and advantages of the technology disclosed herein are set forth in the detailed description that follows, and in part will be readily apparent to those skilled in the art from the description, or may be realized by practicing the technology described herein. The accompanying drawings are included to provide a further understanding of the technology, and are incorporated in and constitute a part of this specification. It is to be understood that the various aspects of the technology disclosed in the specification and drawings can be used individually and in any combination. It is also to be understood that the foregoing general description and the following detailed description are merely exemplary of the invention and are intended to provide an overview or framework for understanding the nature and characteristics of the invention as defined by the claims. [Brief explanation of the drawings]

[0015] [Figure 1] Figure 1 is a schematic diagram of an unflattened expanded metal sheet showing the smooth and roughened surfaces of the sheet, where the roughened surface is characterized by the presence of burrs formed by the expansion process that are large enough to degrade the solid propellant in the inflator via the cheese-grater effect. While burrs have been found to be present on the smooth surface, they are substantially smaller than the roughened surface burrs and therefore do not result in unacceptable levels of degradation of the solid propellant. Figure 1A is a cross-sectional view of the roughened surface of the sheet, and Figure 1B is a plan view. [Figure 2] 2 is a schematic diagram of an airbag filter according to one embodiment of the present disclosure, the filter being in the form of a tube having an inner surface defining a central bore or cavity, an outer surface, and substantially flat ends extending between the inner and outer surfaces, wherein a pyrotechnic composition pellet is disposed within the bore of the filter capable of contacting the inner surface of the filter. [Figure 3]FIG. 3 is a schematic diagram of an embodiment for manufacturing a filter from an expanded metal sheet and a piece formed from the sheet. [Figure 4] Figure 4 is a series of photomicrographs showing the roughened surface of expanded metal sheets that have undergone different amounts of flattening. Figure 4A shows an unflattened sheet, while Figures 4B, 4C, 4D, and 4E show sheets with thickness reductions of 21%, 33%, 45%, and 55%, respectively. For scale, the distance between the left and right corners of perforation 11 in this figure is 33 kJ. This scale also applies to Figures 5-7. [Figure 5] Figure 5 is a series of photomicrographs showing the smooth surface of expanded metal sheets that have undergone different amounts of flattening: Figure 5A shows an unflattened sheet, while Figures 5B, 5C, 5D, and 5E show sheets with thickness reductions of 21%, 33%, 45%, and 55%, respectively. [Figure 6] Figure 6 is a series of photomicrographs showing the effect of different amounts of flattening on pairs of adjacent burrs in an expanded metal sheet. Figure 6A shows an unflattened burr, while Figures 6B, 6C, 6D, and 6E show burrs where the thickness of the sheet that is part of the burr has been reduced by 21%, 33%, 45%, and 55%, respectively. [Figure 7] Figure 7 is a montage of Figures 4 through 6. Specifically, the five panels from Figure 4 are repeated as the left column of Figure 7, the five panels from Figure 5 as the middle column, and the five panels from Figure 6 as the right column. This montage highlights the differences in the surface structure of expanded metal sheets with different amounts of flattening.

[0016] The reference numbers in the figure correspond to the following: 9 Expanded Metal Sheet 11. Perforating expanded metal sheets 13a Rough surface expanded metal burr 13b Smooth surface expanded metal burr 15 Smooth surface of expanded metal sheet 17 Rough surface of expanded metal sheet 21 Substantially cylindrical filter 22 Substantially cylindrical inner surface of filter 21 23 Cavity of filter 21 defined by inner surface 22 24 Substantially cylindrical outer surface of filter 21 25 Substantially flat end of filter 21 26 Solid Propellant 101 Thin metal roll 103 Press 105 Perforator 107 teeth or bits 109 Reinforcement 111 Camera 113 Computer Controlled Devices 115 monitors 121 Roller 123 Cutter 125 Expanded metal pieces 127 Expanded metal pieces 129 Welding Machine 131 cylinders 133 Welding Machine 135 Welded Mesh Cylinder 137 Female 139 Mandrel 141 Wall of drilling hole 11 (smooth surface) 143 Wall of drilling hole 11 (smooth surface) 145 Concave barrier (rough surface) 147 Concave Barrier (Rough Surface) 149 Three-dimensional grooves in concave barriers (rough surfaces) 151 Sharp angle (rough surface) 153 Rounded corners (rough surface) 155 Airbag Assembly 157 Inflator 159 Inflatable vehicle occupant restraint devices DETAILED DESCRIPTION OF THE INVENTION

[0017] As mentioned above, in commonly used airbag configurations, airbag filters have historically always been wrapped with the smooth side facing inward because the inflator filter is in direct contact with the inflator's solid gas-generating propellant pellets (also called tablets). The pellets are susceptible to vibration degradation, and rough surfaces can abrade the pellets, reducing the inflator's performance and / or making it more dangerous. If the filter's expanded metal had the smooth side facing outward and the rough side facing inward, the expanded metal could act like a cheese grater on the solid propellant. For this reason, all pyrotechnic airbag manufacturers have historically prohibited wrapping expanded metal filters with the rough side facing inward.

[0018] During deployment of an airbag inflator, the gas that fills the airbag is generated by solid rocket fuel, most commonly guanidine nitrate-based. This propellant is typically high in copper and other metals, sometimes comprising over 60% of the total composition. During deployment, the metals in the propellant liquefy and become entrained in the gases generated by the propellant's combustion. In this highly dynamic system, the solid-to-liquid phase change occurs within tens of milliseconds.

[0019] During deployment of an airbag inflator, the gas that fills the airbag is generated by solid rocket fuel, most commonly guanidine nitrate-based. This propellant is typically high in copper and other metals, sometimes comprising over 60% of the total composition. During deployment, the metals in the propellant liquefy and become entrained in the gas generated by the propellant combustion. In this highly dynamic system, the solid-to-liquid phase change occurs within tens of milliseconds.

[0020] The function of an inflator filter is to thermodynamically diffuse and cool the hot combustion gases, converting the liquid copper and other metals back into a solid phase and trapping them in the filter, while allowing the cooled gases to escape. Automakers are very concerned about the amount of slug produced by an inflator. If an inflator produces more than one gram of residue and airborne particulates (collectively known as slug), the inflator will be rejected by the automaker as not meeting USCAR standards established by NHTSA and other automotive safety organizations to protect asthmatics and other vulnerable occupants from exposure to airborne particulates.

[0021] According to one aspect of the present disclosure, once formed, the expanded metal sheet is flattened, e.g., by passing the sheet through a set of calender rollers, to smooth the roughened surface of the sheet so that the inflator propellant pellets do not unacceptably degrade as a result of contact with the roughened surface. The flattening should be such that, from the perspective of a cheese grater, the flattened smooth surface and the flattened roughened surface of the expanded metal sheet are approximately the same. That is, the flattened roughened surface should not wear down the inflator pellets faster than would normally be the case on an unflattened smooth surface. Automobile companies require inflator component suppliers to test inflator pellet wear through laboratory vibration environmental testing. This test is designed to replicate long-term vibrations experienced within an automobile. In one embodiment, when so tested, the flattened roughened surface is sufficiently smooth that, when forming the cavity wall of the filter, it does not degrade the inflator pellets faster than would normally be the case for a filter in which the smooth surface forms the cavity wall. In one embodiment, after planarization, both the planarized rough surface and the planarized smooth surface feel smooth to the touch.

[0022] Surprisingly, despite the flattening, having the flattened rough surface facing inward, i.e., toward the aircraft flow, provides more efficient cleaning than having the flattened smooth surface facing inward. While not wishing to be bound by any particular theory of operation, it is believed that the flattened rough-surface expanded metal burrs form slag-trapping pockets in all perforations, allowing them to rapidly trap the slag generated by the burning propellant. In other words, the rough fins (burrs) of the expanded metal, although flattened, act like a horde of tiny internal trapping pockets. Consequently, wrapping the expanded metal filter with the flattened smooth side facing outward and the flattened rough side facing inward provides cleaner gas filtration.

[0023] Referring now to FIG. 3, the manufacture of expanded metal sheets according to certain embodiments of the present disclosure begins with a roll of metal sheet 101 (e.g., approximately 19 inches wide, which can be cut into 2-6 inch widths for passenger airbag filters and approximately 1.5 inch widths for driver airbag filters, although any width can be used depending on the equipment). Stainless steels such as SS304, 309, 310, 409, 410, and 430 can be used for airbag inflator filters. Carbon steels C1006 to C1008 are often preferred for various applications. Other metal compositions available in sheet form can also be used depending on the environment in which the expanded metal will be used.

[0024] The sheet is first fed into a press 103, where a perforator 105 with multiple teeth or bits 107 is moved across the sheet so that the teeth perforate the sheet, after which the perforator is removed, similar to a stamping operation. The bits are preferably identical in shape and are shaped to form slits in the sheet. The bit shape determines the depth of the bit's penetration and the length of the slits formed; the deeper the penetration, the longer the slits, and therefore the more open the final structure after stretching. While a single perforator is shown, multiple perforators can be used to provide different perforation spacing, shapes, and / or depths. For airbag inflator filters, openings are sized based on the airbag manufacturer's specifications for the sheet's open area, sheet porosity, or other parameters required for the filter.

[0025] The sheet is preferably advanced by a servo motor (not shown) or other mechanism capable of precisely controlling the longitudinal advance of the sheet. The advancement of the sheet is preferably in discrete steps so that the sheet comes to rest when punched. Although not preferred, toothed rollers can also be used for continuously moving sheets.

[0026] The perforated sheet produced by the press is then fed to a stretcher 109 where differential rollers stretch the perforated sheet axially (i.e., along the direction of travel) so that the slits form diamond-shaped holes. (Of course, a hexagonal bit or other shaped bits could be used to form hexagonal openings, but diamond-shaped slits are a common shape.)

[0027] While cutting and stretching can be performed as separate operations, it is often preferable to produce expanded metal sheets by cutting and stretching them in the same motion with the same teeth when creating fine patterns. In this process, the material hangs over a flattened lower blade, and angled upper teeth or bits cut and penetrate the sheet. The sheet is then folded, and the angle formed by this folding causes a stretching motion in the sheet relative to the teeth. As a result, the sheet stretches to the depth of the tooth penetration. The amount of stretch achieved in this manner typically ranges from 20 to 25%, and can be as high as 37%. Compared to the cutting and stretching method, the one-step method produces perforations (openings) that are more triangular than diamond-shaped (see Figures 4-5 and 7). Like the separate cutting and stretching method, the one-step method (1) forms a slit in the metal sheet and (2) stretches the slit along the longitudinal axis of the metal to form the opening, but in one step rather than two.

[0028] A video control system including at least one camera 111 is connected to a computer controller 113 running software and an optional monitor 115 to inspect the hole or opening area and determine whether the perforations are within specifications (after parameters are entered into the controller). The controller software checks the size and / or shape of the openings to determine whether the individual opening or opening area (actual, estimated, or calculated) is within specifications. A second camera (not shown) can be installed between the perforator and the stiffener to determine whether the initial punch is within specifications. The video control system optically inspects the expanded metal sheet product to determine whether the product is within specifications. To modify the process to get within specifications, return to specifications, or change specifications, the sheet advance can be changed by adjusting servo motors (via the computer controller) to change the longitudinal spacing of the perforations. The stiffener can also be adjusted to increase or decrease the amount of stretch in the perforated sheet.

[0029] Once formed, the expanded metal sheet is flattened, for example, by one or more pairs of rollers 121. If desired, the expanded metal sheet can be passed through multiple pairs of rollers to achieve the desired degree of flattening. One skilled in the art, based on knowledge of the art and this disclosure, would readily be able to select an appropriate roller configuration to achieve the flattening levels discussed herein. As noted above, in accordance with this disclosure, the amount of flattening is selected to achieve enhanced slag capture without compromising the inflator's useful life as a result of degradation of the inflator's solid propellant due to the cheese-grater effect. Referring again to FIG. 3, a camera 111 of a video control system can be positioned after the flattening process and used to determine whether the degree of flattening is within specifications.

[0030] FIG. 4A is a photomicrograph of the rough surface 17 of an expanded metal sheet produced by the single-step expanding process described above. This view shows the rough surface before flattening. As can be seen, the perforations 11 in the sheet are roughly triangular in shape. During this manufacturing process, the rough expanded metal burrs are largest along the intersecting short sides of the triangle. These rough burrs are labeled 13a in FIG. 4. FIG. 5A is a photomicrograph of the corresponding smooth surface 15 of the sheet, also before flattening. Burrs are also visible on this surface, but they are smaller and located along the long sides of the triangle. These smooth burrs are labeled 13b in FIG. 5.

[0031] The stretching process increases the thickness of the sheet, for example by a factor of two, although the exact amount depends on the specifics of the process, the perforation pattern formed, and the thickness of the substrate. As an example, for the expanded metal of Figures 4-7, the substrate thickness was 0.015 inches (1 in 15,000), and the stretching process increased the thickness of the sheet by more than two times, i.e., after stretching, but before flattening as shown in Figures 4A and 5A, the thickness of the sheet was 0.033 inches (1 in 33,000).

[0032] As discussed above, the thickness of the expanded metal sheet before flattening depends on the thickness of the substrate and the amount the substrate is stretched. The thickness after flattening depends on the amount of flattening applied to the expanded sheet. In one embodiment, the amount of flattening required to achieve enhanced slag capture without compromising the inflator's service life as a result of the cheesegrater effect can be expressed, for example, as a percentage reduction in the pre-flattened thickness t of the expanded metal sheet resulting from the flattening. In one embodiment, the thickness reduction can be in the range of 25 to 45%, which corresponds to a post-flattened thickness in the range of 25,000 to 18,000 for an expanded metal sheet having a pre-flattened thickness of 1 in 33,000, respectively.

[0033] In certain embodiments, the thickness reduction is within a range of 30-45%, or within a range of 30-40%, or within a range of 30-35%. Percentage ranges referred to in this specification and claims include the endpoints of the ranges. The lower and upper limits can be used in other combinations, such as 25-40% and 25-35%. Expanded metal exhibits a springback effect after flattening, and the final thickness of the sheet is somewhat greater than the spacing between the rollers used for flattening. The post-flattening thickness used in calculating the reduction percentage is the final thickness after springback.

[0034] The thickness of the sheet before and after flattening can be measured at multiple locations on the sheet using, for example, a vernier caliper and averaged. Typically, instead of using a vernier caliper across multiple perforations in the sheet, a micrometer can be used on each perforation and the measurements can be averaged again across multiple locations on the sheet. Because the improvement achieved by this technique depends on the rate of reduction in sheet thickness, it typically doesn't matter which measurement technique is used, as long as the same technique is used for both pre-flattening and post-flattening measurements. However, in competitive situations, micrometer measurements are preferred due to their greater accuracy.

[0035] Figures 4B-4E show the effect of different amounts of flattening on the rough burr 13a of Figure 4A, and Figures 5B-5E show the effect on the smooth burr 13b of Figure 5A. Shading is used in these figures to more easily visualize the flattening effect. The thicknesses of the sheets after flattening were 26,000 times smaller for panel B, 22,000 times smaller for panel C, 18,000 times smaller for panel D, and 15,000 times smaller for panel E. These post-flattening thicknesses correspond to 21%, 33%, 45%, and 55%, respectively, of the sheet thickness before flattening (i.e., 33,000 times smaller).

[0036] Figures 6A-E show the effect of the flattening on a pair of adjacent burrs. In this figure, the rough surface of the sheet is on the left in all panels. As in Figures 4 and 5, panel A shows an unflattened burr, while panels B-E show flattened burrs with the same amount of flattening as panels B-E in Figures 4 and 5, i.e., a reduction in sheet thickness of 21%, 33%, 45%, and 55%, respectively. The micrograph in Figure 6 was prepared by cutting a cross section from the sheet and polishing the exposed edge.

[0037] The enhanced slag capture by the filter disclosed herein can be understood from the shape of the adjacent burrs shown in FIG. 6. As shown in FIG. 6A, gas passing from right to left in this figure (i.e., the path through the filter in which the expanded metal is wound with the smooth side facing inward) is guided into a relatively smooth flow by walls 141 and 143 of perforations 11, which give the perforations a funnel-like shape as viewed from the smooth side. On the other hand, when the gas passes through the filter from the rough side, the gas engages concave barriers 145 and 147 (i.e., concave as viewed from the incoming gas). However, unlike the walls 141 and 143 of the smooth side, these barriers do not have a funnel-like shape to guide gas flow. Instead, they have a pocket-like shape in which slag can be trapped. Furthermore, the barriers include three-dimensional grooves 149, which further enhance slag capture capability. As a result of these geometric effects, the rough surface has been found to be significantly better at capturing slag than the smooth surface.

[0038] However, the roughened surface is responsible for the cheese-gratering effect and therefore, as noted above, cannot be turned inward without compromising the inflator's service life. Specifically, as shown in FIG. 6A (when not flattened), the roughened surface includes sharp corners 151 that protrude from the substrate surface at angles that can engage and degrade the solid propellant. The challenge, then, is to minimize the cheese-gratering effect while retaining the geometric features of the roughened surface that have been discovered to provide enhanced slug capture. In accordance with the present disclosure, this combination is achieved by controlling the amount of flattening applied to the expanded metal sheet. Specifically, the amount of flattening is selected so that the reduction in sheet thickness resulting from the flattening is in the range of 25 to 45%, or one of the subranges within that range noted above.

[0039] Panels B–E of Figure 6 show the effects of different amounts of flattening. Specifically, Panels B (21%) and E (55%) show flattening levels outside the 25–45% range, while Panels C (33%) and D (45%) show levels within that range. Starting with Panel C (33% reduction in thickness), the burrs are sufficiently flattened to have rounded corners 153 that can contact the solid propellant without substantial degradation of the inflator's service life, but are not flattened to the point where the concave barriers 145 and 147 and their associated three-dimensional grooves 149 can no longer provide enhanced slug capture. Panel E (55% reduction in thickness) shows the effects of excessive flattening. Both the concave barriers and the three-dimensional grooves have flattened to the point where they can no longer enhance slug capture. Panel D (45% reduced thickness) is intermediate between Panels C and E, retaining enough of the concave barrier and three-dimensional grooves to achieve an improvement in slug capture compared to the existing state of the art, but not as great an improvement as Panel C. Panel B shows the other extreme of under-flattening, with the sheet still containing sharp corners extending out of the plane of the sheet that could contact and degrade the inflator propellant.

[0040] In the manufacture of filters for vehicle airbag inflators, one filter shape is a cylinder with porous walls. To produce such a device, continuing with FIG. 3, the flattened expanded metal sheet is cut into individual pieces 125 using cutter 123. Narrower width pieces can be formed using additional cutters (not shown). Individual pieces, if of sufficient length, can be rolled into a filter, or multiple pieces, possibly with different opening areas, can be arranged in an overlapping relationship as shown at 127 in FIG. 3 and attached to one another (preferably by electric welding) via welder 129. The individual pieces or joined composite pieces are then rolled into cylinder 131, and the mesh ends are secured to the cylinder by welder 133. To produce the appropriate inner and outer diameters, cylinder 135 can be placed into a female mold 137, optionally with a movable inner wall, and an optional expandable mandrel 139 can be inserted into the cylinder's central bore. The desired inner and outer diameters of the final filter can be achieved by cold forming the cylinder into the desired radial shape and dimensions using a combination of the optionally expanding mandrel and optionally contracting mold.

[0041] In one embodiment, when multiple pieces of expanded metal are used to manufacture the filter, the pieces may differ from one another in perforation pattern, including orientation, shape, size, and spacing between the perforations (e.g., pitch between rows of perforations). In a preferred embodiment, at least one piece of variably expanded metal (VEM) having a non-uniform perforation pattern is used. Such a piece may comprise all or substantially all of the filter. Commonly assigned U.S. Pat. No. 10,717,032 discloses a filter employing variably expanded metal, the contents of which are incorporated herein by reference in their entirety. In addition to a layer of expanded metal, the filter may include one or more layers or sections of other materials, such as metal screen, ceramic fabric, etc.

[0042] In a typical application, the expanded metal strip is rolled onto itself to produce a structure with multiple layers, e.g., 3 to 20 layers. For example, the filter may have 10 to 15 layers. The first 360-degree wrap (first layer) is secured with spot welds, and the remaining layers can be successively wrapped around each other until the desired outer diameter is reached. The outermost layer is then secured with spot welds. Once completed, the filter can be installed within an inflator housing having multiple openings that allow gases generated by combustion of the inflator's solid propellant to exit the housing and inflate the airbag secured to the exterior of the housing. Figure 8 is a schematic diagram illustrating the overall structure of an airbag assembly 155, consisting of an inflator 157 containing a filter 21 containing a solid propellant 26 that, upon combustion, inflates an airbag, or more generally, an inflatable vehicle occupant protection device 159. Details of the structure of an airbag assembly are omitted, as such assemblies are well known in the art.

[0043] From the above, it can be seen that the technology disclosed herein can provide a cleaner and safer airbag inflator filter by combining flattening of an expanded metal sheet with sheet wrapping to form a filter with the flattened, rough side of the expanded metal sheet facing inward. This filter can replace existing filter designs without compromising long-established performance standards, including both cooling and ballistic performance. This filter can purify inflator gas at a lower cost, with fewer filter layers and less filter mass. Furthermore, use of this filter can result in cleaner output gas.

[0044] Various modifications that do not depart from the scope and spirit of the present invention will be apparent to those skilled in the art from the foregoing disclosure. The following claims are intended to cover the specific embodiments described herein, as well as modifications, variations, and equivalents of those embodiments.

Claims

1. 1. A method for manufacturing a filter for an airbag inflator, comprising: (1) providing a sheet of expanded metal having a smooth surface and a rough surface, the rough surface having burrs formed during perforation of the sheet, the burrs having corners and concave barriers that can trap slugs during use of the sheet of expanded metal in an airbag inflator; (2) flattening the sheet of expanded metal, the flattening reducing the thickness of the sheet and rounding the corners of the burrs but leaving the concave barrier with a flattened rough surface; (3) forming a piece of flattened expanded metal from the flattened sheet of expanded metal; (4) forming the filter by a process including the step of winding the piece of flattened expanded metal about a shaft; and (a) the filter has a cavity for receiving a solid propellant, the cavity having a surface that can contact the solid propellant in an assembled airbag inflator; (b) the flattened piece of expanded metal has a flattened rough surface; (c) the flattened piece of expanded metal is rolled so that at least a portion of the surface of the cavity has a portion of the flattened roughened surface. method.

2. 2. The method of claim 1, further comprising the step of selecting the amount of flattening in step (2) so as to reduce a cheese-grater effect in which the solid propellant deteriorates due to contact with the rough surface of the expanded metal.

3. 3. The method of claim 2, wherein the expanded metal sheet before flattening has a thickness t, and the amount of flattening selected in step (2) reduces t by 25-45%.

4. 4. The method of claim 3, wherein the amount of planarization reduces t by 30 to 40%.

5. 4. The method of claim 3, wherein the amount of planarization reduces t by 30-35%.

6. The method of claim 1 , wherein the amount of planarization is selected such that at least a portion of the concave barrier has a three-dimensional groove.

7. 1. A filter for an airbag inflator comprising a piece of expanded metal rolled up about an axis to form a plurality of rolled layers, the filter having a cavity for receiving a solid propellant, the cavity having a surface contactable with the solid propellant in an assembled airbag inflator, (a) the piece of expanded metal has a flattened smooth surface and a flattened rough surface, the flattened rough surface having burrs formed during perforation of the sheet, the burrs having rounded corners and a concave barrier capable of trapping slugs when the sheet of expanded metal is used in an airbag inflator; (b) the piece of expanded metal is rolled up such that (1) the flattened roughened surface faces the interior of the cavity, and (2) at least a portion of the surface of the cavity has a portion of the flattened roughened surface. filter.

8. 8. The filter of claim 7, wherein at least a portion of the concave barrier has three-dimensional grooves.

9. 8. The filter of claim 7, wherein the filter releases less slag than a filter having the same structure but in which the piece of expanded metal is rolled up so that the flattened roughened surface is away from the cavity.

10. 10. The filter of claim 9, wherein slag discharged from the filter is reduced by at least 10%.

11. 10. The filter of claim 9, wherein slag discharged from the filter is reduced by at least 20%.

12. 8. The filter of claim 7, wherein the piece of expanded metal comprising the filter has a thickness t before flattening, and after flattening, the thickness t is reduced by 25 to 45%.

13. 13. The filter of claim 12, wherein the reduction in thickness t is 30 to 40%.

14. 13. The filter of claim 12, wherein the reduction in thickness t is 30 to 35%.

15. 1. A device for protecting an occupant of a vehicle, comprising: (1) an inflatable vehicle occupant protection device; (2) an inflator operable to supply inflation fluid for inflating the inflatable vehicle occupant protection device; and The inflator is (A) a solid propellant that is susceptible to degradation due to contact with a rough surface; (B) a filter having a flattened piece of expanded metal rolled up about an axis to form a plurality of roll layers, the filter having an inner surface and an outer surface; and (1) The flattened piece of expanded metal has a flattened smooth surface and a flattened rough surface, the flattened rough surface having burrs formed when the expanded metal is perforated, the burrs having rounded corners and a concave barrier that can trap slugs when the sheet of expanded metal is used in an airbag inflator. (2) the flattened piece of expanded metal is rolled up so that the flattened smooth side of the piece is on the outside and the flattened rough side is on the inside; (3) At least a portion of the inner surface of the filter has the planarized rough surface; (4) at least a portion of the solid propellant and the inner surface of the filter are in contact within the assembled inflator, and the contact occurs before the inflator is activated; Device.

16. 16. The apparatus of claim 15, wherein at least a portion of the concave barrier has three-dimensional grooves.

17. 16. The apparatus of claim 15, wherein the filter has the same structure but has reduced slag shedding than a filter in which the piece of expanded metal is rolled up such that a portion of the planarized smooth surface, instead of a portion of the planarized rough surface, comprises at least a portion of the interior surface of the filter.

18. 18. The apparatus of claim 17, wherein slag discharged from the filter is reduced by at least 10%.

19. 16. The apparatus of claim 15, wherein the piece of expanded metal comprising the filter has a thickness t before planarization and has a thickness t reduced by 25-45% after planarization.

20. 20. The device of claim 19, wherein the reduction in thickness t is between 30 and 40%.

21. 20. The device of claim 19, wherein the reduction in thickness t is between 30 and 35%.

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