Note: Descriptions are shown in the official language in which they were submitted.
2038765
CUSHION ELEMENT FOR DETONATORS AND THE LIKE;
APPARATUS AND METHOD OF ASSEMBLY
FIELD OF INVENTION
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.
BACKGROUND OF THE INVENTION
In blasting operations, a variety of electric and non-electric
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 which amplify and transmit an initating
signal.
Detonators are generally comprised of 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.
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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 compression, 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
housing as consolidation of the explosive occurs. 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.
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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.
OBJECTS OF THE INVENTION
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.
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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 better understanding 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 employed.
SUMMARY OF THE INVENTION
A device for amplifying anci 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, ancl 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.
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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 towards the
closed end of the housing, thereby compacting the explosive material
between the element and the housing closed end.
In further accord with the method of this invention, the housing is
positioned in a vertical orientation with the closed end below the open
end prior to inserting the explosive material into the channel, whereby
residual explosive material adhearing to the housing is dislodged by the
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.
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BRIEF DESCRIPTION OF THE DRAWINGS
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. l;
FIG. 5 is a cross-sectional view of the cushion element taken
on-line 5-5 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 7-7 of FIG. 6.
FIG. 8 is an enlarged view of an alternative embodiment of the
cushion element of FIG. 5;
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 initating charge;
FIG. 10 is a longitudinal, cross-sectional view of a detonator being
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. ll.
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DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS
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 FIG. 1, a cushion element 11 of the invention is shown
disposed within a non-electric, 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
17 opposite the closed end 16. As used herein, the term "channel" is
intended to generally refer to the tubular 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 capabi]ity of the
device. The initating charge 22 comprises a primary explosive such as
lead azide, lead styphnate or diazodinitrophenol (DDNP). 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.
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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
absorbance. The preferred element material is paper board; however,
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 activate 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 element 27 in a delay detonator 28 (FIG. 2), 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
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communicating 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 7~. 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 a thin membrane 43 which acts as a retaining barrier 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 such as cellulose
acetate, or self consuming materials, including high nitrogen 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 figure 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.
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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
of 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. As the
element 11 moves down the bore of the housing 13, the interference fit
between 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 rupture 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
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separation between the press pin 48 and the initiating charge 22, 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. Additionally, 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 assembiing a detonator with the cushion
element of 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 detonator
assembled with a cushion element of the initiation is three (3) times more
resistant to initiation by 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.
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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 inserting 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 the delay
element during insertion of the delay element within the detonator is
substantially eliminated. Additionally, the elimination of loose particles
of explosive material reduces premature initiation of the detonator by
explosive material between the de3ay 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
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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
houslngs.
If the detonators include a delay element 27, delay elements may be
positioned between each cushion element ll and the press pin 48, and the
compression force is thereafter applied directly to the delay element by the
press pin as described hereinbefore.
Each element ll is detachably held in the sheet 52 by one or more
holding tabs 55 which have a reduced material thickness allowing them to
easily break away under the force of a press pin 48 (FIG. lO).
The foregoing tab relationship has been found to work well with sheet
card board but other means can be 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 skillec
in the art that the foregoing and various other changes, ommisions and
additions may be made therein and thereto, without departing from the spiri
and scope of the invention.
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