US 2415045 A
Description (OCR text may contain errors)
' FREDERICK R. SEAVE) F. R. SEAVEY Filed June 2, 1939 f a J atente Jan. 28, i947 Frederick R. Sea'vey, Alton, BL, assignor to 9 Industries, Inc., East Alton, EIlL, a corporation of Delaware Application June 2, 1939, Serial No. 276,967
have generally consisted of an elongated metal plosion of the base charge, a layer of waterproofing compound, and a seal of sulfur for the open end of the metal cup. According to common practice, the electrical lead wires have been generally inserted in a sulfur plug and the bridge wire attached across the terminals of the same, prior to the assembly of the cap. The waterproofing layer has usually consisted of a pitch composition poured in place while in a heated i'luid state, and subsequently rendered relatively non-fluid by cooling to normal temperatures. The final sulfur seal has been usually applied by pouring in the molten material and solidifying by cooling, and is secured in position by the provision of head or embossments for that purpose near the opening of the metal casing.
(oi. lea-2st wire through the sensive explosive, susceptibility to sympathetic premature explosion following the explosion of a neighboring cap, and
the emission of a large number 'of high velocity metal fragments as a result of the explosion of the cap. in addition to the fact that such particles, constitute a source of danger, their presence is at times undesirable in the product, as, for example-in the mining of common salt, ceramic clays, and other products. Furthermore, heated particles of a metal like aluminum,
. which has heenin extensive use for the det- Numerous disadvantages have existed in the' manufacture and use of this type of electric detonator, and improvements have been par-- ticularly desirable with respect to increased safety in manufacture, handling, and use, greater certainty of functioning of the caps under adverse conditions, and simplification of the manufacturing operations. The waterproofing has not been'as thorough and efiicient as desired and its application has been troublesome and timeconsuming, with attendant hazard. The use of metal for'the casing has been considered essential, but the use of metal has been responsible discharge from the metal casing to the bridge onator housing, particularly in caps containing lead aside as the initiating explosive, may cause the explosion oi mine gases.
The object oi the present invention, generally stated, is to provide a detonator or blasting cap which will not be subject to the disadvantages hereinbefore mentioned.
A further object of the invention is to provide such a detonator, or blasting cap, ,constructed and arranged so as to avoid the use of metal casing, and the like, to simplify the process of manufacture, and improve the eihciency and certainty of operation of such devices.
Further objects of the invention are to provide a detonator, or blasting cap, of the type referred to which is safer to use and manufacture than those heretofore employed, which is less susceptible to premature explosion from accidental electrical discharges or mechanical shock, which can be assembled economically at normal room temperatures, which functions satisfactorily even under adverse conditions such as extremely low temperatures, and which is more compact than caps of the conventional type having the same initiating strength. 7
Another object of the invention is to provide such a. blasting cap, or detonator, constructed and arranged so as to admit of inspection of the component parts without disassembly.
Other objects will appear from the following description when read in connection with the accompanying drawing, in which:
Figure 1 is a longitudinal sectional view of an electric blasting cap illustrating one embodiment of the present invention.
Figure 2 is a similar view of another electric blasting cap illustrating another embodiment of the present invention.
" In accordance with tis invention, detonators are provided which not only possess the desired efficiency with respect to the initiation of the explosion of blasting compositions, butwhich at the same time may be manufactured and handied and used with a degree of safety heretofore unattainable. The results are attributable largely to the novel shells or casings which this invention contemplates as well as to certain novel features of manipulation and treatment.
The present invention especially contemplates that the shell, or casing, be formed of moldable organic plastic material, a great variety of which are suitable for the purpose. Especially adapted for the formation of such shells are thermosetting molding compositions based on resins of the phenol-formaldehyde, casein-formaldehyde, urea-formaldehyde, and phenol-furfur'al types, and thermoplastic molding compositions formu lated with cellulose esters, cellulose ethers, polymerized vinyl esters, polymerized esters of acrylic and substituted acrylic acids, polystyrene, chlorinated rubber, modified isomerized rubber, and the like. Each of the above mentioned materials is adapted to meet the condition of blasting can manufacture, and the thermoplastic materials are especially desirable, due to their susceptibility to gelatinization and re-gelatinization at normal atmospheric temperatures by the use of suitable volatile solvents and then resetting upon volatilization of the solvent without impairment of physical properties.
Molding compositions, containing suitable plasticizers, with or without fillers, may be formulated with these and similar materials so that under proper molding conditions a shell may be produced which has the desired physical properties. Shells molded of these compositions likewise have a desirable degree of waterproofness, absorbing less than 2% and the majority less than 0.5% of their weight after 24 hours immersion in water. With caps which are intended for use in initiating the explosion of dynamites of high nitroglycerin content, it is furthermore desirable that the plastic shell be resistant to the solvent or swelling action of this ingredient. Thermoplastics, such as cellulose acetate, which become softened and swollen on contact with nitroglycerin are less suited for such use than the other materials listed, although entirely suitable with explosives containing small percentages of nitroglycerin. The procedure used for preparing the shells may be according to the compression, transfer, or-injection method of molding.
It is of special advantage to employ organic plastic having high impact strength for the manufacture of the shell or casing because shells made of organic plastics having an impact strength of about 0.75 foot pounds or more (Izod or Charpy method as described in the Proceeding of the American Society for Testing Materials, vol. 34, part I, pp. 995-1000 (1934)) are decidedly safer than those of lower impact strength, Accordingly, to obtain full advantage of the safety features of the present invention, an organic plastic having an impact strength of at least 0.75 foot pounds should be employed. Among the organic plastics above mentioned, the following particularly commend themselves for use in accordance with the present invention because they are characterized by impact strength as indicated: ethyl cellulose, chlorinated rubber, modified isomerized rubber, and some varieties of polymerized vinyl resin and phenol-formaldehyde resins.
A further advantageous result is obtained when, in addition to the shell, the plug for carrying the lead and bridge wires is formed of organic plastic, but in this case the plastic need not have the high order of impact strength just mentioned. This not only affords a unit which is a vast improvement in strength and utility over the sulfur plug heretofore in use, but also permits the attainment of a simple waterproof seal of the assembled cap, with the elimination of the usual space-taking layers of pitch composition and sulfur which have previously been required. Such a waterproof seal may be ob tained by providing a molded plug having a close fit with the shell and by moistening the sealing surface with a suitable solvent or cement just prior to the assembly. The plug may be molded of any of the organic plastics mentioned above, thermoplastics being particularly suitable for the large scale production of such small parts by means of the injection molding technic. It is desirable to carry out the molding so that good adhesion is secured between the inserted metal lead wires and the molded plug. Likewise, the initiating or priming charge may advantageously be housed in a sleeve or ferrule of molded plastic.
Such plugs may be provided in any convenient shape so as to be best adapted for use with the particular shell desired, and may, for example, have the usual cylindrical form when intended for use within the shell, or other shapes according to the position in the shell and the method adopted for forming the seal. Improved strength and safety may thus be secured, as evidenced by comparative strength tests wherein plugs molded respectively of plasticized cellulose acetate composition and of polystyrene successfully withstood compressive forces of over 200 pounds while sulfur plugs were broken at pounds, In a series of drop tests, sulfur plugs failed to withstand a inch drop of a 4 ounce ball, while polystyrene and cellulose acetate plugs successfully withstood the blow of a 28 ounce ball dropping 2 inches and 5 inches respectively.
Referring to Figure 1 of the drawing for an illustrative embodiment of the invention, it will be observed that the detonator consists of a cylindrical housing or shell I of organic plastic, in the lower end of which is disposed a suitable base explosive charge, such as 2, 3. Above the base explosive charge a suitable initiating or priming charge, such as 4, may be placed. The charge 4 may be advantageously loaded in suitable carrying element, such as sleeve 5, which if desired may be cemented to shell i. Arranged in spaced relation above charge 4 is an ignition charge 6. The charge 6 may be conveniently arranged in a recess in the lower end of a closure plug 1, also made of organic plastic. As shown, the end wall of shell I, adjacent the base charge, is substantially thinner than the side walls thereof.
A bridge wire 8 is connected between a pair of lead wires 9 to be energized thereby, and actuate ignition charge 6. In the embodiment shown, bridge wire 8 is connected, as by soldering or weld" ing, to the ends of lead wires 9, so that the joints are substantially flush with the end of plug 1. The bridge wire is sufiiciently long that it may be bent into the recess in the end of plug 1, there to of its weight of a plasticizer or mixture of plasticizers such as dibutyl phthalate, triphenyl phosphate, chlorinated diphenyl, butyl stearate, and similar materials. If desii ed the housing may be transparent but coloring and filler materials such as zinc oxide may be added in addition to a small percentage of a. lubricant such as stearic acid.
The plug I may be likewise fabricated of the same composition although the plug may be made of a material different from that of the housing I. Polystyrene has been found especially useful as a plug material because of the ease with which good adhesion, and therefore a strong waterproof seal, of the inserted lead wires can be obtained. Polystyrene plugs are well adapted for use with shells of ethyl cellulose and materials of comparable solubility because of the ease with which the plug may be sealed in the shell.
In order to seal about lead wires .9, it is advantageous to mold the plug about the lead wires. An effective anchorage between the lead wires and the plug may be obtained by kinking those portions of the wires which are to be embedded in the plug i. In molding plug 4', the lead wires may be positioned in the mold and the plug iormed about them. As shown, the insulation on I the lead wires extends well into the body of the plug. If the bridge wire is to be attached to the lead wires by soldering or welding, the recess in the lower end of plug 1 for accommodating the ignition charge 6 may be sumciently large to partially expose the lead wires therewithin. After the plug has set, the ends of lead wires 0' if projecting beyond the inner end of plug 1!, may be cut off so as to leave the ends thereof flush with the end of plug l; and thereafter the bridge wire may be connected to the lead wires, the bridge wire being disposed wholly within the recess as shown. Of course if the bridge wire is to be connected to the lead wires by swaging in accordance with one common practice," the lead wires must be exposed either beyond the end of plug 1 or within the recess so as to permit'the manipulation. If desired, however, the lead wires may be disposed outwardly of the recess for accommodating the ignition charge It, and grooves formed in the inner end surface of plug i for accommodating and protecting the bridge wire and its joint with the lead wires.
The plug l is arranged to completely close and cfiectively seal the end of shell i. It is desirable to form plug i of dimensions such as to have a tight fit in the end of shell I. Suitable means such as shoulder it, may be desirable for facilitating the final location of plug l with reference to the other components of the completed detonator.
In order to further assure proper location and spacing of the components, especially charge t with respect to the ignition charge 6, a spacing device such as washer I i may be interposed. Such a washer may desirably be made of resilient material such as cotton linters paper.
in order to conveniently accomplish an eflec- If desired, or course, in any case, a suitable ceconnection with'the other features 'of the detonator herein disclosed, but in detonators at large. Such preformed pellets may be either hollow, or solid, or both, as shown. In the detonator shown in Figure 1, the base charge consists of seven grains by weight of tetryl inserted in the cap as three preformed tablets 0.16 inch high and 0.22 inch in diameter, prepared by compressing powdered tetryl, containing a small percentage of graphite if desired, in a suitable die so as to result in a final density of 1.60 for the explosive. The bottom tablet 2 is provided having an axial opening 0.10 inch in diameter, and weighs 2 grains, while the two uppertablets 3 weigh 2.5 grains each.
Sleeve 5 which carries the initiating charge a may advantageously be made of an organic plastic composition such as a plasticized cellulose acetate molding compound, but other materials may be used. The charge t may be constituted of any suitable initiating or priming composition but in the embodiment shown consists of about two grains of lead azide.
The charge 6 may be composed of any suitable ignition composition. In the embodiment shown the charge 6 consists of 0.25 grain of fine particles of mercury fulminate. The charge 0 may be retained in position in its recess and about bridge wire 8 by a film of a suitable lacquer such as one of cellulose acetate base. In order to avoid deadening the ignition mixture, care should be exercised to use a lacquer of such consistency that penetration will be avoided.
The further embodiment, shown in Figure 2, is similar to that of Figure 1, like reference characters designating corresponding parts. In this embodiment, the shell I is composed of a molded plasticized compositionof a polymerized vinyl resin such as a vinyl acetal (butyral) resin, while plug 1 consists of a molded plasticized cellulose acetate composition. For improved sealing, lead wires 9 may be coated with a, waterproof casein glue or similar composition prior to the molding operation. The initiating charge i consists of 4 grains of mercuryvfulminate compressed so as to fill a copper ferrule it, having wall thickness of 0.024 inch, open at the bottom, and enclosed at, the top excepting for a central inch opening.
The assembly is accomplished as described in the previous example, acetone being suitable for use as the solvent applied to the sealing surface of the plug just prior to the insertion of the.
latter in th loaded shell.
The marked advantages in safety over prior art caps ofthe electric blasting initiators prepared in accordance with this invention is demonstrated in the results of a series of tests, in which caps A and B, corresponding respectively to Figures 1 and 2, were tested in direct comparison with commercial No. 6 electric blasting caps of different makes, utilizing various combinations of base and priming explosives such as tetryl with lead azide, and a trinitrotoluenepicric acid mixture with mercury fulminate.
(1) Sympathetic detonation (a) Two caps were placed in a paper tube with the base of one facing the base of the second at a known distance of separation. With comaeraoee mercial caps having metal shells, the firing of the first cap caused the explosion of the second even when .the distance between the bases of the two caps was as much as twenty-four inches. With caps of both the- A and B types, the firing of the first cap did not cause the explosion of the second when the bases were only four inches apart.
(b) Two caps were supported in parallel positions at a known distance of separation. With commercial caps having metal shells, the firing of one cap caused the explosion of the second at a, distance of four inches, while with caps of the A and B types, the firing of one cap did not cause the firing of a second similar cap at a space of only one inch.
(2) Projectiles emitted (a) A typical commercial No. 6 electric detonator, having a metal shell, was suspended vertically along the axis of a twelve inch diameter cylinder formed of two thicknesses of 0.25 inch cardboard set within a similar cylinder of fifteen inch diameter. On firing the detonator, it was found that about three hundred perforations had appeared in the outer cylinder indicating the passage of that number of metal fragments through the four thicknesses of cardboard. With a .type A cap, fired under the same conditions, not a single perforation appeared in the outer cylinder, while only sixteen were produced on the firing of a type B cap.
(b) n firing a commercial No. 6 metal cap, it was found that its base had penetrated three and a quarter inches of pine board placed at right angles to the axis of the detonator and thirteen inches from the base. The firing of caps A and B did not even mar the exposed surface of a cardboard sheet similarly placed with respect to the detonator.
(3) Susceptibility to accidental electrical,
discharge i ondary was raised to 25,000 volts.
(4) Sensitivity to mechanical work:
A three pound weight was allowed to fall through a distance of three feet on the top of a cap placed upright on a heavy metal anvil.
Commercial No. 6 detonators were found either to explode under this treatment or to be rendered unfit for subsequent use as by short circuiting of the-firing circuit; subsequent application of the firing voltage across the bridge wire leads failed to produce explosion of such caps as survived the test. Plastic-shell caps of types A'and B did not explode under the conditions of this test and fired satisfactorily on subsequent application of the firing voltage across the bridge wire leads.
Each of the above tests substantiates the fact that detonators made with shells of a molded but also that the increased margin of safety which is obtained provides entirely new standards and criteria of safety in the art. The im- 4 proved resistance to sympathetic detonation, as
evidenced by the decrease from more than 24 to 4 inches in permissible longitudinal separation of two caps and from more than 4 inches to 1 inch in permissible parallel separation, obviously provides increased safety in the manufacture and handling of the caps, but is of greatest significance in diminishing the chances for the occurrence of mass explosions during transportation and in the field.
The elimination of high velocity fragments projected from an exploding cap is an important factor in reducing the danger from accidental explosions of caps. The molded plasticshells of the present invention possess the unprecedented property of providing suitable housing for the explosive charge, while at the same time having the tendency, on the explosion of the cap, to shatter into fine and light particles which remain within a close radius of the site of explosion. Furthermore, it appears that the molded plastic shells, especially those made of material having an impact strength of at least 0.75 foot pounds, provide a cushioning action, which furnishes protection against mechanical and explosion-wave shocks.
The safety from electrical discharge between the shell and bridge wire, as evidenced by the results obtained under test (3) above, is such as to remove this type of occurrence from practical consideration as a potential source of danger. Furthermore, the test results indicate the existence of a definite hazard in this respect with the prior metal-encased caps, in view of the fact that it is common practice in the field to maintain the lead wires shortcircuited before use, in order to minimize chances for the passage of a current through the bridge wire, and thus, contact of the metal case with-a stray'current or sources of electromotive force may cause premature explosion.
The results of test (4) are of practical significance as indicating the superiority of the presvent plastic-encased cap over prior detonators not only with respect to increased safety in manufacture, handling, and transportation, but also in their use in the field. The latter arises particularly because of the commonly established procedure of tamping explosives rather tightly into bore holes, in which type of operation the blasting initiator of this invention provides a highly desirable and necessary extra margin of safety. This is particularly desirable because in such cases there is involved not only the relatively small quantity of explosive within the detonator, but also the large mass of high explosives being tamped into the borehole; premature explosion of the cap will cause a disastrous explosion of the entire charge.
With regard to tests of functioning under normal as well as adverse conditions, caps A and B yielded the following results when placed in direct comparison with two current commercial No. 6 metal-shell detonators, No.'1 having a base charge of four grains of tetryl, an initiating charge of 3 grains of lead azide and a 2.75 grain ignition charge; and No. 2 having a base charge of '7 grains of a TNT-picric acid mixture conc ances teinlng 25% by weight or m. on initiating charge of e grains by weight of mercury-fulminute, and a 4 grain ignition charge:
materials with other explosives or with oxidizing agents such as potassium chlorate; end the ignition composition may be any of the well-known Metal-case No. 6 caps Plastic-case caps No. 1 No. 2 A. B
vemlllengthoicap 1.61:1 1.21m 1.2m. Trauzl block expansion (room temp.) 33.9 c. c 16.7 c. c 18.4 c. c insensitive dynamite tests: Per cent functioning at 12% F9208. 287 o a 70%. Lead piste tests-average diameter in shots:
Temp. C 0.325 in 0.330 in 0.330 in Aiter3Dminutcsat80 Nopcrlorstiom. 0.360 0.340. After5-25min.utl33 ..do 0.360 H.350
The insensitive dynamite test referred to in the above table is one which has been in use for comparing the initigting strength of blasting caps. In this test, cartridges 1%, inches in diameter and d inches long are charged with '75 grams of a mixture of'ierric oxide and trinitrotoluene, in
F8203, were exploded by the blasting cups prepared in accordance with this invention than with a. prior type cap, indicating greater initiating strength "for the new type.
Alter twenty-four hours storage st s.. ternperature of 165 F. (733 8.), no failures were obtalned with A and B type caps, while 75% of the commercial metal-shell cops sublected to the some test failed to explode on passage of the firing current through the bridge wire.
Plastic-shell caps of the A and B types likewise functioned perfectly efter the following severe test of their waterproof qualities. The cops were submerged for 3 hours in water under a. pressure of 5d lbs/sq. in; then, at atmospheric pressure, the wetter bath containing the cups was first heated to 5il C. and then cooled to 10 C.,-
the cycle being repeated nine times; finally, the cops were submerged under water at room temperoture and at 2. pressure oi oil to 70 pounds per suuore inch above atmospheric pressure for fiitecn hours.
Although it is usually desirable to provide electric detonators having both s. shell and plug of molded plastic. advantage over prior cops may also be obtained by the provision of e. molded plastic shell and e. conventional type of plus for carrying the bridge and leg wires, such es of cost sulfur. Even in case n. metal shell is employed, substantial advantage is achieved by the use of a plug, such as l, mode of molded plastic insterleil as hereinbeiore described.
It is to be understood that electric blasting initiators may be prepsred'in accordance with this invention utilizing any suitable explosives for the verlous compo cuts or the charge. Thus the base charge may consist of c. nltro-sromatlc compound such as tetryl, picric sold, trlnitrotoluene, or didzodinitrophenol, of s nltrated polyhydrio alcohol such as penteerythrltol. tetrenitrote, or monnitol hexamtrete, or or e. nitrsmine such es ethylene dlnitremine; the initiating explosive may be lead szide. mercury lulminate,
diozodinitrcphenol. or mixtures or soy of these types based on mercury rulmlnste, diosodinitrophenol, nltrosterch, ocetylides, or motel suits or nitrophenols.
While the detonstor shown in the accompanying drawing, and hereinbeforc especially de= scribed for the purpose of illustration, is of the electrically energized-type. it is to be understood that the feature of making the shell of Organic plastic material is also applicable to detonators of the fuse type. In the case of fuss type detonutors, however. it is desirable to provide some means for securing the fuse in position in the open end of the detonetor case. This may be accomplished by providing :9. suitsble anchorin device in. theopcn end of the detonetor case, as
for example s spring metal ferrule having prongs directed inward from the mouth or the detcnstor shell. so as to permit a. fuse to be inserted but to prevent its being withdrawn. Such a ferrule may be molded in position in the plastic case, or may be driven therein after the explosive ingredients have been inserted. Alternatively, a. metal sleeve may be provided in the open end of the plastic detonetor case. extending suficiently beyond the end of the plastic case to permit the application of a clamping tool for clamping the fuse in position, as has heretofore been done in the case of fuse type detonators having metal cases.
The use of s. transparent molded plastic for the shell may be particularly advantageous as this feature enables inspection of the cops during and after assembly. Furthermore, any desired color may be obtained by including suitable dyes, and pigments, in the molding composition.
This invention thus accomplishes its objects in providing blasting detonators possessing signincsntly improved features of safety and utility over any which have heretofore been known in the art.
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 limitetive except in the light of the appended claims.
Having thus described the invention, what is claimed is:
1. A blasting cup having a. shell of molded organic plastic and an explosive charge composed of a series of preformed tablets of high explosive, the bottom tablet having s. hollow portion adjacent tothe base of the cap.
A blasting cap having at shell of molded organic plastic, 9. base charge of preformed tablets of high explosive therein, and on initiating charge housed in a. molded organic plastic sleeve superposed on the base charge.
FREERICK R. SEAVE Y.