US2980019A - Electric initiator - Google Patents

Description

April 18, 1961 e. A. NODDIN ELECTRIC INITIATOR Filed Sept. 9, 1957 RN Y mm MQ ND E0 '0 MN A E QM o W ELECTRIC INITIATOR George Adelbert Noddin, Sewell, N.J., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed Sept. 9, 1957, Ser. No. 682,768

' Claims. (1. 102-28) The present invention relates to a novel electric initiator. More particularly, the present invention relates to a novel electrically actuated nonviolent initiator stable at high temperatures.

In many applications of explosives, a nonviolent initiator, i.e. an initiator which is substantially noiseless in operation and produces no shattering or disruptive eifects, is highly desirable. For example, in the novel blasting method and arrangement described in detail in copending application Serial No. 618,217, filed October 25, 1956 and now abandoned by G. A. Noddin and assigned to the present assignee, explosive connecting cord, which isessentially noiseless in operation and comprises a core of from 0.1 to grains per foot of length of a high explosive within a metal sheath, is used as the above-surfaceexplosion conducting means, i.e. trunk line, in blasting operations. the conventional Primacord trunk line in order to decrease the noise produced by the operation, because when Primacord is used the noise it produces constitutes a nuisance which on occasion has provoked law suits orinjunctions which impede the operation. The trunk line of explosive connecting cord, however, has been initiated by conventional electric initiators which produce an annoying loud bang or snap, and the elimination or reduction of such noise is much desired.

In addition, in the novel seismic prospecting method described in detail in copending application Serial No.

618,509, filed October 26, 1956 and now abandoned by D. L. Coursen and N. G. Johnson and assigned to the present assignee, improved seismic records are obtained by use of a length of the explosive connecting cord to such an initiator would make its adoption very desirable in many other applications of explosives.

Accordingly, an object of the present invention is to provide a nonviolent electric. initiator. Another object of the present invention is to provide an easily assembled electric initiator characterized by substantially noiseless operation and the absence of shattering elfects. A further object of the present inventionis to provide a nonviolent initiator, suitable for use in numerous applica:

tions of explosives.

I have found that the foregoing objects may be achieved when I provide as an electric initiator a thinwalled metal tube in which is positioned axially a rigid cylnder in the bore of which is confined a small quantity of lead azide, said tube being sealed by an ignition assembly. A

In accordance with the present invention, an initiating charge, comprising 0.03 to 0.6 grain of lead azide disposed within the central bore. of a heavy-walled rigid cylinder whose outer to inner diameter ratio is 2 or more is inserted axially in an open'ended tube and an ignition assembly is sealed within one end of the tube.

The cord is used instead of conduct the initiating impulse from'the surface of the ventional electric initiator on the surface of the ground is used to actuate the cord, such an initiator produces interfering shock waves and is noisy in functioning. The

\ noise produced, aside from the effect on seismic recording instruments, is of little import when the exploration is being carried out in remote regions. However, in populated or industrialized sites, such a disturbance becomes an important problem to the seismic surveyors.

. Moreover, in the method of joining metallic elements by means of a wrapping of high explosives, which method is described in detail in US. Patent 2,367,206, a conventional initiator has been used to actuate the high explosive. Consequently, the violence and metal fragments of such initiators has caused undesirable metal-working and deformation in the area of the metallic elements adjacent the initiator.

Obviously, a need exists for a nonviolent initiator, substantially noiseless in operation and producing no shattering effects, in all of the afore-mentioned applications. Furthermore, the substantially noiseless action of V In order to describe more fully the present invention, reference is made to the accompanying drawings, in which: 1

Figures 1 to 3 are sectional elevations of the initiator I of the present invention, showing various modifications of the ignition assembly.

Figure 4 is a sectional view of a fuse or cord assembly incorporating the initiator of the present invention, which assembly is suitable for use, for example, in the previously mentioned seismic prospecting method.

In all figures, identical parts are indicated by the same symbols. 1 i In Figure l, 1 is a thin-walled tube of metal, plastic, etc., 2 is 'a heavy-walled rigid cylinder, preferably of lead, having a central bore, 3 is a mass of lead azide contained within the bore of cylinder 2, 4 is a high-resistance bridgewire, Sindicates the lead wires, 6 is a resilient sealing plug, for example of rubber, and 7 indicates circumferential crimps which maintain the sealing plug 6 and the cylinder 2 in proper position within the tube.

In Figure 2, 4 is a bridgewire surrounded by a coherent mass, or head, 8, of an ignition composition, and in Figure 3, 8' is a loose mass of an ignition composition which is in contact with the bridgewire 4'.

In Figure 4, the initiator as shown in Figure 2 is combined with a length of explosive connecting cord, a portion of which is indicated by 9. The cord is shown butted against cylinder 2 and held in place by a circumferential crimp 10.

In operation, the current is applied to the lead wires and'passes through the high-resistance bridgewire producing heat which actuates the lead azide contained'within the bore of the rigid cylinder. In this form of the ignition assembly, the so-called exploding" bridgewire assembly, which is shown in Figure 1, the passage of high current through the bridgewire volatilizes it and hot particles of metal are thrown against and ignite the lead azide, which, in this case, acts as a combined ignition-initiation composition. Alternatively,. an ignition composition, such as a boron-red lead mixture, may be positioned, in the form of a bead'or a loose ignition charge, around the bridgewire, as shown in Figures 2 and 3. The passage of low firing currents through the disposed as is shown in Figure3. In all cases,,.the

Patented Apr. 18, 1961 tiator assembly is nonviolent in functioning, which is clearly indicated by the failure of the initiator shell to rupture. The initiator. of the present invention may-be combined readily, as shown in Figure 4, with a length of detonatable explosive cord, for example detonating fuse, or Primacord, or the explosive connecting cord, to form a blasting initiator assembly or the like, the detonation of the lead azide within the cylinder causing initiation of the adjacent explosive cord.

The followlngexamples serve 'to illustrate specific embodiments of the initiator of the present invention. However, they will be understood to be illustrative only an not as limiting the invention in any manner. I

Figure 1 A number of initiators having the bead ignition system shownin Figure 2 were assembled, in which the weight of the lead azide within the cylinder and the length of the cylinder, which was made of lead, were varied as indicated in the following table. Lengths of the connecting cord were crimped in the initiators so that the cord was butted against the end of the heavy-walled cylinder, as shown in Figure 4. In the tests, the initator Iu testing this initiator, no connecting cord was used. The lead azide within the initiator detonated completely without rupture of the shell.

Example 2 A number of initiators having the exploding bridgewire ignition-system were assembled. Inassembling these initiators, the lead azide-containing lead cylinder was inserted in a metal capsule having a 0.005-inch-thick bottom, the end of the cylinder being flush against the bottom of the capsule. The capsule then was inserted in the open-ended tube, one end of which then was sealed by the ignition assembly. A length of Primacord was inserted and crimped in the other end of the shell, the

end of the Primacord being butted againstthe bottom of the capsule. The lead azide-containing cylinder was varied as shown in the following table, and the Primacord was initiated in all cases without rupture of the initiator shell.

TA L II Distribution of Length of 03 1- Total Wt. of PbN (grains/ inder (in) PbN in Cylft.) inder (grains) Exa ple 3 In another series of experiments, anumbjer of initiators wereassembled having a B-graincharge of loose 62/35/3 btsmuth/ selenium/ potassium chlorate ignition composiwere used, and in all cases the Primacord was initiated by the unit.

Distribution of Length of Cyl- Total Wt. of

PbNa rains} inder (in.) PhN in Cylt5 indcr (grains) Example 4 An arc-fired initiator was assembled by inserting a A -inch-long lead cylinder into the center portion of a -inch-long aluminum tube, the cylinder containing 0.6 grain of lead azide. A 1.5-grain charge of loose lead azide was placed on top of the cylinder, and then the ignition assembly was sealed into the tube, the assembly comprising a pair of lead wires maintained in the sealing plug so that there was a 0.010-inch gap between the bared ends of the lead wires, which ends were surrounded by the charge of loose lead azide. Upon application of an electric current, the loose lead azide charge ignited and initiated the lead azide in the cylinder.

As shown in the foregoing examples, a quantity as low as 0.03. grain of lead azide in the initiator of the present invention is suflicient to initiate a high explosive. Heretofore, initiating charges comprising about 5 grains of a high explosive such as lead azide were used in initiators to insure consistent initiation, this quantity of high explosive causing the undesirable violence. The initiator of this invention is basedupon the discovery that a column of lead azide within a rigid cylinder can be ignited by conventionalignition means even at a loading as low asabout 2.5 grains per foot and then when thecolumn is at least Ms inch in length, consistent initiation of detonating fuse or connecting cord can be obtained. Thus, as little as 0.03 grain of lead azide will produce consistent initiation of detonating fuse and connecting cord. However, quantities up to. 0.6 grain of lead azide centrally disposed in a rigid cylinder can be used without loss of the nonshattering and substantially noiseless characteristics desired, provided the lead azide column does not exceed a loading of about 60 grains per foot. .At higher column loadings, regardless of the length of the column, appreciable shattering effects and noise cannot be avoided. The length of the rigid cylinder. containing the lead azide is critical only in the mini mum requirement, i.e., at lengths less than inch, inadequate, priming action is obtained. However, no advantage results from using a cylinder of over /2 inch in length. I

In summation, the lead azide will preferably be present in a quantity of from 0.03 to 0.6 grain at a loading of from 2.5 to 60 grains per foot and centrally positioned in a rigid cylinder A3 to /2 inch in length. Inherent in the foregoing is the fact that the lead azide column is of the same length as the rigid cylinder. The lead azide contained within the cylinder may be admixed with anotherhigh explosive such as PETN or nitromannite.

Thecylinder is constructed of a rigid-materialwhich is nonreactive-with the lead azide. Although. lead has beenexemplified andflis preferred, other metals and their alloys, such as aluminum and steel, can be used. In

quiredrigidity are suitable.

The cylinder is positioned axially in an open-ended thin-walled tube of metal, plastic, etc. in igniting proximity to the ignition assembly. The dimensions of the tube are not critical, the outer diameter of the cylinder being adjusted in relation to the inner diameter of the tube and being so selected that the cylinder can be inserted into the tube and crimped in place without difficulty. When the initiator is to be combined with detonatable explosive cord, the diameter of the tube is selected accordingly. Tubes of about inch in length were exemplified, but shorter tubes which are of a length sufiicient to contain the cylinder and the ignition assembly and longer tubes, i.e. up to about 3 inches in length, are equally suitable. Of course, the tube wall must be of a thickness such that the ignition composition does not shatter it.

To protect the lead azide within the cylinder from water, a thin metal wall may be provided adjacent the cylinder, for example, by providing a capsule of thin metal which surrounds the sides and bottom of the cylinder as illustrated in Example 2. The thin bottom, which can be of a thickness in the order of 0.005 inch, does not interfere with the functioning of the initiator. As an alternative method of protecting the lead azide, a sealant can be applied to the end of the cylinder opposite the ignition assembly without interference with the functioning of the initiator.

A variety of electrical ignition assemblies can be sealed into the tube, for example by a resilient plug, such as a rubber plug, of conventional design, through which the customary lead wires are introduced into the initiator.

These ignition assemblies include a bridgewire and bead arrangement, a bridgewire inserted in a mass of loose ignition composition, the previously described "exploding bridgewire unit, and an arc-firing system in which the bridgewire is eliminated. .The exploding bridgewire. unit is especially advantageous because such a unit can be actuated only by application of very high firing currents. 'The specific ignition or burning composition used intheignition assembly is not critical to the initiator of the present invention, any ignition composition being 'prise a fuel-oxidant mixture, such as boron-red lead (Pb o silicon-red lead, manganese-bismuth trioxide, lead sulfocyanate-potassium chlorate-calcium silicide, bismuth-selenium-potassium chlorate, diazodinitrophenolpotassium chlorate, and thelike, or a single compound, such as lead styphnate, tetrazene, or a loose charge of lead azide. I

The lack of violence of the initiators is apparent. No noise aside from a slight popping sound, was produced by any of the initiators. Visual inspection of the initiators showed that no tube rupture occurred, indicating the absence of shattering eifects. This lack of violence presents an important safety feature. For example, when suitable for use. Such ignition compositions usually comsix of the initiators described in Example 2 were bunched together, actuation of the central initiator occurred without sympathetic actuation of the other five initiators.

The heat stability of the initiator of the present invention likewise was shown by a series of tests. In these tests, one group of the initiators prepared as described in Example 3 was subjected to heat-cycle storage under conditions more severe than those normally encountered in storage of initiators relative humidity and 65 F. to F.) for one month, and another group was maintained at F. for six weeks. All the initiators so treated actuated lengths of the connecting cord. The tests made to illustrate the initiating ability of the present initiator show that the initiator is capable of actuating both the explosive connecting cord, which contains about 0.1 to 10 grain of PETN or RDX per foot, and conventional detonating fuse such as Primacord," even when the cord or fuse is not positioned immediately adjacent the lead azide.

It will be obvious to those skilled in the art that many modifications in the design and use of the initiator of the present invention are possible without departure from the scope of the presentinvention. I intend, therefore, to be limited only by the following claims.

I claim:

l. An electric initiator of high-temperature stability which comprises a tube open at both ends, a thick-walledrigid cylinder inch to about /2 inch in length with an outer-to-inner diameter ratio of at least about 2.0 and having a central bore containing from'0.03 to 0.6 grain of lead azide at a distribution of from 2.5 to 60 grains per foot and positioned axially in said tube, and an electrical ignition assembly sealed within one "end of said tube in igniting relationship to said lead azide, said initiator being of low brisance and resulting in substantially no offensive noise upon initiation.

2. An initiator according to claim 1, wherein said ignition assembly comprises a pair of lead wires connected. at their inner ends by a bridgewire. I

3 An initiator according to claim 2, wherein said bridgewire is surrounded by a mass ofan ignition composition.

t 4. An initiator according to claim. 1, wherein said ignition assembly comprises a pair of lead wires, thev inner ends of which are in contact with a loose mass of an ignition composition.

5. An electric initiator as in claim 1 in combination with a length of detonatable explosive cord in said tube adjacent said rigid cylinder at the end remote from the electrical ignition assembly.

References Cited in the file of this patent UNITED STATES PATENTS Great Britain July 18, '1956

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