WO1996030105A1

WO1996030105A1 - Filter medium formed from a metal sheet and use thereof in an air bag filter

Info

Publication number: WO1996030105A1
Application number: PCT/US1996/004482
Authority: WO
Inventors: Paul C. Koehler; Stephen A. Geibel; Glen Pataja
Original assignee: Pall Corporation
Priority date: 1995-03-31
Filing date: 1996-04-01
Publication date: 1996-10-03

Abstract

A filter element (20) comprising a plurality of layers (22) of metal sheet, each layer having through holes formed therein, wherein a first layer includes a first pattern of through holes and a second adjacent layer includes a second different pattern of through holes, the first pattern of through holes being juxtaposed with the second pattern of through holes. The filter can be used in an air bag inflator module.

Description

FILTER MEDIUM FORMED FROM A METAL SHEET AND USE THEREOF IN AN AIR BAG FILTER

This invention relates generally to filter elements and their use in filtering fluids such as liquids and gases. In a particular application, the invention relates to the filtration of gases such as those used to inflate an air bag in a supplemental restraint sys¬ tem (SRS) for a vehicle.

The filtration of liquids or gases is often problematic, but the filtration of rapidly expanding gases is unusually difficult. Filters used in this role must be capable of withstanding very high pressure differentials as well as providing good filtering effi- ciency. An especially demanding application is the filtration of gases used to inflate an air bag in a supplemental restraint system.

Most known SRS devices comprise an air bag module and associated electronics. The air bag module includes an air bag casing surrounding the air bag and an air bag inflator. The air bag inflator supplies gas to the air bag and is activated to inflate the air bag upon detection of a collision or other vehicle parameter. In order for the air bag module to be effective, the air bag inflator must inflate the bag before the passenger makes contact with the interior of the vehicle. This requires that the air bag be inflated sufficiently to restrain the passenger typically within about 20 to 60 milliseconds from initiation of inflation, although shorter times may be utilized. In addition, it is desirable to deflate the bag as soon as the impact of a crash is completed, so that the passenger is not trapped within the vehicle by the inflated bag.

There are two popular methods for inflating air bags. First, the air bag may be inflated by an air bag inflator that generates quantities of hot gas by igniting a combustible material. This method will be referred to hereinafter as the "hot gas" infla- tion method. Second, the air bag unit may be inflated using a source of compressed gas. This inflation method will be referred to hereinafter as the "compressed gas" inflation method.

The compressed gas inflation method requires a receptacle of gas stored at a very high pressure, which may be discharged into the air bag as soon as a collision is sensed. In order to obtain a sufficient volume of gas for inflating the air bag, however, a relatively large receptacle of gas at pressures of 1,000 psi or more may be required. To insure opening of the gas receptacle in the short time interval required to maintain passenger safety, explosive units are frequently employed for increasing the pressure in the receptacle and thereby bursting a diaphragm or cutting through a structural portion of the receptacle. The explosive units have a number of undesirable effects such as the production of debris accelerated to high velocity during the explosion and extreme heat generation. Any debris must be filtered out to minimize the likelihood of damaging the air bag and endangering passengers. Thus, a typical inflator for use in compressed gas inflation methods includes a filter for removing the debris.

The hot gas inflation method employs a gas generant that typically includes a material which bums very rapidly once it is ignited and generates large quantities of hot gases which must be cooled and filtered before they enter the air bag.

Many different forms of inflators are used in known hot gas air bag modules. Generally, however, hot gas inflators include certain basic elements. For example, as depicted in U. S. Patents 4,865,635, 5,204,068, and 5,215,721, a typical inflator may include 1) a housing, 2) a gas-generating combustible material (gas generant) located within the housing, 3) an initiator to initiate combustion of the gas generant in response, for example, to a collision, and 4) a filter element or a series of filter elements posi¬ tioned between the gas generating combustible material and the housing to filter and/or cool the gas. During the combustion process, the temperature of the generated gases may be

1200°C or greater and the pressure within the housing may be 3000 psi or greater. In addition, undesirable residues may be generated, including high velocity fine molten par¬ ticles of metal and/or reactive oxides which pollute the gases propelled into the air bag. Such part-icles are often referred to as "slag". Accordingly, gas cooling and filtering is of great importance to insure passenger safety and to maintain the integrity of the air bag module.

Filtration of gas in air bag applications, whether of the compressed gas or hot gas inflation method, thus differs markedly from the usual filter application involving the passage of gas or liquid through a filter element at modest temperature and pressure levels. An air bag filter element not only performs the usual filtration function, it preferably also maintains its integrity while being exposed to very rapid temperature and pressure increases and being impacted with particles accelerated to a high velocity during the inflation process. Further, an air bag filter element, in some instances, is preferably capable of cooling hot gases generated, especially if the hot gas inflation method is used. Air bag filters of the prior art generally employ a multilayer and/or multielement design which includes at least one metal mesh layer. As defined herein, multilayer means multiple layers which are immediately or closely adjacent one another and which may or may not be joined together and multielement means discrete filter elements which are spaced from each other. The metal mesh can be used to cool the rapidly expanding hot gases used in the hot gas inflation method, to remove slag and/or as a final filter for removing fine particles from the gas stream. U.S. Patents 5,221,107, 4,131,299 and 4,865,635 are exemplary; each discloses a metal mesh or screen as a filter and/or cooling medium. The use of metal mesh has several drawbacks, however, which lessens its effectiveness as a filtration medium, and particularly as a filtration medium in an air bag.

It is difficult, for example, to produce metal mesh having very uniform through holes, due to the lack of alignment of the weft and weave pattern forming the mesh, and because of variations of the diameter of the wire filaments forming the mesh. Further, a filter made from multiple metal mesh layers may have an unpredictable voids volume and/or permeability due to the random alignment of through holes in one layer with the through holes on adjacent layers. Another source of possible variation is the necessarily three-dimensional character of the woven pattern, which may provide unpredictable increases in voids volume of a wrap depending on how adjacent layers lay against each other. Channels and open void spaces may be formed between adjacent layers of a wrap if the wefts protruding from the planes of the mesh layers touch each other. Metal mesh also has limitations with respect to strength. Because the mesh is made of a plurality of separate filaments, stresses can cause the filaments to migrate and/or stretch, thereby distorting the weft and weave pattern and changing the size of the through holes in the mesh.

It is also difficult to vary the size of the through holes in a given piece of metal mesh. Thus in applications which require a filter having a voids volume or permeability which varies radially, it is necessary to use separate strips of metal mesh to provide such variations. However, a filter made of separate strips may have lower strength than filter made from one continuous strip, particularly in the radial direction or in the circumferential or "hoop" direction. Lastly, the heat transfer characteristics of metal mesh cannot be varied without altering other properties such as through hole size. Hence there is a need in the art for a filter medium for filtering liquids and gases, which has consistently reproducible and highly tailorable voids volume and permeability. There is also a need in the art for a filter medium which has superior strength and heat transfer characteristics.

In one aspect, the invention relates to a filter element comprising metal sheet having one or more apertures or channels formed in the metal sheet. In another aspect, the invention relates to a filter element including a plurality of layers of metal sheet, each layer having through holes formed therein. A first layer includes a first pattern of through holes and a second adjacent layer includes a second different pattern of through holes, the first pattern of through holes being juxtaposed with the second pattern of through holes. In another aspect, the invention relates to a filter element including a plurality of layers of metal sheet, each layer having through holes formed therein and at least one surface of each layer having flow paths formed therein, wherein the through holes in a first layer communicate with the flow paths in an adjacent layer.

In another aspect, the invention relates to a filter element including metal sheet having flow paths formed in at least one surface of the metal sheet and through holes which extend from the flow paths to the opposite surface of the metal sheet, the metal sheet being corrugated in a pleated configuration wherein the flow paths extend between the through holes and an open portion of each pleat.

In another aspect, the invention relates to a filter element including at least first and second adjacent sections of metal sheet and a porous medium. Each section of metal sheet has one or more apertures or channels formed therein. The porous medium is position between the first and second sections of metal sheet.

In another aspect, the invention relates to an air bag inflator including a housing having one or more ports and a filter disposed within the inflator and positioned to permit a gas flow exiting the filter to enter the one or more ports, the filter including at least one metal sheet layer having a plurality of through holes or channels formed therein. In another aspect, the invention relates to an air bag module including a housing having one or more ports, an air bag communicating with the ports of the housing, and an air bag filter disposed within the housing and positioned to permit a gas flow exiting the filter to enter the one or more ports, the air bag filter including at least one layer of metal sheet having through holes or channels formed therein.

In another aspect, the invention relates to an air bag inflator including a housing having one or more ports and a filter disposed within the inflator and positioned to permit a gas flow exiting the filter to enter the one or more ports, wherein the filter includes a plurality of metal sheet layers, each layer including at least one surface having gas flow paths found therein.

In another aspect, the invention relates to an air bag module including a housing having one or more ports, an air bag communicating with the ports of the housing, and an air bag filter disposed within the housing and positioned to permit a gas flow exiting the filter to enter the one or more ports, wherein the air bag filter includes a plurality of metal sheet layers, each layer including at least one surface having gas flow paths formed therein.

In another aspect, the invention relates to a method for making a filter element which includes forming a plurality of through holes in a metal sheet and arranging the metal sheet in layers with through holes in one layer communicating with through holes in an adjacent layer. The forming step includes forming a first pattern of through holes in a first layer and forming a second, different pattern of through holes in an adjacent layer, and the arranging step includes juxtaposing the first and second patterns of through holes.

In another aspect, the invention relates to a method for making a filter element which includes forming flow paths in at least one surface of a metal sheet, forming a plurality of through holes which extend from the flow paths to the opposite surface of the metal sheet, and arranging the metal sheet in layers with through holes in one layer communicating with flow paths in an adjacent layer.

In another aspect, the invention relates to a method for making a filter element which includes forming a plurality of through holes in metal sheet and corrugating the metal sheet to form a pleated structure.

In another aspect, the invention relates to a method for making a filter element which includes juxtaposing a porous medium and a metal sheet having one or more apertures or channels formed therein.

For a full understanding of the invention, reference should be made to the following detailed description and the drawings, wherein: Figure 1 is a plan view of a continuous strip of metal sheet showing first, second, third and fourth through hole patterns;

Figure 2 is a prospective view of the strip of Figure 1 spirally wound to form a filter element;

Figure 3 is a plan view of an alternative embodiment of the continuous strip of metal sheet showing through holes which extend through the metal sheet and feed slot-shaped channels formed in one surface of the metal sheet;

Figure 4 is a cross section view of the continuous strip of metal sheet of Figure 3;

Figure 5 is a prospective view of a metal sheet showing channels, grooves, and through holes prior to corrugation of the metal sheet; Figure 6 is a close-up view of a portion of the metal sheet of Figure 5;

Figure 7 is a top view of a plurality of pleats formed by corrugating the metal sheet shown in Figure 5;

Figure 8 is a close-up cross section view of two adjacent pleats shown in Figure 7;

Figure 9 is an alternative embodiment of a metal sheet similar to the view shown in Figure 6;

Figure 10 is a perspective view of another embodiment of a metal sheet prior to corrugation.

Figure 11 is a perspective view of another embodiment of a metal sheet prior to corrugation; Figure 12 is a top view of an alternative pleated embodiment of the filter element;

Figure 13 is a perspective view of a corrugated metal sheet used in the pleated embodiment of Figure 10;

Figure 14 is a close-up view of a portion of the metal sheet of Figure 11 ; Figure 15 is a plan view of an air bag filter embodying the invention; Figure 16 is a partially cut away elevation view of an air bag inflator embodying the invention;

Figure 17 is a partially cut away perspective view of a housing and an air bag filter element of the present invention;

Figure 18 is a plan view of a continuous strip of metal sheet showing first, second, third and fourth through hole patterns, each pattern having a different voids volume; Figure 19 is a perspective view of an alternative air bag filter element embodying the invention; and

Figure 20 is a partially cut away elevation view of another air bag inflator embodying the invention.

A filter element embodying the invention may be formed from a metal sheet which is etched, engraved, or embossed to provide a plurality of apertures or through holes extending through the metal sheet and/or channels or flowpaths in one or both surfaces of the metal sheet.

The metal sheet may be etched to form the above described grooves and through holes using any one of the well known methods for etching metal. Examples of such processes include mechanical punching, photochemical etching, EDM (electro discharge machining), electron beam and laser etching. Preferred is photochemical etching due to its ability to provide smaller topography on an etched surface compared to, for example, EDM.

The metal sheet may be formed from any suitable material of construction, including various metals, such as nickel, chromium, copper, molybdenum, tungsten, zinc, tin, gold, silver, platinum, aluminum, cobalt, iron, magnesium, and titanium; as well as combinations of metals such as metal alloys, including austenitic, martensitic, and ferritic stainless steels and 17-7 and 17-4 PH stainless steels, maraging steels, the Hastelloys, the Monels, the Inconels, brass, and bronze. Iron is particularly preferred when the metal sheet is used as a filter in a hermetically sealed air bag inflator as described hereinafter. A filter element including metal sheet formed from iron is particularly easy to manufacture. The iron is readily etched, easily bent and shaped, and conveniently joined to itself, e.g. , simply by welding. The metal sheet may be made corrosion resistant, for example, by plating with one or more layers of a corrosion resistant metal.

The dimensions of the metal sheet may vary depending on the intended configuration of the filter element formed from the metal sheet. Thus, the height and width may be any suitable value. The thickness is preferably greater than about .001 inches, more preferably greater than about 0.002 inches and even more preferably in the range from about 0.005 inches to about 0.032 inches, and most preferably in the range from about 0.006 to about 0.015 inches.

The metal sheet can be employed in a number of different configurations as a filter medium to filter both liquids and gases. In one preferred embodiment, the metal sheet is provided with apertures which extend through the metal sheet, and the metal sheet is arranged in one or more layers or "wraps" to form a filter medium in which the fluid to be filtered passes through the apertures in the one or more layers.

In a preferred embodiment, the apertures comprise slots. The apertures may be arranged in any suitable regular or iιτegular pattern. For example, as shown in Figure 1, a strip of metal sheet 10 includes two rows 12 and 14 of slots 13 and 15, respectively, extending along the longitudinal length of the metal sheet 10 in a herringbone pattern. Alternatively, one or more than two rows of slots may be formed in the strip. The size of the slots can vary from about 0.010 or less to about 0.20 or more inches in length and from about 0.002 or less to about 0.020 or more inches in width. The number of slots can vary from about 10 to about 300 per longitudinal inch of metal sheet and preferably from about 25 to about 250 per longitudinal inch of metal sheet. These ranges are exemplary, and metal sheets having patterns of slots outside these ranges are still within the scope of the invention. One skilled in the art can readily ascertain the dimensions and number of slots for a particular application. The slots in adjacent rows 12 and 14 preferably extend diagonally in opposite directions and switch directions along longitudinal increments la, lb, lc, and Id. To arrange the metal sheet 10 in layers, the strip of metal sheet 10 may be spirally wrapped around a mandrel (not shown) having a diameter D. Consequently, approximately every πD increment (preferably taking account of the increase in diameter D due to the thickness of the metal sheet layers), the direction of the herringbone pattern may be changed, e.g., such that each successive layer wrapped over the mandrel has slots facing at an opposite or "complimentary" angle to the longitudinal length of the metal sheet 10. The result is a "criss cross" of patterns of overlapping slots wherein a first pattern of apertures in one layer of the metal sheet is juxtaposed with a second pattern of apertures in an adjacent layer of the metal sheet.

Depending on the length and shape of the slots 13, 15, at least a portion of each slot may intersect a portion of at least one slot on the immediately adjacent layer. However, the slots may not be in "complimentary" register, i.e., the criss cross pattern may not be symmetrical, and the point of intersection of any given slot with the slots on an adjacent layer may vary in both the axial and circumferential directions. Regardless of how the slots are juxtaposed, it is preferably that the result is a "tortuous path" through the filter medium.

The slots preferably terminate prior to the edge of the strip, defining longitudinal ribs 16a and 16b extending along each edge of the strip. A longitudinal rib 18 also extends between adjacent rows of slots and lateral ribs 16c may extend between adjacent increments. These ribs may be configured in a variety of ways. For example, if the filter element is intended to withstand significant hoop stress or absorb a significant amount of heat, it may be preferable to make the ribs wider or more numerous, since the ribs provide sigmficant circumferential support and an effective heat sink. The intermediate longitudinal ribs, i.e. , the ribs between rows of slots, may have a uniform thickness equal to or less than the maximum thickness of the metal sheet, or they may have flow channels or paths which allow fluid to flow between the rows of slots. The longitudinal edge ribs and the lateral ribs preferably have a uniform thickness equal to the maximum thickness of the metal sheet. Consequently, when the filter element is arranged in layers, the edge ribs and the lateral ribs of adjacent layers will seal against one another and prevent fluid from bypassing the apertures. Figure 2 illustrates a filter element formed from the strip of metal sheet shown in

Figure 1. The filter element, indicated generally by the number 20, may comprise one layer but preferably includes a plurality of layers 22a, 22b, 22c, etc, which may correspond to the increments la, lb, lc, etc of Figure 1. The multilayered filter element may be formed by first forming a plurality of successively larger circular bands from the strip of metal sheet. The bands are then concentrically nested with adjacent bands preferably being attached to one another, e.g., by welding. In an even more preferred method of making the multilayered filter element, the continuous strip of metal sheet is spirally wound, e.g., on a mandrel. Adjacent layers may or may not be attached to one another. Adjacent layers of the multilayered filter element preferably have differing juxtaposed patterns of apertures which provide a more tortuous path through the filter element. For example, the layer 22a defining the outer circumference has slots 13a and 15a. Slots 13b and 15b are on a layer 22b immediately adjacent to the outer circumference layer 22a. Slots 13a, 15a face in respective diagonal directions while underlying slots 13b, 15b face in opposing or complimentary diagonal directions. Fluid passing through the filter element thus traverses a far more tortuous path. The metal sheet configurations of Figures 1 and 2 are merely examples of the type of useful slot configurations. It is contemplated for example, that more than two rows of slots be provided, with an intermediate longitudinal rib positioned between each row. The ribs can be the same width or a different width and one or more ribs can have flow channels or paths as described above. The rows of slots may be angled in the same direction or in complimentary directions or arranged in vertical or horizontal configurations. The slots can be varied in direction or shape or location at various lengths, for example, at the irD increments as described above or the outer diameter of the filter medium as successive layers are added.

The slots in Figures 1 and 2 are, moreover, but one example of the type of apertures which could be formed in the metal sheet. Other examples include circular and non-circular through holes, such as square, oblong, and triangular through holes, one or more continuous slots formed along the longitudinal length of the metal sheet and various patterns with through holes and/or discontinuous or continuous slots. Such patterns include a continuous or discontinuous slot extending from one longitudinal edge rib to the other in sinusoidal fashion, optionally including a second continuous or discontinuous sinusoidal rib partially or completely out of phase with the first slot. Such a pattern may preferably be discontinuous to provide for one or more intermediate ribs for increased strength.

Another pattern may comprise longitudinal intervals having apertures alternating with intervals that have no apertures but which have channels or grooves. Such a structure may provide a tortuous path for fluid exiting an aperture and flowing circumferentially along the channels or grooves to the next downstream aperture. The channels or grooves may include sections running transversely or at an angle to the circumferential flow direction, thereby enhancing the inertial capture filtration component. In place of channels or grooves, flow paths may be provided with a series of random or patterned lands extending from an etched "floor" below the lands. The lands may be any well known geometric shape or may be a continuous or discontinuous sinusoidal, zig zag, herringbone or random length shape, or any combination thereof. Patterns which may greatly increase the flow path between apertures include random sinusoidally shaped discontinuous lands some of which may be nested together.

Another example of a pattern on the metal sheet is illustrated in Figures 3 and 4, wherein a metal sheet 10 is provided with a series of through holes 28 connected on one side to one or more channels 27. Figure 3 illustrates a pattern of channels 27 in one surface of the metal sheet. The channels 27 are similar in shape to the slots of Figures 1 and 2, except that the channels 27 do not extend completely through the metal sheet. Through holes 28 connect with the channels 27. As shown in Figure 3, the through holes 28 in each row may be offset, which can enhance fluid flow distribution.

The channels as well as the through holes can vary in configuration and number. For example the channels can be circular, elongated, or one of any known geometric shapes. Although in Figures 3 and 4 a separate channel is provided for each through hole, two or more through holes can connect to one channel, or the through holes can be in a regular or irregular pattern and of variable size and shape as described above.

A metal sheet formed according to this embodiment may be formed into a filter element having a plurality of layers similar to that shown in Figure 2. The channels in each layer are positioned such that they tend to register with through holes in an adjacent layer. This reduces or eliminates blockage of the through holes by the adjacent layers. In another embodiment, a metal sheet having apertures which extend through the metal sheet and/or flow channels which are formed in one or both surfaces of the metal sheet may be corrugated to form a filter element comprised of a plurality of pleats. The pleated filter element may comprise a single corrugated metal sheet or a corrugated composite including two or more layers of metal sheet or one or more layers of metal sheet and one or more additional layers of any other suitable material.

The pleated filter element may have a wide variety of configurations. For example, it may have a generally box-like or parallelpiped configuration or a generally cylindrical configuration. Further, the pleats may extend back and forth in a linear configuration or in a laid-over configuration. In a laid over configuration, the height of a pleat is greater than the distance between the inner and outer peripheries of the filter element. International Publication No. WO94/11082 describes an analogous but different laid-over configuration. One example of a metallic sheet for use in a corrugated filter element is shown in Figures 5 and 6 prior to corrugation. The metal sheet, indicated generally by the number 30, may have any of a wide variety of apertures, patterns of apertures and/or flow paths. For example, the pattern may include longitudinal grooves 32, 34, and lands 40, 42 formed in the first surface or, preferably, both the surfaces 44, 46 of the metal sheet, respectively. Transverse grooves 48, 50 may be formed at regular intervals along both surfaces of the sheet 30 and may be substantially perpendicular to the longitudinal grooves 32, 34. As illustrated, the transverse grooves 48 on the first surface 44 may be offset from the transverse grooves 50 on the second surface 46. Also, the longitudinal grooves 32, 34 on opposite surfaces 44, 46 may be offset and/or on a given surface of the metal sheet, the longitudinal grooves may be offset from one another on opposite sides of a traverse groove.

The transverse grooves 48, 50 preferably extend deep enough into the thickness t of the metal sheet 30 to intercept the grooves on the opposite surface and create cut outs forming apertures in the metal sheet which align with the transverse grooves. Apertures 52 formed in the second side 46 are illustrated in Figure 5 and in detail in Figure 6. Pleats may be formed by bending the metal sheet along the "fold lines" created by the transverse grooves 48, 50. The metal sheet is bent in an alternating direction for every successive transverse groove, which then forms part of the bights of the pleats of the filter element.

As shown in Figures 7 and 8, a pleated filter element 70 embodying the invention may be cylindrical in form and include a pleated filter medium 72 comprising a photoetched metal sheet and having a plurality of axially-extending laid-over pleats 74. A perforated cylindrical core 76 may be coaxially disposed along die inner periphery of the filter element 70 to support the filter element 70 against the forces associated with outside-in flow. The core 76 can be a cylindrical perforated sleeve extending along the entire length of the filter medium 72. Alternately, the inner periphery of the filter element may be unsupported or may be supported by one or more cir-cumferential weld beads or one or more rings which do not extend along the entire filter medium length and afford considerable open area at the surface of the filter medium for fluid flow. A cylindrical perforated cage 77, a circumferential or helical wrap, one or more weld beads or rings, or any other suitable mechanism may be disposed along the outer periphery of the element 70 to support the filter element against the forces associated with inside-out flow and/or to maintain the pleats in a laid-over configuration. Alternatively, the outer periphery may be unsupported.

As shown more specifically in Figure 8, each pleat 74 has two legs 74a which are joined to one another at a bight 74b at one end of the pleat 74 and which open at the other end of the pleat 74. One leg 74a may be longer than the other leg 74a. Each leg 74a of a pleat 74 is joined to a leg 74a of an adjacent pleat 74 at another bight 74c. In the illustrated embodiment, each of the bights 74b and 74c on the inner and outer peripheries includes one or more apertures 75a formed by the intersection of the longitudinal grooves 32, 34 and the transverse grooves 48, 50. Each leg 74a has an internal surface 74d which opposes the internal surface 74d of the other leg 74a of the same pleat 74 and an external surface 74e which opposes the external surface 74e of a leg 74a of an adjacent pleat 74. The surfaces of adjacent legs 74a may be bonded together in any suitable fashion. The lands 40, 42 of the opposing inner surfaces 74d of the legs 74a of each pleat

74 may be in intimate contact with one another over substantially the entire height h of the corrugation 74 and may overlie the opposing longitudinal grooves 32, 34. In addition, the lands 40, 42 of the opposing external surfaces 74e of the legs 74a of adjacent pleats may be in intimate contact over substantially the entire height h of the pleats and may overlie the opposing longitudinal grooves 32, 34. Here, the height h (shown in Figure 7) of the pleats 74 is measured in a direction along the surfaces 74d, 74e and extends from the inner periphery, e.g., from the core 76, to the outer periphery, e.g. , to the cage, of the filter element 70. The condition illustrated in Figures 7 and 8 in which the height h of each pleat 74 is greater than the distance between the inner and outer peripheries of the filter element 70 (i.e. , R-r in Figure 7) defines the laid-over state. In the laid-over state, pleats may extend, for example, in an arcuate or angled fashion or in a straight, non-radial direction.

Fluid flow through the cylindrical pleated filter element 70 may be either inside-out or outside-in. For example, when flow is inside-out, fluid enters the pleats through the apertures 75a in the bights 74c along the inner periphery, passes into and along the longitudinal grooves 32, 34 in the corresponding legs, and exits the pleats through the open ends of the pleats at the outer periphery. Fluid also enters the open ends of the pleats at the inner periphery, flows into and along the longitudinal grooves 32, 34 in the corresponding legs, and exits the pleats through the apertures in the bights along the outer periphery. Particles in the fluid are effectively removed by the filter element by both sieving and inertial capture and/or heat may be transferred from and/or to the fluid by contact with the metal sheet.

A pleated filter element embodying the invention is subject to many variations. For example, in the embodiment shown in Figures 5-8, the grooves and lands are longitudinal and parallel. Fluid flow through the pleated filter element is thus relatively unimpeded and the primary filtration mechanism may be sieving via the apertures 75a and the longitudinal grooves 32, 34. Alternatively, the grooves may be formed as more tortuous paths to enhance the inertial capture mechanism. For example, in Figure 9 a metal sheet is indicated generally by the number 80. Cross grooves 82 in the metal sheet may be at right angles relative to the longitudinal length of the metal sheet and to the longitudinal grooves 84. The cross grooves 82 connect adjacent longitudinal grooves 84. Flow through a longitudinal groove 84 at the location of a cross groove 82 may proceed from the longitudinal groove to an adjacent longitudinal groove. Barriers 83 may be formed across the longitudinal grooves to ensure fluid flow is directed to the cross grooves. It is also possible, depending partly on the fluid and filtration conditions, to leave the longitudinal grooves unobstructed, whereby some fluid flow may still be directed into adjacent longitudinal grooves via the cross grooves.

The cross grooves can also intersect the longitudinal grooves at an angle different than 90 degrees. For example, the cross grooves can be at an angle in the range from 90 degrees to 75 degrees. Further a random pattern of transverse grooves can be used wherein grooves of various angles intersect the longitudinal grooves, yielding, for example, a waffle pattern of grooves. The grooves may also be randomly oriented and of equal or different length, intersecting each other at different angles.

As an alternative to grooves, the flow paths may be defined in the legs of the pleated filter element by an etched surface having a "floor" where the metal sheet has been etched away and having spaced projections or lands extending away from the floor, e.g. , spaced projections which have been unetched or etched to a lesser extent than the floor. The shape of the spaced projection can be any one of well known geometric shapes including squares, circles, rectangles, triangles, etc. The spaced projections can be patterned or positioned randomly. The apertures in the bights communicate with the space above the floor. Consequently flow of fluid proceeds along each leg through tortuous flow paths around the spaced projections.

Where fluid flow is directed along tortuous flow paths by the presence of transverse grooves, spaced projections, and/or any other suitable arrangement, filtration includes not only sieve mechanism but also an enhanced inertial capture mechanism. Thus, filtration of particles smaller than the apertures and/or grooves is possible.

In another embodiment, a pleated filter element may be formed from a metal sheet having discontinuous longitudinal grooves. For example, as shown in Figure 10, the metal sheet 800 includes longitudinal grooves 801, 802 which teiminate prior to every other bight, e.g., prior to every other transverse groove 48, 50. The metal sheet 800 may be corrugated such that through holes 803 are formed in the roots of the pleats along the inner periphery but not in the crests of the pleats along the outer periphery.

Fluid enters the pleated filter element through the through holes 803 in the roots of the pleats, passes into and along one set of longitudinal grooves 802 which terminate prior to the crests of the pleats. The fluid then passes through cross grooves 804, which are preferably smaller than the longitudinal grooves 802, and enters the adjacent longitudinal grooves 801, which terminate prior to the transverse groove 48. From the adjacent longitudinal grooves 802, the fluid exits the pleated filter element at the crests of the pleats. By providing cross grooves having a smaller cross sectional area than the longitudinal grooves, both the sieving and inertial capture mechanisms are enhanced.

In a similar embodiment shown in Figure 11 , the grooves 801a, 802a, 804a in a first leg of a pleat communicate with the through holes 803 in the root of the pleat while the grooves 801b, 802b, 804b in the opposing second leg of the pleat are isolated from the through holes 803 in the root of the pleat. Further, the grooves 801a, 802a, 804a in the first leg of the pleat are isolated from the outer periphery of the filter element at the crest of the pleat while the grooves 801b, 802b, 804b in the second leg of the pleat communicate with the outer periphery at the crest of the pleat.

The metal sheet may be corrugated preferably with a porous medium between the opposing legs of the pleats. The porous medium may be any suitable filter medium and/or support and drainage medium. For example, the porous medium may comprise a porous membrane, a mesh or netting, or a woven or non woven sheet. For higher temperature applications, the porous medium may comprise a ceramic, glass, or metal. For example, the porous medium may comprises a ceramic fiber medium, such as a ceramic paper, a glass fiber medium, such as a resin-bonded glass fiber sheet, a sintered metal fiber or metal powder medium, or a metal mesh or screen. For lower temperature applications, the porous medium may comprise a natural or synthetic polymer. For example, the porous medium may comprise a porous polymeric membrane, such as a nylon or fluoropolymeric membrane, a nonwoven fibrous polymeric sheet, such as a fibrous sheet formed from an aramid, a woven polymeric screen, or a polymeric netting or mesh, such as an extruded or expanded mesh. In the embodiment of Figure 11, fluid enters the pleated filter element through the through holes in the roots of the pleats and enters the grooves 801a, 802a, 804a in each first leg. The fluid then passes through the porous medium into the grooves 801a, 802a, 804a in the opposing second legs and then exits the pleated element at the crests of die pleats. In another embodiment, as shown in Figures 12-14, a pleated filter element formed from a metal sheet indicated generally by the number 100 is designed for fluid flow in the axial direction. In this embodiment, there are no apertures in the bights since fluid flow in and out of the filter element is blocked in the radial direction. Further, the grooves 101 may extend from edge to edge generally transversely of the metal sheet rather than longitudinally, although longitudinal cross grooves 103 may be included to connect adjacent transverse grooves. The transverse grooves 101 are preferably not formed in the exterior surface of the bight of each pleat. Alternatively, any other arrangement of flow paths may be utilized. Adjacent pleats of the pleated filter element are shown in Figure 12. Fluid flows into one annular end of the pleated filter element, axially along the grooves 101, 103 in the legs of the pleats, and out the opposite annular end. Again, filtration is accomplished by both sieving and/or inertial capture. Inner and outer impermeable housing members 124, 125 may be used to support the pleated filter element.

In yet another embodiment, a pleated filter medium is formed from a metal sheet having through holes or apertures, preferably photoetched apertures, in the legs of the pleats. The apertures in the legs may be in addition to or in lieu of the apertures in the bights. The apertures may be any one or more than one geometric shape, such as the slots described above in connection with the spirally wound embodiment. The legs of the pleats may have flow paths formed in one or both surfaces and communicating with the apertures. If the legs of the pleats do not have flow paths, the filter element may include a support and drainage medium corrugated with the metal sheet on the upstream and/or downstream surface of the metal sheet. The orientation of the pleats, core and cage may be similar to that shown in Figures 7 and 8. Flow through d e pleated filter medium of this embodiment may be inside-out or outside-in in the radial direction or in the axial direction, and the mode of filtration is similar to previous embodiments. In another embodiment of a hollow, cylindrical pleated filter element, me pleats are not laid-over. Rather, they extend in a straight radial direction. Because the inner circumference of the filter element is smaller than the outer circumference, the legs of each pleat are preferably tapered. For example, the metal sheet may be photoetched such that the legs of each pleat are thickest at the outer circumference, and the thickness of the legs continually decreases to a minimum at the inner circumference. Other than these differences, this embodiment may be similar to the other embodiments described above.

The filter medium of invention possess as a number of highly advantageous characteristics, especially when used as an air bag filter and in similar applications. In accordance with the invention, the filter medium can be tailored to individual air bag designs. Design parameters can vary greatly, especially with air bags employing the hot gas inflation method. For example, the combustible material employed to inflate the air bag can vary with respect to shape, the velocity of gas generated, die rate of gas generation, the amount of heat produced, and/or the amount of debris formed. The inflator housing, which houses the combustible material and the filter element, can have a variety of geometric shapes, and include one or several gas exit ports which themselves can vary in size and shape. Further, the orientation of the combustible material and the filter with respect to the inflator housing can vary. Changes in these parameters from one air bag inflator design to the next can easily be accommodated by filter media embodying the invention to maintain required temperature and filtration specifications. A filter medium of me invention can easily be tailored to meet me needs of a particular air bag application. For example, if a more energetic type of combustible material is used, generating higher gas temperatures, the mass of the filter medium can be increased to provide greater heat absorption or "quench". For the single or multilayered wrap embodiment exemplified by Figures 1-4, greater heat absorption is brought about by increasing the unetched surface area, such as the edge and intermediate longitudinal ribs and lateral ribs. This increases the mass of the filter which in turn increases total heat capacity. By the same token, if a particular combustible material burns cooler, then the rib area may be decreased. Changes in permeability and/or voids volume may be brought about by varying aperture and/or channel area or the size and shape of the individual apertures and/or channels along one or more linear dimensions, e.g., radially, axially, or circumferentially. For example, if slots are used, they can be increased in width and decreased in length, concentrating more of the open channel area in a smaller region of the metal sheet. Strength may be maintained by virtue of the thicker rib or ribs.

If the orientation of the gas exit ports in the inflator housing is varied, then die voids volume of the filter may also be varied to prevent blow through. For example, if the inflator has three exit ports, then the voids volume may be adjusted, e.g., by adjusting die number and size of the apertures and/or flow paths, to have a low voids volume in the layer or layers adjacent die three exit ports. This may be accomplished by providing that the last or last few increments of the metal sheet have low voids volume portions adjacent the exit ports, i.e., the wrap has an axial, radial and/or circumferential variation in voids volume. It is also possible to tailor die low voids volume area such that only the area of the outer circumference of the filter directly adjacent the exit ports has a lower voids volume.

For air bag inflators using a combustible material which generates significant amounts of slag, i.e., molten material formed as a byproduct of combustion, me wrap can include a layer or layers having very high voids volume which function as a slag filter used to capture the slag without blinding the downstream filter layers. The slag filter layers may include increments with large apertures or a large number of smaller apertures. Optionally me slag filter increments can include one or more channels or other etched configurations on dieir downstream sides, as illustrated in Figure 3 and 4. The pore structure of the filter medium may also be varied in a wide variety of ways to effect one or more desired flow characteristics. The pore structure may be varied along one or more linear dimensions, e.g., radially, axially, or circumferentially, in any suitable manner, including varying the cross-sectional area of the flow channels and/or through holes. For example, the cross-sectional area of the flow channels and/or the through holes may be progressively smaller in successive layers of a multilayer filter element. Thus, the filter element may have a graded pore structure radially through the entire filter element or only through a portion of the filter element.

Different amounts of sieving and/or inertial capture may be achieved by varying the shape and orientation of the apertures and/or channels in the wrap layers. A high level of inertial capture may be achieved by providing a very tortuous path for the gas flow through the filter. One of the advantages of using the multilayer wrap is that tortuous paths may be achieved by multiple changes in gas flow direction with a relatively low pressure drop. This is achieved dirough the use of etchings defining flow paths and lands having a relatively open configuration, which permits generally unobstructed gas flow yet provides significant eddies in the gas flow stream. The tortuousness of the flow paths may also be varied along one or more linear dimensions, e.g., radially, axially, or circumferentially, to effect one or more desired flow characteristics.

It can be seen from the above discussion mat various characteristics can be imparted to the multilayered wrap embodiment by altering the pattern of etching or embossing on the metal sheet increments. Variations in one characteristic can be provided by changing the pattern and/or dimensions of apertures and/or channels in one or several of the appropriate increments or layers without altering the other characteristics of the filter expressed by the remaining increments or layers. Thus, one of the features which distinguishes me use of etched, engraved, or embossed metal sheet from metal mesh in filtration applications is the ability to isolate individual filtration characteristics and cooling effects characteristics in separate wrap layers or increments, and in portions of the same increments, and die ability to vary those characteristics at will.

The same considerations apply when a pleated filter element embodying me invention is used as an air bag filter. In embodiments where me apertures and grooves in the pleats provide sieve filtration, the size of the bight apertures and grooves adjacent the exit ports can be smaller and/or fewer in number than the apertures in the same bights which are spaced from the ports. Slag filtration can be achieved by increasing the number and/or size of die apertures in the bights on the upstream end of me filter medium. Variations of permeability can be achieved radially widiin the pleated structure by radially varying d e number, widd and/or depth of die longitudinal grooves in the legs of the pleats. For example, if a low permeability zone is desired in the center of the pleats, that section may contain a large number of grooves having a smaller cross- sectional area than grooves in a higher permeability zone. The change in groove size may be a gradual tapering of the grooves in an adjacent higher permeability zone, or there may be an abrupt transition in groove size.

In embodiments where the apertures and grooves in the pleats provide inertial capture filtration, the same variations apply with die added factor that the transverse grooves can be varied to increase or decrease die level of inertial capture. Similarly, a pleated filter element having apertures in the legs of the pleats can be tailored for a particular application. The apertures in the legs can be varied in size and number to increase or decrease voids volume/permeability or degree of inertial capture.

A pleated filter embodying the invention can be combined wid a wrap embodiment. For example, a single or multilayer wrap may be positioned on die upstream side of a pleated filter element to function as a slag filter, or on the downstream side to function as a low permeability zone adjacent me exit ports, or as a drainage layer to aid in gas flow exiting the pleated element and into me exit ports. When used as a drainage layer, the wrap embodiment may have slots or odier apertures optionally combined widi channels which direct die flow exiting the apertures into the ports. Filter elements embodying die invention may also be combined widi omer types of filter media, for example, a sintered metal filter medium such as that described in U.S. Patent Application No. 08/506,840 filed March 3, 1995 and entitied "Filter Medium and Use Thereof in an Air Bag Filter", which is hereby incorporated by reference in its entirety. An example of such an arrangement is illustrated in Figure 15 , in which a filter element 120 includes a sintered metal filter medium 122 and a slag filter 124 formed from a wrap of etched metal sheet preferably having a high voids volume/permeability. It is also possible to use a single or multilayered wrap as a stand-alone filter element in a known air bag filter assembly as a replacement for one or more of the metal mesh filters. An air bag inflator is illustrated in Figures 16 and 17. As shown in Figure 16, an air bag inflator, indicated generally by the number 105, includes an ignitor 107 and a generant 109. As shown in both Figures 16 and 17, an air bag filter 110 having a graded permeability is positioned within an inflator housing 112 with a low permeability zone 114 positioned adjacent a gas port 116 and high permeability zones 118 and 120 spaced from the gas port 116, e.g., located on eidier side of die low permeability zone 114. Placement of the filter 110 in me housing 112 such diat the low permeability zone 114 is adjacent to me gas port or ports 116 minimizes the likelihood diat the hot gases will preferentially flow through the filter 110 only in the region adjacent to d e gas ports 116, potentially perforating the filter by melting the metal sheet and/or reducing die level of cooling. With the placement of the low permeability zone 114 adjacent the gas ports 116, the hot combustion gases encounter increased resistance in the most direct path dirough the filter 110 to die ports 116, forcing the gases along longer parallel flow padis dirough the lower resistance, high voids volume zones 118, 120 to the gas ports 116. Thus, die hot gases may be more uniformly distributed throughout die filter than diey would be if the filter had a more uniform voids volume.

Figure 18 illustrates a metal sheet having sections of variable through hole size and spacing and useful for constructing the filter medium of Figures 16 and 17. A first section 14a has very high permeability and can function as a "slag" filter to remove large particles from the gases. Section 14b and 14c have successively lower permeability and/or voids volume while section 14d has a permeability similar to section 14b, i.e. , higher than section 14c. Section 14d functions to direct gases to the ports as described above. It will be appreciated diat more than four sections or layers may be used to form the air bag filter, wi i die criteria being that at least the last applied layer has a lower voids volume dian an adjacent, previously applied layer or layers.

The spirally wound embodiment of the filter medium of the invention also has a very high strength in the radial direction, i.e., high "hoop strength" due to d e plurality of layers formed by winding of die continuous metal sheet. The strength of the filter medium can be further increased by tack welding die layers together to form inter-layer bonding.

It is also possible to provide separate (non-spirally wound) concentric metal sheets which may be secured togedier by, for example, tack welding. This non-spirally wound embodiment may, however, have less hoop strength than a spirally wound filter medium.

Hoop strength of the non-spirally wound embodiment may, however, be increased by employing sheets of varying length secured togedier. Building die filter medium in this manner, provides offsetting points of securement and thereby spreading d e risk of failure along a substantial portion of the filter.

Both the spirally wound and non-spirally wound concentric embodiments may include one or more porous media, as previously described, between die layers. For example, as shown in Figure 19 a strip of metal sheet 130 and a strip of a porous medium 131 may be spirally wound to form alternating layers of metal sheet 130 and porous medium 131. In a preferred embodiment, the filter element may have a graded pore strucmre in one or more linear dimensions, e.g., radially, axially, or circumferentially. For example, the filter element may have a graded pore structure in which me pore structure decreases or increases, in a continuous or step-wise manner with increasing radius over a region or die entirety of the filter element. The graded pore structure may be effected by varying the size of the channels and/or apertures in the metal sheet 130 such that different layers of the metal sheet 130 have differendy sized channels and/or apertures. Alternatively, or additionally, die graded pore structure may be effected by varying the pore size and/or removal rating of die porous medium 131 such diat different layers of the porous medium 131 have different pore sizes and/or removal ratings.

The spirally wound filter element can have a variable mass and/or surface-area-to- mass ratio. The ability to vary the mass and/or surface area-to-mass ratio is particularly advantageous in certain applications, e.g., where die fdter medium serves also as a heat sink. For example, in the hot gas inflation method for inflating an air bag, the type and amount of combustible material used can vary, and, dierefore, die temperature of d e hot gases produced can also vary. However, due to d e flexibility of design afforded by die metal sheet, the surface area-to-mass ratio of the filter medium can be varied to accommodate sufficient cooling of the hot gases regardless of die combustible material used and without significantly compromising the filter permeability.

In a particular example, if the combustible material has a combustion temperature of from about 1200 to about 1600°C and operates for an event time of about 20-60 milliseconds, an air bag filter embodying the invention may be constructed of a continuous sleeve of metal sheet wound about a mandrel such diat it possesses a sufficient mass and/or surface area-to-mass ratio to absorb enough energy so mat me gas exiting the filter has a temperature low enough to protect both d e air bag and vehicle occupants from excessive heat.

One manner of doing this is to increase the widdi of the center rib which functions as a heat sink, since it has the highest heat capacity per unit volume of any portion of the filter medium. The pleated filter element described above can also be used in an air bag filter. For example as shown in Figure 20, a pleated filter element 200, such as any of those previously described, may be disposed in a housing 112 in a manner similar to the wrap filter element 110 shown in Figure 16. Generally when die pleated filter element described above is used in an air bag filter, die inflator housing defines the cage, and die generant and ignitor may be positioned within the pleated filter element.

Claims

What is claimed:

1. A filter element comprising a plurality of layers of metal sheet, each layer having through holes formed therein, wherein a first layer includes a first pattern of through holes and a second adjacent layer includes a second different pattern of dirough holes, the first pattern of through holes being juxtaposed wid die second pattern of dirough holes.

2. A filter element comprising a plurality of layers of metal sheet, each layer having through holes formed dierein and at least one surface of each layer having flow paths formed merein, wherein the dirough holes in a first layer communicate with die flow padis in an adjacent layer.

3. A filter element comprising metal sheet having flow paths formed in at least one surface of die metal sheet and dirough holes which extend from the flow paths to the opposite surface of the metal sheet, the metal sheet being corrugated in a pleated configuration wherein me flow paths extend between the through holes and an open portion of each pleat.

4. A filter element comprising at least first and second adjacent sections of metal sheet, each section having one or more apertures or channels formed therein, and a porous medium positioned between the first and second sections of metal sheet.

5. An air bag inflator comprising: a housing having one or more ports; and a filter disposed within die inflator and positioned to permit a gas flow exiting said filter to enter said one or more ports, the filter including at least one metal sheet layer having a plurality of through holes or channels formed therein.

6. An air bag module comprising: a housing having one or more ports; an air bag communicating with the ports of me housing; and an air bag filter disposed within the housing and positioned to permit a gas flow exiting said filter to enter said one or more ports, said air bag filter including at least one layer of metal sheet having through holes or channels formed merein.

7. An air bag inflator comprising: a housing having one or more ports; and a filter disposed widiin the inflator and positioned to permit a gas flow exiting said filter to enter said one or more ports, wherein the filter includes a plurality of metal sheet layers, each layer including at least one surface having gas flow paths found dierein.

8. An air bag module comprising: a housing having one or more ports; an air bag communicating wim die ports of the housing; and an air bag filter disposed widiin die housing and positioned to permit a gas flow exiting said filter to enter said one or more ports, wherein the air bag filter includes a plurality of metal sheet layers, each layer including at least one surface having gas flow paths found dierein.

9. A method for making a filter element comprising: forming a plurality of through holes in metal sheet and arranging the metal sheet in layers with dirough holes in one layer communicating wim through holes in an adjacent layer, wherein forming a plurality of through holes includes forming a first pattern of through holes in a first layer and forming a second different pattern of through holes in an adjacent layer and wherein arranging the metal sheet in layers includes juxtaposing die first and second patterns of through holes.

10. A method for making a filter element comprising: forming flow paths in at least one surface of metal sheet; forming a plurality of through holes which extend from me flow paths to the opposite surface of the metal sheet; and arranging the metal sheet in layers with dirough holes in one layer communicating wim flow paths in an adjacent layer.

11. A method for making a filter element comprising: forming a plurality of through holes in metal sheet and corrugating the metal sheet to form a pleated structure.

12. A method for making a filter element comprising: juxtaposing a porous medium and a metal sheet having one or more apertures or channels formed dierein.