Patent GB-747935-A (GB)
Title: Improvements in or relating to static resistant electric initiator
IPC: F42B3/18
Filed: 1954-05-12
Abstract: 747,935. Blasting detonators. HERCULES POWDER CO. May 12,1954 [Nov. 9, 1953], No. 13936/54. Class 9(1) A static resistant electric initiator comprises in combination a metallic shell, a semiconductive ignition plug disposed about bared portions of the lead wides within the shell, a bridge wire joining the terminals of the lead wires below the lower surface of the plug and a spacing body of dielectric material disposed about the lower portion of the plug between the plug and the shell wall; the portion of the plug above the spacing body being separated from the shell by a distance less than that between either lead wire and the shell in the locus of the ignition composition and the total resistance between the shell and plug being less above the spacing body than at any other point on the plug. The blasting cap, Fig. 1, comprises a metallic shell 1, base and priming charges 2, 3 respectively, a semi-conductive ignition plug 4; a cardboard sleeve 5, an ignition composition 9, an asphaltic sealing layer 10 and a sulphur layer 11 maintained in position by a bead 12. In a modification (Fig. 2, not shown) the ignition plug is cylindrical and projects into a spacing sleeve having a thickness equal to the width of the gap 6 between the plug and the shell. The ignition plugs of the above described embodiments may be recessed to receive the composition 9 (see Figs. 3 and 4 respectively. not shown). In a fourth modification, Fig. 5, the plug 50 is coated with a dielectric film 52 of nitrocellulose lacquer and a rubber sealing plug 53 is crimped to the shell. In a fourth modification of the device described with reference to Fig. 1 (see Fig. 6, not shown) a rubber sleeve is disposed in the gap 6 and the members 10, 11 replaced by a rubber sealing plug. In a sixth modification. Fig. 7, a delay fuze element 73 is disposed above the priming charge 3 and consists of a lead tube 74 filled with a barium peroxide and selenium mixture. Moreover, the ignition and sealing plug 70 has a semicunductive core 71 and a non-conductive rubber sheath 76; the core 71 consisting of a rubber-graphite composition. In a further modification, Fig. 8, a bead 9 of ignition composition is formed around the bridge wire 8 and a cardboard liner 81, is arranged as shown. The embodiments of Figs. 1-4 and 8 may incorporate a dielectric collar around the upper portion of the ignition plug. Alternatively, dielectric beads or spacing members may be employed or a dielectric spacing disc may be mounted on top of the ignition plug. Again, the dielectric sleeve disposed between the upper portion of the ignition plug and shell in Fig. 6 may be perforated. The bridge wires may be made from a nickel-chrome alloy and the lead wires may be of copper or iron and tinned, if so desired. The lead wires may be insulated with cotton servings, rubber or plastic material. The shells may be made of aluminium, brass, copper, ferrous metals or various alloys. The ignition plug may be made from aluminium powder or conductive carbon; the conductive particles being bonded together by a natural or synthetic wax, rubber, rubber-like material, resinous material or sulphur. Suitable dielectric materials are cardboard, paper, fibreboard, rubber, plastic material, sulphur, asphalt and synthetic resin. The base charge may be formed from pentaerythritol tetranitrate, cyclonite, tetryl, trinitrotoluene &c., and may be cast or pel. leted as well as pressed. The priming charge may be omitted provided a suitable base charge is used. Any primary explosive or mixture may be used such as diazodinitrophenol, diazodinitrophenol - potassium chlorate, lead azide, lead styphnate and mercury fulminate and any known ignition composition may be used such as finely divided diazodinitrophenol-chlorate mixture, fulminates, lead-or-tin-selenium mixtures &c. The ignition assemblies may employ cavity ignition plugs, match heads or loose ignition charges. The shell may be sealed by more than one sealing layer or a single seal such as a rubber sealing plug or a cast or moulded resin plug used. Part or all of the plug may be coated with a dielectric layer or film. Specification 709,142 is referred to.