US2420201A - Blasting cap - Google Patents

Description

May 6, 1947.

F. R. SEAVEY BLASTING CAP Filed March 7, 1942 INYENTUR. FREDERICK R. SEAVEY ATTORNEY Patented May 6, 1947 BLAsTrNe CAP Frederick R. Seavey, Alton, Ill., assignor to Olin- Industries, Inc., a corporation of Delaware Application=March 7, 1942, Serial No. 433.69;

ThisinventiOn relates to explosion initiators, commonly referred to as blasting caps. of the character employed for initiating the explosion oi larger charges of explosives as in blasting operations. This application is in part a continuation of co-pending application, S. N. 276,967, filed June 2, 1939, now U. S. 2,415,045.

, Heretofore, the base charge in blasting caps, of the fuse, electric, or delay electric types, has been added in loose formand was subsequently pressed in. place within the shell.

A disadvantage of this method of forming a base charge is that an undesirable density,

"gradient results, the portion of the charge adjacent the press pin being of substantially higher density than that near the bottom of the cap. This is the reverse of the condition which would explosive used as the base charge and a reducmtion in density usually lowers both the rate of detonation and the brisance.

. A further important disadvantage in the prior procedure of pressing the base charge within the shell is that the magnm'um ,pressure has been limited to below a 'value at which theunsupported shells are bulged or weakened.

Attempts to improve the uniformityand effectiveness by pressing the base charge as a number of successive increments have been attended by a substantial increase in the number of operations in'the manufacturing process, and therefore in the cost, while failing to obviate all the other disadvantages.

The object of the present invention, generally stated, is to provide a blasting cap free from the disadvantages hereinbefore mentioned.

A further object of the invention is to provide a.

blasting cap having improved functioning characteristics.

blasting cap which maybe loaded by an improved and simplified method of assembly. Other objects 'will appear from the following description when read in connection with the accompanying drawings, in which:

present invention;

2 Claims. c1. 102-29) be mosteffective, becausean increase in density renders itmore diflicult to detonate the secondary" Another object of the invention is to provide; a,

. v i 2 l And Figure 4 is a similar view of a blasting cap embodying the principles of this invention.

In accordance with this invention, blasting caps are provided which lend themselves to an improved and simplified method of assembly and in which the detonating effect may be markedly accentuated. These and other advantages are secured by the provision of a novel form of base charge in compound blasting caps having a main charge of relatively insensitive secondary explosive and a superposed initiating charge of primaryexplosive.

r The base or secondary explosive charge is provided, in accordance with this invention, as pre formed self-sustaining tabletsfpreferably with the bottom tablet having a'hollow portion adiacent the base of the shell. Such tablets can be prepared having high density, uniform weight,

and accurate dimensions at high rates of production by means of well-known automatic tabletforming machinery, from powdered or granular secondaryexplosives such as tetryLpentaerythritol tetranitrate, cyclotrimethylene trinitramine, picric acid,trinitrotoluene, or mixtures thereof,which are useful for base charges of. blasting caps. Likewise, the preformed base charge may at times be convenientlyprepared at high'density by casting the liquefied explosive in suitable molds.

When the tablet are preformed by compressing the explosive in a suitable die, the height of each Thus, while preformed tablets of tetryl can readily be prepared having a substantially uniform density of 1.60 to 1.65or higher, the maximum average density of the same granular tetryl compressed in place in a'bla sting cap shell is about 1.40 to 1.50 (compressed at 4,000 to 6,000 lbs/sq. in.).

The preforming of the explosive base charge tors, means are provided for the utilization of the Munroe effect to an extent heretofore impossible in blasting caps. The small cavity, heretofore commonly provided by indenting blasting caps at the base, has been external and tended to bea come filled, thus to have its potential effectiveness impaired, on insertion of the cap into the explosivecharge intended to be detonated thereby. I

In the illustrative embodiment of this invention shown in Figure 1, the blasting cap comprises a cylindrical housing or shell I, molded of suitable organic plastic such as an ethyl cellulose composition, the base being substantially thinner than the side wall.

The base charge consists of '7 grains by weight of tetryl inserted in the cap asa series of three preformed tablets 0.16 inch high and 0.22 inch in diameter, prepared by compressing tetryl, mixed with a small percentage of graphite if desired, in a suitable die so as to result in a final density of about 1.60 for the explosive, and then firmly seated together in the shell I. The bottom tablet 2, contacting the base of shell I, is provided having an axial opening 0.10 inch'in diameter and weighs 2 grains, while the two solid upper tablets 3., weigh 2.5 grains each.

The initiating charge 4 consists of 4 grains of mercury fuiminate compressed so as to fill a copper ferrule 5, open at the bottom and enclosed at the top excepting for a central inch opening.

The cap is sealed by means of plug 1, of organic plastic, carrying lead wires 9, which are electrically connected through the bridge wire 8. The

latter and the surrounding ignition composition tare disposedwithin a cavity in the base of plug 1. A spacing device such as washer ll, desirably made of resilient material such as cotton linters paper, may be interposed for insuring proper location and spacing of the components.

In the embodiment shown in Figure 2, a fuse blasting cap is provided in which the shell I is formed of a suitable metal such as copper, gliding metal, or aluminum. The base charge consists of preformed tablets of secondaryexplosive, the bottom tablet 2 having ahollow portion adjacent theshell base, while the upper tablets 3 are in the form ofsoiid cylinders. A compressed initiating charge 4, such as mercury fuiminate, lead azide, diazodinitrophenol, hexanitromannite, or a suitable mixture, is disposed within metal ferrule 5.

Figure 3 illustrates a further embodiment in which the biasing cap shell I of suitable organic plastic is loaded with a base charge consisting of preformed tablets of secondary explosive, both the top and bottom of the charge being cavitated. Thus, the tablets may be formed by compressing particles of tetryl, coated with up to 2.5% graphite, to suitable dimensions and at any desired controlled density. The cavitated tablets may consist of cylinders of the explosive provided with acentral perforation 0.10 inch in diameter. The upper tablets [2 are advantageous as housing for the initiating charge 4 in that, unlike metal or other inert ferrules, the tablets l2 enter into and contribute to the explosive action. Adequate confinement of the initiating charge 4 is provded for proper functioning, while greater protection against accidental detonation by shock is presented than by the metal ferrules heretofore in use. Another safety feature is that on explosion of the cap, the ferrule of explosive becomes converted into gaseous decomposition products. Tablets 12 are preferably of somewhat lower density than the remainder of the base charge, being thereby rendered more readily detonatable by the explosion of the initiating charge. Thus, in a preferred example, the -tetryl base charge may consist of two tablets l2 having a density of about 1.50 (each tablet weighing 1.8 grains) and the remainder of the charge having a density of about 1.60 (two tablets 3 weighing 2.25 grains each and tablet 2 weighing 2.0 grains).

The shell I is sealed by organic plastic plug 7, carrying a pair of electrical lead wires 9 molded in place therein. The interior terminals of the lead wires 9 are bent in parallel relation at the base of plug 1 and are electrically connected through bridge wire 8, attached across the parallel portions of the lead wires, as disclosed and claimed in co-pending application S. N. 329,650,

filed April 15, 1940, now U. S. 2,382,921.

A suitable ignition composition 6, which con-- tacts bridge wire 8 and which may consist of 0.6 grain of fine mercury fulminate, is interposed between plug 1 and the initiating charge 4.

The initiating charge 4 may consist of the required amount of lead azide, compressed within the cavity of tablets l2. Preferably, however, the initiating charge is added in loose form after the base charge has been seated within the shell'and is then caused to settle in position by tapping or vibrating the assembly.

Particularly effective results may be obtained by employing lead azide of high apparent density in the uncompressed state (settling to a density of at least 1.0 gram per cc. when tapped in a tube). For example, an excellent initiating charge consists of about 1.7 grains of the uncompressed crystalline material, containing 98.0 to

99.8% PbNs and having an apparent density of 1.5 to 2.5 g. per cc., which is produced by the gradual addition over about seventy minutes of 500 cc. of sodium azide solution (containing 26.8 g. NaNa per liter) to 575 cc. of lead nitrate solution (containing 13.3 grams of the salt per liter) maintained at about 40 C. and continuously agitated.

The manufacturing procedure involved in the production of the above-described blasting cap is characterized as an assembly operation from which the hazardous step of pressing an explosive charge, containing relatively sensitive initiato within a shell has been eliminated.

The effectiveness of the resulting blasting cap is indicated by average lead plate perforations of v 0.35 inch diameter when fired at room temperature and of 0.33 inch diameter when fired at liquid oxygen temperature.

Figure 4 shows an arrangement of base and initiating charge, similar to that of the above example, inserted in a, metal case blasting cap. For use as a fuse-ignited cap, the ignition composition 6 may be provided as a compressed pad, covered, if desired, by a retaining film l6, for example of nitrocellulose. For electrical ignition, a bridge-wire plug and sealing assembly may be provided, for example as in the electric blasting cap described in U. S. Patent 1,673,584. Furthermore, a delay element may be interposed in. accordance with the structure disclosed in U. S. Reissue Patent 19,661 or in U. S. Paten 2,133,119.

It will therefore be apparent that a preformed base charge in accordance with this invention is utilizable in all types of blasting caps, thus enabling the complete elimination of the step of compressing the base charge within the case. Also, the loading of the greatly elongated shells manner.

of ventless electric delay blasting caps may be facilitated thereby. Of further significance, however, is the fact that a base charge of greatly increased effectiveness may be secured in this The greater eifectiveness of preformed blasting cap charges in accordance with this invention as 1 compared with conventionally compressed charges is illustrated by the results, obtained by standard tests, listed in Table l. Series A caps were prepared a shown in Figure 2, having a base 8 charge of 6.5 grains of tetryl at a density of about 1.65 inserted in flat-base metal No. 6 blasting cap shells, 0.235 inch in diameter. The charge was loaded as a bottom tablet 0.22 inch in diameter,

0.16 inch high. and having a central axial perforation 0.10 inch in diameter, and two solid cylindrical tablets 3, 0.22 inch in diameter, and 0.14 inch high. Series B consisted of shells, identical with those in Series A except for the provision of a conventional indentation in the base, which were charged with 6.5 grains of granular tetryl, compressed within the shell, in accordance with the prior art practice, at agzpressure of 3300 lbs./sq.,in. Both series of caps were provided with a Ggrain initiating charge of mercury iulminate, compressed in ferrule 5.

Another comparative test involved caps similar to the above, except that the initiating charge consisted of 3.75 grains of diazodinitrophenol compressed into ferrule 5, a superposed ignition charge was used, and the tetryl base charge in Series B was pressed in the shells at 5000 lbs/sq.

in. When fired on lead plates, perforations were produced averaging 0.23 inch in diameter for the prior art type (Series B) and 0.38 inch for the caps (Series A) loaded in accordance with this invention.

Table II lists the results of another series of comparative tests which involved identical metal shells, loaded with a constant weight of base charge (10 grains of tetryl) and the same initiating charge grains of mercury fulminate). These results illustrate clearly the advantages of loading blasting caps in accordance with this invention, particularly the striking improvement due to the provision of an internal cavity of suitable height and width in the charge adjacent the bottom of the case.

The data of Table III illustrate the manner in which the optimum height of a cavity of given diameter at the bottom of the charge can be determined. The results listed were obtained with the use of blasting cap shells, plug and initiating charge as shown in Figure 1, the basechargebeing kept constant at 7 grains of tetryl in the form of preformed tablets? A cylindrical perforation having the indicated height was provided at the bottom of the charge. The data indicate that as the cavity increased in height,.the efiectiveness increasedrapidly between heights of 0.08 to 0.12 inch and .then rose gradually to a maximum.

Table III Height of Cavity (.10 in. diameter) Diameter of Lead Plate Perforation Inches 9. MHHQQQ Manon 99???? come:

As further illustration of the increased effectiveness obtained when the preformed explosive charge is provided with a proper cavity at the base, craters having volumes of 0.30 cc. and 0.50 cc. respectively were produced when caps of types 1 and 5 above were fired on inch thick lead plates.

Thedata of Table IV show the results obtained on varying the diameter of the central axial perforation 'of the bottom pellet. A diameter of about 0.10 inch yields excellent results and is preferred, since although, larger diameters are somewhat more effective, such tablets are more Table II Average Method of Loading Base Charge g g gg Perforation Inches Pressed in shell 0. 22

- Preformed tablets (all solid) 0.25 Preformed tablets (bottom tablet perforated) 0.36

difficult to manufacture and may be rather fragile.

Table IV Diameter of Cavity (0.16 Diameter of in. high) in Lead Plate Bottom Perforation Tablet Inches Inches 0.0 0 22 0.04 0.24 0.06 0.28 0.075 0.36 0.10 0.37 0.12 0.38 0.15 0.39

Table V shows the effect of variations in the height of an 0.075 inch diameter'cylindrlcalperforation at the bottom of the charge of preformed explosive tablets.

Table V Height of ew tiiiiiti ggg fig i Perforation Inches Inches The foregoing data establish the fact that in order to obtain the most efiective concentration of the explosive force (which is believed to be accomplished through acceleration of the explosion gases), it is necessary to provide a cavity at the base of the explosive charge having dimensions of height and diameter each above a minimum; In the case of tetryl tablets, 0.22 inch in diameter and compresseg to a density of about 1.60, the optimumgcylindrical cavity is established as one at'least about 0.12 inch high and 0.075 inch in diameter. I

The fact that the volume of the cavity is not the controlling factor is shown by the obtainameter resulted. Furthermore, the use of a bottom tablet with a larger cavity (volume0.0019 cubic inches) in the form of a prolate spheroid 0.14 inch in diameter and 0.08 inch in height, yielded a lead plate perforation of 0.30 inch.

The objects of this invention are therefore accomplished by the provision of blasting cap base charges which are preformed as tablets prior to insertion in the cap, the bottom tablet preferably being provided with a cavity adjacent the inner surface of the shell base. The tablets are charge.

The results show far greater initiating strength for the blasting caps prepared in accordance with this invention.

It will be observed that this conclusion is paralleled by the results of the lead plate test; it has been found that such concordance is to be expected in tests involving similar blasting cap shells loaded with the same initiating and ignition charges and having base charges consisting of the same amount of a given secondary explosive.

The Trauzl block test involves the insertion of ablasting cap in a cylindrical axial cavity in an otherwise solid lead cylinder, firing the cap, andimeasuring the resulting expansion of the cavity.

Since variations in the specific embodiments which have been described may be made within the spirit and scope of this invention, the detail description is not to be considered as limitatlve 1 except in the light of the appended claims.

Having now described the invention, what is claimed as new and is desired to be covered by Letters Patentis: I

throughout the charge, the preferred condition is to provide tablets having density approaching the maximum at the base, and tablets of substantially lower density in the proximity of the initiating charge. The cavity in the bottom tablet is preferably of controlled dimensions of height andwidth so as to provide a desired concentration of the explosive force. While it is evident that no limitation exists to any particular form of cavity, a cylindrical perforation is preferred since such tablets are readily made in symmetrical form and can be easily controlled to produce the desired results.

The lead plate tests, referred to in the foregoing description, were carried out by placing the blasting cap under test in vertical position with its base in contact with the center of a lead plate,

' thicker lead plates, with the exception that the final measurement consisted in determining the volume of the resulting cavities.

The "insensitive dynamite test is a standard test which has been in common use for comparing the initiating strength of blasting caps. In this test, cartridges 1% inches in diameter and 4 inches long are charged with 75 grams of a mixture of ferric oxide and trinitrotoluene, in the form of powder passing through a 20 mesh screen, and compressed to a density of 0.93. The blasting cap under test is inserted in a cartridge and fired, a record being made of whether or not explosion of the TNT-F6203 mixture is secured. The data in Table I under this heading were obtained by loading a number of cartridges with a mixture containing 14% iron oxide, inserting then firing these caps in alternate succession.

1. A blasting cap having a shell, an explosive initiating charge therein, and a base charge of secondary explosive in contact with the initiating charge, the said base charge being composed of a series of preformed tablets of explosive wherein each tablet has a substantially uniform high density, the top and bottom tablets are perforated so as to have the form of annular rings, and at least one solid tablet is disposed between the said perforated tablets, the initiating charge being disposed within the top perforation of the said base charge.

2. A blasting cap having a shell, an explosive initiating charge therein, and a base charge of secondary explosive in contact with the initiating charge, the said base charge being composed of a series of preformed tablets of explosive wherein each tablet has a substantially uniform' high density, the top and bottom tablets are perforated so as to have the form of annular rings, and at least one solid tablet is disposed between the sai'd perforated tablets, the initiating charge being disposed within the top perforation of the said base charge and the top tablet surrounding said initiating charge being of substantially lower density than the remainder of the base charge.

FREDERICK R. SEAVEY.

REFERENCES CITED r file of this patent:

UNITED STATES PATENTS Number Name Date 1,516,009 Grant Nov. 18, 1924 1,950,019 Biazzi Mar. 6, 1934 905,336 Lheure Dec. 1, 1908 1,785,529 Pratt Dec. 16, 1930 356,064 France Jan. 11, 1887 2,275,569 Snelling Mar. 10, 1942. 1,713,838 Large May 21, 1929 1,010,430 Jandrier et al Dec. 5, 1911 392,922 Johnson et al Nov. 13, 1888 FOREIGN PATENTS Number Country Date 447,458 Ger. July 21, 1927 17,443 Brit. Dec. 13, 1915 310,048 Ger. Sept. 9, 1921 7,763 Ger. Oct. 22, 1879

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