WO1996008302A2

WO1996008302A2 - Filter medium and use thereof in an air bag filter

Info

Publication number: WO1996008302A2
Application number: PCT/US1995/012331
Authority: WO
Inventors: Paul Charles Koehler; Stephen Arthur Geibel
Original assignee: Pall Corporation
Priority date: 1994-09-15
Filing date: 1995-09-15
Publication date: 1996-03-21
Also published as: AU3642895A; WO1996008302A3

Abstract

A filter medium comprises metal particulates including at least about 10 % non-linear metal fibers. The metal particulates are sinter bonded to one another and the filter has a voids volume of at least about 35 %. The filter medium may be used in an air bag inflator (14) which includes an inflator housing (22) cooperatively arranged with an air bag to direct gas from the housing (22) into the air bag. A combustible material (24) and an ignitor (26) are disposed within the inflator housing (22). An air bag filter (28) is positioned in the gas flow path between the ports (20) and the combustible material (24) to filter and/or cool the gases before they are propelled into the air bag.

Description

FILTER MEDIUM AND USE THEREOF IN AN AIR BAG FILTER

This application is a continuation-in-part of United States Patent Application Serial Number 08/506,840 which was filed on March 3, 1995, which is a continuation-in-part of United States Patent Application Serial Number 08/306,496 which was filed on September 15, 1994, now abandoned.

Technical Field

This invention relates generally to filter media and their use, for example, in filtering rapidly expanding gases. In a particular application, the invention relates to the filtering of gases used to inflate an air bag in a supplemental restraint system (SRS) for a vehicle.

Background Art

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 efficiency. 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. Rapid deflation is also desirable so that, in case of accidental inflation, the restraint upon the operator of the vehicle is sufficiently short that the operator does not lose control of the vehicle. 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 air or other 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 d e debris.

The hot gas inflation method employs a gas generant that typically includes a material which burns 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 particles of metal and/or reactive oxides which pollute the gases propelled into the air bag. 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.

Prior art air bag inflators generally incoi Orate at least one π-ultilayεred filtεriniz element including two or more metal fine mesh screens formed as a laminate. Additional filtering elements may also be employed upstream of the laminate to remove larger debris. An example of such an assembly of filter elements is described and illustrated in U. S. Patent 5,215,721. Such multilayer/multi-element filter assemblies are often expensive to manufacture and in some instances present difficulties in installation in an air bag inflator housing. Moreover, multi-element filter assemblies can be a problem from an inventory standpoint since there is a risk that one or more of the components could be found defective, resulting in scrapping of the entire assembly.

Another problem associated with both multilayer and multi-element filter assemblies is their size and/or weight. This is of particular concern in the case of a driver's side air bag located on the steering column. Excess size or weight in this area can contribute to steering wheel bulkiness and heaviness. The weight and complexity of prior art air bag filter assemblies are due in part to the above-mentioned need to effectively filter and, simultaneously in the case of hot gas inflation, to cool the gases prior to their entry into the air bag itself. In multi-element assemblies, the gas cooling may be aided by spacing the various discrete filter elements to provide adequate flow and residence time for cooling. Such spacing, however, adds to the size and weight of the inflator assembly and requires a larger housing. Size and weight could be reduced if filter elements were more effective at cooling. For cooling of the involved high temperature, high velocity gases, the characteristics of the filter element with respect to heat transfer and heat capacity are important. In general, the rate of heat transfer is significant in that it is difficult to transfer sufficient heat to the filter assembly from the gases in the short period of time available during the actual inflation process.

The heat transfer rate could be increased by increasing the filter surface area, but this has been thought to require increasing the size and hence the weight of the filter element, thereby defeating the original goal of smaller size and lower weight.

In addition to being complex and heavy, multilayer air bag filters can fail when subjected to the high stresses encountered during the deployment of the gases into the air bag. In the case of the hot gas inflation method, these stresses arise primarily from the high temperatures and/or large pressure differentials encountered. Each of these indi¬ vidually, or combined, can work to cause loss of filter function.

If the multilayer filter is formed of a laminate, heat stresses can cause delamination, particularly if the individual layers of the laminate have different coefficients of thermal expansion. Hot gases and possibly debris can then bypass or "blow through" one or more of the delaminated layers prior to exiting the filter housing. This is of particular concern for structural laminates, in which one or more filter elements are supported by reinforcing layers which maintain filter integrity, or where the filter itself is composed of multiple structural elements joined together.

Delamination or more catastrophic failure of the laminate can also be caused by the shock wave of combustion. As combustion is initiated, a large pressure differential is set up which radiates outward to create hoop stresses which in turn cause circumferential expansion of the filter. In this context, hoop stress is defined as circumferential stress, such as that applied to the air bag filter and caused by the shock wave of combustion in a hot gas inflation method or by the large pressure differential due to release of compressed gas in the compressed gas inflation method.

A large pressure differential causes no difficulty when the filter is supported by the housing, and in fact as set forth in U. S. Patent 4,865,635, this expansion can have the alleged beneficial effect of sealing the filter against the housing to prevent gas leakage around the filter. However, where the filter is unsupported by the housing, particularly at the gas discharge apertures or ports, the hoop stresses may deform the filter sufficiently to cause delamination or even fragmentation. Again, this can result in blow through of the combustion gases.

Blow through of another sort can occur even in the absence of delamination. In this type of blow through, hot combustion gases bypass those areas of the filter abutting the filter housing, and preferentially take the path of least resistance directly from the combustion chamber through those areas of the filter adjacent to the discharge ports, and then outward into the discharge ports. Those portions of the filter abutting the housing are thereby underutilized and unavailable for cooling and filtration. The hot gases passing along the more direct paths through the filter and into the discharge ports overheat the filter portions adjacent the ports, which can then disintegrate and result in blow through.

Multilayer filters of the prior art frequently also suffer from a lack of flexibility in design shape. Generally, manufacturing methods limit available geometries to circular and oval shapes having a uniform wall thickness. In summary, the filter elements currently used in air bag gas filtration are deficient in the areas of cost, complexity, size, weight, structural integrity, geometric flexibility, and/or gas cooling performance. There is hence a need in the art for a filter medium for use in an air bag filter, and in similar applications, which provides that the filter is simpler and less expensive to manufacture, can be formed into a variety of shapes, is less prone to failure and/or quality control problems, and combines the features of smaller size, lighter weight and adequate cooling capacity.

Disclosure of the Invention

In one aspect, the invention relates to a porous metal filter medium for filtering a fluid. The filter medium has a mass of metal fibers which are formed by a melt overflow casting technique or process and then worked in a hammermill or equivalent device to bend at least about 10 percent of the fibers into a non-linear shape. The fibers have a nominal diameter in the range from about 30 to about 1500 microns and a nominal length in the range from about 100 microns to about 20 millimeters. The fibers are mechanically interlocked and sinter bonded to one another, defining voids between the fibers of the mass. The voids have geometric pore sizes in the range from about 30 microns to about 250 microns, and the mass of fibers has a voids volume in the range from about 35 percent to about 95 percent.

In another aspect, the invention relates to a porous metal filter medium formed from a mass of fibers at least about 10 percent of which are non-linear. The fibers have, in cross-section, a generally crescent shape defining an arc. A substantial portion of the crescent-shaped fibers define an arc generally no greater than about τ radians. The fi¬ bers have a nominal diameter from about 30 microns to about 1500 microns, and a nominal length in the range from about 100 microns to about 20 millimeters. The fibers are substantially mechanically interlocked and sinter bonded to one another, defining voids between the fibers of the mass. The voids have nominal geometric pore sizes in the range from about 30 microns to about 250 microns and the mass has a voids volume in the range from about 35% to about 95%. By substantially mechanically interlocked is meant that at least 20%, desirably at least 45%, and preferably at least 70% of the fibers are mechanically engaged or secured to or intertwined with other fibers to form the medium.

In another aspect, the invention relates to a porous metal filter medium comprising a mass of metal fibers. The fibers are formed by a melt overflow process including conveying a molten metal onto a rotating casting wheel by overflow from a container of the molten metal, solidifying the metal on the wheel to form metal fibers, and projecting the fibers from the rotating wheel onto a recovery surface. The metal fibers are also worked in a hammermill to bend at least a portion of the fibers into a non-linear shape. The fibers have a nominal diameter in the range from about 30 microns to about 1500 microns and a nominal length in the range from about 100 microns to about 20 millimeters. The fibers in the mass are mechanically interlocked and sinter bonded to one another, defining voids between the fibers. The voids of the mass have a nominal geometric pore size in the range from about 30 microns to about 250 microns and the fibrous mass has a voids volume in the range from about 35% to about 95%.

In a further aspect, the invention relates to a process for forming a porous metal medium comprising arranging metal fibers into a porous mass. The fibers are formed by a melt overflow process which includes conveying a molten metal onto a rotating casting wheel by overflow from a container of the molten metal, solidifying the metal on the wheel to form metal fibers, and projecting the fibers from the rotating wheel onto a recovery surface. The fibers are also worked in a hammermill to bend at least a portion of the fibers into a non-linear shape. The fibers have a nominal diameter in the range from about 30 microns to about 1500 microns and a nominal length in the range from about 100 microns to about 20 millimeters. The process further comprises compressing the porous metal mass and then sintering the compressed mass to provide a mass of mechanically interlocked metal fibers sinter bonded to one another having a voids volume in the range from about 35% to about 95 % .

Filter media embodying these aspects of the invention are superior to known filter media. For example, a filter medium embodying the invention is unexpectedly strong and yet may have any suitable voids volume, including a voids volume as high as 95% . Further, a filter medium embodying the invention may also have superior heat transfer characteristics enabling it to not only remove contaminants from a fluid flowing through the filter medium but also cool the fluid.

In another aspect, the invention relates to an air bag module comprising a housing, an air bag, and an air bag filter. The housing has one or more ports and the air bag communicates with die ports in the housing. The air bag filter is positioned within the housing to permit a gas flow exiting d e filter to enter the ports. The air bag filter includes at least two zones, one zone having a lower permeability than the odier zone. The zone of the filter element having the lower permeability is positioned adjacent to the ports and the zone having the higher permeability is spaced from the ports.

In another aspect, the invention relates to a method for inflating an air bag. The method comprises providing a high pressure gas; passing the gas through a filter having at least two zones, one zone having a lower permeability than the other zone. Passing the gas through die filter includes passing in parallel a first portion of the gas through the low voids volume zone and a second portion of the gas through the higher voids volume zone. The metiiod further comprises directing me gas from the filter into the air bag. ^τn another aspect, the invention relates to an air bag module which comprises a housmg, an air bag, and an air bag filter. The housing has one or more ports, and the air bag communicates with the ports in the housing. The air bag filter is positioned within the housing to permit the gas flow exiting the filter to enter the ports. The air bag filter has an outer surface which includes at least one drainage channel. The drainage channel communicates with the ports between the housing and the outer surface of the air bag filter.

In another aspect, the invention relates to a method for inflating an air bag. The method comprises providing a high pressure gas; passing die gas through a filter element having an outer surface which includes at least one drainage channel on an outer surface; directing the gas along die drainage channel and through a port; and directing the gas from me port into the air bag.

Air bag modules and methods embodying diese aspects of die invention are particularly advantageous because they insure uniform distribution of gas flow through the filter. This greatly enhances the ability of d e filter to remove harmful contaminants from me gas and cool d e gas before it enters the air bag.

In another aspect, die invention relates to an air bag module comprising a housing, an air bag, and a filter. The housing has one or more ports and the air bag commu¬ nicates with die ports in the housing. The filter is a unitary sintered metal structure which is positioned widiin die housing to permit a gas flow exiting the filter to enter die ports.

In anodier aspect, die invention relates to a method for inflating an air bag which includes me steps of providing a high pressure gas; passing me gas dirough a unitary, sintered metal filter; and directing die gas from the filter into me air bag. In still anodier aspect, die invention relates to an air bag inflator comprising a housing having one or more ports, an integral air bag filter disposed within die housing and a means for supplying gas to an interior of the housing. Debris is also generated as a byproduct of supplying the gas. The integral filter has at least first and second zones, die first zone having a voids volume higher dian the second zone. The first zone func- tions as a slag filter to capture the debris generated by the gas supply means and is disposed in die gas flow path between the supply means and me second zone. The second zone is disposed in the gas flow path between the first zone and die ports of the housing.

In another aspect, the invention relates to an air bag module comprising a housing, an air bag, and an air bag filter. The housing has one or more ports, and die air bag communicates with the ports and housing. The air bag filter is positioned within the housing to permit the gas flow exiting the filter to enter the ports. The air bag filter has an outer surface which includes a plurality of standoffs positioned at spaced intervals and forming flow channels communicating with the ports between the housing and die outer surface of the air bag filter.

In yet anodier aspect, the invention relates to a method for inflating an air bag which includes die steps of providing a high pressure gas, passing die gas through a filter having an outer surface which includes a plurality of standoffs positioned at spaced intervals, directing d e gas between and along die standoffs and through at least one port, and directing die gas from the port into the air bag.

Air bag modules, inflators, and methods embodying these aspects of die invention are very economical and highly reliable. The unitary sintered filters are very strong, enabling diem to widistand die stresses involved in air bag deployment and resist failure in service. When used in die hot gas inflation method, the filters can simultaneously provide superior gas cooling.

Brief Description Of The Drawings

Figure 1 is a representational view of me cross-section of metal fibers of a filter medium embodying die invention;

Figure 2 is a partially cut away elevation view of an air bag module embodying die invention;

Figure 3 is a cross-sectional view of an air bag inflator embodying me invention; Figure 4 is a partially cut away perspective view of a housing and an air bag filter element of the present invention;

Figure 5 is a perspective view of a second air bag filter element embodying die invention;

Figure 6 is a perspective view of a third air bag filter element embodying die invention;

Figure 7 is a plan view of a fourth air bag filter element embodying die invention;

Figure 8 is a plan view of a fifth air bag filter element embodying d e invention; and

Figure 9 is a perspective view of a sixth air bag filter element embodying die invention. Detailed Description Of Preferred Embodiments

A filter element according to d e invention includes a filter medium formed from paniculate material, preferably metal particulates, i.e. , metal fibers and/or metal powder. Metal fibers, or metal fibers in combination with powder metal, are especially preferred. The term "fibers", as used herein, is intended to describe elongated metallic bodies having longitudinal dimensions, i.e. , nominal lengths, which are greater than the diameters of me bodies. The nominal lengm, in the case of fibers which are bent or non-linear as described hereinafter, is the length the fiber would have if straightened. That is, d e measurement of nominal lengm follows and includes die bends of die non- linear fibers.

The term "diameter" is intended to refer to me average cross-sectional dimension of die body across a narrow dimension, regardless of die cross-sectional shape of the body. Thus, die cross-sectional shape may be circular, oval, rectangular, ribbon-shaped, quasi cruciform, elliptical, dendritic, acicular, or any odier regular or irregular shape. As set forth more fully hereinbelow, highly preferred is a crescent shape.

The metal powder material may include particles of any regular or irregular shape. For example, die metal powder may include generally spheroidal particles.

The metal paniculate composition used to formulate the filter medium according to die invention preferably includes metal fibers of a variety of shapes, including linear or straight. However, in accordance wi one aspect of the invention, the metal paniculate composition preferably includes at least about 10% non-linear metal fibers. Non- linearity may be imparted to die fibers by substantially "working" linear fibers in a hammermill or equivalent device which randomly bends the fibers by the application of mechanical energy. The resulting non-linear fibers have a "kinky" appearance, charac- terized by at least one change in direction and more often by a plurality of random directional changes which give the fibers a curved, twisted, hooked, corkscrew, crimped, or otherwise bent or undulated appearance. The worked fibers are also more uniform in nominal length.

Non-linear metal fibers of me type described above may be manufactured in a variety of ways well known to those skilled in die art. For example, a mat of metal fibers may be produced by a melt overflow casting process. This process is described, for example, in U. S. Patents 5,213,151, 4,977,951, 4,930,625, and Re. 33,327 which are hereby incorporated by reference in their entirety. In this process, a container of molten metal is positioned adjacent a rotating wheel or drum. Molten metal flows over the edge of die container and onto the rotating wheel where it is solidified to form metal fibers. The momentum of rotation causes the fibers to be projected onto a recovery surface.

Further working of the fibers until they meet die desired size and shape cha¬ racteristics can be carried out in a device such as a G5HFS hammermill manufactured by Prater Industrial Products, Inc. Preferably, the fibers are hammermilled until at least about 40% of me fibers each have a plurality of bends.

Fibers produced by the melt overflow casting process and then worked in a hammermill are not only bent and kinky, but they frequently also have a generally cres¬ cent shape in cross section, as shown in Figure 1. The cross-sections of the crescent shaped fibers may have tapered tips, giving die cross-section the appearance of a "quarter moon". The thickness of die fiber cross-section may be either uniform or tapered along die edges. The arc defining the crescent is generally no greater than about - radians (180°). Preferably, the arc is from about τ/2 to about - radians.

While d e above-described crescent shaped fibers manufactured according to the melt overflow casting process are preferred, odier suitable processes for producing bodi crescent and non-crescent shaped fibers may be employed, including, for example, extru¬ sion. Many types of steel wool and chopped wire produced by diese alternative processes may also be further worked to provide non-linear fibers for the filter medium in accordance with the invention. These fibers are less preferred, however, since diey typically do not have a crescent shaped cross-section and the resulting bent fibers typically do not have die same characteristics of bends or kinkiness of die preferred crescent shaped fibers produced by die melt overflow casting process.

Preferably, die metal particulates include from about 30% to about 100% non-linear metal fibers. The metal fibers are preferably rather coarse, e.g., in one embodiment die nominal diameter of me fibers may range from about 30 to about 700 microns. In another embodiment, the nominal diameter may range from about 30 to about 1500 microns. Preferably, die nominal diameter ranges from about 50 to about 900 microns. More preferably, e fibers have a nominal diameter of between about 100 and about 150 microns. The nominal length of d e metal fibers may range from about 100 microns to about 20 mm, and preferably from about 2mm to about 8mm. Generally, the aspect ratio (nominal length: nominal diameter) for at least 50% of the fibers is from about 3: 1 to about 700: 1 and preferably from about 150: 1 to about 700: 1. The metal particulates, including die metal fibers and the metal powder, may be of any of a variety of metal materials including alloys of various metals such as nickel, chromium, copper, molybdenum, tungsten, zinc, tin, gold, silver, platinum, aluminum, cobalt, iron, and magnesium, as well as combinations of metals and metal alloys, including boron-containing alloys. Brass, bronze, and nickel/chromium alloys, such as stainless steels, die Hastelloys, the Monels and die Inconels, as well as a 50 weight percent nickel/50 weight percent chromium alloy, may also be used.

A filter medium incorporating the metal particulates described above can be formed using a variety of suitable processes which can broadly be characterized as eidier "wet" or "dry". In wet processes, the metal particulates are suspended in a liquid medium and dien formed into a structure which is men sintered to form the filter medium. In dry processes, no liquid medium is used, die structure being formed by pressing the metal particulates togemer so that die resulting structure has sufficient "green" strength to permit sintering.

In a preferred embodiment for wet processes, a stabilized suspension is prepared including a liquid medium such as tiiat disclosed in U. S. Patents 4,822,692, 4,828,930, or 5,149,360. The stabilized suspension of metal particulates may also include a stabilizing agent and/or a binding agent. More preferably, a single constituent, i.e., a stabilizing/bonding agent, serves both to stabilize the dispersion of metal particulates and, upon drying of die suspension, to bond me individual particulates to each od er and provide die requisite unsintered or green strength.

Typically, die stabilized suspension of metal particulates is prepared using a carrier formed by combining die stabilizing/binding agent with d e liquid medium which is preferably water for ease of use and disposal. Odier liquids, such as, for example, an alcohol or a light oil, can be used if desired, provided diat d e resulting suspension can be readily filtered. A preferred stabilizing binding agent comprises from about 0.1 % to about 2% of die total weight of die carrier. A variety of stabilizing/binding agents may be used. Poly acrylic acids are particularly desirable. In general, polyacrylic acids widi molecular weights from about 1,000,000 to about 4,000,000 are suitable. Examples of such acids are CARBOPOL^® 934 and CARBOPOL^® 941, manufactured by B. F. Goodrich Chemical Co. CARBOPOL^® 934 has a molecular weight of about 3,000,000 and CARBOPOL^® 941 of about 1,250,000. Other materials which can be used include carboxyediyl cellulose, polyediylene oxide, sodium alginate, carboxymethyl cellulose, guar gum, memyl cellulose, and locust bean gum. In general, when water is used as the liquid medium, water compatible stabilizing/binding agents which volatize and/or de¬ compose substantially completely prior to or during sintering may be used.

By way of example, in a carrier formed from a CARBOPOL^® 934/ water mixture, where die CARBOPOL^® 934 comprises about 1.4% by weight (based on the weight of die carrier), the viscosity of die earner is approximately 1250cp at 20°C. Mixtures of CARBOPOL^® 934 and water are preferred carriers because the combination provides compositions having substantially consistent viscosities and viscosities which are readily reproducible. Based on d e diameter and die lengdi of die largest metal fibers of the metal paniculate to be suspended, die viscosity of the carrier that will render the sus¬ pension sufficiently stable can be determined. The desired viscosity of die carrier is that which is capable of holding the metal particulates in suspension and diereby maintaining a substantially uniform dispersion prior to lay down in a formation step to be described hereafter. For example, a viscosity in the range from about lOOOcp to about 4000cp is suitable for die prefened non-linear fibers previously discussed. Viscosity is adjusted by varying die amount of stabilizing/binding agent in die carrier. In general, a lower carrier viscosity is desired and hence less stabilizing binding agent may be used widi finer metal particulates since there is a reduced tendency for finer particulates to settle out. The carrier is preferably mixed until uniform dispersion of die stabilizing/binding agent is obtained. The metal particulates are men added and mixed widi d e carrier to provide a uniform stabilized dispersion or suspension of die particulates in the carrier. The weight ratio of the particulates to the carrier is typically from about 1:20 to about 1:2. An additive such as a ceramic material, for example, mullite, may be mixed widi die stabilized suspension to aid in protecting me filter medium during exposure to high temperatures encountered, for example, in an air bag inflator using the hot gas inflation mediod.

In a first "lay down" type wet process for forming the filter medium, me suspension of particulates dispersed in the carrier is injected into a mold defining a cavity. To enhance uniformity of die suspension, die mold may be rotated during die injection. Preferably, the mold is rotated at about 3000 rpm or less. The cavity may be of any shape and corresponds to die desired shape of die filter. For example, the cavity may be an annulus widi a relatively small height, yielding a filter shaped like a washer, or an annulus with a relatively large height, yielding a filter shaped like a hollow cylinder. The bottom of die cavity is provided with drainage ports, such as a mesh having openings, which allow the earner to drain from the cavity while retaining the metal particulates. The suspension is men pressure filtered or vacuum filtered to remove the carrier from the suspension. Pressure filtration may be carried out by inserting an annular ram into the mold at die top of die cavity and driving the ram such that die liquid is forced from the cavity dirough the drainage ports. Consequently, die ram com¬ presses the metal particulates and mechanically interlocks them widi one anodier while d e carrier is expelled from the mold dirough die drainage ports. The preferred amount of pressure exerted on die structure to adequately compress die structure is from about 10,000 to about 80,000 psi, more preferably from about 20,000 to about 60,000 psi. Vacuum filtration involves applying a vacuum to the drainage ports and wididrawing die liquid from the cavity. Depending on die level of vacuum, die metal particulates may be compressed and mechanically interlocked as with pressure filtration. Generally, however, the degree of compression is less than widi die pressure filtration.

The resulting compressed metal paniculate structure is removed from the mold and dried. Drying is preferably conducted in a convection oven at a temperature from about 100 to about 210°F for about 30 to about 45 minutes or longer. The individual particles of die structure are further bonded to one anodier by die stabilizing/bonding agent during drying, such d at die structure has sufficient strength to maintain its integrity and shape during further processing. The metal paniculate structure is dien further processed by sintering to yield die filter medium. Sintering may be performed by placing the structure in a furnace, such as a vacuum furnace, an inert ataiosphere furnace, or a reducing atmosphere furnace. Sintering removes volatile material and fuses die individual panicles of die metal par¬ ticulates to each other at the junctions of die metal fibers, and if present, the metal powder, widi voids being defined between the metal particulates.

Sintering is preferably carried out at a temperature high enough to promote solid state diffusion of metal atoms from one particle to another to form the sintered bonds. For stainless steel metal fibers, a temperature in the range of from about 1600 to about 2580°F, more preferably from about 1900 to about 2525 °F, for a period of time from about 1/2 hour or less to about 8 hours or more has been found adequate. When lower melting materials are used, die sintering temperature is adjusted accordingly. For example, widi bronze particulates, temperatures in die range from about 1300 to about 1900°F are adequate.

When sintering is complete, the resulting filter medium is cooled and then removed from the furnace. Upon cooling, the medium may be easily handled due to die formation of sinter bonds at die junctions of die particulates. In a second "seamless" type wet process for forming the filter medium, as exemplified by U. S. Patent 4,822,692, a mold in the form of a hollow cylindrical con¬ tainer is at least partially filled, preferably completely filled, widi die stabilized sus¬ pension of dispersed metal particulates. During addition of suspension, die mold may be rotated as explained in die above description of die lay down process. The mold is dien rotated at a high speed to form a metal particulate structure, and dien placed in a sintering furnace.

Prior to adding die suspension, one end of the mold is preferably sealed wid , for example, a rubber stopper or equivalent device. Preferably, a sufficient amount of the suspension is added to fill the mold. A full mold is preferred because it provides more uniform distribution of die metal particulates which in turn results in a product widi a more homogeneous pore distribution. Additionally, a completely filled mold aids in start-up because me center of gravity is more nearly coincident widi die longimdinal axis of die mold.

After adding die suspension, d e mold is sealed and mounted on a structure capable of rotating the container about its longitudinal axis, preferably widi the mold in a substantially horizontal position. For example, a machine lathe, such as a Clausing ladie, or a spindle may be used. The mold is rotated at a high enough rate to provide a centrifugal acceleration at the interior wall of the formed strucmre equal to or greater than about 60, more preferably from about 70 to 80 G's (gravities) or greater, and possibly as high as 200 or more G's to produce porous articles.

The rate of rotation required may vary inversely widi the diameter of the strucmre being formed. For example, to generate a centrifugal force of 70 G's in a two inch diameter strucmre, the rate of rotation may be about 1,575 rpm. Similarly, for a one inch diameter article, die rate of rotation may be about 2,225 rpm, and for a one-half inch diameter article, the rate of rotation may be about 3, 150 rpm. Rotation of the mold compresses the metal fibers, mechanically interlocking them and forming a generally cylindrical compressed metal particulate strucmre.

The mold is generally rotated at die desired rpm level from about 3 to about 5 minutes, following which it is stopped. Longer times may be used if necessary. Preferably, the mold is allowed to slow down widiout being stopped abruptly, more preferably it is allowed to spin until its momentum runs out. The mold is dien removed from the rotating strucmre, supernatant fluid is removed, and die compressed metal particulate structure is dried in die container, preferably while in a horizontal position. Drying is preferably conducted in a convection oven, at from about 100 to about 210°F for about 30 to about 45 minutes or longer. The compressed metal particulates are bonded to one anodier by the stabilizing/bonding agent during drying, forming a self-sup- porting strucmre.

The mold widi die strucmre still inside is dien sintered in a furnace as described above. Most preferably a vacuum furnace is used to remove volatile material and to fuse me individual particles of metal particulate to each odier to form die filter medium. Sintering is best done widi die mold in a vertical position to avoid distortion of d e re- suiting filter medium due to high creep rates of the particulate material at elevated temperatures.

The strucmre is sintered and cooled at similar temperatures and conditions described above for die "lay down" type wet process. As in the lay down type wet process, sinter bonds are formed at the junctions of die metal fibers and, if present, die metal powder widi voids being formed between the metal particulates. Upon cooling, the resulting filter medium can be easily removed from die container or mold due to die formation of the sinter bonds. The mold may be formed of any material capable of withstanding die sintering temperatures employed in die process. Examples of materials that can be used include silicon carbide, silicon nitride, molybdenum and various ceramics. An advantage of us¬ ing a ceramic tube as the mold is diat diere is no need to employ a releasing agent, e.g. , a carbon mold releasing agent, to prevent the metal particulate strucmre from binding to die mold. The use of such releasing agents may contaminate the sintered filter medium. Further, die releasing agents may be difficult to remove. Accordingly, it is highly preferred to prepare the filter medium without the use of a releasing agent.

In a dry process for forming the filter medium, in which no liquid suspension or carrier is used, dry metal particulates are introduced into a mold dirough a sifter.

Preferably, die mold is filled widi die particulates. While the particulates are being introduced into the mold, the latter may be rotated as explained above regarding the lay down step for wet processes.

In a highly preferred procedure for introducing the metal particulates, the mold in- eludes a core section and a sleeve section. The core section has a conical upper portion and a cylindrical lower portion. The sleeve section surrounds die cylindrical lower portion of the core section, defining an annulus between die sleeve section and the cylindrical lower portion of the core section. The metal particulates are introduced into die mold by sifting mem onto die conical upper portion of the core section. The particulates fall by gravity onto the conical upper portion, slide down die conical surface of the conical upper portion and accumulate in a random fashion in the annulus between the cylindrical lower portion of die core section and die sleeve section. This procedure provides a more even distribution of particulates in me mold dian odier procedures since, among odier dungs, d e uniformity of die distribution is not a function of die rate at which d e particulates are introduced into die mold.

As an example of a non-preferred procedure, die tip of a funnel may be positioned over the annulus in die mold and particulates may be sifted into die funnel as die tip of d e funnel travels around die annulus. Multiple layers of die particulates d en accumulate in die mold. Sometimes referred to as "felting in planes", this procedure can produce uneven particulate distribution, especially if the speed of travel of the funnel and die rate of introduction of the particulates are not closely controlled. However the particulates are introduced into die mold, once d ey are deposited in die annulus, d ey are compressed in die mold to form a compressed particulate strucmre. Compression may be earned out by inserting an annular ram into the mold at die top of die annulus, die shape of die ram generally corresponding to die shape of die annulus. The ram is driven downward into die annulus at any suitable pressure for providing a desired permeability. Alternatively, to reduce die density gradient over the height of the filter, the particulates may be compressed by driving a ram downward dirough the top of the annulus and driving a second ram upward so diat die particulate material is compressed from two directions. The force applied to die particulates is similar to that described above for die "lay down" wet process.

In addition, die permeability of the resulting compressed strucmre may be rendered more uniform by adding a dry lubricant to e metal particulates before compression. Stearic acid and zinc stearate are prefened dry lubricants, however, other lubricants well known to those skilled in die art may also be used. Where the metal particulates comprise bent fibers, there is considerable mechanical interlocking during die dry laying and compressing steps.

After compression, the resulting compressed metal particulate strucmre is removed from me mold. The strucmre has enough green strength so diat is can be readdy han¬ dled and transported. It is dien sintered as described above. During sintering, the dry lubricant is burned off and sinter bonds are formed at die junctions of die fibers and, if present, the metal powder widi voids extending between the metal particulates.

Regardless of which of the above processes are used to form the medium, it may be further processed by mechanical means either before or after sintering. For example, the medium may be machined, cut, rolled, coined, swaged, welded, brazed and/or resin- tered.

The filter media of the invention possess a number of highly advantageous characteristics, especially when used as an air bag filter and in similar applications. One benefit of using the non-linear fibers is an unexpected increase in the strength of die filter medium. The kinkiness of die fibers greatiy increases the number of junctions or contact points between the fibers, and this in turn increases die number of sinter bonds formed. The result is a highly sintered medium having additional strength derived from me rigid, interlocking metal particles. By comparison, a sintered article formed exclusively from linear fibers has comparatively few sinter bonds, and die average length of fiber between each bond may be much greater.

The filter medium of the invention also has a greater strengdi in the axial direction due to d e more random orientation of me bent fibers. Linear fibers tend to orient in d e radial plane and hence a medium formed from linear fibers has relatively high strengdi in die radial direction (hoop strength) and relatively low strength in the axial direction. By contrast, bent fibers have less preferential orientation in the radial direction and hence have a more even geometric strengdi distribution.

Another benefit of using non-linear fibers is die capability of having extremely high voids volume, e.g., as high as 95%. Preferably the voids volume is within the range from about 35 % to about 95 % . While die invention is not to be bound by a particular dieory, it is speculated diat the irregular shapes of the non-linear fibers contribute to the high voids volume because irregularly shaped fibers tend not to pack as tightly when they are compressed as do regularly shaped linear fibers or powder material. That is, the packing density of die non-linear fibers is very low and tiiere is relatively little nesting of the non-linear fibers. Consequently, a filter medium including die above described non-linear fibers has relatively large nominal geometric pore sizes, ranging from about 30 to about 250 microns. More preferably, me nominal geometric pore size ranges from about 50 to about 150 microns. Accordingly, d e permeability of the filter medium may be very high.

Permeability may be measured as me velocity of air exiting the filter for a predetermined pressure drop (DP) across die filter, measured, for example, in inches of water and at a filter medium thickness of, for example, 0.1 to 0.18 inches. For exam¬ ple, at a Δp of 0.5 inches of water, die superficial velocity of air exiting the outer diame- ter of the filter medium may be between 5 and 200 ft./min. Permeability is directly related to voids volume for a given metal particulate. That is, permeability increases as voids volume increases, and vice versa. Permeability is also directly related to die size of die metal particulates. Permeability decreases as the metal particulates decrease in size. The filter medium of die invention may have, due to die non-linear fibers, a large voids volume and high permeability. Consequently, large adjustments in d e mass and/or die surface area-to-mass ratio of the filter medium can be made widiout unduly reducing voids volume or permeability. For a filter medium constructed from a batch of non¬ linear fibers having nominal diameters ranging between about 50 and about 250 microns, die surface area-to-mass ratio may range between about 115 and about 20 cm²/g. More preferably, the surface area-to-mass ratio may range between about 55 and about 25 cm²/g.

Alternatively, the mass and/or die surface area-to-mass ratio of die filter medium may be adjusted by admixing various quantities of small linear metal fibers and/or small metal powders widi d e non-linear fibers. The large voids volume and high permeability may not be significantly affected by die admixed materials. However, the surface area- to-mass ratio and/or die heat capacity can be varied by die addition of die small fibers or particles. For example, the addition of dendritic particles having a very high surface area-to-mass ratio may increase the overall ratio and increase die heat capacity, while d e addition of spherical particles may decrease the overall ratio but increase heat capacity. The ability to vary the mass and/or surface area-to-mass ratio is particularly advantageous in certain applications, e.g., where die filter medium serves also as a heat sink. For example, in the hot gas inflation metiiod for inflating an air bag, the type and amount of combustible material used can vary, and tiierefore die volume and temperature of the hot gases produced can also vary. However, due to die flexibility of design afforded by die non-linear fibers, 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 witiiout significantly compromising the filter permeability.

In a particular example, if die combustible material has a combustion temperature of from about 1200 to about 1600°C and operates for an event time of about 10-60 milliseconds, an air bag filter embodying die invention may be constructed of non-linear fibers such that it possesses a sufficient mass and/or surface area-to-mass ratio to absorb enough energy so diat die gas exiting die filter has a temperature low enough to protect both die air bag and vehicle occupants from excess heat.

As mentioned above, in a highly preferred embodiment, the invention relates to the use of crescent shaped metal fibers for forming a porous metal filter medium. It has been discovered diat a crescent shape provides exceptional heat transfer from the hot gases to die filter medium, even at very high filter permeability. Again, while not being bound by a particular theory, it is speculated diat die follow¬ ing factors may contribute to the increased cooling effect of me crescent shaped fibers. First, a crescent shaped cross-section provides a much higher surface area-to-weight ratio than, for example, a circular cross-section. Since the rate of heat transfer is proportional to the surface area of the fibers, die higher ratio in theory would allow more heat to be transferred from me hot gases in a given period of time. While mis is true for crescent shaped fibers generally, the increase in cooling effect is particularly pronounced for crescent shaped fibers which are tapered at the tips, i.e. , where the cross-sectional area decreases from the centerline of the crescent to each of the two tips, e.g. , forming a "quarter moon" . The thin tips may act as "cooling fins" and efficiently transfer heat to the relatively thick center area of the crescent, which functions as a heat sink.

A second factor believed to contribute to the increased cooling effect of die crescent shaped fibers is die ability of such fibers to promote better gas circulation by providing some degree of directional flow. Combustion gases are believed to be directed through the "trough" formed by die crescent shape. Since the fibers are arranged in a random fashion, the gases are also directed in a random fashion and hence distributed diroughout die filter medium in a more even manner. This allows more of the surface area of the crescent shaped fibers to be utilized for cooling which in turn allows the use of a lesser number of fibers to effect sufficient cooling of hot gases. Because it is possible to cool gases sufficientiy widi fewer fibers, the size and consequently die weight of the filter element can be reduced, which is a desired goal in many applications such as an air bag filter, especially a driver's side air bag, as men¬ tioned above.

It is also speculated diat die preferred crescent shape of the filters increases the strengdi of die filter medium by enhancing me efficiency of die sintering process. The crescent shape increases the surface energy, and hence die sintering efficiency, at the tips of die crescents. This increased surface energy is a result of the very small effective radii at tiiese tips. Small radii contact points, because of their high surface energy, result in more rapid sintering when in contact with odier small or even large radii surfaces. This is because die sintering process tends toward die lowest energy state. The result is a strong sinter bond at die majority of contact points in a reasonable sintering time and/or temperature. A filter medium formed from these kinky, crescent shaped fibers thus has unique strength, fluid flow and heat transfer characteristics. The kinkiness imparted to the crescent shaped fibers by the hammermilling aids strengdi, fluid flow, and heat transfer by providing much of me randomness in direction to die mass of fibers, which in turn results in die random directional flow of the hot gases. The use of non-linear fibers having crescent shaped cross-sections allows an even higher surface area-to-mass ratio as discussed above. Thus for any given fiber length die surface area-to-mass ratio of a crescent shaped fiber will generally be higher than the comparable round cross-section fiber. Provided diat die filter has sufficient heat capacity to cool the hot gases to an acceptable degree, or in applications where hot gases are not generated, a very light weight filter medium can be produced using tiiese fibers.

A highly preferred application for the filter medium of the invention, whether or not it incorporates crescent shaped fibers, is in an air bag module. The voids volume of air bag filters is preferably between about 45% and about 95%. An exemplary air bag module operating according to die hot gas inflation method is depicted in Figure 2 and is generally indicated by die number 10. An air bag 12 is positioned over an inflator 14. The air bag 12 and d e inflator 14 are botii disposed witiiin an air bag module casing 16. In the hot gas inflation method, upon detection of a collision or other designated vehicle parameter, the inflator 14 is energized and die air bag 12 is supplied wid hot gases from the inflator 14 along a gas flow patii indicated by arrows 18 dirough gas ports

20. As the air bag 12 begins to inflate, it exerts pressure on the air bag module casing 16. The casing 16 has a sufficientiy low tensile strength such diat die force of d e air bag 12 bearing against it causes the air bag to break dirough. The air bag 12 then expands into die vehicle cabin (not shown). The internal construction of die inflator 14 is shown in Figure 3. The inflator 14 includes an inflator housing 22 cooperatively arranged widi d e air bag to direct gas from the housing into die air bag. A combustible material 24 and an ignitor 26 are disposed within die inflator housing 22. An air bag filter 28 is positioned in the gas flow path between the ports 20 and d e combustible material 24 to filter and/or cool d e hot gases before they are propelled into the air bag. In the exemplary embodiment the filter 28 is generally cylindrical in shape and has an outer diameter corresponding to d e inner diameter of die inflator housing 22. The air bag filter 28 has a permeability, voids volume and/or pore size which allows the gas to expand dirough die filter to quickly inflate the air bag while effectively removing harmful contaminants such as combustion products. In some embodiments, die permeability of the filter may be uniform. In other embodiments, die permeability of the filter may be selectively adjusted axially, radially, or circumferentially as desired. The filter may also have a mass which can be adjusted to absorb a significant portion of the heat from the expanding gas widiout significantly sacrificing permeability. Further, the filter may have a surface area-to-mass ratio which may be adjusted to enhance heat transfer and die condensation of combustion products on the filter, again without significantly sacrificing permeability. Consequently the expanding gas quickly inflates the air bag and is not only free of harmful combustion products but is also relatively cool.

According to anodier aspect of die invention, and as shown, for example, in Figure 3, the filter 28 can have a unitary construction which facilitates assembly of the air bag module. The unitary construction also provides an enhanced structural integrity which more reliably withstands die force of rapidly expanding gases produced by die inflator. By "unitary" is meant that die filter has the same or substantially the same composition diroughout. Thus, a unitary strucmre has only one structural "layer" but may have different zones or regions widi differing characteristics, such as permeability, voids volume, fiber diameter, and/or surface area-to-mass ratio.

The strength of the unitary construction is believed due at least in part to the combination of interlocking of the kinky fibers and die subsequent sintering as explained above. In a highly preferred embodiment using crescent shaped fibers produced by die melt overflow casting process, the resulting unitary filter element is exceptionally strong and can be substituted for previously-used multilayer laminates in a number of existing filter designs, wiΛ little or no additional reinforcement.

The filter medium constructed in accordance wi i die invention may have a variety of geometric configurations. Hence, an air bag filter comprising die filter medium may be an elongated hollow cylinder, a truncated hollow cylinder, an annulus, a disk, a hollow cone or any segment of me foregoing such as a half or quarter cylinder. Alternatively, the air bag filter can have an elongated rectangular or triangular shape or any other geometric configuration depending upon die geometry of the inflator and/or the inflator housing.

In some embodiments of die invention, the filter medium has a permeability and/or voids volume diat varies. For example, the voids volume in a first zone of die filter may be less than die voids volume in a second zone. By way of example, die high voids volume zone may have a voids volume ranging between about 70% and about 90% and d e low voids volume zone may have a voids volume ranging between about 45% and about 75%. Similarly, me permeability ratio of the high permeability zone to the low permeability zone may be in die range from about 2: 1 to about 5: 1. That is, the high permeability zone may have a permeability which is from 2 to 5 times the permeability of the low permeability zone. Generally, however, permeability and/or voids volumes are adjusted to a particular application. The permeability and/or voids volume of die filter medium may be graded radially, axially, or circumferentially or in any combination of these directions. For example, the permeability and/or voids volume of die filter medium may be axially varied. In an exemplary embodiment, a low permeability zone may be a cir¬ cumferential region which extends along a first portion of the axial lengdi, while a high permeability zone may be a circumferential region which extends along a second portion of the axial lengdi. A low permeability zone may be disposed between first and second high permeability zones, or the low permeability zone may be positioned at die upper or lower end of die medium such diat only die top or bottom of the low permeability zone is adjacent to d e bottom or top of die high permeability zone, respectively.

Preferably, a filter medium having graded permeability and/or voids volume is manufactured by further processing a filter medium having a uniform permeability and/or voids volume, such as die medium produced using a wet or dry process as discussed above. For example, a uniform permeability medium having a uniform thickness may be further processed by compressing a region thereof to change the permeability in that region and create a low permeability zone. When compressed, die metal fibers become deformed and pack more densely, thereby decreasing die permeability. The uncompres- sed region of the medium retains the higher permeability and dius comprises me high permeability zone. For example, where the filter medium has a hollow cylindrical geometry including inner and outer surfaces of uniform diameter, compression may be performed by roll pressing the uniform permeability medium. The medium is placed on a mounting roller having a uniform outer diameter such that die inner surface of the medium contacts d e mounting roller. A pressing roller having a protruding rib circumscribing its outer diameter may be rotated and forced against the mounting roller, compressing die filter medium between die rollers and pressing the rib of the pressing roller against the outer surface of the medium. Preferably, d e protruding rib extends axially by a distance less dian the height of the medium and is elevated from the surface of the roller. The com- pressed region is formed dirough rotational contact between the rib of the pressing roller and die outer surface of die medium. However, the compressed region may be formed on the inner surface of the medium by mounting the inner surface of the filter medium on the pressing roller. After the low and high permeability zones have been formed, the filter medium may be sintered or, if previously sintered, may be resintered to restore and/or replace any sinter bonds broken during formation of the compressed region.

An air bag filter 28 which includes a unitary filter medium 30 having axially spaced high and low permeability zones formed by pressing is illustrated in Figure 4. In this embodiment, the low permeability zone 32 is positioned between upper and lower high permeability zones 34, 36, and each of die zones 32, 34, 36 comprise a circumferential band extending all die way around die filter 28.

Permeability can also be varied circumferentially. As shown in Figure 5, a unitary filter medium having circumferentially varying permeability is indicated generally by the number 40. Low permeability zones 42 are circumferentially spaced in die medium be¬ tween high permeability zones 44. Such a medium may be formed according to a modification of die roll pressing process described above. The mounting roller used in me modified roll pressing process may be the same as d e mounting roller used in die previously described roll pressing process. However, the pressing roller may be modified by replacing the circumferential rib by a series of circumferentially-spaced projections. The projections may extend axially along the pressing roller a distance less tiian the height of die filter medium, yielding die limited low permeability zone 42 shown in Figure 5. Alternatively, the projections may extend axially along the pressing roller a distance greater than or equal to die height of die filter medium, yielding a low permeability trough which extends from the top to die bottom of the filter medium. The projections may have a circumferential dimension which may be determined according to the desired widtii of die low permeability zone.

When forming a low permeability zone by compression as described above, d e shape of die medium is altered such diat die wall tiiickness of the filter medium is non- uniform. It is also possible to provide a graded permeability filter medium which has uniform wall thickness. Likewise, it is possible to provide channels or other alterations in shape without altering permeability.

One way to provide a filter medium having a graded permeability and uniform wall thickness is to compress a uniform permeability medium having a non-uniform wall tiiickness. For example, the medium structure may be a hollow cylinder initially molded to have a circumferentially-extending rib or one or more axially-extending, circumfcrcn- tially-spaced projections on the inner or outer surface. The inner and outer surfaces are then rolled between two rollers having uniform outer diameters until the rib or projections are compressed to form an air bag filter having a uniform wall thickness. The highly compressed regions, i.e., die compressed rib or the compressed projections define zones of lower permeability, while the less compressed regions, i.e. , the regions beyond die rib or between the projections, define zones of higher permeability. The di¬ mensions of the rib and die projections may be varied in order to change die voids volume in the lower permeability zone or to increase the area of the lower voids volume zone.

A filter medium having high and low permeability and/or voids volume zones may also be consttucted using a modification to die above-described wet process. In this modification, a plurality of suspensions are used, each resulting in a different perme- ability. For example, one suspension may contain metal particulates of smaller nominal particle size, e.g. , smaller fiber diameter than die particulates contained in die odier sus¬ pension^) and/or d at are of a different shape than die particulates contained in the other suspension(s). In addition, die suspensions may contain concentrations of particulates which differ from each other. Alternatively, a ceramic material may be added to die particulate material used in one of the suspensions.

In any event, the size, shape and/or amount of particulates used in die different suspensions should be such that upon formation of d e filter medium, it includes a first zone or series of first zones having a first permeability and a second zone or a series of second zones having a second permeability different from the first permeability.

A filter medium made according to die modified lay down process preferably includes a circumferential region which comprises the low permeability zone. The axial position of the low permeability zone may be controlled by die quantity of die various suspensions initially injected into the mold. That is, if it is desired to place a low permeability zone near one end of die medium, then a large amount of a first suspension resulting in a permeability volume may be injected into the mold initially, followed by a low permeability suspension. Conversely, if it is desired to position the low permeability zone near the odier end of the filter element, a small amount of the first suspension is injected into the mold initially. Multiple rings of low and high perme¬ ability zones may be created by alternately layering die first and second suspension in die mold.

A filter medium having high and low permeability and/or voids volume zones may also be constructed by modifying die dry process in a manner similar to that described above widi respect to the wet process. That is, the dry process may be modified by adding quantities of metal particulates in alternate regions having different voids volume.

Filter media having zones of differing voids volume and/or permeability are frequently particularly advantageous when used as air bag filters in air bag modules em- ploying the hot gas inflation method. More particularly, such filters are especially ef¬ fective when installed in inflators or gas generators which include housings having a plu¬ rality of gas ports, such as exit ports.

In accordance widi anodier aspect of the invention, an air bag filter is disposed in die housing with a low permeability and/or voids volume zone positioned adjacent die ports and a high permeability and/or voids volume zone spaced from the ports. This arrangement may provide a far more uniform distribution of gas flow dirough die filter medium, especially where the average permeability of die filter medium is relatively high.

As shown in Figure 4, an air bag filter 28 having a graded permeability is posi- tioned witiiin an inflator housing 22 widi a low permeability zone 32 positioned adjacent a gas port 20 and die high permeability zones 34 and 36 spaced from die gas port 20, e.g., located on either side of die low permeability zone 32. Placement of the filter 28 in the housing 22 such diat die low permeability zone 42 is adjacent to the gas port or ports 20 minimizes the likelihood diat die hot gases will preferentially flow through the filter 28 only in the region adjacent to d e gas ports 20, potentially perforating the filter by melting the metal particulate and/or reducing die level of cooling. Widi the placement of d e low permeability zone 32 adjacent the gas ports 20, die hot combustion gases encounter increased resistance in the most direct patii dirough the filter 28 to the ports 20, forcing the gases along longer parallel flow paths dirough the lower resistance, high voids volume zones 34, 36 to die gas ports 20. Thus, die hot gases may be more uniformly distributed throughout the filter than they would be if die filter had a more uniform voids volume.

To ensure uniform distribution of gas flow dirough die filter, it is also important to provide adequate drainage of die gas from die substantially entire downstream surface, e.g., outer surface, of the air bag filter to the ports of die housing. In a typical air bag inflator, the ratio of die total area of die filter to die total area of the gas ports in the housing is in die range from about 13: 1 to about 20: 1, depending upon die height of die filter and die number of gas ports and tiieir diameter. In tiiis environment, extremely high sonic or supersonic flow rates may be created.

In accordance widi anodier aspect of die invention, one or more channels are provided to drain the rapidly expanding gases from the entire downstream surface of the filter to die gas ports. Although die channels may be formed in die inner surface of the housing, diey are preferably disposed in the downstream surface of the air bag filter. For example, one or more grooves may be formed on die outer surface of die air bag filter during die roll pressing process. The channel can also be formed by using an ap¬ propriately shaped mold for forming the filter medium. In this case, a uniform permeability may be maintained widi no graded permeability in the area of the channel.

In the embodiment illustrated in Figure 4, a compressed region defines not only d e low permeability zone 32 but also a single, large drainage channel 46. The channel 46 is positioned axially along die outer diameter of the medium and includes first and second side walls 48, 50 and a base 52, respectively. The base may be flat or con- toured, or the base may comprise an intersection point of the first and second side walls.

The first or second side walls 48 or 50 or both side walls may be inclined widi respect to d e base 52 by an angle of less than 90°. For example, in cross section, the channel may have a V-shaped geometry, a U-shaped geometry or a substantially rectangular geometry. Alternatively, die channel may have a geometry in which the inclination angle of die first side wall 48 differs from the inclination angle of die second side wall 50. That is, die geometry of die channel may be asymmetric or iπegular. Widi respect to die dimensions of the channel, die channel has a widdi W defined as the perpendicular distance between die side walls measured at die maximum outer diameter of the filter medium. The channel also has a deptii D defined as die perpendic¬ ular distance from the point of die base closest to die inner diameter of die filter medium to the maximum outer diameter. The channel dimensions (width and deptii) may vary depending on die particular application, the desired permeability, and d e desired drai¬ nage characteristics of a particular filter/ housing combination. For example, the widtii W of the channel, or the aggregate width of a plurality of channels, is preferably in the range from about 15% to about 80% of the height H of die filter. The deptii D of d e channel is preferably in die range from about 10% to about 45% of the wall thickness. In alternative embodiments, the drainage channels may comprise a continuous or dis¬ continuous pattern about the outer surface of the medium. For example, the channels may comprise a helix, a continuous triangle wave pattern, a continuous sign wave pat¬ tern, or a knurled or waffled pattern providing drainage from the downstream surface of the filter medium to the ports in the housing. These patterns are preferably roll pressed into die outer surface of the filter medium.

Figure 6 illustrates a filter medium having a waffled pattern. The filter medium, indicated generally by the number 80, has regularly spaced projections or "standoffs" 82 projecting from a base area 84. Combustion gases flow along die base area 84 to exit ports in the inflator housing (not shown). The above descriptions of drainage channels and axial and circumferential low permeability and/or voids volume zones relate primarily to the aspect of improving the gas flow characteristics of an air bag filter as the gases exit the filter and pass dirough exit ports in d e inflator housing.

In another aspect, the filter medium can have a voids volume which is graded in the radial direction. Radially graded permeability and/or voids volume is particularly useful when the filter medium is required to perform multiple filtration functions. For example, prior art multi-element air bag filters often have a separate "slag" filter which is spaced from and upstream of the final filter. A slag filter is used to capmre large particles of debris, including molten debris, generated upon ignition of the combustible material in the hot gas inflation method or generated upon release of the compressed gas in the compressed gas inflation method. Compared to unitary filters, multi-element filters are more expensive to produce and install in an inflator housing and moreover result in a larger and heavier air bag module. By providing a unitary filter with a high permeability and/or voids volume zone on the upstream side, and a lower permeability and/or voids volume zone on the downstream side, it is possible to combine the functions of a slag or prefilter and a final filter in a single filter element.

Figure 7 illustrates a unitary filter medium suitable for use as a combination slag/final filter. As shown in Figure 7, a filter medium having radially graded voids volume is indicated generally by the number 60. A higher voids volume zone 62 is located adjacent to die inner diameter 64 of the filter 60, and a lower voids volume zone 66 is located adjacent to die outer diameter 68. The high and low voids volume zones 62, 66 preferably extend along die entire axial length of the filter medium.

A zone 70 of intermediate voids volume can also be present and is located between die high and low voids volume zones 62, 66, and it too extends along die entire axial length of the medium. Alternatively, a sharp step in voids volume can be provided between die higher and lower voids volume zones widiout the presence of the intermediate zone.

The filter medium of Figure 7 can be used as a replacement for separate slag and final filters in an air bag module. The upstream side of die filter (shown as die inner diameter in Figure 7) can have a very open strucmre with a high voids volume, e.g. , in the range from about 70 to about 90% . This enables the capmre of large particles of de¬ bris such as slag. The open strucmre prevents blockage of the filter by the debris. Hot gases dien pass into die lower voids volume zone for final filtration and/or cooling. The lower voids volume zone may have a voids volume of from about 45% to about 75%. A filter medium having radially graded permeability and/or voids volume can be produced in a number of ways. For example, me radially graded voids volume can be formed prior to die sintering step by varying me coarseness of die fibers added to die mold in one of die wet or dry processes described above. Highly preferred is die teaching of varying fiber size in U. S. Patent 4,822,692, i.e. , the second "seamless" pro¬ cess. Using this process, a graded strucmre is formed by laying down a first suspension on the interior of the mold at a specified rate of rotation. The supernatant is then re¬ moved and the metal particulates laid down are dried. A second suspension of metal particulates is men introduced having a nominal particle size different from the first suspension, followed by drying. This process can be repeated if desired to produce addi¬ tional layers having different voids volume. The resulting "green" strucmre is sintered as described above.

To produce a filter medium having a unitary construction and a dual function as a slag and final filter, the first suspension may include metal particulates which provide a filter medium having a voids volume of from about 45 % to about 75 % . The second suspension could include coarser metal particulates which provide a filter medium having a voids volume of from about 70% to about 90%. The second layer, which in this example forms the inner diameter of the seamless filter, functions as a slag filter. Air bag filters having graded or uniform, but overall relatively high, voids volume and/or permeability are preferred in many applications. In other applications, however, a relatively low permeability and/or voids volume may be more advantageous. For example, air bag filters having uniform permeabilities in the range from about 5 to about 50 ft/min at a Δp of 0.5 inches of water may be preferred. A filter medium having a relatively low permeability can be produced by compressing a higher permeability medium or by using metal particulates having a particle size distribution which provides die lower permeability.

Increased pressure drop across die filter medium can be achieved by eitiier a uniform or graded permeability. Generally, if the filter medium has a graded permeability, the permeabdity values in both die high and low permeability regions are lower compared to filter media having an overall relatively high permeabdity.

While many of the preferred embodiments of the air bag filter are unitary filter elements, the air bag filter can be a multilayered strucmre. The multilayered strucmre is, in accordance wid another aspect of the invention, most preferably an integral structure, i.e., one in which distinct layers or structures are attached to one anodier in any suitable manner to form a unit. In addition to one or more layers or structures formed from the metal particulates of the invention, the multi-layered filter can include layers of wire wrap, metal mesh, ceramic and/or distinct layers of fibers.

An integral air bag filter 90 comprising both a final filter 91 and a slag filter 92 is shown Figure 8. The slag filter 91 and die final filter 92 comprise separate layers attached to one anodier in any suitable manner, for example, by welding. The slag filter 91 may be formed from a variety of materials including, for example, one or more layers of a wire mesh or a layer of sintered metal particulate preferably having a voids volume in die range from about 70% to about 90% . The final filter 91 is preferably formed from a sintered metal particulate similar to the filter medium 30 shown in Figure 4. An integral air bag filter combining both a final filter and a slag filter has many of die same advantages as the air bag filter 60 previously described with respect to Figure 7.

Figure 9 illustrates a filter medium 100 which includes a plurality of sintered studs or "standoffs" 102. The standoffs 102 may be formed together with the main body 104 of die medium to form a unitary strucmre, for example, in a lay down process as described earlier and as illustrated in Figure 6. Preferably, however, the standoffs 102 are affixed to a green or previously sintered main body 104 to form an integral filter medium which may subsequentiy be sintered. The standoffs of an integral filter medium may thus be formed as separate structures affixed to die main body of die filter medium in a separate step.

The standoffs may be formed by first preparing a stabilized suspension of metal particulates having a relatively high viscosity. The stabilized suspension preferably includes a carrier which comprises a liquid medium, preferably water, and a stabiliz¬ ing/binding agent. The stabilizing/binding agent is combined with d e liquid medium in an amount such as to provide a desirable concentration of agent in the liquid medium. The combination is mixed until dispersion of the stabilizing binding agent is obtained. A desired amount of metal particulate material is d en added and mixed to provide a uni¬ form dispersion or suspension of die metal particulates in the liquid medium. The stabi¬ lized suspension forming the standoffs may be applied to die surface of die main body widi a syringe or a pressurized apparatus to form standoffs, such as "dots", at prede¬ termined intervals. Alternatively, the stabilized suspension can be applied to form standoff structures other than dots. For example, the stabilized suspension can form a continuous or discontinuous bead or series of beads which may circumscribe the main body. The beads may define a straight line path coincident widi or at an angle to the axis of die main body or die beads may bs zig-zagged eitiier randomly or in a pattern.

The various stabilizing/binding agents discussed above with respect to the wet laydown process may be used to form the stabilized suspension of standoffs. Preferred stabilizing/binding agents include CARBOPOL^® 934 and CARBOPOL^® 941. For many applications it is preferred to add an additional component to set up the stabiliz- ing/binding agent. For example, with CARBOPOL^® 934, the addition of a neutralizing base, ammonium hydroxide, serves to neutralize the polyacrylic acid and increase the viscosity substantially, e.g. , to about 10,000 to 50,000 centipoise.

Alternatively, stabilizing/binding agents may be used without the addition of another component to set up the suspension. For example, CARBOPOL^® 941 may be used without the addition of a neutralizing base. Viscosities in the range of from about 200 to about 50,000 centipoise, preferably in the range of 200 to 10,000 centipoise, measured at the temperature of application, may be used.

The metal particulates which form the standoffs preferably comprise a metal powder or a combination of metal powder and fiber. Preferably, the metal particulates are finer than the metal particulates used to foπn die main filter medium and hence die standoffs may have a lower voids volume and/or permeability. In one embodiment, die standoffs may have a voids volume or permeability sufficiently low to significantly reduce or completely block gaseous flow dirough die standoffs.

The standoffs may be applied preferably along the outer surface of the main body and function in a manner similar to the drainage channels described above. Spacing of die standoffs can be in a regular or irregular pattern and may encompass all or part of the outer surface of die main body. For example, and as shown in Figure 9, die standoffs 102 can be applied at regular intervals of from 15° to 20° along the outer surface of a circular main body 104 in a single axial plane. Alternatively, die standoffs can be applied along d e axial lengdi of die filter medium at regular or irregular intervals or may form a regular or irregular pattern similar to the waffle pattern Ulustrated in Figure 6. While die main body of die filter medium is preferably cylindrical, other shapes can also be used. For example, the standoffs can be applied to a flat sheet which is subsequently formed into a cylindrical shape by welding or otherwise securing the ends.

After sintering, the dimensions of die filter medium may need to be varied slightly by compressing the main body of die medium or the standoffs or by removing material from die standoffs by, for example, filing, etc.

The dimensions of die individual standoffs may be me same or different and may vary widi the size of the filter medium and its intended use, for example, in high pressure or low pressure applications. Generally, as hoop stresses on the medium in- crease, the number of standoffs used may increase. Generally, the standoffs may having a height of about 0.1 inches or less and more preferably from about 0.06 to about 0.08 inches. In a highly preferred embodiment, the height is about 0.07 inches. The diameter of the standoffs may be a function of die desired standoff height and/or die size of die filter medium and may vary from about 0.05 to about 0.20 inches and preferably from about 0.10 to about 0.15 inches. In a highly preferred embodiment, the standoffs are about 0.125 inches.

In another embodiment, an integral air bag filter element can include a reinforce¬ ment layer in the form of a support strucmre to increase hoop strengtii in applications where hoop stress is unusually high. Preferably, the support strucmre may be one or more wires, and die wires may be wrapped around die filter medium by one or more turns and nested in die grooves. Preferably, the wires have a diameter ranging between about 810_/ιm to about 1600 m, and die ends of die wires are fixed to each odier, preferably by welding. In die case of a filter medium having a channel on its outer sur¬ face, the support structure may comprise wrapped wires. However, preferably the support strucmre comprises a "C" ring nested in die channel. The ends of die C ring may be fixed or they may be free. The C ring serves to support the filter medium and resist radial expansion of the filter medium during combustion.

Alternatively, one or more metal mesh layers may be provided along die outer surface, as well as die inner surface, of die filter medium. The mesh layers may function as a support structure, a drainage medium, or a prefilter. For example, a rela¬ tively open mesh may be provided along die outer surface of die filter medium, for example, between the outer surface of the filter medium and die inner surface of the housing. Gases, expanding radially dirough the filter medium easily pass edgewise dirough die open mesh to die ports of the housing, so neither the outer surface of the fil¬ ter medium nor the inner surface of the housing need have any channels formed in the surface. Further, a metal mesh may be provided along die upstream surface, e.g. , the inner surface, of the filter medium to serve as a prefilter. Large products of combustion would dien be retained by die metal mesh prefilter.

Air bag filters having graded voids volume and/or permeability are not restricted to unitary or integral metal particulate structures. The air bag filter may be a multi-layered element comprising layers of porous media in which the voids volume and /or permeability is graded in the radial, axial and/or circumferential direction. More par¬ ticularly, the air bag filter may comprise multi-layered metal mesh structures, fibrous mat strucήires, powder metal structures or any combination thereof. For example, a graded permeability filter may also be formed by adding a metal mesh layer to a sintered metal particulate filter or by forming the air bag filter entirely from one or more layers of a metal mesh. The metal mesh layer may be woven or knitted. In order to create the low permeability zone, the metal mesh layer may be woven with varying mesh counts. In addition, d e metal mesh layer may be compressed to create the low permeability zone.

The strucmre of the filter medium and processes for making an air bag filter may be varied and/or supplemented in a variety of ways. For example, a "seamed" cylindrical filter medium may be formed by producing a flat sheet of a porous metal medium having die non-linear fibers described above. The flat sheet may men be rolled into a cylinder and die adjoining edges of die rolled sheet may be joined to form a longimdinal seam. Seamed filters are less preferred, however, since me presence of the seam decreases filter area and may reduce filter strength. The particulate material of any of the above filter media may also be joined to one anodier to form a porous medium in any suitable manner other than sintering. For example, the metal particulates may be joined by brazing, resin bonding, or ceramic bonding. In addition, any of die above filter media may be treated in a variety of ways to enhance die ability of d e metal particulates to absorb combustion products from the gas flowing dirough the filter.

Claims

1. A porous metal filter medium for filtering a fluid comprising: a mass of metal fibers formed by a melt overflow casting process and worked in a hammermill to bend at least about 10% of die fibers into a non-linear shape, the fibers having a nominal diameter from about 30 to about 1500 microns and a nominal lengm of from about 100 microns to about 20 millimeters, said mass of fibers being substantially mechanically interlocked and sinter bonded to one anodier, defining voids between die fibers of die mass, die voids having nominal geometric pore sizes of from about 30 microns to about 250 microns and die mass of fibers having a voids volume in me range from about 35% to about 95%.

2. A porous metal filter medium for filtering a fluid comprising: a mass of metal fibers, at least about ten percent of which are non-linear the fibers having, in cross-section, a generally crescent shape defining an arc of less than or equal to about π radians, a nominal diameter of from about 30 to about 1500 microns and a nominal lengm of from about 100 microns to about 20 millimeters, said mass of fibers being substantially mechanically interlocked and sinter bonded to one anodier, defining voids between die fibers of die mass, die voids having nominal geometric pore sizes of from about 30 microns to about 250 microns and die mass having a voids volume of from about 35% to about 95%.

3. A porous metal filter medium comprising a mass of metal fibers (i) formed by a melt overflow casting process including (1) conveying a molten metal onto a rotating casting wheel by overflow from a container of die molten metal, (2) solidifying die metal on die wheel to form metal fibers, and (3) projecting die fibers from the rotating wheel onto a recovery surface and (ii) worked in a hammermill to bend at least a portion of die fibers into a non-linear shape; die fibers having a nominal diameter in me range from about 30 microns to about 1500 microns and a nominal lengdi from about 100 microns to about 20 millimeters, said mass of fibers being substantially mechanically interlocked and sinter bonded to one anodier, defining voids between the fibers, the voids having nominal geometric pore sizes in the range from about 30 microns to about 250 microns and die mass of fibers having a voids volume of from about 35 % to about 95 % .

4. A process for forming a porous metal filter medium comprising the steps of:

(a) arranging into a porous mass metal fibers (1) formed by a melt overflow casting process including conveying molten metal onto a rotating casting wheel by overflow from a container of die molten metal, solidifying the metal on the wheel to form metal fibers, and projecting the fibers from the rotating wheel onto a recovery surface and (2) worked in a hammermill to bend at least a portion of the fibers into a non-linear shape, the fibers having a nominal diameter of from about 30 microns to about 1500 microns and a nominal lengdi of from about 100 microns to about 20 millimeters;

(b) compressing die porous metal mass of step (a); and

(c) sintering the compressed mass of step (b) to provide a mass of substantially mechanically interlocked metal fibers sinter bonded to one anodier and having a voids volume in die range from about 35% to about 95%.

5. An air bag inflator comprising: a housing having one or more ports; and a filter disposed witiiin the inflator and positioned to permit a gas flow exiting said filter to enter said one or more ports, die filter including a porous, metal filter medium as claimed in claim 1, 2, or 3.

6. An air bag module comprising: a housing having one or more ports; an air bag communicating with die ports of the housing; and an air bag filter disposed within die housing and positioned to permit a gas flow exiting said filter to enter said one or more ports, said air bag filter having an outer surface, the outer surface including at least one drainage channel communicating with said one or more ports between die housing and die outer surface of the air bag filter.

7. An air bag module comprising: a housing having one or more ports; an air bag communicating with the ports of the housing; and an air bag filter disposed witi in the housing and positioned to permit a gas flow exiting said filter to enter said one or more ports, said air bag filter having at least first and second zones, die first zone having a lower permeability than the second zone, wherein the first zone of the air bag filter is disposed adjacent said one or more ports and e second zone of d e air bag filter is spaced from said one or more ports.

8. An air bag module comprising: a housing having one or more ports; an air bag communicating with the ports in the housing; and a unitary sintered metal filter disposed witiiin tiie housing and positioned to permit a gas flow exiting said filter to enter said one or more ports.

9. A method for inflating an air bag comprising: (a) providing a high pressure gas;

(b) passing the gas through a porous metal filter medium as claimed in claim 1, 2, or 3; and

(c) directing d e gas into die air bag.

10. A method for inflating an air bag comprising: (a) providing a high pressure gas;

(b) passing the gas through a filter having an outer surface, die outer surface including at least one drainage channel;

(c) directing die gas along the drainage channel and dirough a port; and

(d) directing die gas from die port into d e air bag.

11. A mediod for inflating an air bag comprising:

(a) providing a high pressure gas;

(b) passing the gas dirough a filter having at least first and second zones, die first zone having a lower permeabdity than the second zone, wherein passing die gas through the filter includes passing in parallel a first portion of the gas through the first zone and a second portion of die gas dirough the second zone; and (c) directing the gas into the air bag.

12. A method for inflating an air bag comprising: (a) providing a high pressure gas;

(b) passing the gas through a unitary, sintered metal filter; and

(c) directing the gas from the filter into the air bag.

13. An air bag inflator comprising: a housing having one or more ports; a means for supplying gas to an interior of said housing, said supply means also generating debris as a byproduct of supplying the gas; and an integral air bag filter disposed witiiin the housing and positioned to permit a flow of the supplied gas exiting said filter to enter said one or more ports, said air bag filter having at least first and second zones, die first zone having a voids volume higher than the second zone, wherein die first zone comprises a slag filter for capturing the debris and disposed in die gas flow patii between the gas supply means and die second zone, and wherein the second zone is disposed in d e gas flow path between the first zone and die ports of the housing.

14. An air bag module comprising: a housing having one or more ports; an air bag communicating wid d e ports of the housing; and an integral air bag filter disposed within die housing and positioned to permit a gas flow exiting said filter to enter said one or more ports, said air bag filter having an outer surface, the outer surface including a plurality of standoffs positioned at spaced intervals and forming flow channels communicating widi said one or more ports between the housing and d e outer surface of the air bag filter.

15. A method for inflating an air bag comprising: (a) providing a high pressure gas; (b) passing the gas through an integral filter having an outer surface, the outer surface including a plurality of standoffs positioned at spaced intervals;

(c) directing die gas between and along die standoffs and dirough at least one port; and (d) directing the gas from the port into the air bag.