GB2249373A - Cushion element for detonators - Google Patents

Abstract

An improved detonator has a cushion element 11 disposed within a channel of the detonator housing 13. The element is provided with resilient, pliable and shock absorbant surface for contact with, and retention of explosive material 20, 22 in the detonator housing. A signal communicating surface such as a membrane or hole is formed on the element for communicating an initiating signal to the explosive material; as shown this is a hole 35 which may be covered by a mesh (40) or membrane (43). The element 11 may be of paperboard, polyethylene, rubber or polyurethane. A cushion element having a diametric interference fit with the wall of the housing 13 is inserted into the channel, and pressed towards the enclosed end of the housing for compacting the explosive material between the element and the housing closed end. A plurality of cushion elements may be detachably supported by a sheet for simultaneous insertion into a plurality of detonator housings. <IMAGE>

Description

CUSHION ELEMENT FOR DETONATORS AND THE LIKE; APPARATUS AND METHOD OF ASSEMBLY This invention generally relates to devices for amplifying and transmitting a blast initiation signal, and more particularly to an improved detonator construction and method of assembly.

In blasting operations, a variety of electric and non-eRectric devices may be used to amplify and transmit a blast initiation signal including detonators, ignitors, delay detonators, initiators and the like. As used herein, the term "detonator" is intended to generally refer to the variety of devices wiiici amplify and transmit an initating signal.

Detonators are generally comprised oE an elongated tubular housing closed at one end and open at the other end, and contain one or more pyrotechnic or explosive charges positioned within the housing adjacent the closed end.

The output of the detonator is proportional to the density and quantity of the explosives as well as their chemical composition, and, therefor, a small diameter press pin is used to compress the explosives within the housing. To achieve the desired explosive density, pressing forces as high as 300 pounds (6,000 psi on a 0.254 inch OD pin) and higher may be used.

It has been found that many explosives are more sensitive to initiation by impact or friction if compaction energy is applied to the surface of the explosive using a hard surface. The degree of sensitization is a function of the hardness, i.e., increasing hardness causes increased sensitivity. Because of the high pressures required to achieve the desired detonator explosive density, pressing of explosive within the housing is usually accomplished using a press pin, typically steel in direct contact with the explosive, thereby causing sensitization of the explosive and increasing the hazard associated with pressing the explosive.

The combination of the hardness of the press pin, alignment of the tooling, the breaking of the explosive into smaller particles during compression, and the explosive being under high conpresslon, provide conditions which, unless very carefully controlled, can lead to inadvertant and unwanted activation of the detonator.

Another potential mechanism for inadvertently initiating the detonator during compression of the explosive is displacement of small particles of explosive into the annular space between the pin and the hosing as 3S consolidation of the explosive occtirs. As the press pin is in motion while being inserted into the housing, while pressing the explosive material or while being withdrawn from the housing, particles of explosive material trapped between the housing and the press pin surfaces are subjected to a substantial amount of friction which could lead to unwanted detonation.

After compression of the explosive within the housing, the detonator is usually shipped between manufacturing facilities as well as to the use site, and during such shipment, the detonators are subjected to vibration, shaking and shock which tends to loosen the compressed explosive material. The loose particles of explosive are typically very sensitive to initiation by friction and static electricity and therefor present a safety hazard during transportation and subsequent handling of the detonator at the new location.

It is therefor a primary object of the invention to provide an improved detonator having increased resistance to inadvertent detonation during assembly of the detonator and which retains explosive materials compacted within a housing of the detonator during shipment and storage of the detonator.

It is another object of the invention to provide a cushion element for use with a detonator which reduces the opportunity for friction initiation of the detonator during compression of explosive materials within a housing of the detonator and which thereafter retains the explosive materials in the compressed location within the housing.

It is a further object of the invention to provide a cushion element for use with a detonator having a membrane-type signal communicating surface for reliable communication of an initiation signal to explosive materials of the detonator while, at the same time enhancing the desired qualities of improved resistance to inadvertent detonation and improved ease of manufacture.

It is a still further object of the invention to provide an improved method of assembling one or more detonators which provides improved resistance to shock during manufacture, shipping, and other external sources and friction initiation of explosive materials during compression of the materials within the housing of a detonator and improved retention of the materials compressed within the housing.

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

A netter understandlng of the objects, advantages, features, properties and relations of the invention will be obtained from the following description and accompanying drawings which set forth certain illustrative embodiments and are indictive of the various ways in which the principals of the invention are empoyed.

device for amplifying and transmitting an initiating signal constructed according to the present invention comprises a tubular housing having an axial channel formed therein, the housing having a closed end and an open end opposite the closed end; compressed explosive material received in the channel and positioned against the closed end; a cushion element disposed within the channel in juxtaposition with, and substantially covering the explosive material, the element having a pliable and shock absorbant surface facing the material, and in contact therewith for retaining the explosive material against the closed end; and a barrier-type signal communicating surface formed on the cushion element for communicating the signal to the explosive material.

In further accord with the present invention, the cushion element has a diametric interference fit with the housing interior side walls for retaining the compressed explosive material against the closed end.

In accordance with this invention, a new and improved method is disclosed which includes inserting explosive material into an axially extending channel of a tubular housing; inserting a cushion element having a signal communicating surface into the channel, the element covering the cross-section of the channel and having a diametric interference fit with the housing interior side walls; and pressing the element to,ards the closed end of the housing, thereby comFacting the explosive material between the element and the housing closed end.

tn further ccord with tie method of this invPittiori, tlo h,)llRing ic positioned in a vertical orientation with the closed rnl helow tho npn end prior to inserting the explosive material into the channel, wherebv residual explosive material adhearing to the housing is dislodged by th element during pressing, and the materials thereafter fall by gravity towards the closed end.

In still further accord with the method of this invention, a plurality of accurately spaced, pre-cut or otherwise formed cushion elements are detachably supported by a sheet and each element is registered to align with the open end of one of a plurality of detonators for simultaneous insertion of the elements into the detonator housings.

Specific embodiments of the present invention are now described, by way of example only, with reference to the accompanying drawings, in which: FIG. 1 is a longitudinal, cross-sectional view of a non-electric, instantaneous detonator having a cushion element of the present invention; FIG. 2 is a longitudinal, cross-sectional view of a non-electric, delay detonator having the cushion element of the present invention; FIG. 3 is a longitudinal, cross-sectional view of an electric, instantaneous detonator having the cushion element of the invention; FIG. 4 is an enlarged view of the cushion element taken on-line 4-4 of FIG. 1; FIG. 5 is a cross-sectional viek of the ushi-- ernent taken on-line S-5 of FIG. 4;; FIG. 6 is an enlarged view of an alternative embodiment of the cushion element of FIG. 4; FIG. 7 is a cross-sectional view taken on-line - of FIG. 6.

FIG. 8 is an enlarged view of an alternative embodiment of the cushion element of FIG. z; FIG. 9 is a cross-sectional view taken on-line 9-9 of FIG. 8; FIG. 9A illustrates the aperture in each cushion element filled by the imitating charge; FIG. 10 is a longitudinal, cross-sectional view of a detonator teing assembled with a cushion element of the invention, a press pin being shown in phantom; FIG. 11 is a top plan view of a sheet containing a plurality of cushion elements of the invention; and FIG. 12 is an enlarged view of detail A of FIG. 11.

The cushion element of the present invention is particularly suited for use with a device for amplifying and transmitting a blast initiation signal, e.g., a detonator. The element provides a pliable and shock absorbant surface for contact with explosive material within the housing of the detonator which minimizes friction initiation of the material during compression within the housing, i.e., during manufacture, and which thereafter retains the material compressed within the housing and reduces the opportunity for shock initiation of the detonator during subsequent handling.

Referring to rIG. 1, a cushion element 11 of the in -rt ion is shok disposed within a non-eleztric instantaneous detonator 12. The detonator comprises a generally tubular shaped housing 13 having an axially extending channel 14 formed therein, with a closed end 16 and an open end 1 opposite the closed end 1t. As used herein, the term "channel" is intended to generally refer to the tubul3r enclosed passage defined by the interior surfaces or side walls of the tubular housing. A First charge (base charge) 20 of a secondary explosive is positioned within the channel charge) 22 is positioned within the channel 14 in juxtaposition with the base charge 20.

The base charge 20 comprises a secondary explosive such as pentaerythritol tetranitrate (PETN) or cyclotrimethylenetrinitramine (RDX) to provide the principal output or signal amplifying capability of the device. The initating charge 22 comprises a primary explosive such as lead azide, lead styphnate or dia odinitrophenol (DEEP1. Such primary explosives are characteristically very sensitive to initiation by heat in the form of flame, spark, friction or impact, and serve to detonate in response to an initiating signal to activate the base charge.

The cushion element 11 is positioned in juxtaposition with the initiating charge 22, and is dimensioned to provide a slight diametric interference fit with the housing interior side walls. In the preferred embodiment, a 0.003 to 0.005 inch interference is considered adequate; however, the interference may vary depending on the material selected to form the cushion element to thereby provide the desired retention of the explosive materials within the channel.

The cushion element 11 should be made of an easily deformable form-sustaining material having a soft, pliable consistency for shock ahs'r'are. The preferred element material is paper board; -eer, polymers such as polyethylene, rubber and polyurethane are also suitable element materials.

The normal functioning of the detonator requires that a signal transmission device transmit an initiating signal to be applied to the initiating charge 22 to cause it to detonate and thereby 3Cti3tP the base charge 20. The signal can take the form of a detonation shock wave from a shock tube 25, a deflagrating flame front from a deflagrating type tube, or detonating cord in a non-electric, instantaneous detonator 12 (FIG.

1). The signal can also take the form of a thermal pulse, such as from a pyrotechnic time delay elemcnt 2' in a delay detonator 8 tFIG. ), or from an ignition charge 30 activated by a bridge wire 31 in both instantaneous and delay-type electric detonators 32 (FIG. 3), or an electric match.

For reliable communication of the initiating signal to the initiating charge 22, a signal communicating surface 35 is formed in the cushion element. Referring to FIGS. 1, 4, 5, 6 and 7, the signal commnicating surface comprises a structure having at least one hole with sufficient open space to allow the initiating signal to pass through to, and cause initiation of, the initiating charge 22. Additionally, the pattern should have sufficiently small hole size to thereby act as a barrier to retain the explosive materials 20, 22 against the closed end 16.The pattern may be formed by providing the element 11 with at least one central through hole 38 (FIGS. 4 and 5), or by providing the element 11 with a central through hole 38 covered by a mesh or screen 40 (FIGS. 6 and '). Referring to FIGS. 8 and 9, the signal communicating surface 35 may also be formed by providing the element with a central through hole 38 covered by thin membrane 43 whicht acts as retaining @@@@@@@ or membrane while allowing the initiating signal to pass through with sufficient ease that the reliability of initiating the explosive materials is not substantially reduced.Suitable membrane materials Include thin, porous tissue paper adhered to a surface of the element 11, or other non-metallic woven materials capable of passing the initiating signal Other membrane materials include non-porous inert films isch as cel li:lose acetate, or self consuming materials, including high nitrugen content nitrocellulose, which decompose rapidly upon exposure to the initiating signal. All such membrane elements provide the desired improved shock resistance.

For completeness, it is noted that hole or aperture in cushion element 11, whether closed by screen 40 or membrane 43 or if these are a plurality of smaller holes in cushion element 11, the space is filled by the initiating charge 22 typically, lead azide. Enlarged Eigure 9A shows that configuration with the memebrane embodiment; such a "filled hole" feature was omitted from the other figures for purposes of clarity in the other figures.

Assembly of a detonator with the cushion element of the present invention greatly minimizes the inadvertent initiation of the detonator explosive material during assembly. Referring to FIG. 10, the base charge 20 and the initiating charge 22 are first positioned within the channel 14. The cushion element 11 is then inserted in the channel 14 and is pushed into position in juxtaposition with the initiating charge by action oE a press pin 48.A small, annular channel 49 is formed between the press pin 48 and the housing 13 because of the clearance required to allow the pin 48 to be easily inserted and withdrawn from the housing. ts the element 11 moves down the bore of the housing 13, the interference fit Wet-ee the element and the housing dislodges any residual explosive material adhering to the housing within the channel, thus preventing any particles of explosive from becoming lodged in the annular channel 49 between the press pin 48 and the housing 13. The pressing operation occurs with the housing 13 oriented in a vertical or upright position with the closed end 16 positioned below the open end 17, and any loose explosive dislodged by the cushion element 11 during the pressing operation falls by gravity onto the yet unpressed explosive material.

As the cushion element 11 is inserted into the channel and pressed, the signal communicating surface 35 allows entrapped air to escape thereby preventing backpressure which could buckle or ruptire the element.

Additionally, the element acts as a filter, capturing small particles of explosive entrapped in the air stream, thereby preventing the explosive from contaminating the space above the element.

Continued insertion of the press pin 48 within the channel 14 will cause the cushion element 11 and the explosive material 20, 22 to become compressed against the closed end 16. The cushion element 11 maintains a separation between the press pin 48 and the initiating charge ', and provides a pliable surface which minimizes friction while contacting the explosive material to achieve the high explosive density without the explosive material significantly contacting the hard surface of the press pin 48. Similarly, when use with a delay element, similar separation and protection between the delay element and powder is provided.

Once the desired compression force has been applied to the explosive material, the press pin 48 is withdrawn from the channel 14, and the cushion element 11 maintains the explosive material compressed within the housing. Additional lv, no loose and unpressed explosive is left adhering to the sides of the housing within the channel, thereby eliminating the need to clean the housing internal surfaces and discard collected loose explosive material. The cushion element also minimizes the loosening of the explosive material caused by vibration and shaking of the detonators during shipment and storage.

It has been found that assembling a detonator with the cushion element oE the invention greatly improves the resistance of the detonator to shock initiation, i.e., initiation.caused by exernally applied forces such as the shock wave from the detonation of an adjacent borehole, striking the detonator, etc. Test results indicate that a ,!tenator assembled with a cushion element of the initiation is three (3) times more resistant to initiation hy shock or impulse relative to a detonator assembled without a cushion element. Shock resistance is further improved where the cushion element is provided with a membrane signal communicating surface 43 (FIG. 9).Test results further indicate that the cushion element of the invention with a membrane provides improved detonator shock resistance over prior art detonators specifically designed for improved shock resistance.

Referring to FIG. 2, it is often desirable to provide the detonator with a delay element 27. The delay element may be inserted into housing 12 immediately after insertion of the explosive materials and the cushion element 11. Compressive forces applied to the delay elements is transmitted through the delay elements to the cushion element and in turn to the explosive to cause consolidation of the explosive materials. Conversely, a detonator may be made by pressing the explosive via the cushion element to cause consolidation of the explosive material and then insertirg the delay element which can be pressed onto the cushion element, further seating the delay element against the cushion element and simultaneously consolidating the explosive material.Because loose particles of explosive material are substantially eliminated during the pressing process by the cushion element 11, inadvertent initiation of the detonator caused by friction ignition of particles of explosive trapped between the housing and sides tulip delav element during insertion of the delay element within the detonator is substantially eliminated. Additionally, tulle elimination of loose particles of explosive material reduces premature initiation of the detonator by explosive material between the delay element and the housing which would allow the initiating signal to bypass the delay element.

For ease of assembly at a reduced cost, a plurality of detonators typically are simultaneously assembled in an automated assembly process using a process block (not shown) which can accommodate from 50 to 500 housings. After placement of the housings in the process block, explosive material is inserted into the channel of each housing for subsequent compression. The cushion element of the present invention is well suited for use in such an automated assembly process. Referring to Figs. 11 and 12, a plurality of cushion elements 11 may be die cut from fiber board or molded of plastic in a sheet pattern 52 wherein each element is maintained and positioned in the sheet in a pattern that conforms to the pattern of the process block (not shown).The sheet 52 is then placed above the detonators with each element in the sheet registered to align with an open detonator end allowing the simultaneous compression of all the elements within the housings.

If the detonators include a delay element 27, delay elements may be positioned between each cushion element 11 and the press pin 48, and the compression force is thereafter applied directlv to the delay element by the press pin as described hereinbefore.

Each element 11 is detachably held in the sheet 52 by one or more holding tabs zD which have a reduced material thickness allowing them to easily break away under the force of a press pin 48 (FIr. 101.

The foregoing tab relationship has been found to work well with sheet card hoard but ostler meats can he provided for detachability and other sheet materials can be used.

Although the invention has been illustrated and described with respect to exemplemary embodiments thereof, it should be understood by those skilled in the art that the f o r ego i n g v a r i o us o t h e r changes , ommisions and additions may be made therein and thereto, without departing from the spirit and scope of the invention.

Claims (19)

CLAIlS

1. A detonator comprising: a tubular housing having interior sidewalls which define an axially extending channel therein, said housing having a closed end and an open end opposite said closed end; explosive material received in said channel and positioned against sald sed el a cushion element disposed within said channel, in form sustaining juxtaposition with said material, said element substantially covering said material and having a pliable and shock absorbant surface facing said material and in contact therewith for retaining said materia against said closed end; and a barrier-type signal communicating surface formed on said element for communicating an initiating signal to said material, whereby said signal initiates said material for amplifying and transmitting the signal.

2. The detonator according to claim 1 further comprising a delay element having a pyrotechnic composition for delayed transmission of the initiating signal between a first and second side of the delay element, said delay element being positioned within said clannel with said second side in juxtaposition with said signal communicating surface.

3. The detonator according to claim 1 further comprising a signal transmission device for transmitting the initiating signal, said device being disposed within said channel with an end of the device adjacent said signal communicating surface.

4. The detonator according to claim 2 further comprising a signal transmission device for transmitting the initiating signal, said device being disposed within said channel with an end of the device adjacent said delay element first side.

5. The detonator according to any preceding claim wherein said signal communicating surface comprises a menbrane which is consumed by the blasting initiation signal.

6. The detonator according to any preceding claim wherein said signal conrunicating surface comprises a pattern having a least one hole, said pattern having sufficient open space to allow the blasting initiation signal to pass through to, and cause ignition of said explosive material, said pattern having sufficiently small hole size to retain said explosive raterial compacted against said closed end.

7. The detonator according to any preceding claim wherein said cushion element has a diametric interference fit with said housing interior side walls for retaining said explosive material against said closed end.

8. A shock resistant detonator for amplifying and transmitting an initiating signal transmitted by a signal transmission device comprising: a tubular housing having an axially extending channel formed therein, said housing having a closed end and an open end opposite said closed end; explosive material received in said channel and positioned against said closed end; a cushion element disposed within said channel between said material and an end of the transmission device, said element being in form sustaining juxtaposition with said material, said element substantially covering said material and having pliable surface facing said material and in contact therewith for retaining said material against said closed end; and a signal communicating surface formed on said element for communicating the initiating signal between the end of the transmission device and said material, whereby the signal initiates said material for amplifying and transmitting the signal.

9. The detonat5. according to claim 8 further comprising a delay element having a pyrotechnic composition for delayed transmission of the initiating signal disposed within said housing between said element and the end of the transmission device.

10. A method of making a detonator, which comprises: providing a tubular housing having interior side walls which define an axial channel therein, said housing having a closed end and an open end opposite said closed end; inserting explosive material in said channel open end; inserting a cushion element having a signal communicating surface into said channel open end, said element covering the cross-section of said channel and having a diametric interference fit with said housing interior side walls; and pressing said element towards said closed end whereby said element provides a pliable surface for compacting said explosive material against said housing closed end.

11. The method of claim 10 including holding said housing in a vertical orientation with said closed end positioned below said open end prior to inserting said explosive material into said channel wherebv anv residual amount of explosive material adhearing to said housing is dislodged by said element during pressing said element towards said closed end by the diaZetric interference between said element and said housing, the 'dislodged explosive material falling by gravity towards said closed en.

12. The method of claim 10 or claim 11 including: providing a delay element having a pyrotechnic composition for delayed transmission of the blasting initiation signal between a first and second side of said delay element; positioning said delay element within said channel with said second side in juxtaposition with said signal communicating surface; and pressing said delay element towards said closed end.

13. The method of any one of claims 10-12 wherein said signal communicating surface comprises a membrane which is consumed by the blasting initiation signal.

14. The method of any one of claims 10-13 wherein said signal conunicati surface conprises a pattern having at least one hole, said pattern having sufficient open space to allow the blasting initiation signal to pass through to, and cause ignition of said explosive material, said pattern having sufficiently small hole size to retain said explosive material compacted against said closed end.

'5. A te.h~4 cf raking detonators which cerises: providing a plurality of tubular housings each having an axially extending channel, said housings having a closed end and a open end opposite said closed end; holding each of said housings in a vertical orientation with said closed end positioned below said open end, each housing being held in spaced relation to other of said housings; inserting explosive material ,into the channel of each of said housings; positioning a plurality of cushion elements above said housing open ends, each element being registered to align with one of said open ends; and pressing said elements towards said closed ends for compacting said explosive material against said closed ends.

16. The method of claim 15 wherein each element is provided with a signal communicating surface for communicating an initiating signal to said explosive material.

17. The method of claim 15 or claim 16 further comprising the steps of: providing a plurality of accurately spaced, pre-cut cushion elements detachably supported by a sheet; and positioning said sheet with each element being registered to align with each of said housing open ends.

18. A detonator substantially as hereinbefore described with reference to and as shown in one or more of the accompanying drawings.

19. A method of making a detonator substantially as hereinbefore described with reference to and as shown in anv one or more of the accompanying drawings.