GB664583A - Blasting initiator - Google Patents

Abstract

664,583. Blasting fuzes. DU PONT DE NEMOURS & CO., E. 1. Nov. 14, 1949. [June 11, 1949] No. 29095/49. Class 9 (i). An electric blasting cap comprises a shell closed by a rubberlike holding the leg wires, the plug being made of a composition containing sufficient carbon black for the electrical resistivity of the plug to be not more than 3 x 10<SP>4</SP> ohms per c.c. The metal shell 1 contains a base charge 2 of compressed tetryl with a primer charge 3 of compressed lead azide within a cavity in the charge 2. An ignition charge 8 of lead styphnate mixed with smokeless powder in proportion 80-20 and grained with gum arabic is superposed on the primer charge. The shell is closed by a plug 4 moulded about leg-wires 5 connected by a resistance wire 7 so that on passing a current through the wire 5 the charge 8 is ignited. The leg-wires are free from insulation for a part of the length 6 within the plug, which is a polymerised chloroprene composition containing 75 parts by weight of carbon black per 100 parts of neoprene. This composition is of sufficiently low conductivity to allow the firing current to pass through the bridge wire while the conductivity is sufficient for static discharges of the order of 10,000 volts at 0.0003 mfd capacity to be grounded to the shell. In a modification the loose ignition charge may be replaced by a bead 9 of lead styphnate surrounding the bridge wire. In this case the plug has a carbon black content of 75 parts per 100 parts of rubber to increase the conductivity. In another initiation the base charge 2 is a 50-25-25 blend of smokeless powder, a double salt of lead nitrate with a lead salt of dinitro-orthocresol and potassium chlorate. The primer charge is red lead and boron. The plug comprises 50 parts by weight of carbon black with 100 parts of rubber. As additional protection against static discharge a body of dielectric material may be placed between the exposed portion of the leg and bridge wires and the shell wall. Fig. 4 shows a sleeve 11 of dielectric material extending from a wedge-shaped extension of the plug. The dielectric material may be a solid mass almost completely enclosing the ignition bead and exposed leg wires or it may be used as a coating on the exposed leg wires and exposed part of the bridge wire. The dielectric material may be a synthetic resin composition or a sulphur-base cement. Synthetic or natural rubbers may be used for the plug and the carbon black content is preferably between 30 and 100 parts. The carbon black may be acetylene black.