GB2191566A - Electrical igniter - Google Patents

Abstract

A construction for the electrical ignition of an ignitable material comprises an insulating pellet 5 having two separate conducting pins 1,3 embedded in and extending through the interior of the pellet, each pin extending at both ends outside the pellet, at least one ignition filament 7,9 forming a conducting bridge between the two pins outside the insulating pellet and a deposit of an ignitable material (11) coating the ignition filament. The deposit may be protected by an outer binder layer and contained within a rubber sleeve. <IMAGE>

Description

SPECIFICATION Constructions for the electrical ignition of an ignitable material The present invention relates two constructionsforthe electrical ignition of an ignitable material.

Constructions in which an ignitable material is ignited by the energy obtained from an electric current are used in several applications. For example, in an electrical detonator for a charge of a main explosive material such a construction comprises a fusehead employed to provide initiation of the detonation. The fusehead comprises a detonating material, comprising an explosive material, having a suitable conductor passing through it and an arrangementforapplying an electriccurrentthrough theconductorto ignitethe detonating explosive. The conductor, which is typically very thin to provide a suitable electrical resistance, is known as an ignition filament.

One known fusehead construction incorporates a sandwich having metallised outer layers formed on an insulating inner layer in a structure similarto a printed circuit board sandwich structure. An ignition filament is soldered between the upper and lower metallised layers of the sandwich and is coated with an explosive composition containing material by dipping in a slurry containing the explosive composition. External conducting leads are attached by soldering to the upper and lower metallised layers of the sandwich to enable an electric current two be passed through the ignition filament to ignite the explosive composition.

The known construction is relatively expensive to produce owing to the number of soldered joints required and the need to turn the sandwich over during the soldering sequence to permit soldered joints to be madeto the upper and lower metallised layers. The known construction is relatively unreliable owing to the number of soldered joints present. Its construction does not facilitate mechanical attachment to a conventional electrical circuit construction, e.g. a printed circuit board.

It is desirableto monitorthe amount of explosive containing material coated overthe ignitionfilamentand a convenientway of carrying this out is to weigh the construction. However, the weight of the assembly prior to dipping in the slurry can vary owing to the variable contribution to the weight of the soldered joints.

According to the present invention there is provided a construction for the electrical ignition of an ignitable material which comprises an insulating pellet having two separate conducting pins embedded in and extending through the interiorofthe pellet, each pin extending at both ends outside the pellet, at leastoneignition filament forming a conducting bridge between thetwo pins outside the insulating pellet and a depositofan ignitable material coating the ignition filament.

Preferably, the construction includes at leasttwo ignition filaments bridging the pins (for greater reliability), desirably located close together, e.g. from 0.5 to 1 .5mm apart, typically 1 mm apart.

The pellet may have any shape but is conveniently a shape approximating a spheroid orelipsoid ora flattened spheroid orcylinder, e.g. a button or disc shape, with the pins extending through thethinnest region oftheshape.

The pellet may be formed of any mouldable insulating material, e.g. glass our a suitable plastics material, e.g. polymethylmethacrylate, polyester, polyvinyl chloride, polyamide, polytetrafluoroethylene or polyurethane.

The pins, which are preferably stiff, may be made of any suitable conducting material, but are preferably made of a material similartothatofthe ignition filament(s). Forexample,the pins and the ignitionfilament(s) may be made of nickel alloy materials.

The pins preferably have a cross-sectional diameter of from 0.5mm to 1.5mm, typically 0.8mm.

The pins are preferably aligned substantially parallel to one another and preferably have a distance be tween their cross-sectional central axes of 1 to 5mm, typically 2.5mm.

The pins desirably have atone end flattened portions where the or each ignition filament bridges them to facilitate attachment of the ignition filament(s). Preferably, the flattened portions have a thickness of from 0.1 to 0.6mm, typically 0.4mm. The flattened portions may be parallel or inclined toward one another at their ends remote from the pellet.

The pins may be made ofa nickel/iron alloy,forexample, nickel/iron alloy having a nickel to iron weight ratio of 48:32 has proved convenient.

The or each ignition filament may be made of a nickel/chromium alloy, e.g. an alloy having a nickel to chromium weight ratio of 80:20 has proved convenient.

Preferably, the or each ignition filament has an electrical resistance in the range 0.1 to 100 ohms, e.g. 0.8to 2.0 ohms, depending on the material to be ignited. For a pin centre-to-centre spacing of 1 to 5mm the diame terofthe ignition filament is typically 0.005 to 0.1 mm, e.g. 0.01 to 0.4mm.

Desirably, each ignition filament is resistance welded in a known way to the pins to form a conducting bridge between them.

The ignitable material may be contained in a composition also comprising a binder, e.g. an organic binder.

Preferably, the ignitable material is coated as a globule over one end ofthe pins and the ignitionfila- ment(s). The globule may be formed by dipping the assembly comprising the pellet, pins and ignitionfilament(s) in a viscous solution of a solvent containing the binder,which together with the required ignitable material which is mixed therewith, forms a suitable coating slurry. Two or more dips may be required to form the required layers of ignitable material overthe pins and filament(s). Another material excited by ignition of the ignitable material may be coated in a similar manner, e.g. in a slurryformed with the same solvent/binder solution, overthe ignitable material.In any case, the globule formed may be given a final dip in a viscous binder/solvent solution containing no ignitable or other excitable material to form an outer protective coating. The required construction is obtained by allowing the solvent two dry and the binder to harden thus leaving the ignitable material (possibly coated with other excitable material) contained in a dry solid globule.

The viscous solvent composition may comprise polyvinyl alcohol, nitrocellulose or ethyl cellulose as binder.

For example it may comprise from 1 to 10% by weight of ethyl cellulose binder in a non-viscous solvent, e.g.

amyl alcohol oramyl acetate.

The ignitable material is chosen according to the particular application of the construction but may be one or more of the materials listed below.

The construction according to the present invention has the advantages that is is simpler and cheaperto make than the known construction (described above), it is more reliable and easierto use. For example, the pins, at the ends remote from the ignition filament(s), may if the pins are of appropriate length be plugged directly into a printed circuit board for rapid connection to electrical components assembled thereon.Altern- atively, the pins may be plugged into a known socket to which are attached flexible conducting or'flying leads'.

Since the ignition filament(s) and joints to the pins make a negligible contribution to the weight ofthe assembly comprising the pins, the pellet and the filament(s) the blank comprising the pins and the pelletcan conveniently be weighted prior to attachment of thefilament(s). Any blank not having a weight within a predetermined range may be rejected at this early stage in the production process. The appropriate size of the globule may then be subsequently measured by a weight determination.

The pellet provides a convenient abutmentfora protective sleeve, e.g. of rubber, which may be fitted over the ignitable material after production of the construction.

The pins may be embedded in the pellettoform a blank by known techniques, for example where the pellet is glassthefollowing known procedure may be used.

A revolving table carries a series of stations spaced apart at each of which a blank is formed. Each station comprises a jig which holds a pair of the pins spaced apart with the required separation between them. A glass ring is dropped over each pair of the pins. The table is rotated so that each station passes in turn adjacentto one or more heating jets. The rings aregraduallysoftened by the jets and are melted into shape (by conventional electronics valve producing techniques) to form the required pellets embedding the pins by the application of a gentle pressure. The pins and pellets are then left to cool and are subsequently removed from the jigs.

The ignitable material may comprise one or more of a number of possible materials many of which are known forthis purpose. Examples of known materials include lead mononitroresorcinol (LMNR), potassium chlorate, lead styphnate, LMNR/tetrazene mixtures, LMNR/potassium chlorate mixtures, potassium chlorate/ charcoal mixtures.

Depending on the application of use ofthe construction according to the present invention the ignitable material may, as mentioned above, optionally be coated with a material different from the ignitable material which is excitable by energisation from ignition of the ignitable material. For example, the coating may comprise a flash producing material, a primary explosive or shock producing material; or a gas producing material or a material producing a combination of two or more of flash, shock, and gas.

Examples of suitable known flash producing materials are potassium chlorate/charcoal mixtures, cerium/ red lead mixtures, boron/lead oxide mixtures and lead styphnate.

An example of a known shock producing material is lead azide.

An example of a known gas producing material is lead4,6-dinitroresorcinate.

Fuseheadswhich are constructions embodying the present invention may be used for initiation in the following applications: granular pyrotechnics, flares, safety fuses, tracers, smoke producing and incendiary charges, explosive charges.

It has been found that a composition comprising from 2 to 8%, especially 4to 5%, by weight of boron and amorphous lead oxide is especially suitable for providing both ignition and flashing in fuseheads which are constructions embodying the present invention.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure lisa front elevation of a partly formed electrical fusehead construction; and Figure2 is a front elevation of the construction shown in Figure 1 when completely formed.

Figure 3 is a front elevation of an alternative fusehead construction fitted in an electrical socket.

In the construction shown in Figure 1, stiff conducting pins 1,3 are flattened atone end to provide flats 1 a, 3a. The pins 1,3 are embedded in a button shaped pellet 5 of glass by one ofthe methods described above so that the ends ofthe pins having the flats 1 a, 3a project above the pellet and the opposite ends of the pins 1,3 project belowthe pellet 5. The pins 1,3 are set parallel to one anotherwithin the pellet 5.

Two parallel wire filaments 7,9 are resistance welded to the flats 1 a, 3a to form conducting bridges between them.

The assembly (or partly cpmpleted construction) comprising the pins 1, 3, the pellet 5 and the filament 7,9 as shown in Figure 1 is dipped at its end containing the flats 1 a, 3a twice in a slurry comprising an explosive composition comprising 5% by weight of boron and 95% by weight of amorphous lead oxide mixed with a 7% byweightethyl cellulose 95% by weight amyl alcohol solution the slurry comprising 50mg explosive composition per 10ml of ethyl cellulose/amyl alcohol solution. The dipped portion is then further dipped in the ethyl cellulose/amyi alcohol solution without the explosive composition to complete formation of a globule 11 aroundtheflats 1 a, 3a and the filaments 7,9 as shown in Figure 2.The maximum diameterofthe globule 11 is controlled to be not (significantly) greater than the diameter of the pellet 5. The globule 11 is allowed to dry and solidify.

When the globule isfullyformed a rubber sleeve (notshown in Figures 1 and 2) isfitted overthe globule and the ends of the sleeve are sealed, e.g. with a plastics plug, to provide protection of the globulefrom accidental ignition.

Suitable materials and dimensions for the components ofthefusehead construction shown in Figures 1 and 2 are as follows.

Material of the pins 1,3 : Ni/Fe alloy containing 48% by weight Ni Diameterofthepins1,3 : 0.8mm Centre-to-centre spacing : 2.5mm of the pins 1, 3 Thicknessoftheflats : 0.4mm 1a,3a Length oftheflats : 3.5mm 1a,3a Material of the pel let 5 :Glass ofthe composition name ME3supplied by Mullard Maximumdiameterofthe : 7mm pellet5 Thickness ofthe pellet5 : 2mm Separation distance : 3mm between the flat 1 a (3a) and the pellet 5 Material ofthefilaments : Ni/Cralloycontaining 80% by weight of Ni 7,9 Diameterofthefilaments : 0.04mm 7,9 Separation distance : 1 mm between the filaments 7,9 In an alternative embodiment (not shown) a mouldable plastics material, e.g. polymethylmethacrylate, may be used as the material for the insulating pellet 5 instead ofglass.

In operation an electric current of 0.8 Ampsfor a period of 10 msec is applied around the circuit comprising the pin 1,theflat 1a,thefilaments 7, 9,theflat3a and the pin 3 by a known pulse generating circuit(not shown). The heating effect of the current ignites the explosive composition contained in the globule 11 to provide the required effect.

Figure3 shows an alternativefusehead formed in a similarway but having a different shape of pellet Sand globule 11. In this case the pins 1,3 are inserted into a plastics socket 21 in which connection to them is made by conducting wires 23, 25 respectively fitted into the socket 21.

Claims (9)

1. A construction for the electrical ignition of an ignitable material which comprises an insulating pellet having two separate conducting pins embedded in and extending through the interior of the pellet, each pin extending at both ends outside the pellet, at least one ignition filament forming a conducting bridge between the two pins outsidethe insulating pellet and a deposit of an ignitable material coating the ignition filament.

2. A construction as claimed in claim 1 and wherein the pins are aiigned substantially parallel to one another.

3. A construction as claimed in claim 1 or claim 2 and wherein the ignition filament has been resistance welded to the pins.

4. A construction as claimed in claim 3 and which includes at least two ignition filaments arranged in electrical parallel.

5. A construction as claimed in any one of the preceding claims and wherein the deposit of the ignitable material is a globule which comprises also an organic binder.

6. A construction as claimed in claim 5 and wherein the globule contains also a material coating the ignitable material which is excited by ignition ofthe ignitable material.

7. A construction as claimed in claim 5 or claim 6 and wherein the globule comprises a protective outer layerformed of binder material.

8. A composition as claimed in claim 5 or claim 7 and wherein the ignitable material comprises a boron/ lead oxide mixture comprising from 1 to 10 per cent by weight of boron and 99 to 90 per cent by weight of amorphous lead oxide, and the binder contained in the said globule is ethyl cellulose.

9. A construction as claimed in claim 7 and which issubstantiallythe same as any one of the specific constructions described hereinbefore.