US2773447A - Ventless delay electric initiators - Google Patents

Description

Dec. 11, 1956 c. R. L. HALL ETAL 2,773,447

VENTLESS DELAY ELECTRIC INITIATORS Filed Feb. 18. 1953 r INVENTORS CECIL ROBERT LOVETT HALL IDWAL OWEN LEWIS M MY ATTORNEYS United States Patent VENTLESS DELAY ELECTRIC INITIATORS.

Cecil R. L. Hall, Saltcoats, and Idwal 0. Lewis, Ardrossan, Scotland, assignors to Imperial Chemical Industries Limited, a corporation of Great Britain Application February 18, 1953, Serial No. 337,622 Claims priority, application Great Britain April 21, 1952 3 Claims. (Cl. 102-28) The present invention is concerned with improvements in or relating to delay electric initiators and is particularly concerned with ventless short delay electric detonators.

Hitherto the delay elements in ventless short delay electric detonators have usually comprised a length of rigid metal tubing into which a powdered delay composition that yields little or no gas or vapour as it is consumed has been introduced in increments each of which is highly compressed into said length of tubing before the next increment has been introduced. In this type of delay element this incremental loading is considered essential in order to give the composition the required mechanical strength and to ensure that the packing is of a uniform density along the whole length of the column so that a rigid tube of a given length loaded with the same fuse composition will always give the same delay period. The rigid metal tube so charged with the gasless delay composition is pushed into the tubular container of the detonator in which it must fit tightly so that no annular space is left by which flame can travel from the igniting charge to the initiating charge and so that it will be secure from any sliding motion in the free space. Ventless short delay electric detonators are also produced in which the delay element is produced by filling a lead tube with a delay composition, for example, a mixture of silicon and red lead, rolling or swaging the tube to a suitable diameter to give a good fit in the ventless container and cutting off the length required. Both these types of ventless short delay electric detonators, however, when adapted to give short delay intervals, for example, of the order of 25 or 50 milliseconds have a tendency to cause ignition of an inflammable gas mixture, for example, 9% methane/air mixture, when fired freely suspended therein.

The object of the present invention is to provide a series of ventless short delay electric detonators having delay intervals between successive delay numbers, for example, of the order of 25 or 50 milliseconds which, when the ventless container consists at least essentially of copper, have the advantage that single detonators of any such practical series when fired freely suspended in an inflame mable gas mixture, for example, 9% methane/air mix' It was recognised that if it were possible it would be} cheaper to make ventless short delay electric detonators having a short delay composition enclosed in a lead tube,

of substantially reduced thickness and it was also considered that by such a reduction it might be possible to reduce the incendiary properties of a ventless short delay electric detonator if the ventless container were to consist at least essentially of copper.

It was found that improved ventless short delay electric detonators suitable for short delay firing can in fact be produced much more economically than those previously known by using as the delay element a length cut off from a fuse comprising a delay fuse composition enclosed in a lead sheath of such thickness that it is consumed or melts 2,773, 14? Patented Dec. 11, 1956 at substantially the same rate as that at which the flame front advances along the delay element on reaction of its delay composition and a covering for said lead sheath of a thickness which will give the delay element a total thickness substantially equal to the internal diameter of the ventless container. It was also found that such ventless short delay electric detonators have the additional advantage of not causing ignition of inflammable gas mixtures, for example a 9% methane/ air mixture, when fired freely suspended therein if the ventless containeris of copper or a copper alloy.

According to the present invention the ventless shortdelay electric detonator comprising an. electrically actuated igniting charge, an initiating charge and an interposed short delay element enclosed in a ventless container is characterised in that the sheath of said short delay element is of lead of such thinness as to be consumed or melted at substantially the same rate as that at which the flame front advances along the delay element on reaction of its delay composition and in that the sheath has a covering of a thickness to occupy the space between said sheath and container. Preferably the ventless container consists essentially of copper or of a copper alloy rich in copper.

Preferably the covering is friable and permeable to the.

small amount of gas evolved during the reactionof the so-called gasless delay composition and may consist for:

example of paper or textiles which if desired maybe provided with a coating of a thermoplastic resin.

The delay fuse composition for the core may consist of a conventional fast burning fuse composition for example a mixture of silicon and red lead in amount from 5 to 35% by weight of silicon and from to 65% by weight of red lead.

The so-called gasless delay compositions produce a little gas on reaction and when these are used in a conventional manner in a lead sheath which has comparatively thick walls which are not melted or consumed until some time after the flame front has passed, the pressure developed by the gaseous products of reaction is sufiicient to cause the hot slag remaining after reaction to be ejected forcibly as soon as the ventless container is disrupted. Any tendency for the hot slag to be forcibly ejected is substantially removed in the ventless short delay electric detonators of the present invention.

The lead sheath is preferably of a thickness less than the diameter of the core of the short delay element, for

example, the core may be 0.050 inch in diameter and the lead sheath may be 0.015 inch thick, whereas in the conventional lead fuse delay element the diameter of the fuse an internal diameter of 0.050 inch. The thin lead fuse thus produced is then covered so as to bring it to a size to fit the ventless container by for example wrapping it with paper or by spinning textiles around it and applying i a finish of, for instance, a thermoplastic resin.

It is also preferred to secure the delay element in posi-' tion by pressing an inverted perforated metal cup over the end thereof remote from the initiating composition instead of relying on a friction fit between the delayfelement and the wall of the ventless container. The useof such a cup also prevents the covering, if ignitible, from being. ignited by the igniting charge.

The delay element of the ventless short delay electric detonator of the invention can be readily inserted into i y the container without the application of so high a pressure as to damage the structure of the delay'ele'ment'because the soft and somewhat compressible nature of the covering permits thenecessary tight fit to 'beieasily attained.

Anexpansionspace should be left between the end of the delay element and the igniting charge. Furthermore a small charge of supplementary, preterably'ga'sless, igniting composition may be interposed'between the end of thedelay element'and the initiating charge. It is also. sometimesdesirable to press a small charge of gasless igniting composition directly on top of the delay element, for example, by pressing itinto the pr'forated metal clip prior to assembly in order to ensure the ignition "of the delay composition from the electrically actu ated igniting charge which is 'usually spaced from the delay 'element. The electrically actuated'igniting charge maybe an electric fus'ehead. r r I,

"one embodiment of the'invention is illustrated in the diagrammatic drawing accompanying the specification in which 1 is "copper detonator tub e,:2*is a detonating xplosivech'arge of tetryl compressed into the end of the tube, 3 is a more sensitive deton'atin'gcharge functioning as the initiating charge compressed on top of charge} and consists essentially of a mixture of aluminiu'm 'lead azide andlead styphnate, 4 ,is'a lea'd tube, Sis a textile covering, 6 is a polythene 'sheath'and 7 isa del'aycornposition. 8 and '9 are layers of gasless igniting compositionconsisting'ot silicona'nd red lead in the'weight ratio 30 to 70, 10 isan inverted perforated metal cup pressed over the compositio'n 9 andover the top of the delay element so as to secure, it longitudinally in the'tube and eifectively seal off the space above this cup from the composition 9, 1 1, is an electric fusehead, 12 is an axially perforated sealing plug of rubber-like material crimped between the copper tube 1 and the insulated electric'leadingfwires 13 and13 asshown at 14 whereby there 'is attainedfa seal capable of withstanding an'internalpressure greater than the maximum pressure that can beproduced as a result of the combustion of the contents of the "detonato'r tube with the exception of the detonating explosive charges 2 and 3. The space ISJbetwe'en the I end 'of the plug 12' and the'inverted cup 10 is sufiicient to ensure that the maximumpressure that can even temporari lybe'developedin the initiator by'the successive consumption of the compositions of the electric fusehead 11,

the igniting charge 9, the composition 7 of the delay element, t'o'getherwith any gases which may arise from char- -ring of its'cov ei'ings, and the igniting charge 8 will not 7 unseal thetube. I The. invention 'is also illustrated by the followingex amples'inwhich allpercentages are by weight.

The lead delay elements in Examples 1 to 7 have "an I externaldiameter of 0.08" and internal diameter of'0.05

These are coated with paper to a diameter to fit a commercial copper'detonator in'which the ,basechargeis tetryl and the upper charge 'is an aluminum/lead 'aiide/lead styphnate mixturewithout any supplementary igniting charges top andbo't'tom.

4 Table 2 Delay time in Delay composition milliseconds Length of delay element, mm.

Example Red lead, Mean Scatter percent;

Silicon, percent The assemblies used inExamples 8 and 9 are the 'same as those of Examples 1 to 7 except that the 'lead'fu'se instead of having a paper covering has a spun jute covering and an outer coat of polythene.

A comparison of a number of types of detonator for incendiveness is carried out by exploding singleidetona; tors suspended freely ma 9% methane/ air mixture using a delay composition as in Example land an "element jar Table 3 Ignitions per 10 tests Mean delay Delay element time Incrementally loaded tyme brass tube of internal diameter 0.130 Do Incrementally loaded type in 5 tube of ternal diameter 0.08 Lead fuse of internal diameter 0.170 and external diameter to fit detonator tube Lead fuse of internal diameter 0.050 and external diameter to fit detonator tubc Lead fuse of internal diameter 0.035 and external diameter to fit detonator tube Lead fuse-0f internal diameter 0.05 and external iliasmeter 0.08 packed with paper to fit detonator u e Lead fuseoi internal diameter 0.05 and external diameter 0.08 with spun jute covering and outer covering of polythene It will be seen from Table 3 that copper delay detonatoi's according to the invention are non-incendive whereas copper delay detonators accordingto the prior art are not.

What we claim is: r 7

1. A ve'ntless short delay electric detonator comprising an electrically actuated igniting charge, 'an initiating charge and an interposed short delay element comprising a core of red leadand silicon and a lead'sheath of a thickness less than the diameter ofthe core enclosed in a "ventless container consisting essentially of copper,

wherein thelead sheath :of said short delay elementis of such thinriss as to be melted at substantially the same rate asthat at which r, the flame front advances along the delay element on reaction of its core of delay composition 7 and wherein the said sheath has agas-permeable textile covering of afthickness to occupy the space between the said sheath and ventless container. I I v 2. A ventles s short delay electricdetonator as claimed claim 1 wherein the core is 0.050 inch in diameter and the lead sheathis 0.015'lI1Ch thick.

3. A ventlesslshort delay electric detonat-or as claimed inclaii'nl wherein the covering consists of textiles provided with a coating of a'thermoplastic resin.

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