NO145787B - LOW ENERGY-DETONING LUNCH AND PROCEDURE AND MAKING PRODUCT - Google Patents

Description

The present invention relates to an improved explosive connecting fuse for use in transmitting a detonation wave to an explosive charge, and more particularly to an explosive connecting fuse of the type known as a "low energy detonating fuse".

The invention also relates to a method and device for producing detonating fuses.

The dangers associated with the use of electrical initiation systems for detonating explosive charges in mining operations, i.e. the dangers of premature initiation by stray or extraneous electricity from such sources as lightning, static, galvanic action, stray currents, radio transmitters and transmission lines, are well known. . For this reason, non-electrical initiation using a suitable detonating fuse has been considered a widely acceptable alternative. A typical high-energy detonating fuse has a uniform detonation rate of approx. 6000 m/sec and comprises a fuse with 6-10 g/m pentaerythritol tetranitrate (PETN) covered with various combinations of materials, such as textiles, water impregnating materials, plaster, etc. The strength of the sound produced when a fuse with such PETN core charges is detonated on the ground surface, such as in mains, are however often unacceptable for blasting operations in built-up areas. Moreover, the explosiveness (crushing force) of such a fuse can be sufficiently high that the detonation impulse can be transferred laterally to a nearby section of the fuse or to a mass of explosives such as, for example, the fuse comes into contact with along its length. In the latter case, the fuse cannot be used to initiate an explosive charge in a borehole at the bottom ("bottom hole ignition method"), which is sometimes desirable.

The Low Energy Detonating Fuse (LEDC) was developed to overcome the problems of sound and high explosiveness associated with the 6-10 g/m fuse described above. LEDCs have an explosive core charge of only 0.02 - 2 g/m of fuse length, and often only approx. 0.4 g/m. This fuse is characterized by low brisance and the production of little sound, and can therefore be used as a main line in cases where the sound must be kept to a minimum, and as a lowering line for bottom hole initiation of an explosive charge.

US patent 2,982,210 describes a low-energy detonating fuse comprising a continuous core of a granular, detonating capsule-sensitive high explosive such as PETN of such a diameter as to contain 0.02 to 0.4 g/m of explosive, enclosed by a metal sleeve capable of be covered with a textile wrap or a plastic coating. The metal sleeve is indicated to be essential to the propagation of detonation in detonating fuses with such low charges.

Because LEDCs with a metal sleeve cannot be manufactured continuously of unlimited length, and because the fuse is electrically conductive along its length due to the conductivity of the metal sleeve, attempts have been made to eliminate the metal sleeve by taking other precautions to overcome the effect of its absence. Such attempts have not always been entirely successful, especially with fuse charges of 0.4 g/m or less. For example, it is stated in US patent 3,125,024 that a uniform detonation velocity can be obtained even without a metal sleeve with a granular PETN core in charges of 0.3 2 to 2 g/m length provided that the specific surface of PETN is from 900 to 3400 cm<2>/g and the granular core is enclosed by a woven textile sleeve surrounded by a protective and reinforcing cover, i.e. a thermoplastic layer or a series of water impregnating and reinforcing materials including another textile sleeve. However, the woven or wound sleeve is relatively expensive to <p>apply, both in terms of the type of equipment required and the limitations thereby imposed on the fuse production rate. Also, even with the high PETN specific surface area and containment provided by the woven textile sleeve and thermoplastic cover, reliable high-velocity detonation is not achieved when the PETN core charge is lower than the LEDC range.

British Patent 815,534 and US Patent 3,311,056 describe low-energy detonating fuses with an explosive core enclosed in a polymer sleeve. The British patent describes a fuse with

a granular core of finely divided explosive in charges from 0.4

to 3 g/m surrounded by a flexible sleeve of a thermoplastic polymer, which may be wrapped in woven fabric and string for strength and abrasion resistance. The detonating fuse described in US patent 3,311,056 is a non-explosive type of fuse due to a thick expandable sleeve of elastomeric polyurethane surrounding the explosive core, the ratio of the amount of explosive in grams per meters until the sleeve thickness in centimeters is less than 11/1, and preferably from 0.8/1 to 8/1. Detonating fuse charges of 0.2 to 80, preferably 0.4 to 20 g/m are described and the fuse thus includes high-energy as well as low-energy detonating fuses. The 0.4-4 g/m fuse according to the patent has a PETN core surrounded by a lead sleeve. Although detonating fuses made from self-supporting preparations of the type used in disc explosives, e.g. those shown in US patents 2,992,087 and 2,999,743 are also discussed as having low-energy detonating fuse charges of 1 and 2 g/m lead sleeves, and low ratios of explosive charge to polyurethane sleeve thickness (4 and 1.7 g/m m per cm sleeve thickness).

US patent 3,384,688 describes the manufacture of a textile-surrounded sleeve with increased sensitivity to side initiation and the ability to propagate detonation at lower charge densities using a specially finely divided granular PETN core at a charge of 2 g/m. US patent 3,382,802 prescribes a maximum particle size of 100 µm, with at least half of the particles below 50 µm, for a core of granular primary explosive in low charges, e.g. 1-2 g/m, enclosed in a sheath of helically wound wire-like elements made of metal or thermoplastic, helically wound fibrous sheaths, and a thermoplastic outer shell.

As will be seen from the above mentioned patents, only granular explosive cores have been used in detonating fuses with core quantities of 2 g/m or less. In addition, metal or heavy textile sleeves have generally been necessary, especially when the content falls below 0.4 g/m. Self-sustaining explosive compositions in which a crystalline high-explosive compound is mixed with a binder can be rapidly extruded in the form of fuses and would enable higher fuse production rates to be achieved as opposed to production rates achieved with granular core fuses. Also, bonded explosive preparations have a higher density and can detonate at a higher rate at a given diameter in contrast to low-density explosives. However, since the usual bonded explosive preparations contain less sensitive materials, such preparations are less sensitive to initiation than fully explosive granular preparations and would not be expected to detonate under all the same conditions as such granular preparations. Thus, while US patent 3,311,056 describes certain detonating fuses with bonded explosive cores, the low-fill cores therein are granular PETN and lead azide/aluminum, and even these are lead jacketed. It is also known that the fuse diameter and the explosive content must be sufficiently large if self-supporting plate-shaped explosive preparations are to propagate a detonation at a uniform high speed. The above-mentioned US patent 2,992,087 states that a fuse made by extruding a nitrocellulose-based PETN sheet explosive to a PETN charge of 4 g/m detonates at a velocity in excess of 6400 m/sec,

and the aforementioned US patent 3,311,056 discloses bonded explosive cores in PETN charges of 3.7 and 4 g/m. However, fuses with bonded explosive cores in explosive charges of 2 g/m or less have been avoided despite the fact that such charges have been found to be usable with granular PETN explosives. US patents 3,338,764, 3,401..215, 3,407,731 and 3,428,502 state the manufacture of detonating fuses with an explosive charge of 10-40 g/m by extruding a flexible elastomer-bound explosive preparation, preferably around an axially located reinforcing yarn or wire. Wrapping reinforcing yarns or threads around the extruded core, e.g. as in a braided structure, and bonding the yarns to the core with a latex or liquid polymer is stated to be less desirable than an internally placed reinforcing device.

In the manufacture of detonating fuses, threads have also been used to facilitate the wrapping of powdered explosive cores. For example, US patent 3,683,742 describes passing one or more roughened threads circularly through a funnel that feeds dust-like explosives into a sleeve that is continuously produced at the lower end of the funnel, the thread or threads being bent away from the vertical axis of the funnel and introduced into the sleeve together with the explosives. The thread or threads carry the dust-like explosive and lead it into the sleeve, whereby a granular explosive core is formed around internal threads.

British Patent 1,416,128 and Belgian Patent 815,257 describe enclosing a column of dry, powdered explosive within a region of united axial threads, and drawing the column/wire assembly through a compressing die under a tension applied to the threads so as to form a core of a detonating fuse. The thus formed core, in which the threads envelop and form a sleeve around the explosive, is shown enclosed by a reinforcing layer of wound textile material, which is coated with plastic for water impregnation.

US patent 2,687,553 describes the use of longitudinal threads in fuse manufacture for the purpose of reinforcing a thermoplastic coating to overcome the elasticity of the latter. The resulting fuse has an explosive core surrounded by a sleeve of thermoplastic material in which strong threads are embedded in the longitudinal direction. The entire circumference of the explosive core is in direct contact with the thermoplastic sleeve, and the threads are surrounded by the thermoplastic.

The present invention relates to an improved low-energy detonating fuse as stated in claims 1-11.

A particularly preferred fuse according to the invention is one in which the crystalline high-explosive compound in a bound preparation is pentaerythritol tetranitrate (PETN), the PETN charge in the fuse being 0.4 to 2 g/m in length, the plastic material being a poly-olefin which is extrudable at a temperature of approx. 175°C, and at least four reinforcing strands of a polyamide or polyester yarn are substantially uniformly distributed on the periphery of the core.

The present invention also relates to a method for producing a detonating fuse as stated in claims 12-16.

In a preferred embodiment of the method, the core is extruded inside the continuously moving yarn cage, and the contained core assembly, with or without peripheral support, is then moved into and through a plastic coating extrusion die.

nozzle in which the plate material is formed into a sleeve over the enclosed

core unit. In another embodiment, the yarns and core are moved separately inside a plastic coating extrusion die, and the formation of the cage, the containment of the core, and the formation of the sleeve all occur within the die, either simultaneously or with the sleeve formation after the containment. In each case, there is essentially no reduction in the diameter of the core as a result of compression.

The present invention also relates to a device for producing a detonating fuse as stated in claim 17.

The first extrusion device is preferably associated with an extrusion chamber having an opening for applying a vacuum, and particle screening device to exclude excessively large foreign particles from the core. The thread orientation device can be a separate guide plate or a component of the other extrusion device.

In the drawings illustrating particular embodiments of the explosive connecting fuse, the method and the device according to the invention are

Figs. 1 and 5 are perspective views in partial longitudinal section of parts of different embodiments of the connecting fuse according to the invention, Fig. 2 is a schematic view of the device according to the invention, and Figs. 3 and 4 are sections of different embodiments of parts of the device shown in Fig. 2.

In Fig. 5, which shows a section of a low-energy detonating fuse (1), the section shows a continuous solid core (2) of a deformably connected detonating explosive preparation, e.g. superfine PETN mixed with a binder such as plasticized nitrocellulose, the diameter and explosive content of the core being such that explosives are present in an amount of 0.1 to 2 g/m of length, and a protective plastic sleeve (4), e.g. 0.013 to 0.191 cm thick, which encloses the core (2). In the section of the fuse shown in Fig. 1, shown is core strengthening means (3) consisting of a mass of filaments obtained from multifilament yarn around and in contact with the periphery of the core (2) running parallel to the longitudinal axis of the core (2), and a sleeve (4) which encloses the core (2) and the core-reinforcing filaments (3). In another part of Fig. 1 the sleeve (4) is removed to show the peripheral appearance of the core (2), and in other parts of Fig. 1 the sleeve (4) is removed to show the peripheral appearance of the filaments (3 ) on the core (2), and the filaments (3) removed to show the peripheral appearance of the core (2).

The low energy detonating fuse of the invention combines the features of a continuous solid (ie non-hollow) core of a deformably bonded detonating explosive preparation with a low charge, ie 0.1 to 2 g/m of length, and crystalline high-explosive in a binder for this, and only a light protective plastic sleeve enclosing the core. A further feature, which is preferred, is longitudinal fiber reinforcement of the core outside it. It has been found that, contrary to the prior art teaching of low energy detonating fuses, a deformable bonded detonating explosive in the form of a fuse can be made to reliably plate a detonation even in charges below 1-2 g/m, at a rate which is useful in blasting operations, e.g. above 4000 m/sec., without containment in a metal or woven textile sleeve, spirally wound textile, plastic or metal wires or filaments, or a thick plastic sleeve. It has been found that the above containment is unnecessary if the core is a continuous solid rod of bonded explosive, e.g. a plastic bonded explosive, containing at least 55% explosive by weight, and a "superfine" crystalline high explosive component (which will be described below), and where any reinforcement devices for the core are external to this. In the fuse according to the invention, the explosive particles in the core are held together with a binder, e.g. an organic polymer preparation, and this has been shown to have a beneficial effect in ensuring a uniform, high core density and consequently reliable detonation, high density being an important factor especially in small-diameter, low-charge, low-brisance fuses. With respect to the bonded core, it has been found that despite the fact that central, internal reinforcement has been stated to be preferable in high-charge fuses made from self-sustaining explosives (US patent 3,338,764), external reinforcing filaments are essential for the proper effect of low-charge fuses made with this type of core. Furthermore, when the connected explosive core of the fuse according to the invention is reinforced, the external reinforcement device, e.g. textile yarn, preferably longitudinal, running essentially parallel to the fuse axis. Such a core is well suited to be manufactured by high speed continuous manufacturing methods as opposed to the low energy detonating fuses previously known which employ woven or wound textile reinforcement.

The bonded explosive preparation that makes up the explosive core of the fuse contains at least one finely divided blasting cap-sensitive crystalline high-explosive compound, which can be an organic polynitrate such as PETN or mannitol hexane nitrate, or a polynitramine such as cyclotrimethylenetrinitramine (RDX) or cyclotetramethylene-tetranitramine (HMX). PETN is the most readily available of these compounds and is satisfactory for use under the conditions most common in blasting, and for these reasons it is the preferred crystalline explosive in the bonded explosive core. The crystalline high explosive compound is mixed with a binder which can be a natural or synthetic organic polymer, e.g. the soluble nitrocellulose described in US patent 2,992,087, or the mixture of an organic rubber and a thermoplastic terpene hydrocarbon resin described in US patent 2,999,743. The preparations described in these patents can be used for the core of the present fuse, and the description of these patents is incorporated herein by reference. Other components may be present in the preparation, such as additives used to plasticize the binder or make the preparation denser. Other preparations that can be used are those described in US patents 3,338,764 and 3.4 28,502, which descriptions are also incorporated herein by reference.

The detonating fuse according to the invention is a "low energy" fuse, i.e. one which, when detonated, produces relatively little noise and exhibits relatively low explosiveness. For a given core preparation, the core diameter is therefore such that from 0.1 to 2, preferably at least 0.4 grams of the crystalline high-explosive compound is present per meter core. With mains containing cores with higher charges, noise levels can be a problem in certain areas. Below 0.1 g/m the reliability of complete propagation of detonation is low unless a high energy binder and/or plasticizer is used in the core preparation. With such a preparation, e.g. with a preparation having a high viscosity nitro-cellulose binder plasticized with trimethylolethane trinitrate, as described in US patent 3,943,017, charges of particulate high explosive in the core down to 0.02 g/m can be used. Charges of 0.4 to 2 g/m have been found to be particularly advantageous for branch and main fuses. In the case of low-charge explosive cores, such as in the present fuse, it is important that the crystalline high-explosive component is in the "superfine" particle size range, i.e. the maximum dimension of the particles should be in the range 0.1-50 pm, and in in general, the average maximum dimension should not be greater than 20 pm.

Larger explosive particles, extreme variations in particle size and particulate foreign material are undesirable as they interfere with the smooth propagation of detonation in the core. A preferred explosive for use in the core is one having microholes, as produced by the method described in US Patent 3,754,061, which description is incorporated herein by reference.

The explosive charge of the core is a function of the content of crystalline high explosive in the bonded preparation and the core diameter. The content of crystalline high explosive can vary, e.g. from 55% up to 90% by weight of the core preparation. Although a low explosive content can to some extent be compensated by a large core diameter, it is more efficient and the propagation of the detonation is more reliable if, for a given charge, the explosive content is as high as possible, preferably at least 70% by weight of the core preparation. Explosive content in the range of 55-90%, core diameters of approximately from 0.025 cm to 0.152 cm will be used to achieve core charges of 0.1-2 g/m core length. A diameter of approx. 0.069 cm is used to achieve the preferred nuclear charge of 0.4 g/m. The explosive preparation also contains 1-10%, preferably 2 to 5%, by weight of a binder, and also, if necessary, a plasticizer, to make the preparation extrudable, and provide cohesion in the core.

The density of the core varies with the particular particulate explosive and binder used and the amounts thereof, and the nature and amount of any other additives. In general, cores based on the preparations described in the above-mentioned US patents 2,992,087 and 2,999,743 will have a density of approx.

1.5 g/cm^. A core density of this size, in contrast to densities of o only approx. 1.2 g/cm 3 obtained with particulate cores have the advantage that they provide a better transmission of the detonation wave and therefore a higher detonation speed for a given diameter. Although the shape of the core's cross-section is not critical for the fuse to function properly, it is generally preferred to use a core of substantially circular cross-section to facilitate the production of fuses of the circular configuration commonly used.

The bonded explosive core is enclosed in a sleeve as a means of protecting it from abrasion or other damage that may occur during handling and preparation for detonation. As the sleeve is primarily protective, it is relatively thin, i.e. in the area approx. 0.013 to 0.191 cm, except that a sleeve up to 0.318 cm thick may be used if the fuse is to be exposed to very stressful conditions, such as those encountered in open pit operations. Uniform protection is difficult to obtain with sleeves thinner than 0.013 cm. A sleeve that is thicker than 0.318 cm is not necessary in the present fuse, and in any case will unnecessarily increase the thickness and price of the fuse, limit its flexibility and may be difficult to make in small diameter boreholes. A sleeve thickness of 0.051 - 0.127 cm is preferred in view of the ease of applying it to the core and the degree of protection obtained. With preferred core charges of 0.4 to 2 g/m and preferred sleeve thicknesses of 0.051 to 0.127 cm, the ratio of core charge (g/m) to sleeve thickness (cm) is 3/1 to 39/1.

Within the useful sleeve thickness range, it is often advisable to use a thicker sleeve when the explosive charge in the core is near the lower end of the core charge range, as in such cases this can ensure reliable initiation and propagation of

the detonation. Likewise, since the nuclear explosive charge

, increases, increasing the casing thickness ensures a continuous detonation through knots and half-sticks.

The sleeve alone consists of one or more plastic layers. This means that any layer from which the sleeve is made consists essentially of plastic and that no enclosing metal or woven textile layer is present in the sleeve, either next to or separate from the core.

The sleeve is made of a plastic, i.e. deformable, material that is able to flow, e.g. is extrudable, at a temperature not much above the melting point of the explosive in the core, i.e. no more than 75°C above the melting point of the explosive. This makes it possible for the plastic sleeve to be applied to the core, e.g. by extrusion or other conventional coating method, without causing a harmful transformation of the explosive. The plastic should be flexible and robust when it has hardened. Although the temperature of the plastic to be used during the application of the sleeve to the core will vary depending on the contact time between the core and the overlying soft plastic, on the heat exchange rate between the core and the plastic, and on the stability of the binder in the core, the plastic in a PETN- containing core be liquid at a temperature not exceeding 2CO°C. The plastic material can be a heat-setting material such as a rubber or other elastomer, or a thermoplastic material such as wax, asphalt or one or more polyolefins, e.g. polyethylene or polypropylene, polyesters, e.g. polyethylene terephthalate, polyamides, e.g. nylon, polyvinyl chloride, ionomeric resins, e.g. ethylene/methacrylic acid copolymer metal salts, etc. Thermoplastic sleeves are preferred, and most preferred is polyethylene, on the basis of availability, ease of application, etc.

To enable the fuse to retain its structure and dimensions during field use, it is preferred that reinforcing agents are used to increase the tensile strength of the fuse and prevent the core from constricting to the point of failure under forces that normally occur in borehole charging. Although such reinforcement may be provided by a material suspended in the plastic layer or layers of the protective sleeve, e.g. of pieces or strands of yarn placed therein, e.g. as shown in the U.S. patent 2,687,553, or on the outer periphery of the sleeve, it is preferred

that the core is reinforced by at least one, and usually preferably four or more, continuous threads of yarn which are substantially in contact with the circumference of the core and run substantially parallel to the longitudinal axis of the core.

The presence of the yarn threads between the core and the sleeve is preferred over yarn threads inside the plastic layer of the sleeve because

heat does not transfer as easily from the plastic to the core when hot plastic is extruded onto the core. The term "yarn" is used here in the sense given in "Standard Definitions of Terms Relating to Textile Materials", ASTM reference D 123-74a, where "yarn" is defined as a generic term for a continuous thread of tek -style fibres, filaments or material which occurs as a number of twisted fibres, a number of filaments joined together without twist, a number of filaments joined together with more or less twist,

a single filament (monofilament) with or without twist, or one or more strips produced by longitudinally splitting a sheet of material such as a natural or synthetic polymer with or without twist. Different yarns included in this definition are single yarn, twin yarn, double twisted yarn, twine, thread, effect yarn, etc. The thread or strands of yarn are held in place around the core by the plastic sleeve which encloses the core and peripheral threads. Any yarn can be used which has a sufficiently high tensile strength to prevent the core from constricting under forces that normally occur in borehole charging to such an extent that it fails to propagate a detonation. This usually requires the core to be given a tensile strength of at least 44N. Too increased

security that the fuse will withstand more extreme forces, a tensile strength of at least 88N in the reinforced core is preferred. The yarn material, the filament number and the denier, and the number of yarns, are selected so as to provide the required tensile strength. Multifilament yarns may be preferable as these, unlike monofilaments, tend to spread out around the core,

and provide an insulating effect during the coating operation and a more extensive enveloping effect. In addition, fewer threads and lower deniers can be used with stronger fibres. Yarns greater than 2000 denier are not preferred as these contribute unnecessarily to the thickness of the fuse. Although any natural fiber may be used in the yarn, synthetic fibers of the polyester, polyamide and polyacrylic types are preferred on the basis of their greater strength. Particular preference is given to nylon, polyethylene terephthalate, and fully aromatic polyamide produced by condensation of terephthalic acid and p-phenylenediamine. These fibers in deniers of 800 or higher have a tensile strength of at least 88N and thus a single thread or yarn thereof is sufficient

similar to the current fuse. However, multiple yarns give increased strength, and are therefore preferred. Moreover, they can be used in lower deniers, e.g. down to 400 denier. In the preferred fuse, at least four multifilament yarns are placed substantially evenly around the periphery of the core, which leads to an even distribution of the reinforcement around the core. There is no significant advantage to be gained by placing multifilament yarns next to each other in the sheath before applying the plastic sleeve to them, ±! The overwrapping and plastic coating operations in any case cause a spreading or diffusion of the filaments in multifilament yarn which can cause the yarn to distribute around the core. For this reason, and considering the circumference of the core and the denier of the yarns, the use of more than approx. 12 yarn over-flowing. Generally, the layer of filaments will not be thicker than 0.025 cm.

Textured and multiplex yarns (as described in U.S. Pat

3,338,764) are particularly effective core strengthening agents as they can be firmly bonded to the plastic sleeve that surrounds them. <p>application of an adhesive coating, e.g. a soft wax to the threads also improves the bond between the threads and the plastic sleeve, reducing yarn mobility and possibly leading to interference with the core, as well as increasing the sleeve's stain removal strength.

The method and apparatus according to the invention will now be explained with reference to fig. 2-4- On fig. 2 and 3, 5 denotes a piston-type extruder with a piston 6, and a cylindrical chamber or cylinder 29, which is surrounded by heating coils 7. The extruder chamber 29 is provided with a vacuum opening 25, and a sieve 26, which is placed on one side of a multiperforated carrier plate 27. A mass 28 of a deformably bonded detonating explosive preparation is shown in the extruder chamber 29 and in the openings of the plate 27- The plate 27, on the other hand, has the nozzle portion of the chamber 29 of reduced diameter into which the explosive mass 28 is forced by the impact of the piston 6 and is formed into a solid rod or core 2.

Next to the nozzle part of the extruder 5 is the thread orienting plate 8 which is a device for orienting the threads of yarn including 9 and 10 in a substantially parallel annular arrangement. The plate 8 has an axial channel and thread-receiving radial grooves in a surface which is in connection with the axial channel, the surface of the plate with grooves being curved since it meets the axial channel. The plate 8 is placed in such a position that its grooved surface meets the surface of the extruder 5 so that the axial channel of the plate is coaxial with the core 2 which emerges from the nozzle part of the extruder 5 under the action of the piston 6. The threads 9 and 10 are pulled off from respective coils 11 and 12 of the pulling device 13> which constitutes a device for pulling or pulling the threads under tension sufficient to shape them into a movable cage 14- The core 12 coming out of the extruder 5 is enclosed inside the cage 14 and is guided by it. The pulling device 13 pulls the cage 14 (containing the core 2) through the extrusion die 15 of another extruder, whereby a plastic material is applied around the cage in the form of a sleeve 4- The extrusion die 15 has an annular outer part 17 and an inner tubular part 16, arranged as follows: that a soft, plastic material 30 supplied to the nozzle 15 through the wall of 17 in a known manner (not shown) is formed into a tube between opposing surfaces of the outer part 17 and the inner tubular part 16, and the cage 14 moves through the axial channel in the tubular part l6. The vacuum opening 18 passes through the wall of the tubular part 16 and opens into the latter's axial channel. The tubular part 16 and the wire orientation plate 8 are held in a spaced apart, coaxial relationship, and are joined to each other by the connecting tube 19, which surrounds the cage 14 in the area between the plate 8 and the tubular part 16.

The sleeve-enclosed core-containing cage (fuse 1) formed at the outlet of the nozzle 15 moves through the vessel 20, e.g. a water vessel, which is a device for hardening the plastic sleeve material. The fuse, after passing over the pulling device 13, is then collected on the winding device 22, the winding of the fuse being facilitated by its guidance over the stretch control device 21. The extruder piston 6 is connected to a recording device 23 which registers the speed of the piston and emits a signal in -hold this to the signal processor 24> which is connected to the drive device for the pulling device 13 and to the drive device for the winding device 22 and adjusts their speeds in accordance with the signal received from the recording device 23.

Fig. 4 shows an alternative extrusion nozzle 15 which can be used in the present apparatus together with an extruder for forming the explosive core. This particular nozzle includes a device for orienting yarn threads in a substantially parallel annular order and can thus be used in the apparatus shown in fig. 2 without the wire orientation plate 8- In this embodiment, an axial channel in the extrusion die 15 has a cylindrical part 31 and a conical part 32. A hollow conical insert 33 is placed so that its tip is fitted into a conical die part 32 with a small distance between opposing surfaces . The pulling device 13 pulls yarn threads 9 and 10 through openings in the yarn guide ring 34, and then along the inner surface of the adjacent conical insert 33. The threads approach each other in the conical part of the insert 33 and are then oriented substantially parallel to each other and shaped to a cage when passing through a cylindrical part of the post 33.

The explosive core 2 enters the cylindrical part of the insert 33, where it is enclosed by the yarn cage formed therein. Plastic material 30 is introduced into the annular space formed between the walls of the conical insert 33 and the extrusion nozzle 15. This annular space is connected to the cylindrical nozzle part 31 via the space between the conical nozzle part 32 and the pointed part of the insert 33. The cylindrical part of the insert 33 is in coaxial connection with the cylindrical nozzle part 31. The core-containing cage 14 formed in the cylindrical part of the insert 33 is drawn through a stream of plastic material 30 which flows through the cylindrical nozzle part 31 into which it has entered from the space between the conical nozzle part 32 and the tip part of the insert 33. Plastic material 30 is formed into a sleeve around the enclosed core 14 to form the fuse 1.

The production of a preferred fuse according to the invention is illustrated by the following example.

Example 1

A. With reference to fig. 3, the mass 28 in the extruder chamber 29 is a 455 9 plug of a deformable bonded explosive preparation consisting of a mixture of 76.5% superfine PETN, 20.2% acetyl-tributyl citrate <p>g 3>3% nitrocellulose produced by the method described in U.S. patent 2,992,087- The super-fine PETN is of the type containing dispersed microholes produced by the method described in U.S. patent 3,754 o6l and has an average particle size of less than 15 µm, all particles being smaller than 44 • The temperature of the chamber 29 is maintained at 63°C by heating coils 7 to assist in maintaining the extrudability of the explosive composition therein. After the plug of explosive is placed in the chamber 29, the piston 6 is advanced to seal off the chamber 29, and a vacuum is applied through the opening 25- A vacuum level of -742 mm of mercury is maintained for 1 minute .This is done to prevent entrapment of air in the explosive preparation, a condition which may cause discontinuity in the sp the purge core, adversely affecting its ability to propagate a detonation. The piston 6 is then advanced further until the explosive mass 28 is compressed, but not yet to such an extent that extrusion occurs.

Wires 9 and 10, and a further four wires (not shown), are fed into the radial grooves in the plate 8> and drawn through the axial channels in the plate 8 and the tubular piece 16 by initiating the operation of the pulling device 13. Each of the six threads are a lOOO denier thread of polyethylene terephthalate yarn, and their tension is regulated at 113 g (each) by the tension regulation device 21. At the same time, the operation of the winding device 22 is set

started, and the device for moving the plastic material 30 is started. The plastic material 30 is a low-density polyethylene with a

temperature of 150°C. The vessel 20 is a two-chamber trough containing water at 81°C in the first chamber through which the fuse passes, and water of 21°C in the second chamber. This "two-zone cooling" helps to produce a more uniform cooling of the plastic sleeve and produces a tighter fit of the sleeve to the cage. The diameter of the part of the extruder 5 where the core 2 is formed is 0.076 cm. The distance between the opposing faces of the outer part 17 and the inner tubular part 16 of the nozzle 15 is such that it produces a polyethylene sleeve 4 with a thickness of 0.089 cm.

After the pulling device 13, the tension regulating device 21, the winding device 22 and the tub 20 are in operation, the piston 6 is advanced at a speed of 1.270 cm per minute. The explosive mass 28 is forced through the sieve 26, which sieves

out particles larger than 0.025 cm, and through the openings in the plate 27> and is formed into a solid core 2 with a diameter of 0.076 cm. The core exits the extruder 5 at a speed of 75.6 m/min, and the speed of the cage which is drawn forward by the pulling device 13 and wound up on the winding device 22 is matched to the core extrusion speed by signals received from the signal processor 24 - A vacuum is applied through the opening 18 to assist in collapsing the plastic sleeve onto the core containing cage 14 passing through the tubular member 16. A vacuum level of -150 mm of mercury is maintained in the tubular member 16.

The fuse 1 collected on the winding device 22 has an outer diameter of 0.254 cm, a core diameter of 0.076 cm and a polyethylene coating of 0.089 cm in thickness. The PETN charge in the core is 0.533 g/m (g/m PETN per cm coating = 6/1), and the core density is 1.5 g/cmJ. The filaments of the yarns surround the core substantially completely as shown in fig. 1. The fuse is flexible and light, and has a tensile strength of 441N.

The core, initiated by a No. 6 fuze cap, the end of which abuts axially against the free end of the fuse, detonates at a velocity of 690O m/sec. The fuse does not initiate itself from one part that is joined side to side with another. Detonation of a continuous length of fuse is propagated through knots of various types. The fuse is also difficult to initiate if the cap-to-fuse contact is not coaxial.

B. The same fuse is produced by the method described in section A above, except that the extrusion die 15 shown in fig. 4, is replaced with the nozzle 15 and the wire orientation plate 8 shown in fig. 3. In this method, the pulling device 13 pulls four strands of yarn through the cylindrical portion of the insert 33 under a tension sufficient to form them into a movable cage of longitudinally substantially parallel threads, the cage encloses the core, and the core-containing cage is pulled through the current of polyethylene flowing through the cylindrical part of the axial channel of the nozzle whereby a sleeve of soft polyethylene is applied around the cage. As with the method described in section A, essentially no reduction in the diameter of the core occurs during the operation.

Fuses with different core diameters, sleeve thicknesses,

and numbers of reinforcing yarns can be produced by the methods described above by suitable modification of die sizes and extrusion speeds.

The use of the low-energy detonating fuse according to the invention, and the effects of various parameters such as core charge and diameter, sleeve thickness and preparation, and the number and type of reinforcing yarn, are shown by the following examples.

Example 2

0.26 g of the superfine PETN described in example 1 is placed in a 0.08 mm thick aluminum sleeve with an embossed bottom, the end of which is butted against the side of a 3 m length of the fuse described in example IA, with the exception that the fuse in this case has a 0.127 cm diameter core with a PETN charge of 1.49 g/m. This fuse serves as the main fuse. One end of a 1.5m length

of the fuse described in example IA (descending fuse) is introduced into the aluminum housing (the carrier) so that it comes into contact with the PETN. The other end of the descending fuse is placed with its side against the percussion-sensitive element of a percussion-type delay cap. The main wire is detonated by means of a No. 6 trap cap with its end in coaxial connection with the free end of the fuse. The detonation is transferred from the main fuse to the transferor, from the transferor to the descending fuse, and from the descending fuse to the percussion delay cap.

The same results are obtained with main fuses with 2.13 g/m and 0.938 g/m, i.e. 0.152 cm and 0.102 cm diameter cores; and with descending fuses with 0.638 g/m and 0.469 g/m, i.e. 0.084 cm and 0.07 cm diameter cores.

Example 3

The following tests show the types of abuse with regard to knotting, stretching and rubbing which the fuse according to the invention is able to withstand.

A. One end of an 18 m long descending fuse described in Example IA is placed with its side against the percussion-sensitive element of a percussion-type delay cap. The cap is set into a 0.9 kg cartridge (flexible film tube with narrowed, sealed ends), 5 cm in diameter and 4l cm long containing a non-explosive preparation that simulates a water-gel explosive. The cap and fuse are held in position in the film cartridge with two half-pins. The cartridge is lowered into a simulated 15 m deep borehole under different loading conditions that could occur in field use, the simulated hole being inside a vertical 13 cm diameter steel pipe. The other end of the 0.533 g/m descending fuse is connected to the transferor, and the 1.49 g/m main fuse as described in Example 2. After the tube was charged under the conditions described, the main was detonated as in Example 2. The descending fuze detonates completely, and the percussion-sensitive pre-zinc cap detonates within its calculated time, after the descending fuse-cap cartridge assembly has been subjected to the following charging conditions: I. The cartridge is allowed to fall freely along the entire length of the descending fuse. II. The free fall of the cartridge is suddenly stopped every 4.6 m. III. The fuse moves along the rough edge of the steel pipe as the device is lowered into the pipe.

IV. Conditions II and III occur simultaneously.

V. A 3>2 kg sandbag is dropped into the pipe, removed and dropped again for a total of five times, on the device placed in the pipe in each of cases I, n, m and IV, and the sandbag scrapes against the fuse as it falls.

B. A knot is tied on the fuse described in Example IA, and a 3.2 kg weight is suspended from the end of the fuse. The weight is dropped into a 15 m tube described in part A above, while the free fall of the weight is stopped 5 times, thereby exerting increasing tension on the knot. Five fuses treated in this way, afterwards detonate completely, without any punching at the knots.

Example m pe1 4

The use of the fuses described in examples 1 and 2 to transmit detonation waves to the bottom charge of a column of explosive charges in the boreholes is as follows: 6 pcs. 7.6 m deep, 7.6 cm diameter boreholes 2.4 m apart are each charged with three aligned 5 x 4l cm cartridges, of a water gel explosive described in U.S. patent 3,431,155, wrapped in polyethylene terephthalate film. Inserted into the bottom cartridge in each hole is a percussion type delay cap, connected to the fuse (the descending fuse) described in Example IB in the manner described in Example 2. The other end of each descending fuse is connected to the main fuse described in Example 2 ( except that it has four yarn threads) in the manner described in Example 2. No framing is used. Detonation of the main fuse leads to subsequent detonation of the charge in the holes beginning with the bottom charge, as terminated by the delay time of the caps used. There is no sign of column interruption.

Oak sample 5- IO

The fuses are produced as described in example 1. The core of the explosive preparation is 76.1% by weight superfine PETN, 20.3% by weight acetyl-tributyl citrate and 3.6% by weight nitrocellulose. Four strands of the same yarn as described in example 1 are used. The same plastic coating preparation as described in example 1 is used. The core is extruded to different diameters, and coatings of different thicknesses are applied. The detonation characteristics of the fuses (initiated as described in example 1) are indicated in the following table:

<a>^This core is initiated and propagates a detonation in 50% of them

conducted trials, and all other cores detonate reliably.

turning diameter in cm.

These examples show that the detonation speed of the examined fuses is within the 6900 m/sec -5% range regardless of the PETN charge and the thickness of the plastic coating. However, with this particular core composition and a coating thickness of 0.112 cm, the reliability of the detonation is somewhat compromised by the minimal PETN charge and core diameter.

Example 11-14

The detonating fuse described in Examples 5-10 was tried

in three different core charges and diameters on initiation reliability and continued propagation at minimum coating thicknesses.

These examples show that as the core diameter and PETN charge increase, the plastic coating has a diminishing effect on the fuse's ability to initiate and propagate a detonation.

Example 15

The fuse described in examples 5-10 with a 0.076 cm core is produced with different coating materials and thicknesses. All samples (at least 46 m long) of the fuse with 0.051 cm, 0.071 cm and 0.084 cm thick coatings of low-density polyethylene, high-density polyethylene and a metal salt of a copolymer of ethylene and methacrylic acid (an ionomer resin) reliably detonate at a rate of about. 7200 m/sec with four threads as well as with eight threads of yarn. The extrusion die temperature is 175°C for the application of high density polyethylene, and 135°C for the application of the ionomeric resin.

The minimum tensile strength is 314N for all samples made with 4 threads of the yarn, and 628N for all samples made with 8 threads of the yarn. All samples, regardless of coating material thickness and type, detonate after the following treatment: A 2.7 kg weight is tied to one end of the fuse. The weight is allowed to drag the fuse by gravity over the edge of a concrete block, and then the fuse is pulled back to its starting point. This treatment is repeated 5 times.

Example 16-19

The effect of core charge and sleeve coating thickness on the behavior of the fuse described in examples 5-10 when tied, as may occur in field application, is shown in the following table:

These examples show that the indicated fuses propagate a detonation through knots instead of interruption at knots due to high explosiveness. They also show that as the explosive charge is increased, an increase in casing thickness will ensure propagation of the detonation through knots.

Example 2Q- 24

The fuse described in Examples 5-10 with a 0.076 cm diameter core is made with different numbers of multifilament threads of polyethylene terephthalate yarn (PET) and an aramid yarn made from the condensation polymer of terephthalic acid and p-phenylenediamine (all lOOO denier per thread). The effect of these variables on the fuse strength and the ability of the fuse to propagate a detonation through knots is shown in the following table:

These examples show that the tensile strength of the fuse for a given number of threads of the same denier varies with the tensile strength of the yarn. In this case, aramid provides a fuse with higher tensile strength with fewer threads than the polyester. The examples also show that a greater number of strands of a given fiber, or a stronger fiber, will increase the fuse's ability to propagate a detonation through tighter knots.

Example 25

A continuous solid core of a bonded explosive composition consisting of 75% by weight superfine PETN and 25% by weight of a binder consisting of a butadiene-acrylonitrile-methacrylic acid copolymer (described in the above-mentioned U.S. Patent 3,338,764) is attached to a single strand of aramid yarn produced from the condensation polymer of terephthalic acid and p-phenylenediamine. The core and supporting wire are drawn together through a tubular coating die which applies a 0.064 cm thick sleeve of low density polyethylene around them. The resulting fuse, which has a PETN charge of 1.49 g/m, detonates with approx. 7000 m/sec when initiated by the method described in example 1, and has a tensile strength of approx. 333N.

Example 26

The deformable bonded explosive preparation described in example 1 (except that the superfine PETN content is 76%, the acetyl-tributyl citrate is 20%, and the nitrocellulose is 4%) is extruded to form 10 pcs. 1.2 m long fuses, of which 5 have a 0.076 cm diameter (0.533 g/m PETN) and 5 have a 0.127 cm diameter (1.49 g/m PETN). The extruded cores are inserted into a low density polyethylene tube with an inner diameter of 0.152 cm and an outer diameter of 0.20 cm. The ratios of explosive charge to wall thickness in these cores are 18/1" respectively. 50/1, in g/m charges and thicknesses in cm. All fuses have tensile strengths of approx. 44N.

The fuses are initiated by a No. 6 fuse cap, the end of the fuse being coaxial to the free end of the fuse. All the fuses detonate without interruption, consuming all the plastic coating. The average detonation velocity for all 10 fuses is 7300 m/sec.

With the method according to the invention, essentially no reduction of the core diameter is produced after the core has been shaped. The method provides a high-density core without the need for a reduction in the diameter of the core as required e.g. by methods which produce fuses with a particulate explosive core. The elimination of a change in core diameter during manufacture facilitates process control in achieving a final core explosive charge and avoids penetration of the core by the surrounding yarns.

In detonating fuses of small diameter, low core charges, the presence of foreign particles, e.g. sand, metal, etc., interfere with the detonation of the fuse if the particles are sufficiently large. For this reason, an important feature of the present process is the production of a core explosive preparation which is free of such particles due to the methods and conditions used in its preparation, or the presence of particle-releasing materials in the extruder used to produce the core. For cores with diameters of 0.076 cm and larger, particles larger than 33% of the core diameter should be excluded. For cores of smaller diameter, particles larger than 0.013 cm should be excluded.

In the present process, as strands of yarn and the explosive core move separately into a plastic coating extrusion die, the cage formed therein will typically envelop the core, and the sleeve will then be formed on the core-enclosed unit. However, the cage formation, the core enclosing and the sheathing can take place essentially at the same time. Also, the two extruding devices of the apparatus, i.e. the core forming die and the sleeve forming die, may be components of separate extruders, or may be housed together in a single coextrusion unit.

Claims (21)

1. Low-energy detonating fuse (1) with a continuous, massive core (2) of explosive, containing 0.1 - 2 g/m of cap-sensitive, crystalline high explosive from the group consisting of the organic polynitrates and polynitramines, and with a mantle (4) which surrounds the core (2), characterized in that a) the explosive is a deformable explosive mass (2, 28) containing at least 55% by weight of the crystalline high explosive mixed with a binder, the particles of the high explosive has its maximum dimension in the range from 0.1 to 50 µm, and b) the jacket (4) consists only of at least one layer of plastic, which is able to flow at a temperature that does not exceed the melting point of the crystalline high explosive by more than 75°C. 2. Detonating fuse according to claim 1, characterized by a reinforcement (3) which surrounds the core (2). 3. Detonating fuse according to claim 2, characterized in that the reinforcement (3) essentially consists of one or more continuous yarn threads (9,10), which run on the circumference of the core (2) essentially parallel to the longitudinal axis of the core (2), and the reinforced core exhibits a tensile strength of at least 44 N. 4. Detonating fuse according to claim 3, characterized in that the reinforcement (3) consists of at least four yarn threads (9, 10) which are distributed essentially evenly around the outside of the core (2). 5. Detonating fuse according to claim 4, characterized in that the yarn is a multifilament yarn whose threads are distributed around the core (2). 6. Detonating fuse according to claims 1-5, characterized in that the explosive mass (2, 28) consists of at least 70% by weight of pentaerythritol tetranitrate and at least 0.4 g of pentaerythritol tetranitrate per m length, and that the mantle (4) consists of a thermoplastic. 7. Detonating fuse according to claims 1-6, characterized in that the binder is plasticized nitrocellulose. 8. Detonating fuse according to claim 6 or 7, characterized in that the thermoplastic is a poly-olefin which flows at a temperature below 200°C. 9. Detonating fuse according to claims 6-8, characterized in that the jacket (4) consists of polyethylene and the thickness of the jacket (4) is in the range from 0.013 to 0.318 cm. 10. Detonating fuse according to claims 4-9, characterized in that the reinforced core (2) has a tensile strength of at least 88 N. 11. Detonating fuse according to claim 10, characterized in that the jacket (4) is 0.013 to 0.191 cm thick. 12. Method for producing a detonating fuse according to claims 2-11, in which a continuous, massive core (2) is produced, yarn threads (9, 10) are pulled under tension and arranged essentially in parallel, longitudinally continuous strings, which include the core (2)r and a mantle (4) is placed, characterized in that the yarn threads (9, 10) form an advancing cage (14), and there around the advancing cage (14) a layer of soft plastic is placed, without thereby causing a significant change in the cross-section of the core (2) after it has been entrained in the cage (14), and the plastic is hardened. 13. Method according to claim 12, characterized in that the core (2) in the advancing wire cage (14) is strand pressed and then together with the wire cage (14) is fed into and through a strand press mold (15) in which the plastic is formed into a mantle (4) around the wire cage (14) with the horizontal core (2). 14. Method according to claim 13, characterized in that a tubular strand press mold (15) is used and that - the plastic tube formed therein is allowed to shrink on the core (2) surrounded by the cage (14) by applying a vacuum to the strand press mold. 15. Method according to claim 12, in which the yarn threads (9, 10) and the core (2) are supplied separately, characterized in that the yarn threads (9, 10) and the core (2) are fed to a strand press mold (8) in which the threads (9, 10) are arranged into a cage (14) and that the plastic sheath (4) is formed by passing it off the cage (14) ) surrounded core (2) through a plastic flow (30). 16. Method according to claim 12, characterized in that the mixture is formed into the massive core (2) under vacuum. 17. Device for producing a detonating fuse according to claims 2-11 with a mechanism for continuously forming a massive core (2) of high explosive, a mechanism for applying a jacket (4) to the core (2), a mechanism for orient the yarn strands substantially parallel to each other in an annular arrangement, and a mechanism for pulling off the substantially parallel strands under tension, characterized by) a first strand press (5) for forming a mass (28) of deformable, binder-containing high explosive to a continuous massive core (2), b) a thread orientation mechanism (8; 33), which in connection with the first strand press (5) has such a position that the threads (9, 10) as advancing cages (14) take with them and further passes the core (2) emerging from the first strand press (5), c) another strand press (15) for applying a soft plastic (30) in the form of a mantle (4) to a carrier running through it, the other string press (15) has such a position in relation d to the first strand press (5) and the thread orientation mechanism (8; 33) that the core (2) surrounded by the cage (14) runs as a carrier for applying the mantle (4) through the second string press (15), without the cross-section of the core (2) being reduced before, simultaneously with or after, and d) a device (20) for passing through the core (2) provided with the mantle (4) and the cage (14), to harden the plastic (30). 18. Device according to claim 17, characterized in that the cylinder (29) of the first string press (5) has an opening (25) for applying a vacuum. 19. Device according to claim 17 or 18, characterized in that the cylinder (29) of the first string press (5) has a sieve (26) to retain particles that are larger than 25% of the cross-section of the core (2). 20. Device according to claims 17-19, characterized in that the first string press (5) is a piston string press whose cylinder (29) is provided with heating hoses (7). 21. Device according to claims 17 - 20, characterized in that organs (23, 24) are connected to the first string press (5) and to the string pulling mechanism (13) which monitor the pressing from the first string press (5) and can accordingly regulate the speed of the string pulling mechanism (13) .