Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to low energy explosive
shock tubing of the NONEL (Reg. TM) type. In particular, the
invention relates to an explosive shock tube which is capable
o the lateral initiation of a second, attached length of
shock tube.
Explosive shock tubing as disclosed in Canadian Patent
No. 878,056 granted August 10, 1971 is now widely known and
used inthe blasting art. This shock tubing or detonating
fuse consists of small diameter, for example, 3 millimeters
outside diameter tubing of a pliable plastic, such as,
polyvinyl chloride, polyethylene, SURLYN (Reg. TM) or the
like having an inner diameter of about 1.3 millimeters. The
inner walls of the tubing has adhered thereto a thin layer
of powdered explosive or reactive material, such as PETN
(pentaerythritol tetranitrate), HMX (cyclotetramethalene-
tetranitramine) or powdered metal mixtures with these.
When initiated at one end by means of an appropriate device,
such as a detonating cap, a percussion or impact wave is
propagated within and along the tubing to activate a blasting
cap attached at the remote end of the tubing. When initiated,
substantially all of the percussion wave energy is confined
within the tube and little or no damage to the tube wall
occurs. Indeed, it has been stated that the absence of any
lateral energy allows an initiated shock tube to be held in
the hand without great risk o~ injury. Explosive shock
tubing may be employed in most instances as a replacement for
conventional detonating cord in non-electric blasting and has
the advantage of low noise, safe handling and low cost.
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While the characteristic of lack of sideways or lateral
bursting of conventional shock tube is advantageous from a
safety viewpoint, the absence of the characteristic of
lateral energy output preclud~s the use of shock tubing as
a trunk line in non-electric blasting networks. Shock
tubing heretofore has been limited for use as branch lines
which are initiated by attachment to conventional, solid-core
detonating fuse trunk line. Such conventional detonating
fuse or detonating cord has a strong lateral energy output
which is capable of initiating any properly attached shock
tube branch lines.
In the preparation of non-electric, multiple-change
blasting networks, a conventional detonating cord trunk line
is laid along the ground surface or blasting face surface
with the attached shock tube branch lines passing into the
boreholes to set off the cap/charge combinations within the
borehole. A large noise factor is associated with such
circuits from the detonation of the surface trunk line
which noise is objectionable, particularly on construction
projects in built-up areas. There is a need, therefore,
for a substantially noiseless, cost-effective alternative
to detonating cord trunk line.
SUMMA _ OF THE INVENTION
A modified explosive shock tube is provided which is
capable of lateral initiation of an attached length of a
second shock tube, which modified shock tube may be employed
as a substantially noiseless trunk line in a non-electric
blasting network.
The modified shock tube of the invention comprises a
hollow, elongated flexible tube, the inner surface of which
has a thin layer of powdered, energy-producing material
distributed thereon. The shock tube has, at periodic
intervals along its length, areas which are capable of
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bursting to release energy laterally to the axis of the
tube upon initiation of the energy-producing material
within the tube. In particuIar, the invention comprises
an explosive shock tube, the walls of which are perforated
at intervals along its length by means of elongated cuts
or slits, the cuts or slits being protected with a -thin
outer layer or covering of waterproofing material. When the
shock tube is initiated, the area of the slits will open and
energy from the deflegrating material within the tube will
be delivered through the opened slits at right-angles to the
tube axis. This lateral energy is used to initiate a second
length of shock tube the end of which second tube is
positioned in alignment with the slits.
A more detailed explanation of the invention is provided
in the following description in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a partly cut-away perspective view of the
shock tube of the invention showing the slit areas;
Figure 2 is a cross-section of the shock tube of
Figure 1 taken along the line A-A; and
Figure 3 shows a partly cut-away perspective view of
a connection between the shock tube of the invention as a
trunk line and a conventional shock tube as a branch line.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to Figures 1 and 2, there is shown a shock
tube generally designated 1, which contains an inner core
hollow tube of plastic material 2. The inner wall of tube 2
has adhered thereto a thin layer of powdered explosive or
reactive material 3. A thin overcovering material 4 is shown
enveloping tube 2. Tube 2, as shown in the cut away section,
contains a series of cuts or slits 5 which penetrate the tube
walls. In the Figures, the thickness of overcovering 4 as
shown is exaggerated for clarity.
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Tube l is manufactured by first preparing a shock tube
in a conventional manner, that is, plastic tubing 2 is
extruded through a die with the simultaneous application of
powdered explosive material to the inner wall thereof.
The plastic of tubing 2 is preferably made from SU~,YN, a
salt-containing polyethylene ionomer, although any flexible
plastic having good properties of adherence for the powdered
explosive material can be used. The size of tube 2 is
typically about 3 mm. outside diameter and 1.3 mm. lnside
diameter. After extrusion, tube 2 is passed lengthwise
through a reciprocating cutter device where a series of slits
5 from about .5-l cm. in length are made, preferably two to
four in number, through the tube walls and spaced equally
around the circumference of tube 2. Slits 5 may be made at
any chosen linear interval along tube 2. After being slit,
tube 2 is passed through a second circular die where a thin,
external layer or coating of flexible waterproofing material
4 is bonded to the outer surface of tube 2. Alternatively,
a spray coating of waterproofing material may be applied or
a short length of adhesive tape, for example, vinyl tape can
be applied over the area of the slits. A suitable water-
proofing material 4 is, for example, a polyethylene/ethylvinyl
acetate blend. To assist in locating the position oE the
slits 5 in the finished tube 1, the slit area of tube 2 may
be coloured with an ink or dye prior to overcoating with
waterproof layer 4.
With reference to Figure 3, there is shown a generally
T-shaped connector device lO mounted on or around trunk line
shock tube l. Connector 10 may comprise any configuration
which permits the location of an end 11 of branch line shock
tube 12 close to the slits 5 in trunk line 1. Connector 10,
as shown, is typical of several known T-shaped connectors
used in assembling blasting fuse circuits. In the depiction
shown in Figure 3, connector lO which is hinged and sized
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to fit tightly around trunk line 1, is mounted so that its
cen-tral area is adjacent to slits 5 in trunk line 1. Le~
portion 13 of connector 10 is si~ed to receive in tight-
gripping relationship the end of a shock tube branch line 12
which is inserted into leg portion 13 until tube end 11 is
close to or abutting slits 5, When trunk line 1 is initiated
at a remote end (not shown), the energy pulse within tube 1
opens the slits 5 and bursts the o~ercoating layer 4 to
initiate the energetic material 14 within branch line 12.
A similar trunk-to-branch connection can be made at each
slit location alongtrunk line 1, each branch line leading to
a separate explosive charge in a blasting network.
EXAMPLE I
A NONEL shock tube containing 25 mg/m of a mixture
comprising 8~ powdered aluminium and 92% powdered HMX
explosive was prepared for us~ as a trunk line in a 200 meter
length. At one meter intervals along its length, two
diametrically opposite slits .6 cmO in length were made in
the shock tube trunk line. The location of each slit was
indicated with an inked mark. The slit NONEL was overcoated
by an extrusion process with a clear polyethylene/ethylvinyl
acetate sleeve 0.2 mm. in thickness. Fifty standard NONEL
receptor tube lengths were positioned adjacent the marked
slits in the trunk line using the connector shown in Figure
3 and the trunk line was initiated at one end by means of a
NONEL-type initiator. All 50 receptor tube lengths were
initiated without failure. In a similar test using un-slit
NONEL having the same explosive content as the trunk line,
no branch line initiation occurred.
EXAMPLE II
The test described in Example I was repeated at
temperatures of -40C employing ten branch line connections.
No failures occurred.
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It has been observed that the material employed as
the overcovering or waterproofing material 4 should be chosen
so as to avoid fragmentation during initiation since small
fragments of material could cause blockage of the end of the
receptor or branch line tune and interfere with energy
transfer between trunk line and branch line. It will be
obvious to those skilled in the art, that the open end of
the receptor branch line must be kept dry to ensure initiation.
It will also be obvious that the amount of energetic material
employed on the inner wall of tube 2 must be sufficient to
provide energy adequate to open slits 5 and to rupture
overcovering 4. The amount of energetic material used will
depend on the length of slit 5, flexibility of the material
of tube 2 and the thickness and resistance to rupture of the
overcovering material 4.
