US 3907605 A
A flame barrier vent filter for a battery cap is disclosed. The filter has an internal domed shape configuration which makes the cross-sectional area in the center of the filter thinner than that of the edges so as to tend to confine any flame that results from ignition of vented gases to the center of the filter. The filter is made of a material having a low heat conductivity such as polyvinyl flouride which resists melting of the cap if a flame does break out. The filter may be locked into place by an annular bead formed in the cap, or it may be spun weld to the cap by applying torque and pressure to the filter. In order to form a seal between the filter and the cap on the spin welded embodiment, a bead, preferably of a triangular cross-sectional shape, is formed on the cap so as to melt when it engages the spinning filter.
Description (OCR text may contain errors)
United States Patent [m Melone  Sept. 23, I975 l BATTERY Al Wl'lll FLAME BARRIER VIIN'I FILTER |75| Inventor: Robert Richard Melone, [)es llaines, Ill.
Illinois Tool Works Inc., (hicago Ill.
 Filed: Mar. [8, I974 [Zl I Appl. No.'. 452,l72
4/ I968 Japan 13(1/l7) 14,800 3/[950 (iermany .7 l3o/l77 lrimury Iiuunim'r --l)onald l.. Walton AHUI'IH), Agent, or Firm Robert W. Heart; (ilenn W. Bowen  ABSTRACT A flame barrier vent lilter for a battery cap is diselosed. The filter has an internal domed shape contiguration which makes the cross-sectional area in the center of the filter thinner than that ol the edges so as to tend to confine any flame that results from ignition of vented gases to the center of the filter. The filter is made of a material having a low heat conductivity such as polyvinyl llouride which resists melting olthe cap if a flame does break out. The filter may be locked into place by an annular bead formed in the cap, or it may he spun weld to the cap by applying torque and pressure to the filter. In order to form a seal between the filter and the cap on the spin welded embodiment, a bead, preferably ol a triangular crossseetional shape, is formed on the cap so as to melt when it engages the spinning filter.
5 Claims, 4 Drawing Figures US Patent Sept. 23,1975 3,907,605
BATTERY CAP WITH FLAIVIE BARRIER VENT FILTER BACKGROUND OF THE INVENTION vent, there is a danger that a'spark in the vicinity of the battery will ignite the gas and cause the flameto spread inside the'batt'erythereby causing an explosion. Porous materials have been used as filters in the vent openings of the battery caps to act a flame barrier which confines the flame to the outside of the battery should a spark ignite the gas as long as the cap remains intact.
Silicon carbide has probably been the ohiefmaterial used for such a purpose. This material, however, is quite hard and tends to fracture very easily. In addition, silicon carbide is very hard to work with because of its hardness which requires special handling techniques that create a number of problems in forming the filters. Silicon carbide also is a good heat conductor; and when a flame has been ignited on the outside of the battery, the heat from the flame tends to spread reapidly to the outer edges of the filter so as to cause the battery cap to melt after several seconds. Once the battery cap melts, a danger of explosion again exists. The prevention of the melting of the cap may provide enough time for the flame to be extinguished; and it is, therefore, an important consideration in the use of such caps.
It is, therefore, an object of the present invention to provide a vent filter for a battery cap which is shaped so that if a flame is ignited on the outside of the battery,
the flame will tend to be restricted to the central area' of the battery cap.
It is a further object of the present invention to provide a battery cap having a vent filter of a flame retardent, low heat conductivity material such as polyvinyl fluoride which, if desired, may be secured in the cap by spin welding so that a bead formed in the cap is melted by heat generated by the spinning vent filter thereby forming a seal between the cap and the filter.
Other objects and advantages of the present invention will be apparent from the following description and accompanying drawings.
DESCRIPTION OF THE DRAWINGS TECHNICAL DESCRIPTION OF THE INVENTION Referring to the drawings there is shown in FIG. 1 a
battery cap 10 of a generally typical construction. The cap 10 is formed of a cylindrical skirt portion 12 which extends into the battery and a large diameter outer cylindrical portion 14 serves to close the battery cap opening. The interior of the inner portion 12 leads to.
a centrally located aperture 16 in the partition wall 26 which is in communication in the interior of the battery. The top of the cap 10 .has a cylindrical upwardly open cavity 19, formed in it exteriorly of the wall 26 for receiving the vent filter, 20 which is also in communication with the aperture 16.
The filter 20 is shaped to have the smaller thickness at its central area 24 than atits outer area 22 so that the possibility of a flame beingv ignited at theouter edges of the filter will be minimized. This reduced wall thickness in the central area 24 is provided by an in- 'wardly concave domeshaped recess 18 which is also in communication with the aperture 16. The filter is pref erably formed of a heat retardent plastic so that it acts as a flame barrier if the gas being vented should be ignited. One suitable plastic is polyvinyl fluoride, but other suitable plastiematerials, including polyethylene, are suitable if satisfactory conventional flame retardents are added. Polyvinyl fluoride also has a low heat conductivity so that if a flame is ignited, heat conduc' tion to the plastic battery cap 10 will be retarded, and the additional time may be enough to allow for the extinquishment of the flame in a crucial situation.
FIG. 2 shows an alternate embodiment in which the same reference numerals as used in FIG. 1 are being used to indicate the same general parts but the numbers in FIG. 2 are designated by a prime mark. The only dif ference between the embodiments of FIGS. 1 and 2 being with respect to the manner in which the filters are secured into the cap 10 and 10'. In the embodiment of FIG. I, the upper surface of the wall 26 has a bead 28 preferably having a generally triangular cross-sectional shape. The purpose of the bead 28 is to allow for the filter 20 to be secured into the cap 10 by means of spin welding process illustrated in FIG. 4. When the filter 20 is spun at a high rate, the bead 28 melts and forms a seal for the filter 20 and the cap 10 upon solidification.
The manner in which the spin welding embodiment of the present invention may be achieved is illustrated in FIG. 4, in which the holding jig 30 has a lower cylindrical base 32 and an upper smaller diameter cylinder 34 with a plurality of slots 36 in it. The slots 36 receive the ribs 38 of the outer portion 14 of the cap 10 to hold the cap in place. The filter 20 is then inserted into the cap 10 and a spinning shaft 42 with a resilient spinning cushion 44 is brought into contact with the surface 48. The cushion 44 is preferably formed of a resistant material such as rubber and has a lower surface 46 which conforms substantially to the domed upper surface 48 of the filter 20. The filter 20 is thus caused to spin at a high rate of speed in the cap 10 which is held stationary by the jig 30. By spinning the filter 20 at a high rate of speed, the bead 28 melts, as shown in FIG. 1, and thereby forms a seal between the filter 20 and the cap 10.
It has been found that in a typical battery cap a rib having a triangular crosssectional shape with a base to apex dimension approximately 0.025 inches may be sealed when a rotational speed of 3,000 to 3,300 rpm of the shaft 42 is utilized and a vertical downward displacement of approximately 0.050 inches is employed. The vertical drive rate should be slow and uniform to allow frictional heat to generate to melt the plastic 0f the cap which is generally formed of polyethylene. The porosity of the filter 20 is preferably in the range of 200 microns or less.
While spin welding provides a highly satisfactory lilter and battery cap assembly, a cap and filter assembly may also be produced by merely providing the cap 10 with an annular bead 52 which mates with the filter 20 and when the filter 20 is press fit into the cap 10 so as to achieve a snap-fit of the filter 20 in the cap I".
The invention is claimed as follows:
I. A battery cap comprising a skirt portion and a transverse partition wall thereacross defining an outwardly open cavity and an interior portion of the cap, said partition wall having a substantially centrally located aperture thcrethrough providing communication between the cavity and the interior of the cap, and a filter secured in said cavity and having an outer surface exposed to the surrounding atmosphere and an inner surface provided with a centrally located inwardly concave dome-shaped recess in communication with the interior of the cap through said aperture, the interior concave upper surface portion ofsaid recess and the adjacent outer surface of the filter defining thcrebetween a reduced thickness wall portion in the central area of the filter of a substantially smaller thickness than in the surrounding area of the filtcr which is in contact with said cap.
2. A battery cap as set forth in claim I wherein said filter is made of polyvinyl fluoride 3. A battery cap as set forth in claim I wherein said filter is secured in said cavity by interengaged rib and groove means.
4. A battery cap as set forth in claim I wherein said cavity has initially formed rib means fused with the filter as the result of spin welding motion thcrcbetween to secure the filter in the cavity.
5. A battery cap as set forth in claim 4 wherein said filter is made of polyvinyl fluoride.