US3320847A - Method for manufacturing explosive devices - Google Patents

Description

May 23, 1967 s. R. KELLY ETAL METHOD FOR MANUFACTURING EXPLOSIVE DEVICES Filed March 7, 1966 G W E w w 3.I|E| I Wow fl W W H 0 IE C H a M V N VI E O N R S E U O O T P U0 M L G E WW F E WLM S D O EXAMINATION OF OPTIONAL--- SEMI-FUSE APPLICATION OF MOISTURE BARRI FULL ACTIVATION OPTIONAL-7 INVENTORS STAN LEY R. KELLY JOHN M. SMITH B zim%,%7 nwn/M7%w ATTORNEYS 3,320,847 METHUD FGP; MANUFAtZTURING EXPLGSKVE DEVICES Stanley R. Kelly and John M. Smith, Simsbury, Conn, assignors to The Ensign-Bickford Company, Simsbury, (loud, a corporation of Connecticut Filed Mar. 7, 1966, er. No. 532,285 9 Claims. (Cl. 86-1) This application is a continuation-in-part of co-pending application Ser. No. 511,554, filed on Dec. 3, 1965, by Stanley R. Kelly et al., and entitled, Method for Manufacturing Explosive Devices.

The present invention relates to explosive devices and is principally concerned with a new and improved process for their manufacture. More particularly, the invention is directed to a new method for producing explosive devices such as detonating fuses, cords and the like.

As is well known, detonating fuses, cords and the like are generally produced in continuous lengths as flexible explosive columns, consisting essentially of a central core of high explosive material encased in a protective sheath. The amount of explosive within the core and the make up of the sheath may vary somewhat; however, detonating fuses generally contain 40 to 60 grains of explosive per linear foot and sul'ficient coverings to minimize whatever damage might be encountered prior to use, such as abrasion, water penetration and the like.

Heretofore, it has generally been the practice in the explosives industry to manufacture detonating fuses in accordance with either a dry or a wet process. In the dry procedure, the raw core of a detonating fuse is usually formed as a column of dry high explosive about which is spun a plurality of textile yarns or the like. These yarns ciroumscribe and confine the explosive column in a longitudinally oblique or spiraling manner but do not usually provide a complete barrier or sheath for the column of dry explosive. As mentioned hereinbefore the core load of the dry spun fuse is about 40 to 60 grains per foot or higher. At the moment the column is formed and throughout all of the subsequent fabricating operations the explosive is usually present in a fully active state, i.e., at all times it possesses full sensitivity and explosive strength, i.e., the ability to perform a useful explosive work function. However, when core loads are reduced to about grains per foot and less, the dry process frequently results in a fuse exhibiting certain performance deficiencies.

In accordance with the wet process, the core is formed from a wet slurry of explosive material and is wrapped in a textile covering prior to subsequent operations such as inspection and final wrapping. This process is readily adapted to the manufacture of fuses and the like having core loads of 25 grains and less without the deficiencies: associated with the dry process. Unfortunately, the more complex machinery necessitated by the wet operation together with its greater maintenance and slower production rate constitute a substantial disadvantage from a commercial point of view.

Accordingly, it is a principal object of the present invention to provide a manufacturing technique for the production of detonating devices which is capable of utilizing the advantages of both the wet and dry methods while eliminating many of the disadvantages thereof.

Still another object of the present invention is to provide a new and improved method for manufacturing detonating fuse and the like which method facilitates greater control over the core load of the fuse, provides increased production rates and other substantial economic and manufacturing advantages while requiring a reduced number of process cuts in the fuse as well as reduced labor, machinery and maintenance costs.

3,326,847 Patented May 23, 1967 ice A further object of the present invention is to provide a new and improved process for the dry manufacture of detonating fuses and the like which permits the rapid assemblage, examination and storage of a detonating fuse and under appropriate conditions obviates the necessity for isolating and barricading the entire operation yet is capable of providing in a facile and economical manner a detonating fuse or the like having excellent reliabili ty and performance characteristics.

Other objects will be in part obvious and in part pointed out more in detail hereinafter.

The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others and the article possessing the features, properties, and the relation of elements, which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims:

In the drawing:

FIG. 1 is a flow diagram indicating the typical process steps in the method of the present invention; and

FIG. 2 is an elevational view, partly broken away and partly in section, of a portion of a dry spinning machine suitable for use in the process of the present invention.

As used herein the expressions full activity or fully active refer to explosive cores which possess the characteristics of sensitivity and explosive strength. Broadly, sensitivity is the ability to consistently and reliably receive and propagate detonation.

In accordance with the present invention the above and related objects are, in general, accomplished by first forming a core for an explosive device which exhibits less than full activity and subsequently fully activating the explosive within the core. It will be appreciated that the initially formed core may be so devoid of activity as to be dormant or completely inactive, or it may be only partially active. For example, it may be completely incapable of initiation and propagation or it may propagate at steady state velocity but be lacking in sensitivity or only propagate at low velocity with occasional failure in initiation and propagation. However, in either event it is an advantage of the present invention that activation can take place after fabrication of the device and without removing the explosive core therefrom, the core being characterized by an ability to be activated to a high ve locity detonating state in a facile, effective and reliable manner. It will also be appreciated that the dormant explosive core may be safely handled during fabricating operations and stored in its inactive or passive state until such time as it is necessary to activate the explosive within its core to provide the desired characteristics.

Referring now to the drawing in greater detail and particularly to the flow diagram of FIG. 1, there is illustrated in a general manner the various manufacturing steps in the process of the present invention. In accordance with that process there is initially provided a suitable supply of explosive material, as indicated by the numeral 10. This mate-rial might be any of the conventional solid high explosives and should be in a form suitable for the preparation of the desired explosive core or column.

In the preferred embodiment of the invention the explosive column is formed by gravity feed of the dry explosive, followed by confinement in a textile wrapping. In such an operation it is essential that the explosive material exhibit proper flow characteristics. This is particularly important in fuse manufacturing operations where it is necessary to maintain close control over the core load of the fuse. In general, better flow characteristics are obtained from the coarser grades of explosive. In fact, in many instances explosives of the finer grade will not produce the desired effects and should not be employed. For example, the necessary flow properties for a center thread gravity feed operation are obtained from the explosive pentaerythritol tetranitrate (PETN) only at coarse grain sizes. More particularly, grain sizes which are coarser than those normally used in conventional PETN detonating fuses of core loads from 40 to 60 grains per foot are preferred. Accordingly, cap grade PETN has been used with repeated success. Cap grade PETN generally conforms to a screen analysis (Tyler Standard Sieves) of:

1 percent maximum retention on a ZO-mesh screen, 35 percent minimum retention on a 48-mesh screen, and 20 percent maximum passed through a IOU-mesh screen.

The coarse high explosives may in certain instances be used as received from the supplier; however, as indicated generally at 12, best results are obtained when the explosive is suitably conditioned in order to enhance its characteristics, particularly its free flow properties. In the case of PETN, the flow properties of the explosive may be beneficially affected by suitable chemical treatment. This treatment may take the form of a rinse with an aqueous solution of an anti static or wetting agent, such as the saturated, long chain or fatty alcohol sulfates sold by E. 1. du Pont de Nemours & Company, Inc. under the trade name Duponol G. Such antistatic or wetting agents tend to enhance and promote the flow characteristics of the dry explosives treated therewith. The washed explosive is, of course, filtered to remove the excess treating liquid and then dried prior to its formation into the core of a fuse.

Exemplary of the high explosives which may be efii caciously employed in the present invention are, pentaerythritol tetranitrate (PETN), cyclotrimethylene trinitramine (RDX), cyclotetramethylene trinitramine (HMX), trinitrotoluene (TNT), tetryl and the like or suitable mixtures thereof.

As indicated by the numeral 14, the dry explosive material possessing the requisite size and flow properties is formed into a column at the desired core load. This may be accomplished by forming the dry explosive material into a column having a core load close toor below the critical detonating core load for the particular explosive and particle size utilized. It will of course be appreciated that the column may contain a suitable mixture of explosives or a mixture of high explosive material plus adulterants, the latter being of either a combustible or noncombustible nature. Accordingly, the critical core load will vary depending on the composition of the explosive mixture employed. The critical deto nating core load for the particular core composition employed is the weight of explosive or mixture per unit of length below which the explosive will not initiate or propagate when primed with conventional detonators or boosters.

Where a partially active fuse is initially desired, the core load may be slightly above the critical core load for the explosive employed; however such a fuse is, as mentioned hereinbefore, deficient in either sensitivity or explosive strength and should be subjected to a subsequent activating operation. This deficiency may be evidenced by an inability to initiate and propagate when wet, to shoot through either lap or knot connections, or to exhibit uniform steady state high velocity. Among these the most predominant deficiency of low core load fuses appears to be inability to shoot or propagate detonation through a lap or knot connection.

The critical detonating core load for the particular composition will vary directly with the grain size or screen analysis of the explosive and is different for each explosive material or mixture. For example, the critical core load for cap grade PETN is between about 35 and 42 grains per foot when the core is made up entirely of PETN. At the higher core load cap grade PETN will initiate and propagate while at the lower core load it is incapable of initiation and propagation and would result in a dormant fuse. If a mixture of 72 percent cap grade PETN and 28 percent sodium carbonate is used, the critical core load is increased to above 75 grains per foot (54 grains per foot of PETN). However, in both instances as the grain size is reduced, the critical core load is also lowered. Further variation in the critical core load can be effected by using other explosives, such as RDX which is less sensitive than PETN and at the same grain size has a higher critical core load.

As indicated by numeral 16, the column of explosive is formed into a semi-fuse or raw core by providing the column with a textile covering, such as by the conventional dry spinning operation. This may be accomplished by utilizing a center thread, gravity feed, fuse making apparatus, such as that illustratively depicted in FIG. 2 and generally designated 36). In operation, the coarse, granular explosive 32 is fed into the tapered funnel-like hopper 34 together with the single center thread 36 and, via gravity, flows into and through the neck 38 of the hopper. As the explosive flows through neck 38 it is formed into a column 40 and is circumscribed by the paper tape 42 which has been suitably curved along its longitudinal axis under the influence of the tubular mold 44. The tape 42 is overlapped, as shown, to form a cylindrical sleeve 46 enclosing a portion of neck 38, the column of explosive 40 and the center thread 36.

.As illustrated, the textile spinning machine 30 is further provided with a pair of flat, vertically spaced, counterrotating platforms 48, 50 provided with central apertures 52, 54, respectively, in which is positioned the tubular mold 44. Each of the platforms are adapted to support a plurality of freely rotating textile bobbins 56 adjacent the periphery thereof for feeding trands of yarn 58 through the slots 60, 62 to the interior of the mold.

.As the paper-enclosed explosive column moves downwardly through the tubular mold 4-4, the platforms 48, 50 revolve in opposite directions causing the yarns 53 to wrap the explosive column in a continuous spiraling manner thereby forming the semi-fuse 64. As mentioned hereinbefore, the textile covered semi-fuse, also referred to as the raw core, is not at this stage of the manufacturing operation fully active. In fact, if dormant, it is completely safe for handling since it is incapable of undergoing initiation or propagation.

The textile covered semi-fuse 64- is next fed to an examination station, as indicated by the numeral 18. There the entire length of the fuse is physically examined to ensure that the core weight of all portions of the fuse falls within the desired range. In the case of a dormant fuse the core weight would be below the critical load for the explosive material utilized. This examining process may be effectuated in accordance with standard fuse making techniques. In accordance with the preferred method, a fuse diameter inspection is made by passing the raw core between a pair of sensing rollers which measure the dimensional characteristics of the raw core. Since the examination is by mechanical means, there is necessarily some reduction in the particle size of the explosive column. However, care is generally taken so that such size reduction is maintained at a minimum. Additionally, the size of the explosive material initially employed should take into consideration such reduction in order to prevent premature full activation of the fuse.

Following examination, the raw core is provided with a suitable moisture barrier, as indicated by the numeral 20. ,The barrier may take a number of forms but a covering of plastic or the like is generally preferred. Such coverings or layers of plastic can be easily applied by extrusion techniques. Since the application of hot plastic over the raw core tends to increase the ambient temperature of the explosive material in the fuse, it is particularly advantageous to apply the plastic prior to full activation of the explosive within the semi-fuse.

The continuous fuse may at this stage of the process be Wound on a take-up spool for storage, see numeral 22 of FIG. 1. Alternatively, the fuse may be fed directly to the activation station. In any event, it will be appreciated that the entire fuse making operation from the column formation to the application of a moisture barrier has involved the handling of an inactive or only partially active explosive column. The fuse may be fully activated, as indicated by the numeral 24 of the flow diagram of FIG. 1, in a relatively facile and economical manner. Although various types of activation might be employed it is preferred in accordance with the present invention to activate the explosive column by causing a change in the size distribution and an increase in the surface area of the explosive material and thereby effectively lower or decrease the critical core lead of the prefabricated fuse. The revised size distribution and increased surface area is efiectuated without changing the core load or dimensional configuration of the ultimate fuse. The method of producing the desired activation which has been most successfully employed is to pass the fuse through a series of crushing rollers. These have the effect of reducing the particle size of the explosive in the raw core, resulting in a substantial shift in the size distribution of the explosive without adversely affecting the encasing layers of protective material. The result is a grain size which not only renders the fuse capable of initiation and propagation at the core load employed but also ensures the sensitivity and explosive strength so vital to reliable operation. It will be appreciated that the core load or other dimensional characteristics of the fuse are not permanently altered during the activation operation and in accordance with the preferred method of the present invention, should not be so altered. It is, of course, absolutely necessary that all portions of the explosive fuse be subjected to the activating operation in order to ensure the proper operation of the resultant product along its entire length.

The fully activated fuse may finally be encased, if desired, in further textile coverings or sheaths depending upon the environmental conditions under which the detonating fuse will be used. Such coverings provide not only a protective function but also act to reinforce the fuse and protect it from abrasion during usage. Additionally, such added wrapping tends to round-out the fuse after the crushing operation and beneficially affects the velocity of detonation of the explosive column. The wrapping may comprise countering layers of textile wrapping covered by Wax or by other suitable protective materials. It will, of course, be appreciated that the final reinforcing and protecting coverings may be applied to the fuse prior to the activation operation. Generally, however, it is preferred to activate the fuse prior to finishing, since the final countering and waxing operations have the beneficial effects mentioned heretofore.

In order to provide a fuller understanding of the present invention, the following specific examples are given by way of illustration and are not intended to constitute any limitation thereon.

Example I A supply of pentaerythritol tetr-anitrate (PETN) conforming to E. I. du Pont de Nemours specification for cap grade PETN and having the screen analysis set forth in column A of Table I was washed with Duponol G, filtered and dried. The dry explosive was placed in the hopper of a gravity feed, center thread, dry spinning apparatus and formed into a column having a core load of approximately 25 grains per foot. The semi-fuse was then examined and covered with an extruded plastic jacket. A screen analysis of the explosive core of the jacketed fuse is set forth at column B of Table I.

Initiation of the plastic jacketed fuse thus formed was attempted by end priming with SO-grain reinforced Primacord detonating fuse and by side priming with Gelex No. 2 dynamite initiated by reinforced Priniacord. In each instance, the jacketed fuse failed to initiate, indicating its dormant or inactive character.

A portion of the dormant fuse was subjected to the activation step of passing the cord through a series of barrel rollers. A screen analysis of the explosive within the fuse following the activation step is set forth in colurnn C of Table I. The resultant fuse exhibited the ability to be initiated by blasting caps and other detonating devices as Well as the ability to initiate itself by a lap connection. It further evidenced the ability to initiate all cap sensitive explosives and to initiate and propagate when wet or dry.

TABLE I.SCREEN ANALYSIS OF IE'IN 7 Percent Retained Screen Size (Mesh) A B I C l 4. 2 o. 9 i 53. 3 13. 8 5. 5 36. 5 29. 4 f 19. S 4. 4 32. 7 32. 5 l. 6 23. 2 42. 2

Propagation of the explosive signal was tested by activating in the same manner as above portions of two dormant fuses as made hereinbefore. First, one end of a 10-foot piece of dormat fuse was fully activated along a length of 5 feet. Second, a 4-foot length in the middle of a 10-foot piece of dormant fuse was activated. The fuses were marked at the division points between the activated and unactivated portions. The activated portions of the fuses were then initiated. In each instance the fuses failed to propagate beyond the marked borders.

Example II A fuse was prepared in accordance with the procedure of Example I except that a core load of 17.5 grains per foot was used. The fuse was incapable of initiation and propagation prior to the activation step. After full activation by crushing, the fuse exhibited sensitivity and explosive strength comparable to the activated fuse of Example I.

Example III A partially active fuse was formed from dry PETN. The fuse had a core load of about 25 grains per foot and a particle size distribution in the explosive core as set forth in Column A of Table II. The fuse failed to exhibit reliable propagation through knot and la connections, showing ten failures in ten lap connection tests although it did initiate and propagate when end initiated with a No. 6 blasting cap. The velocity of detonation was measured at 7194 meters per second.

A second 25-grain core load fuse was formed dry from PETN having the particle size distribution given at column B of Table II. In five initiation tests employing end initiation with a No. 6 blasting cap, the fuse failed to initate in two of the tests and initiated but failed to propagate in two of the tests. The fuse also exhibited a low velocity of detonation of only 2320 meters per second.

TABLE II Percent Retained Screen Size (Mesh) Upon full activation by crushing both fuses exhibited excellent sensitivity and explosive strength.

7 Example IV A fuse was prepared in accordance with the procedure of Example I except that the explosive core was formed from a mixture of 72 percent by weight cap grade PETN and 28 percent by weight sodium carbonate at a core load of 75 grains per foot (54 grains per foot of PETN). Prior to activation by crushing the fuse would not initiate or propagate. After crushing the explosive column initiated and propagated at a detonation velocity of 5300 meters per second and exhibited good sensitivity and strength.

Example V Example I was repeated using RDX as the explosive material. The starting material had the screen analysis given in column A of Table III. After being spun into a fuse having a core load of 51.6 grains per foot, it was covered with a plastic jacket. At this stage the fuse was tested and found to be incapable of initiation and propagation. It exhibited the screen analysis set forth in column B of Table III. The fuse was then activated by crushing, giving a screen analysis after activation as shown at column C of Table III. The activated fuse possessed excellent sensitivity and strength.

As can be seen from the foregoing detailed description, the present invention provides a new and improved method for the dry manufacture of detonating fuses, cords and the like. The method obviates the deficiencies heretofore associated with the dry fabrication of fuses while at the same time retaining the advantages in a facile and economical manner. From a commercial standpoint the new and improved method facilitates greater control over the core load than was previously possible in accordance with the wet braiding method, while requiring a reduced number of processed cuts and lower labor, machinery and maintenance costs.

As will be apparent to persons skilled in the art, various modfications and adaptations of the above described process and product will become readily apparent without departure from the spirit and scope of the invention, the scope of which is defined in the appended claims.

We claim:

1. A method of producing a consistently reliable, fully active explosive device possessing both sensitivity and explosive strength comprising the steps of roviding high explosive material in solid particulate form; forming the explosive material int-o a core possessing less than the full actvity required for reliable operation of the device; circumscribing the core with a covering; and subsequently fully activating the explosive material while confined within the covering by increasing the surface area of the explosive material without substantially altering the weight per unit of length and cross-sectional area of the explosive material to thereby produce a device having both sensitivity and explosive strength.

2. The method of claim 1 wherein the device is activated by crushing the explosive material within the core while confined within the covering, said covering being flexible.

3. The method of claim 1 wherein the covering of the dry core includes a filamentary covering and the circumscribing operation is effectuated by spinning the filaments of the covering about the core in a spiraling manner.

4. A method of claim 1 wherein the explosive device is a high velocity detonating fuse, cord or the like and the explosive core is an elongated column of less than full activity, the method including the step of forming a moisture barrier over the elongated explosive column along the entire length thereof, said barrier including a plastic jacket.

5. The method of claim 1 wherein the solid particulate explosive material comprises cap grade PETN.

6. The method of claim 1 wherein the explosive material includes PETN and the initially formed core of explosive is partially active.

7. The method of claim 1 wherein the explosive device is a high velocity detonating fuse, core or the like and the explosive core is an elongated column containing PETN at a core load of about 25 grains per foot and less.

8. The method of claim 4 wherein the solid explosive material comprises coarse, granular PETN suitable for being formed into a compact column; the covering being flexible and including a sheet material and yarn covering and the activating step comprises crushing the granular PETN while confined within the covering; the method additionally including the steps of treating the solid PETN prior to column formation to improve the flow characteristics thereof and examining the covered column by passing it between a pair of sensing rollers.

9. The method of claim 2 wherein the explosive device is a high velocity detonating fuse, cord or the like having a core load of 25 grains per foot and less; the initially formed explosive core is an elongated column comprising cap grade PETN; and the crushing operation results in a particle size of PETN within the column wherein substantially all of the explosive passes a 30-mesh screen, less than 10 percent is retained by a SO-mesh screen, about 25 percent ($10 percent) is retained by a -mesh screen and more than 25 percent passes a 325-mesh screen.

References Cited by the Examiner UNITED STATES PATENTS 1,923,761 8/1933 Snelling et a1. 86-,1 3,155,038 11/1964 Smith 102--27 3,216,307 11/1965 Grffith 86-20 FOREIGN PATENTS 857,621 1/1961 Great Britain.

References Cited by the Applicant UNITED STATES PATENTS 45,572 12/1864 Andrews.

51,679 12/1865 Andrews et al.

328,172 10/1885 Andrews.

868,876 10/1907 Lheure. 2,993,236 7/1961 Brimley et al. 3,125,024 3/1964 Hicks et al. 3,155,038 11/1964 Smith. 3,241,489 3/1966 Andrews et al.

FOREIGN PATENTS 226,015 11/1922 Canada.

BENJAMIN A. BORCHELT, Primary Exami er,

P. A- SHANLEY, Assistant Examiner.

Claims (1)

1. A METHOD OF PRODUCING A CONSISTENTLY RELIABLE, FULLY ACTIVE EXPLOSIVE DEVICE POSSESSING BOTH SENSITIVITY AND EXPLOSIVE STRENGTH COMPRISING THE STEPS OF PROVIDING HIGH EXPLOSIVE MATERIAL IN SOLID PARTICULATE FORM; FORMING THE EXPLOSIVE MATERIAL INTO A CORE POSSESSING LESS THAN THE FULL ACTIVITY REQUIRED FOR RELIABLE OPERATION OF THE DEVICE; CIRCUMSCRIBING THE CORE WITH A COVERING; AND SUBSEQUENTLY FULLY ACTIVATING THE EXPLOSIVE MATERIAL WHILE CONFINED WITHIN THE COVERING BY INCREASING THE SURFACE AREA OF THE EXPLOSIVE MATERIAL WITHOUT SUBSTANTIALLY ALTERING THE WEIGHT PER UNIT OF LENGTH AND CROSS-SECTIONAL AREA OF THE EXPLOSIVE MATERIAL TO THEREBY PRODUCE A DEVICE HAVING BOTH SENSITIVITY AND EXPLOSIVE STRENGTH.

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