Spiral wrapped gas generator filter and method of production

Abstract: Gas generator filter (36) is formed by spirally winding a sequence of elongate elements onto a rotating mandrel (18). First perforate support material (12) is wound onto the mandrel and opposing edges of the perforate material are joined to form a cylindrical core. One or more filter layers each comprising a layer of filter media (22) overwound with a layer of gas permeable support screen material (24) are wound on to the core. The filter layers are held in place by one or two outer layers of gas permeable screen material, the outermost layer being connected to that immediately beneath it. Filter units (36) of the required length are cut from the filter (32) formed by this continuous process. The filter media (22) is ceramic filter paper. The screen materials are made of carbon steel, nickel-plated carbon steel or 300 series stainless steel.

Description: This invention relates generally to inflatable type modular occupant restraint systems for passenger vehicles or, as they are more commonly known, air bag restraint systems. Such a system may be installed in an automobile or other vehicle, at least in part on the steering wheel for the protection of the driver and also in the dashboard or passenger side instrument panel for passenger protection in the event of a collision. More particularly, this invention relates to an improved filter unit for use in passenger side air bag restraint systems and a method for manufacturing the filter unit. The air bag restraint system is located forward of the occupants and normally is in a deflated condition. In the event of a crash, however the air bag is inflated in a matter of 20 to 40 milliseconds by a gas produced by the burning of a gas generant material in an inflator. Common features of such inflators that are used for the inflation of air bag cushion restraint systems are the inclusion in a housing of a gas generant composition containing a pelletized alkali metal azide contained in a basket and a filter unit external to the basket to cool the gas positioned between the basket and gas discharge orifices, as defined by the housing. The gas generant is ignited by an igniter. The gas generant burns rapidly usually with a temperature of about 1200°C and develops pressure in the housing in excess of 13.8 MN/m 2 (2000 psia). Further, the gas that is generated contains combustion residue including fine molten particles of metal and/or reactant oxides. The pressure of combustion gases resulting from the rapid burning of the gas generant material causes gas to rush into the bag to effect a rapid rate of inflation thereof. The cooling and filtration of the gas to remove the fine particles for the safety and comfort of the passengers is of great importance. Filter units typically available in the art include alternating layers of screen and ceramic paper. For example, Paxton et al., U S Patent No. 4,998,751 (EP-A-449 506), on which the pre-characterising part of claim 1 is based, discloses a filter which includes two wraps of nickel coated carbon or stainless steel having 11.8 holes/cm (30 mesh), then a metal filter having for example 31.5 x 275.6 or 19.7 x 98.4 holes/cm (80 x 700 or 50 x 250 mesh) stainless steel or 15.8 x 70.9 holes/cm (40 x 180 mesh) nickel coated carbon steel, then a single wrap of ceramic filter paper 2mm (0.08 in) thick, followed by two wraps of stainless steel or nickel coated carbon steel filter having 11.8 holes/cm (30 mesh). A wrap of stainless steel or nickel coated carbon filter having 2.0 holes/cm (5 mesh) may be provided adjacent the wall of tubular portion to allow space around apertures therein for the free passage of generated gases. The filter units are, typically, manufactured by rolling the different filter materials circumferentially around the mandrel similar to rolling wrapping paper circumferentially around a core. For example, Cunningham, U.S. Patent No. 4,878,690, which patent is assigned to the assignee or the present invention, disclose a filter unit of this type of manufacture. The filter units are manufactured as single units by hand and with the aid of machine. The geometry of the filter unit is determined by the vehicle into which the filter unit will be installed. The individual filter elements for each filter unit are cut to size and are placed by hand on the mandrel of a automatic wrapping machine which wraps the filter and spot welds the outer layer. The filter unit then is removed from the wrapping machine by hand and the cycle is repeated for the next filter unit. The cutting of each piece of filter material and the hand placement of each piece of filter material on the wrapping machine significantly increases the time of manufacture, cycle time, for each filter unit. This manufacturing system is incompatible with the high volume production requirements necessary to meet the demands of the automotive industry. The consistent and economical manufacture of filter elements for passenger side gas generators which will efficiently cool and clean gas from the gas generant is of great concern. This must be done in light of the fact that the size of the passenger side generator, and therefore the filter element, is strongly dependant on the geometry of the automobile's interior and therefore varies significantly from automobile to automobile. The use of spiral wrapping is disclosed in Merrill et al., U.S. Patent 4,101,423 for microporous layer over a porous inner layer for filtering bacteria from fluids. Shevlin, U.S. Patent No. 4,398,931, discloses spiral wrapping a woven ceramic filter material over a rigid tubular cage for radially-inward filtration of hot industrial gases. The Shelvin invention is designed for a pressure drop of from 51 to 152 mm (2 to 6 ins) of water at a flow rate of 3.2 to 24.2 per minute per dm 2 of filter area. An object of this invention is to provide an improved method of manufacturing air bag inflator filter units which reduces the time required to manufacture a filter unit for a passenger side automotive air bag inflator. Further, an object of this invention is to provide an improved method of manufacturing air bag inflator filter units which provides for the rapid change in the length of the filter unit. These objectives have been achieved by a novel filter element according to claim 1 and a means for manufacturing the same. The filter element includes, a hollow cylindrical core open at both ends, at least one filtering layer, and an outer layer of gas permeable material which holds the filter layer in a cylindrical form about the hollow cylindrical core without the application of any external forces. The hollow cylindrical core is made by spirally winding a first gas permeable support material which has a first edge and a second edge. The material from which the first gas permeable support material is made is bendable such that the material will conform to and retain the shape of the form over which it is pressed. The second edge of the first gas permeable support material is joined to the first edge of the adjacent wrap such that the hollow cylindrical core retains a cylindrical form without need for additional clamping. The second edge may overlap the adjacent first edge of the second edge may abut the first edge. The filtering layer is spirally wound radially outward of the hollow cylindrical core. The filtering layer further comprises at least one layer of spirally wound filter media, such as a ceramic paper, and at least one layer of spirally wound second gas permeable support material such as a metal screen radially outward of the filter media. The second gas permeable support material having an inner surface and outer surface. The layers of filter media and gas permeable support material may contain one or more layers of the respective materials. Optionally, at least one layer of a third gas permeable material, such as a coarse mesh metal screen can be spirally wrapped radially outward of the hollow cylindrical core and radially inward of the filter media. The outer layer is spirally wound radially outward of the filter layer and contains at least one layer of spirally wound fourth gas permeable support material, such as metal screen having inner and outer surfaces, a third edge and a fourth edge, and an end portion. The inner surface of the most radially outward third edge and fourth edge are joined to the outer surface of the next most radially inward layer of the fourth gas permeable support material or to the outer surface of the radially most outward second gas permeable material of the filter layer by bonding means, such that the outer layer retains the outer layer and the filter layer in a cylindrical relation about the hollow cylindrical core without further structural support. The respective layers of filter material; filter media, first gas permeable support material, second gas permeable support material, third gas permeable support material, and fourth gas permeable support material are spirally wound in such a way that the seam of one layer does not coincide with the seams of the other layers of filter material. The overlap of individual filter materials at the seams of adjacent wraps should be kept to a minimum such that thickness of the filter is not increased without increasing the filtering capacity of the filter unit. A slight overlap is necessary so that no gaps are formed which permit the unfiltered flow of generated gas. According to the features of claim 11, the invention also comprises a method for continuous fabrication of filter units for use in automotive air bag inflators comprising spirally winding a first gas permeable support material, which material is flexible, around a rotating mandrel to form a hollow cylindrical core having internal and external surfaces, spirally winding at least a filter layer radially outward of the hollow cylindrical core, and finally spirally winding at least an outer layer radially outward of the filter layer. The first gas permeable support material has a first edge and a second edge. The first gas permeable sheer is spirally wound such that the first edge of each wrap abuts the second edge of the adjacent wrap or the wrap linearly downstream of the spiral wrapping process. The first edge of the first gas permeable material is joined to the second edge of the first gas permeable material of the adjacent spiral wrap by means such that the hollow cylindrical core retains a cylindrical form without the application of external forces. Next, at least one filter layer is spirally wound radially outward of the cylindrical core. Each filter layer comprises spirally winding at least one layer of filter media having a first edge and a second edge and inner and outer surfaces radially outward of the cylindrical core, and spirally winding at least one layer of a second gas permeable support material radially outward of the filter media. The second gas permeable support material has an inner and an outer surface and third and fourth edges. Optionally, at least one layer of a third gas permeable support material is spirally wound radially outward of the cylindrical core and radially inward of the first filter layer. Following the last filter layer an outer layer is spirally wound radially outward of the filter layer. The outer layer comprises at least one layer of a fourth gas permeable support material. The fourth gas permeable support material having inner and outer surfaces, fifth edges and sixth edges, and an end portion. The inner surface of the fifth edge and sixth edge are joined to the next most radially inward layer of the fourth gas permeable support material or to the outer surface of the radially most outward second gas permeable support material of the filter layer by joining means such that the outer layer retains the outer layer and the filter layer in a cylindrical relation about the core without further structural support. Thus, a continuous spiral wound filter unit is formed. Finally, the continuous spiral wound filter unit is cut transversely at intervals to form individual spiral wound filter units of the desired length. The invention will now be described by way of example only with reference to the following drawings in which like parts are designated by the same reference numbers, and in which: Fig. 1 is a front plan view illustrating the manufacturing of a spiral wrapped filter unit; Fig. 2 is a series of views showing the individual filter elements broken out of the filter unit; and Fig. 3 is an fragmented view illustrating the placement of the filter unit in a passenger side automotive air bag inflator. As best seen in Fig. 1, a method for the continuous fabrication of a passenger side air bag inflator filter is shown generally at 10 . A first gas permeable support material 12 having a first edge 14 and a second edge 16 , is placed on a rotating center mandrel 18 . The first edge 14 of the first gas permeable support material is joined to the second edge 16 of the adjacent wrap. The first gas permeable support material 12 forms a hollow cylindrical core 20 , as shown in Fig 2., which maintains its cylindrical shape without external clamping. Next, a layer of first filter media 22 is spirally wound radially outward of the first gas permeable support material 12 . Next, a layer of second gas permeable support material 24 is spirally wound radially outward of the first filter media 22 . Additional layers of filter media (not shown) and gas permeable support sheets (not shown) are added to build sufficient layers of and filter media and second gas permeable support material to produce a filter with the appropriate cooling and filtering capacity. Optionally, a third gas permeable support material 26 can be spirally wound radially outward of the first gas permeable support sheet 12 prior to the application of the first filter media 22 . An outer layer 28 of a fourth gas permeable support material is spirally wound around the filter media and gas permeable support material and is joined by spot welds 30 or the like, so as to retain the shape of the continuously fabricated falter unit 32 which is manufactured. The continuous fabricated filter unit 32 is advanced along the length of the mandrel by conventional means as the several wraps are wrapped about the mandrel. Each layer of filter material; first gas permeable support material, first filter media, second gas permeable support material, third gas permeable support material, and fourth gas permeable support material, are held on to the center mandrel 18 by individual compression rollers (not shown). These compression rollers can be mounted such that the axis of the compression rollers are perpendicular to the paths of the individual filter materials which they hold to the center mandrel 18 . The compression rollers are located at the point where the respective filter materials start to wind about the center mandrel 18 . The continuously fabricated filter is then cut to size by a rotary cutting blade 34 or the like to form individual filter units 36 of the desired length which maintain their shape without the application of external forces. The respective filtering materials are fed on to the center mandrel in such a way that the overlap of adjacent wraps is maintained at a minimum. Sufficient overlap is required such that no gaps form in the respective filter layers which would permit the unfiltered flow of generated gas. A preferred filter unit as manufactured by the method of this invention is shown in a series of views in Fig. 2. The hollow cylindrical core 20 is shown with spot welded seams 38 which join first edge 14 and second edge 16 of adjacent wraps of first gas permeable sheet 12 . The first gas permeable sheet can be made from coarse screening. More preferably the material from which the first gas permeable sheet is made is perforated sheet carbon steel. Stainless steel can be used, however, it is not required for good performance of the resulting filter unit. The perforations should be such that at least 25 per cent by area of the sheet is permeable to gas. More preferably the perforations should be such that from about 25 per cent to about 65 per cent of the sheet is permeable to gas in order to allow for the rapid dispersions of gas through the filter unit. The hollow cylindrical core 20 is spirally wrapped with an optional third gas permeable support material 26. Preferably at least two layer of third gas permeable material are used. This gas permeable support sheets can be made of any suitable high temperature conductive material such as metal screen made from carbon steel, nickel-plated carbon steel, stainless steel, or compressed non-woven material. Stainless steels which can be used with this invention include, but are not limited to the 300 series stainless steels, ie, 301, 304, 316 and 330. The third gas permeable support material 26 should preferably be of coarse mesh having from about 7.1 to about 11.8 holes/cm (18 to 30 mesh) (per Fed. Std. RR-W-360). The first filter media 22 is spirally wound about the hollow cylindrical core 20 and the option third gas permeable support sheet 26. The first filter media and all subsequent layers of filter media can be any high temperature ceramic filter papers available for use in air bag inflator filters. This ceramic filter paper is able to withstand temperatures up to about 1500°C and a volumetric flow rate of between from about 76 to about 570 m 3 /min/m 2 (250-1870 cfm/ft 2 ), with a permeability of between about 12 and about 61 m 3 /min/m 2 (40-200 cfm/ft 2 ) at a pressure drop of 12.7 mm (0.5 in) of water. Such filter medias are manufactured by Lydall of New Hampshire and Carborundum of New York. A second gas permeable support screen 24 is spirally wound radially outward of the first filter media 22. The second screen 22 like the optional third gas permeable support screen 26 can be made of carbon steel or stainless steel having about 7.1 to about 11.8 holes/cm (18 to 30 mesh). A coarser or finer mesh screen can be used depending on the amount of cooling and filtration required from the filter element being manufactured. Preferably, in order to support the ceramic filter media which normally is of low tensile strength the ceramic filter material and a layer of gas permeable support material can be combined after feeding from their respective roller and spirally wound around the center mandrel 18 simultaneously. The preferred filter unit is further constructed using second filter media 40 and fourth gas permeable support screen 42. The second filter media 40 and fourth gas permeable support material 42 are as described herein above. Multiple layers of ceramic filter media and screen may be used as needed by the ultimate requirement of the filter element. A third filter media 44 and a fifth gas permeable support material 46 are included in the preferred embodiment of this invention as shown in Fig 2. Also, each layer of screen and ceramic filter media may contain more than one wrap of the respective filter material. The last layer of filter screen, prior to the outer wrap should be of finer mesh, having for example 9.5 x 43.3 to 17.7 x 66.9 holes/cm (24 x 110 to 45 x 170 mesh), commonly known as Dutch woven, in order to more completely cool and filter the gas. A non-woven metallic or metallic/ceramic filter media may also be used. This is similar in nature to compressed steel wool. Finally, the outer layer 28 is formed by spirally winding at least one layer of fourth gas permeable support material, the outer layer the edges of which are spot welded 30 or fused to the underlying layer of gas permeable support so as to hold the continuously fabricated falter unit 32 in cylindrical form. To spot weld the outer layer 28 third edge 48 and fourth edge 50 , as shown in Fig. 1, of adjacent spiral may be overlapped slightly, approximately one half the length of the mesh opening, and the radially most outward layer of third screen can then be spot welded 30 . The overlapping of adjacent wraps is not preferred as this adds to the overall thickness of the filter unit 36 without adding to the filtering efficiency of the filter unit 36 . The inflator 52 as shown in Fig. 3 can be any of a number of known constructions, including the construction illustrated in U.S. Patent No. 4,296,084 to Schneiter. The inflator 52 includes generally an outer housing 54 into which is inserted the spiral wound filter unit 36 of this invention. An inner basket 56 can than be inserted radially inward of the filter unit 36 , however, with this invention such a basket may not be needed. An igniter 58 containing igniter granules and the appropriate ignition system is then inserted in to the center of the inflator 50 . Gas generant 60 is then loaded in to the inflator 50 which is then sealed in a conventional manner known to those in the art. Thus, in accordance with the invention, there has been provided an improved method for the continuous fabrication of a filter unit for use with automotive air bag inflators. There has also been provided an improve method for the fabrication of filter units for use in automotive air bags which reduces the time need to manufacture individual filter units. Additionally, there has been provided an improved method for the fabrication of filter units for use in automotive which reduces the cycle time in the manufacture of the filter units. Metric mesh sizes in the specification are given in holes per linear centimetre. Reference in the specification to non-metric mesh sizes refer to the number of holes per linear inch.

Claims: A wound gas generator filter element (36) comprising: a hollow cylindrical core having first and second open ends and internal and external surfaces, comprising a first gas permeable support material (12); at least one filter layer (22, 24) wound radially outward of said core, wherein each filter layer comprises at least one layer of wound first filter media (22), having an inner and outer surface, and at least one layer of wound second gas permeable support material (24); and an outer layer radially outward of said wound filter layer (22, 24) comprising at least one layer of wound third gas permeable support material (28) having inner and outer surfaces, characterised in that: said first gas permeable support material (12) constituting said core is bendable and spirally wound and has a first edge (14) and a second edge (16), forming adjacent wraps of said first gas permeable support material (12), wherein said first edge (14) of said first gas permeable support material (12) is joined to said second edge (16) of said first gas permeable support material of said adjacent wrap by joining means such that said hollow cylindrical core retains a cylindrical form without need for additional clamping; said at least one filter layer (22, 24) is spirally wound with said second gas permeable support material (24) outward of said first filter media (22); and said third gas permeable support material (28) constituting said outer layer is spirally wound to form adjacent wraps and has a third edge (48) and a fourth edge (50), and an end portion, wherein said third edge (48) and said fourth edge (50) of the most radially outward layer (28) of said third gas permeable support material is joined to the most radially outward gas permeable support material (24) by spot welding such that said outer layer (28) retains said outer layer and said filter layer (22, 24) in a cylindrical relation about said core (20) without further structural support. The gas generator filter element of claim 1 further comprising a fourth gas permeable support material (26) radially outward of said hollow core (20) and radially inward of said first filter media (22). The gas generator filter element of claims 1 or 2 characterised in that said first gas permeable support material (12) is in the form of a perforate sheet. The gas generator filter element of any preceding claim characterised in that any of said gas permeable support material (24, 26, 28) other than said first support material (12) comprises screen material. The gas generator filter element of any preceding claim characterised in that said core (20) is made from carbon steel or 300 series stainless steel. The gas generator filter element of any preceding claim characterised in that said second gas permeable support material (24) is made from carbon steel, nickel-plated carbon steel or 300 series stainless steel. The gas generator filter element of any preceding claim characterised in that said second gas permeable support material (24) is made from screen material having from about 7.09 to about 11.81 holes/cm (18 to 30 mesh). The gas generator filter element of any preceding claim characterised in that said second gas permeable support material (24) is made from screen material having 17.72 x 66.93 holes/cm (45 x 170 mesh). The gas generator filter element of any preceding claim characterised in that said first filter media (22) is made from a ceramic filter paper, wherein said filter paper is able to withstand temperatures up to about 1500°C and a volumetric flow rate of between from about 76 to about 570 m 3 /min/m 2 (250 - 1870 cfm/ft 2 ) with a permeability of between about 12 and about 61 m 3 /min/m 2 (40 - 200 cfm/ft 2 ) at a pressure drop of 12.7 mm (0.5 in) water. The gas generator filter element of claim 2 comprising a plurality of fourth gas permeable support materials (26) spirally wound radially outward of said hollow cylindrical core (20), three filter layers spirally wound radially outward of said fourth gas permeable material (26), wherein each of said filter layers comprises a spirally wound first filter media (22, 40, 44) and a spirally wound second gas permeable support material (24, 42, 46) radially outward of said first filter media, and an outer layer radially outward of said spirally wound filter layers, comprising two spirally wound third gas permeable support materials (28) forming adjacent wraps of said third gas permeable support materials, wherein said third gas permeable support material (28) having inner and outer surfaces, third (48) and fourth (50) edges, and an end portion, wherein said third edge (48) and said fourth edge (50) of the most radially outward layer of said third gas permeable support material (28) is joined to the next most radially outward gas permeable support material (28) by bonding means such that said outer layer retains said outer layer and said filter layers in a cylindrical relation about said core (20) without further structural support. A method for continuous fabrication of filter units (36) for use in automotive air bag inflators comprising: spirally winding a first gas permeable support material (12) of bendable material around a rotating mandrel (18) to form a hollow cylindrical core (20) having internal and external surfaces, said first gas permeable support material (12) having a first edge (14) and a second edge (16) forming adjacent spiral wraps of said first gas permeable support material (12) wherein said first edge (14) of said first gas permeable support material is joined to said second edge (16) of said first gas permeable support material of said adjacent spiral wrap by means such that said cylindrical core (20) retains a cylindrical form without the application of external forces; spirally winding at least one filter layer radially outward of said cylindrical core (20), wherein each filter layer comprises spirally winding at least one layer of first filter media (22) radially outward of said hollow cylindrical core (20) and spirally winding at least one layer of a first gas permeable support material (24) layer radially outward of said first filter media (22); spirally winding an outer layer radially outward of said filter layer comprising spirally winding at least one third gas permeable support material (28) forming adjacent spiral wraps of said third gas permeable support material (28), wherein said third gas permeable support material having inner and outer surfaces, third (48) and fourth (50) edges, and an end portion, wherein said third edge and said fourth edge of the most radially outward spiral wrap of said third gas permeable support material (28) is joined to the next most radially outward layer by means such that said outer layer retains said outer layer and said filter layer (22, 24) in a cylindrical relation about said cylindrical core without further structural support, thereby forming a continuous spirally wound filter unit (32); and transversely cutting said continuous spiral wound filter unit at intervals to form individual spiral wound filter units (36) of desired lengths. The method of claim 11 further comprising winding a fourth gas permeable support material (26) radially outward of said hollow core (20) and radially inward of said first filter media (22). The method of claims 11 or 12 characterised in that said first gas permeable support material (12) is in the form of a perforate sheet. The method of any of claims 11 to 13 characterised in that any of said gas permeable support materials other than said first support material comprises screen material. The method of any of claims 11 to 14 characterised in that said first and/or second gas permeable support material (12, 24) is made from carbon steel, nickel-plated carbon steel or series 300 stainless steel. The method of any of claims 11 to 15 characterised in that said second and/or third gas permeable support material (24, 28) is screen material having from about 7.09 to about 11.81 holes/cm (18 to 30 mesh). The method of any of claims 11 to 16 characterised in that said first filter media is made from a ceramic filter paper, wherein said filter paper is able to withstand temperatures up to about 1500°C and volumetric flow rate of between from about 76 to about 570 m 3 /min/m 2 (250 - 1870 cfm/ft 2 ) with a permeability of between about 12 and about 61 m 3 /min/m 2 (40 - 200 cfm/ft 2 ) at a pressure drop of 12.7 mm (0.5 in) water.

Inventors: RINK LINDA M, SMITH BRADLEY W, RINK, LINDA M., SMITH, BRADLEY W.
Applicants: MORTON INT INC, MORTON INTERNATIONAL, INC.
Application year: 1993
Main classification: B60R21
IPC codes: B60R21, B01D39, B01D46