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 fire-fighting mats to filters in airbag inflators. Expanded metal can be made in a variety of ways. For example, expanded metal can be made by taking a metal sheet, piercing the sheet to create a number of slits, and pulling the sheet perpendicular to the direction of the slits to stretch the slits and provide openings in the sheet. Another common method of making expanded metal is by piercing and cold forming the openings, often referred to as "diamonds" because of their final shape. The final length of the sheet, along with the associated holes, will be longer than the original, and so will the openings that are formed.

[0003] Thus, although details vary depending on the particular process, expanded metal sheets are typically produced by forming perforations in the sheet using a row of teeth or bits of a perforator. The side of the sheet facing the perforator (the smooth side) has indentations 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), whereas the burrs on the smooth side are too small to exhibit this effect at a commercially unacceptable level. To distinguish between the two types of burrs, the burrs on the rough side are referred to herein as "rough expanded metal burrs" and the burrs on the smooth side are referred to herein as "smooth expanded metal burrs."

[0004] As used herein, the side of an expanded metal sheet that does not include a rough expanded metal burr is referred to as the "smooth side" of the sheet, and the side that includes a rough 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 are perforations in the expanded metal sheet, 13a are rough expanded metal burrs, 13b are smooth expanded metal burrs, 15 is the smooth side of the expanded metal sheet, and 17 is the rough side of the expanded metal sheet. As discussed and quantified below, in certain embodiments, the planarized rough side of the expanded metal of the filters of the present disclosure is at least somewhat rougher than the planarized smooth side.

[0005] B. Filter for airbag inflator Airbag inflator filters must meet many stringent standards. These filters are responsible for trapping a wide range of debris (slugs) that are generated when the solid propellant in the airbag rapidly burns. The slugs can damage the airbag and, if released from the airbag, can injure occupants of the vehicle in which the airbag is deployed. In addition, the slugs are often chemically hazardous to humans. To address these concerns, airbag inflator manufacturers have developed strict standards regarding the amount of slugs that may be released from the airbag inflator upon activation. In the United States, for all types of airbag assemblies, the maximum amount of total particulates (total slugs) that may reach the airbag cushion upon deployment of the airbag assembly is 1,000 milligrams.

[0006] For an airbag inflator to meet this criterion, the filter must be very effective in its filtering function, but at the same time, the gases generated by the combustion of the solid propellant must be able to reach and inflate the airbag quickly, i.e., the filter must not create excessive back pressure. Furthermore, the filter must meet the opposing criteria of providing effective filtering at low back pressure in the face of the high forces generated by the rapid combustion of the solid propellant. In addition to these criteria, the filter also acts as a diffuser to equalize the flow of the 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 has an advantage in this respect compared to other filter materials, since the process of expanding the metal 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 in terms of improving fuel economy and therefore reducing vehicle emissions.

[0008] As shown in FIG. 2, a typical configuration of an airbag filter 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, defining a central bore or cavity 23. In an assembled inflator, the bore of the filter 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 bore of the filter 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. The pellets are very tough, but are subject to at least some physical degradation, for example, by 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 vibration for many years. To avoid significant deterioration of the solid propellant due to this vibration, it is an absolute rule for airbag filters using expanded metal that the rough surface of the expanded metal faces outward and only the smooth surface is in contact with the solid propellant. It has been feared that the rough surface will act as a "cheese grater" and deteriorate the solid propellant over time. In fact, to prevent any mistake regarding the orientation of the rough and smooth surfaces, the smooth surface of the expanded metal is colored, so that in the completed filter, it is easy to visually confirm that the smooth surface is on the inside and forms the inner surface of the filter. Summary of the Invention

[0010] As will be described in more detail below, it has been surprisingly discovered that, contrary to conventional wisdom, a roughened surface of an expanded metal can form the inner surface of the airbag filter without causing unacceptable levels of degradation of the solid propellant in an inflator, provided that the expanded metal is subjected to a sufficient amount of planarization to reduce the roughness. Even more surprisingly, it has been discovered that, when the planarized roughened surface faces inward, the filter is substantially more favorable for capturing the slag produced by combustion of the solid propellant, so long as the expanded metal is not over-planarized. As will be 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 useful life of the inflator.

[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) planarizing the sheet of expanded metal, the planarizing reducing a thickness of the sheet; (3) forming a piece of flattened expanded metal from the sheet of flattened expanded metal; (4) forming the filter by a process including the step of winding the flattened piece of expanded metal about a shaft; having (a) the filter has a cavity for receiving a solid propellant, the cavity having a surface capable of contacting the solid propellant in an assembled airbag inflator; (b) the planarized piece of expanded metal has a planarized rough surface; (c) the flattened piece of expanded metal is rolled such 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 such that (1) the planarized rough surface faces the inside of the cavity and (2) at least a portion of the surface of the cavity has a portion of the planarized rough 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 protection device; (2) An inflator operable to supply an 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 planarized piece of expanded metal has a planarized smooth side and a planarized 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, said contact occurring prior to activation of the inflator.

[0014] Additional features and advantages of the technology disclosed herein are described in the following detailed description, and in part will be readily apparent to those skilled in the art from the description, or will be recognized 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 should be understood that the various aspects of the technology disclosed in the specification and drawings can be used individually and in any combination. It should also be understood that the above 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 description of the drawings]

[0015] [Figure 1] 1 is a schematic diagram of an unflattened expanded metal sheet showing the smooth and rough sides of the sheet, the rough side being characterized by the presence of burrs formed by the expansion process large enough to degrade the solid propellant of the inflator via the cheese grater effect. Burrs have also been found to be present on the smooth side, but these are substantially smaller than the rough side burrs and therefore do not result in unacceptable levels of degradation of the solid propellant. FIG. 1A is a cross-sectional view of the rough side of the sheet, and FIG. 1B is a plan view. [Diagram 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, a pellet of pyrotechnic composition being disposed within the bore of the filter capable of contacting the inner surface of the filter. [Diagram 3] FIG. 3 is a schematic diagram of an embodiment for manufacturing a filter from an expanded metal sheet and a piece formed from said sheet. [Figure 4]FIG. 4 is a series of micrographs showing the rough surface of expanded metal sheets that have undergone different amounts of flattening. FIG. 4A shows an unflattened sheet, while FIG. 4B, FIG. 4C, FIG. 4D and FIG. 4E show sheets with a thickness reduction 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 k. This scale also applies to FIGS. 5-7. [Diagram 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 that have had their thickness reduced by 21%, 33%, 45%, and 55%, respectively. [Figure 6] Figure 6 is a series of photomicrographs showing the effect of different amounts of flattening on a pair of adjacent burrs in an expanded metal sheet: Figure 6A shows a burr that has not been flattened, 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 4-6. 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 as follows: 9 Expanded Metal Sheet 11 Perforation of 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 filters 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 Propellants 101 Metal sheet roll 103 Press 105 Perforator 107 Teeth or Bits 109 Reinforcement 111 Camera 113 Computer Controlled Devices 115 Monitor 121 Roller 123 Cutter 125 Expanded metal piece 127 Expanded metal piece 129 Welding Machine 131 Cylinder 133 Welding Machine 135 Welded Mesh Cylinder 137 Female 139 Mandrel 141 Wall of 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 barrier (rough surface) 151 Acute angle (rough surface) 153 Rounded corners (rough surface) 155 Airbag Assembly 157 Inflator 159 Inflatable vehicle occupant protection device DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[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 pills (also called tablets). The pills are easily degraded by vibration, and rough surfaces can chip the pills, reducing the inflator's performance and / or making the inflator more dangerous. If the smooth side of the filter's expanded metal faces outward and the rough side faces inward, the expanded metal can act just 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 of the expanded metal facing inward.

[0018] During the deployment of an airbag inflator, the gas that fills the airbag is produced by solid rocket fuel, most commonly based on guanidine nitrate. This propellant is usually high in copper and other metals, sometimes making up more than 60% of the total composition. During the deployment of an airbag inflator, the metals in the propellant liquefy and become entrained in the gases generated by the combustion of the propellant. In this highly dynamic system, the phase change from solid to liquid occurs in tens of milliseconds.

[0019] During deployment of an airbag inflator, the gas that fills the airbag is produced by solid rocket fuel, most commonly based on guanidine nitrate. This propellant is usually high in copper and other metals, sometimes making up more than 60% of the total composition. During deployment of an airbag inflator, the metals in the propellant are liquefied and mixed with the gas produced by the combustion of the propellant. In this highly dynamic system, the phase change from solid to liquid occurs in tens of milliseconds.

[0020] The function of the inflator filter is to thermodynamically diffuse and cool the hot combustion gases, converting the liquid copper and other metals back to a solid phase, trapping them in the filter, and allowing only the cooled gases to escape. Automotive companies are very concerned about the amount of slag produced by the inflator. If an inflator produces more than 1 gram of residue or airborne particulates (collectively known as slag) in total, the inflator will be rejected by the automakers as it does not meet USCAR standards established by NHTSA and other automotive safety groups to protect asthmatics and other vulnerable occupants from exposure to airborne particulates.

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

[0022] Surprisingly, despite being flattened, having the flattened rough surface facing inward, i.e., toward the aircraft flow, results in more efficient cleaning than having the flattened smooth surface facing inward. Without wishing to be bound by any particular theory of operation, it is believed that the flattened rough expanded metal burrs form slag trapping pockets in all perforations and are capable of trapping the slag generated by the burning propellant at high velocities. That is, the rough expanded metal burrs, although flattened, act like a horde of tiny internal trapping pockets. As a result, cleaner gas filtration can be achieved by rolling the expanded metal filter with the flattened smooth side facing outward and the flattened rough side facing inward.

[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., about 19 inches wide, which can be cut into 2-6 inches to produce passenger airbag filters and about 1.5 inches to produce driver airbag filters, although any width can be used depending on the equipment). Stainless steels such as SS304, 309, 310, 409, 410, 430, etc. 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 is to be used.

[0024] The sheet is first fed into a press 103 where a perforator 105 having multiple teeth or bits 107 is moved over the sheet such that the teeth perforate the sheet, after which the perforator is removed, similar to a stamping operation. The bits are preferably identical in shape to one another and are preferably shaped to form slits in the sheet. The shape of the bits determines the depth of the bit's penetration and the length of the slits that are formed; the deeper the penetration, the longer the slits and therefore the more open the final structure after stretching will be. Although 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 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 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 advancement of the sheet. The advancement of the sheet is preferably in discrete steps so that the sheet comes to rest when it is punched. Although not preferred, rollers having teeth could be used for a continuously moving sheet.

[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 are punched into diamond-shaped holes. (Of course, a hexagonal bit could be used to form the hexagonal openings, or bits of other shapes could be used, but diamond-shaped slits are a common shape.)

[0027] While the cutting and stretching operations can be performed as separate operations, when fine patterns are to be formed, it is often preferable to produce the expanded metal sheet by cutting and stretching in the same motion with the same teeth. In this operation, the material hangs over a flattened lower blade, and an angled upper tooth or bit cuts and penetrates the sheet. The sheet is folded, and the angle formed by this folding causes a stretching motion of the sheet in relation to the teeth. As a result, the sheet stretches to the depth penetrated by the teeth. The amount of stretching thus achieved typically ranges from 20-25% and can be as much 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 forms openings by (1) forming slits in the metal sheet and (2) stretching the slits along the longitudinal axis of the metal, but in one step instead of 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 holes or opening area and know (after parameters are entered into the controller) if the perforations are within specification. The controller software checks the size and / or shape (geometry) of the openings to determine if the individual openings, or opening areas (actual, estimated, or calculated) are within specification. A second camera (not shown) can be placed between the perforator and the stiffener to determine if the first punch is within specification. The video control system optically inspects the expanded metal sheet product to determine if the product is within specification. To change the process to get within specification, get back within specification, or change specification, 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 a pair 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 of ordinary skill in the art, based on knowledge of the art and this disclosure, would be able to readily select an appropriate roller configuration to achieve the flattening levels discussed herein. As discussed above, in accordance with this disclosure, the amount of flattening is selected to achieve enhanced slag capture without compromising the useful life of the inflator 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 may be positioned after the flattening process and used to determine whether the degree of flattening is within specifications.

[0030] FIG. 4A is a micrograph of the rough surface 17 of an expanded metal sheet produced by the one-step expansion process described above. This view shows the rough surface prior to flattening. As can be seen, the perforations 11 in the sheet are generally triangular in shape. In this production 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 micrograph of the corresponding smooth surface 15 of the sheet, also prior to flattening. Burrs are also visible on this surface, but they are smaller and located on 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 by, for example, two times, the exact amount depending on the particularity 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 / 15,000) and the stretching process increased the thickness of the sheet by more than two times, i.e., after stretching and before flattening as shown in Figures 4A and 5A, the thickness of the sheet was 0.033 inches (1 / 33,000).

[0032] As mentioned above, the thickness of the expanded metal sheet before flattening depends on the thickness of the substrate and the amount by which 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 useful life of the inflator 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 25-45%, which for an expanded metal sheet having a pre-flattened thickness of 1 in 33,000 corresponds to a post-flattened thickness in the range of 1 in 25,000 to 1 in 18,000, respectively.

[0033] In certain embodiments, the reduction in thickness is in the range of 30-45%, or in the range of 30-40%, or in the 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 to calculate 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, for example with a caliper, and averaged. Instead of using a caliper, which would normally span multiple perforations in the sheet, a micrometer can be used on each perforation and the measurements averaged again for multiple locations on the sheet. Since the improvement achieved by this technique depends on the rate of reduction in sheet thickness, it usually does not matter which measurement technique is used, as long as the same technique is used for the pre-flattening and post-flattening measurements. However, in cases of conflict, micrometer measurements are preferred due to their greater accuracy.

[0035] 4B-4E show the effect of different amounts of planarization on the rough burr 13a of FIG. 4A, and FIG. 5B-5E show the effect on the smooth burr 13b of FIG. 5A. In these figures, shading is used to make the effect of the planarization more visible. The thicknesses of the sheets after planarization were 1 / 26,000 for panel B, 1 / 22,000 for panel C, 1 / 18,000 for panel D, and 1 / 15,000 for panel E. These thicknesses after planarization correspond to a reduction of 21%, 33%, 45%, and 55%, respectively, of the sheet thickness before planarization (i.e., 1 / 33,000).

[0036] Figures 6A-E show the effect of the planarization 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 unplanarized burr, while panels B-E show a planarized burr with the same amount of planarization as panels B-E of Figures 4 and 5, i.e., a reduction in sheet thickness of 21%, 33%, 45%, and 55%, respectively. The micrograph in Figure 6 was made by cutting a cross section from the sheet and polishing the exposed edges.

[0037] The enhanced slag capture by the filter disclosed herein can be seen from the shape of the adjacent burrs shown in FIG. 6. As shown in FIG. 6A, gas passing from right to left in the figure (i.e., the path through the filter in which the expanded metal is wound smooth side on the inside) is guided in a relatively smooth flow by the walls 141 and 143 of the 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 the concave barriers 145 and 147 (i.e., concave as viewed from the incoming gas), which, unlike the walls 141 and 143 of the smooth side, do not have a funnel-like shape to guide the gas flow, but instead have a pocket-like shape in which slag can be captured. Additionally, the barrier includes three-dimensional grooves 149 which further enhance the slag capture capability. As a result of these geometric effects, the rough surface has been found to be much better at slag capture than the smooth surface.

[0038] However, the roughened surface is responsible for the cheese grater effect and therefore, as discussed above, cannot be turned inward without compromising the useful life of the inflator. 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 is then to minimize the cheese grater effect while retaining the geometric features of the roughened surface that have been discovered to provide enhanced slag capture. In accordance with the present 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 such that the reduction in sheet thickness caused by the flattening is in the range of 25-45%, or one of the subranges of that range discussed above.

[0039] Panels B-E of FIG. 6 show the effect 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), we illustrate a burr that has been flattened sufficiently to have rounded corners 153 that can contact the solid propellant without substantial degradation of the inflator's useful life, but has not been flattened to the point where the concave barriers 145 and 147 and their associated three-dimensional grooves 149 can no longer provide enhanced slag capture. Panel E (55% reduction in thickness) shows the effect of excessive flattening. Both the concave barriers and the three-dimensional grooves have been flattened to the point where they can no longer provide enhanced slag 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 improved slag 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 that extend 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 by cutter 123. Pieces having narrower widths 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 open areas, can be placed in 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 a cylinder 131, and the ends of the mesh are secured to the cylinder by welder 133. To produce the appropriate inner and outer diameters, the cylinder 135 can be placed into a female mold 137, optionally with a movable inner wall, and an optionally expandable mandrel 139 can be inserted into the central bore of the cylinder. 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 die.

[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 patterns, 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-constant 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 layers of expanded metal, the filter may include one or more layers or portions of other materials, such as metal screens, ceramic fabrics, etc.

[0042] In a typical application, the expanded metal strip is rolled on itself to produce a structure having multiple layers, for example, 3 to 20 layers. For example, the filter may have 10 to 15 layers. The first 360 degree turn (first layer) is secured by spot welding, and the remaining layers may be successively wrapped around each other until the desired outer diameter is reached. The outermost layer is then secured by spot welding. Once completed, the filter may be mounted 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 about the outside of the housing. FIG. 8 is a schematic diagram showing the overall structure of an airbag assembly 155 consisting of an inflator 157 including a filter 21 that contains a solid propellant 26 that, when burned, inflates an airbag, or in more general terms, an inflatable vehicle occupant protection device 159. Details of the structure of an airbag assembly are omitted since such assemblies are well known in the art.

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

[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 the steps of: (1) providing a sheet of expanded metal having a smooth surface and a rough surface; (2) planarizing the sheet of expanded metal, the planarizing reducing a thickness of the sheet; (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; having (a) the filter has a cavity for receiving a solid propellant, the cavity having a surface capable of contacting the solid propellant in an assembled airbag inflator; (b) the planarized piece of expanded metal has a planarized rough surface; (c) the flattened piece of expanded metal is rolled such 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 an amount of planarization in step (2) to reduce a cheese-grater effect in which the solid propellant deteriorates due to contact with the roughened surface of the expanded metal.

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

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

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

6. 2. The method of claim 1, further comprising the step of selecting the amount of planarization in step (2) such that the planarized rough surface has a concave barrier capable of trapping slag during use of the filter in an airbag inflator.

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

8. 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, said filter having a cavity for receiving a solid propellant, said cavity having a surface available for contacting said 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 such that (1) the planarized 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 planarized roughened surface. filter.

9. 9. The filter of claim 8, wherein said planarized rough surface comprises said concave barrier capable of trapping slugs during use of said filter in an airbag inflator.

10. The filter of claim 9 , wherein at least a portion of the concave barrier has three-dimensional grooves.

11. 9. The filter of claim 8, wherein the filter has less slag shedding than a filter having the same construction but in which the piece of expanded metal is rolled up so that the planarized rough surface is away from the cavity.

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

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

14. 9. The filter of claim 8, wherein the filter reduces degradation of solid propellant due to the cheese grater effect to substantially the same extent as or less than a filter having the same construction but in which the piece of expanded metal is rolled up so that the planarized rough surface is away from the cavity.

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; having The inflator includes: (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; having (1) the planarized piece of expanded metal has a planarized smooth side and a planarized 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; Device.

16. 16. The apparatus of claim 15, wherein the planarized rough surface comprises a concave barrier capable of trapping slag.

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

18. 16. The apparatus of claim 15, wherein the filter has reduced slag shedding than a filter having the same construction but 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.

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

20. 16. The apparatus of claim 15, wherein said filter reduces degradation of solid propellant by a cheese grater effect to substantially the same extent as or less than a filter having the same structure but in which said piece of expanded metal is rolled up such that a portion of said planarized smooth surface comprises at least a portion of an inner surface of said filter, instead of a portion of said planarized rough surface.

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