Cartridge end-closure

Abstract

 A tubular explosives cartridge (12) having at least one end closed by a transverse closure disc (11) of resilient closed-cell foamed synthetics plastics material, for example, foamed polyethylene, the disc having an uncompressed diameter greater than the internal diameter of the tubular cartridge case. The closure provides a waterproof seal without preventing communication of detonation between cartridges and may be readily pierced for the insertion of a detonator. When the closure consists of nitrogen blown polyethylene, it does not increase the risk of firedamp ignition in coal mines.

Description

[0001] This invention relates to explosive cartridges and to a method of closing an end of an explosive cartridge. The invention is especially advantageous for cartridged nitroglycerine gelatinous explosives and aqueous slurry explosive compositions.

[0002] Explosive compositions have for many years been packaged in rigid tubular cardboard or tough plastics containers, the ends of the containers being closed by inwardly crimping a portion of the container wall or by covering the end with a thin cardboard or plastics disc. This form of closure is not water resistant and the cartridges were not suitable for use in wet surroundings unless the explosive composition was itself water resistant.

[0003] When employed in cartridged form in a wet drill- hole for rock blasting, deformable, water sensitive explosive composition such as aqueous slurry explosives are usually packed in closed waterproof bags of thin flexible synthetic plastics such as polyethylene and the bags are placed inside rigid or tough tubular containers of cardboard or plastics material. Because of the need to seal the plastics inner bags and plug the ends of the rigid containers there is often excessive space between the explosive contents of adjacent cartridges so that with relatively insensitive explosives in small diameters the detonation may fail to propagate between adjacent cartridges in file. There is therefore a requirement for a cartridge case to protect the explosive composition from ambient water without unduly interfering with the communication of detonation between cartridges. Moreover for cartridges to be used in coal mines it is desirable that the materials used in the cartridge case should not increase the risk of igniting firedamp in the vicinity of the explosion.

[0004] Accordingly it is an object of this invention to provide an improved waterproof end-closure for a rigid cartridge case.

[0005] In accordance with this invention at least one end of a tubular cartridge case is closed by a transverse closure disc of resilient closed-cell foamed synthetic plastics material, said disc having an uncompressed diameter greater than the internal diameter of the tubular case. The dimensions of the closure disc may be readily chosen such that the disc resilience causes sufficient outward radial pressure against the case that the case is sealed against the ingress of ambient water.

[0006] The cartridge case end may be plain but more reliable retention of the closure disc is obtained if the end rim of the tubular case is folded over internally to form an end bead, or if one or more internal projections, for example, one or more cannelures are formed in the end portion of the case to engage the closure disc and hold it in position.

[0007] The degree of foaming and the thickness of the closure disc are not narrowly critical, the practical requirement being that the foamed material should be sufficiently resilient to provide the required seal. Thus a foamed material having a density of 30 to 70 kg/m³ is generally satisfactory and discs having a thickness to uncompressed diameter ratio of 1:2.5 to 1:5 are generally sufficiently robust to provide an adequate seal, to resist displacement and to be readily pierced for the insertion of a,detonator in the field.

[0008] For cartridges to be used in mines where incendive gases are present, it is preferred that the closure disc should be made from foamed polyethylene, preferably blown with an inert gas, for example, nitrogen. A convenient, easily made closure disc is one shaped by stamping from a sheet of foamed plastics material, for example, closed-cell foamed polyethylene commercially available under the name "Plastazote" (Registered Trade Hark) from Bakelite Xylonite Limited.

[0009] The uncompressed diameter of the disc is preferably 10 to 25% greater than the internal diameter of the cartridge case. Thus for a case having an internal diameter of 30 mm a disc having a diameter of 33 to 37.5 mm, preferably 35 mm is convenient. The thickness of the disc in this case is conveniently 7 to 12 mm.

[0010] The cartridge case may conveniently be made from any of the materials commonly used for that purpose, for example, cardboard, helically wound multi-ply paper or extruded plastics material.

[0011] The invention is further illustrated by the cartridge end-closures illustrated in the accompanying drawings wherein

[0012] Figs. 1 to 4 inclusive are longitudinal medial sections of cartridge ends closed in accordance with the invention. In the drawings like parts are depicted by the same numeral.

[0013] In the cartridge end of Fig. 1 a disc 11 of resilient closed cell polyethylene is inserted into an end portion of a cartridge case 12. In its normal state the disc 11 has a greater diameter than the internal diameter of the cartridge case 12 and is compressed to permit its insertion into the cartridge case. Because of its resilient nature the disc 11 presses tightly around its periphery against the case 12 and seals the case against the ingress of water, so that the explosive contents of the. cartridge, which may advantageously be aqueous slurry explosive or a powdered explosive based on ammonium nitrate, are protected from ambient water. The other end of the cartridge case 12 may if desired be closed with a similar closure although any other waterproof closure may be used.

[0014] In Fig. 2 the end of the cartridge case 12 is turned over to form a top bead 13 against which the disc 11 abuts so that the disc is more difficult to displace from the case 12 by distortion of the case.

[0015] In the cartridge end of Fig. 3 the cartridge case is shaped at the end by two cannelures 14 and 15 between which the disc 11 is positively located.

[0016] The cartridge end of Fig. 4 is a modification of the cartridge end of Fig. 3 in which the mouth end of the cartridge case 12 is extended beyond the cann- elure 14 to provide an internally threaded portion 16 which is adapted to receive an externally threaded portion of an end of an adjacent cartridge. By screwing the cartridge ends together a file -of cartridges may be readily assembled for loading in a borehole.

Claims

1. A tubular cartridge case closed at least at one end by a transverse closure disc characterised in that the closure disc is made from resilient closed-cell foamed synthetics plastics material, and has an uncompressed diameter greater than the internal diameter of the tubular case.

2. A cartridge case as claimed in Claim 1 wherein the closure disc has dimensions such that the disc resilience causes sufficient outward radial pressure against the case that the case is sealed against the ingress of ambient water.

3. A cartridge case as claimed in Claim 1 or Claim 2 wherein the end rim of the tubular case is folded over internally to form an end bead to retain the closure disc in the cartridge case end.

4. A cartridge case as claimed in any one of Claims 1 to 3 inclusive wherein one or more cannelures are formed in the end portion of the case to engage the closure disc and hold it in position.

5. A cartridge case as claimed in any one of Claims 1 to 4 inclusive wherein the foamed material has a density of 30 to 70 kg/m³.

6. A cartridge case as claimed in any one of Claims 1 to 5 inclusive wherein the closure disc has a thickness to uncompressed diameter ratio of 1:2.5 to 1:5.

7. A cartridge case as claimed in any one of Claims 1 to 6 inclusive wherein the synthetics plastics material of the closure disc is foamed by blowing with an inert gas.

8. A cartridge case as claimed in any one of Claims 1 to 7 inclusive wherein the closure disc has been shaped by stamping from a sheet of foamed plastics material.

9. A cartridge case as claimed in Claim 8 wherein the closure disc has been stamped from a sheet of closed-cell foamed polyethylene.

10. A cartridge case as claimed in any one of Claims 1 to 11 inclusive wherein the uncompressed diameter of the closure disc is 10 to 25% greater than the internal diameter of the cartridge case.

11. An explosive cartridge comprising gelatinous nitroglycerine explosive composition, aqueous slurry explosive composition or ammonium nitrate powder explosive composition encased in a tubular cartridge case as claimed in any one of Claims 1 to 10 inclusive.

Drawing