WO2008074037A2

WO2008074037A2 - Rigid time delay element

Info

Publication number: WO2008074037A2
Application number: PCT/ZA2007/000084
Authority: WO
Inventors: Michael David Edwards White; Cornelius Du Plooy Conradie
Original assignee: African Explosives Limited
Priority date: 2006-12-11
Filing date: 2007-12-04
Publication date: 2008-06-19
Also published as: AR064808A1; CL2007003586A1; ZA200710622B; WO2008074037A3

Abstract

A rigid time delay element formed from a metallic tube with a reduced bore at a first end, an igniter composition inside the bore at the first end, a first segment of a pyrotechnic composition inside the bore, abutting the igniter composition, and a second segment of the pyrotechnic composition inside the bore abutting the first segment at an interface surface, and wherein the pyrotechnic composition adjacent a portion of the interface surface is compressed to a lesser extent than the remainder of the pyrotechnic composition.

Description

RIGID TIME DELAY ELEMENT

BACKGROUND OF THE INVENTION

[0001] This invention relates to a rigid time delay element for generating a precise time delay between a firing signal and subsequent initiation of a detonator.

[0002] A known time delay element of the aforementioned kind includes a lead tube which is filled with a particulate timer composition and then subjected to a drawing/rolling process which reduces the tube diameter and increases its length. Simultaneously the particulate timer composition is compressed.

[0003] The tube is cut into a plurality of sections. Each section is of the same relatively short length and contains a starter element with a substance which is more sensitive than the timer composition and an adjacent igniter/sealer element containing a composition which, in practice, is exposed to a signal from a shock tube. The igniter/sealer composition is more sensitive but less energetic than the starter substance.

[0004] The use of lead in a time delay element is expensive and can give rise to toxic side effects. Attempts have therefore been made to use aluminium instead of lead.

[0005] The aluminium has a heat sink effect which drains energy, produced during the combustion process, particularly from the relatively slow burning timer composition. This is due to the high heat conductivity factor of aluminium, compared to lead, and has the outcome that the propagation of the burning process inside the bore of the tube is unreliable and in some instances does not occur. [0006] It is desirable to eliminate the starter substance. This means that the igniter/sealer would be in direct contact with the timer composition. It is necessary to compact the timer composition, inside the aluminium tube, to obtain good propagation characteristics and this can result in the interface between the igniter/sealer and the timer composition, or between adjacent timer increments which make up the timer composition, being rendered unreliable in transferring combustion from the igniter/sealer composition to the timer composition, or between adjacent timer increments.

[0007] It is an object of the present invention to provide a rigid time delay element which, at least partly, addresses the aforementioned factors.

SUMMARY OF INVENTION

[0008] The invention provides a method of making a rigid time delay element which includes the steps of incrementally placing at least a particulate pyrotechnic timer composition in a plurality of doses, into a bore of a tube and, after each dose, compacting the composition within the bore by applying different pressures to different portions of an exposed surface of the dose within the bore.

[0009] Successive doses may be placed into the bore at a single work station. Alternatively the tube may be moved to successive work stations and, at each work station, a respective dose of the timer composition may be placed into the bore of the tube and then compacted.

[0010] The exposed surface may include first and second regions which are axially displaced within the bore and which are each substantially at a right angle to a longitudinal axis of the bore, and an intermediate region between the first and second regions.

[0011] The method may include the step of configuring the tube so that it has a diameter and wall thickness which cause the tube to act as a heat sink and which prevent combustion of the pyrotechnic composition at an inner wall surface of the tube. This allows a core of the pyrotechnic timer composition surrounded by a non- combusted portion of the pyrotechnic composition at the wall surface to bum in a reliable manner.

[0012] An igniter composition may be placed initially into the tube and this may be followed by the aforementioned pyrotechnic timer composition. The igniter composition may be compacted by applying different pressures to different portions of an exposed surface of the igniter composition within the tube.

[0013] The tube may be preformed to a desired shape before placing the pyrotechnic timer composition into the bore of the tube. The tube may be formed so that at one end which, during the placement of the composition is a lower end, the bore is of a reduced cross section.

[0014] The invention also provides a rigid time delay element which includes a tube and at least a pyrotechnic timer composition in a bore of the tube wherein the pyrotechnic timer composition comprises at least two segments with an interface surface between the segments with at least part of the interface surface lying in a plane which is not at a right angle to a longitudinal axis of the bore of the tube.

[0015] The tube is made from any suitable material, e.g. aluminium. [0016] The pyrotechnic timer composition may include a plurality of the segments. Adjacent segments may abut at respective interface surfaces of the kind referred to.

[0017] Viewed axially i.e. along the length of the bore the respective interface surfaces may be angularly displaced relatively to each other, or they may be aligned.

[0018] An outermost surface of a segment at a mouth of the bore and the tube may be flat or plane.

[0019] The pyrotechnic composition may abut an igniter composition at an interface surface of the kind referred to.

[0020] The interface surface may have first and second regions which are axially displaced from each other. Each region may be substantially at a right angle to a longitudinal axis of the tube, and an intermediate region may be located between the first and second regions.

[0021] The thickness of each segment in an axial sense, i.e. in a longitudinal direction of the bore of the tube, is preferably at least twice the length of the axial displacement of the first region from the second region.

[0022] The tube may have a diameter and bore thickness so that the tube acts as a heat sink and prevents combustion of the pyrotechnic timer composition at an inner wall surface. This allows a core of the pyrotechnic timer composition, which is surrounded by a non-combusted portion of the composition at the inner wall surface, to combust effectively and reliably. [0023] At one end the tube may be shaped so that its bore is of a reduced cross section.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The invention is further described by way of example with reference to the accompanying drawings in which:

Figure 1 is a side view partly sectioned of a rigid time delay element according to the invention;

Figure 1(a) shows a modification to a tube used in the time delay element;

Figure 2 is a side view of part of the element shown in Figure 1; Figure 3 is a cross sectional view of the time delay element taken on a line III - III in

Figure 2;

Figure 4 is a side view of a punch for use in the manufacture of the time delay element of the invention; and

Figure 5 is an enlarged side view of a lower part of the punch which is enclosed in a circle marked "5", in Figure 4.

DESCRIPTION OF PREFERRED EMBODIMENT

[0025] It has been noted if an attempt is made to replace a lead tube in a typical rigid time delay element with a tube made from some other metal, such as aluminium, that the propagation of the burning process of the timer composition can be unreliable and in some instances does not occur. Aluminium has a higher thermal conductivity than lead and this can cause the aluminium tube to display a heat sink effect which drains energy during the combustion process, particularly from the relatively slow burning timer composition. This can interfere with and sometimes prevent the propagation of the burning process.

[0026] If the wall thickness of the aluminium tube is decreased, in order to reduce the heat sink effect, the aluminium may melt or burn together with the timer composition.

[0027] One aspect of the present invention is based on the adverse effect of the heat sink characteristic of the aluminium tube. It has been found if the diameter of the bore of the tube is increased, above the norm, that a layer of unbumed timer composition is predictably and reliably formed at an inner surface of the aluminium tube. This layer is not efficient at conducting heat and effectively isolates a core of the timer composition, which is surrounded by the non-heat conductive layer, from the heat sink effect of the aluminium tube. The inner core of the timer composition then burns reliably and consistently at a predictable and repeatable burning rate. Typically the dimensions of the aluminium tube are: outer diameter approximately 6,5mm; and wall thickness approximately 1 ,5mm.

[0028] The insulating effect which is produced by the non-heat conductive layer of unbumed timer composition, adjacent the inner wall surface, is put to use in a second way. Traditionally the igniter/sealer composition is the last ingredient loaded into a lead tube. The reason for this is that the igniter/sealer composition, in powder form, can contaminate an inner surface of the tube. When the igniter/sealer is ignited the contaminant can propagate the combustion process inside the tube, essentially in parallel to the combustion process which takes place as the slow timer composition burns. This effect is undesirable for the propagation process should depend on combustion of the timer composition only. [0029] In the time delay element of the invention the igniter/sealer composition is the first ingredient placed into the aluminium tube. This has certain manufacturing advantages.

[0030] In a lead-based system, as noted, the igniter/sealer composition is uppermost. At least part of this composition, when ignited, can be ejected from the tube. To stop this the lead tube is restricted e.g. by being crimped or capped. With the invention the igniter/sealer composition is placed first into the tube. This is possible due to the thermal insulation effect produced by the layer of unbumed timer composition at the inner wall surface of the tube. This layer covers the contaminant and prevents ignition thereof.

[0031] The combustion process, in a lead-based time delay element, is normally transferred from the igniter/sealer composition to the timer composition by a starter composition which is less sensitive and more energetic than the igniter/sealer composition, yet more sensitive and less energetic than the timer composition. The manufacturing process can be facilitated if the starter composition is eliminated.

However if the igniter/sealer is placed first in a tube, followed by the timer composition, a problem is encountered in that the propagation of ignition across the interface between the igniter/sealer and the timer composition is unreliable. It is necessary to compact the timer composition to achieve a uniform and predictable burning rate. The high pressure which is exerted by the composition step, on the timer composition, which is in powder form, causes the density of the composition to increase and at the interface with the igniter/sealer the timer composition becomes "glassy", a term used to describe a smooth and dense surface which, at times, is unable to transfer the ignition process reliably into the timer composition. [0032] Figures 1 , 2 and 3 are two side views and a cross sectional view respectively of a time delay element 10 constructed in accordance with the principles of the invention.

[0033] The element 10 includes a rigid aluminium tube 12 with an inner diameter 14 of the order of 3,6mm and a wall thickness 16 of the order of 1 ,5mm.

[0034] An igniter/sealer composition 18 is placed into a bore 20 of the tube from an upper end 22 and pushed to a lower end 24 which is suitably shaped (as referred to hereinafter) to prevent the loss of material so that the lower end can be exposed to a shock tube, not shown, using appropriate techniques.

[0035] Thereafter a timer composition 26 is placed in the bore 20. This is done incrementally, in a plurality of doses, with successive segments or doses 28, 30, 32 and 34 being placed one after the other into the bore. After each dose is placed in the bore it is compacted using a punch 40 of the type shown in Figures 4 and 5. This punch includes a stem 42 with a stepped operative end 44 and a chuck 46 which allows the punch to be coupled to a mechanised device, not shown, which can apply a precisely controlled force to the punch. This aspect is known in the art and therefore is not further described.

[0036] The composition 18 may be inclined to flow from a lower end of the tube 12, particularly during the filling and compacting steps. Loss of the composition can be countered in any suitable way e.g. by placing a cap (not shown) over the lower end.

A convenient technique is, however, shown in Figure 1 (a) which depicts a modified tube 12(a) with a lower end 24 which is preformed to be of reduced diameter 14A. The diameter 14A is sufficiently small to prevent the composition from flowing through the lower end 24, but sufficiently large to allow an effective transfer of energy from the composition 18 to a shock tube (not shown) in use of the element. Another benefit of the small diameter hole is that, in use, it prevents the burned, almost melted, residue of the composition inside the tube from uncontrollably venting back into the shock tube giving erratic timing results.

[0037] Another advantage associated with the modification of Figure 1 (a) is that filling of the tube can take place from one side only, and there is no need to invert the tube between successive dosage placement and compaction steps.

[0038] An interface surface 50 is formed between the igniter/sealer 18 and the first dose 28 of timer composition, and further interface surfaces designated 52, 54 and

56 are formed between the doses 28 and 30, 30 and 32, and 32 and 34, respectively.

[0039] The shaped operative end 44 of the punch is designed to apply different pressures to different regions of each interface surface.

[0040] The punch has planar surfaces 44A and 44B respectively displaced from each other, by a distance 44C, by an intermediate sloping surface 44D. The thickness 58 (in an axial direction) of each dose of timer composition (see Figure 1) is at least twice the distance 44C. This ensures that the integrity of each dose is not destroyed by operation of the punch i.e. the dose is not broken into two or more parts.

[0041] The punch has rounded or radiused edges 60, at the operative end 44. This eliminates sharp corners at the operative end which could possibly gouge small pieces from the inner surface of the aluminium tube. Such pieces would be contaminants and could interfere with the combustion process of the timer composition.

[0042] The shape, in an angular sense, and the axial length, of the surface 44D are important. If these parameters are not correct, for example if the surfaces 44A and 44B are small and the surface 44D effectively goes from one inner side of the tube to a diametrically opposed inner side of the tube, then the application of the punch to the timer composition could force the composition outwardly, and cause the wall of the tube to bulge outwardly. This is not desirable.

[0043] Figures 2 and 3 illustrate the interface surface 52, merely by way of example. This surface has first and second regions 62 and 64 which are axially displaced from each other in a longitudinal direction of the bore and which are generally at a right angle to a longitudinal axis 66 of the bore. An intermediate region 68 extends between the regions 62 and 64 and lies in a plane which is inclined at an acute angle 70 to the longitudinal axis.

[0044] The force which is applied to the punch is transmitted to the underlying surface of the composition over the three regions 62, 64 and 68. The regions 62 and 64 have surfaces which are at a right angle to the force application direction i.e. the direction of the longitudinal axis 66. The areas of these regions are relatively large. On the other hand the projected area of the region 68 in the longitudinal direction, i.e. the area which is shown in the cross sectional plan view of Figure 3, is comparatively small compared to the true area of the region 68. Thus the force exerted by the punch on the region 68 is less than the forces which are exerted on the regions 62 and 64. As a result the pressure on the region 68, which determines the degree of compaction of the underlying material in the region 68, is meaningfully reduced. In addition the total contact area between the upper and lower segments of the tinner composition is increased, compared to the situation which would result if a flat, non- profiled punch were to be used. These factors allow effective propagation of the combustion process to take place from the igniter/sealer to the timer composition, particularly via the region 68, and thereafter within the timer composition which can then burn reliably.

[0045] The successive doses of timer composition can be introduced into the bore of the tube at a single work station and can then be compacted by the same punch. In this case the interface surfaces 52, 54, 56 etc. would be aligned with one another in a longitudinal sense. It is possible though to move the tube to successive work stations and, at each station, to introduce a dose of the timer composition and then to compact the dose using a respective punch located at that work station. In this case it is likely that the interface surfaces, viewed in an axial direction down the bore of the tube, would be angularly displaced from one another depending on the angular orientation of the respective punch, relatively to the tube, at each work station.

[0046] The timer delay element of the invention holds important benefits in that it permits reliable use to be made of an aluminium tube or a tube which is made from a metal which has similar heat conducting properties to aluminium, wherein the igniter/sealer composition can, if required, be placed first into the tube and wherein the need for an intermediate starter composition is obviated. A greater choice of timer compositions is possible primarily because the timer delay element is more thermally stable due to the lower heat sink effect which results from the insulating layer at the inner tube surface. Compositions which are less sensitive can be reliably used. More precise timing can be achieved.

Claims

1. A method of making a rigid time delay element which includes the steps of incrementally placing at least a particulate pyrotechnic timer composition in a plurality of doses, into a bore of a tube and, after each dose, compacting the composition within the bore by applying different pressures to different portions of an exposed surface of the dose within the bore.

2. A method according to claim 1 wherein the exposed surface includes first and second regions which are axially displaced within the bore and which are each substantially at a right angle to a longitudinal axis of the bore, and an intermediate region between the first and second regions.

3. A method according to claim 1 or 2 which includes the step of configuring the tube so that it has a diameter and wall thickness which cause the tube to act as a heat sink and which prevent combustion of the pyrotechnic composition at an inner wall surface of the tube, whereby a core of the pyrotechnic timer composition surrounded by a non-combusted portion of the pyrotechnic composition at the inner wall surface is allowed to burn in a reliable manner.

4. A method according to any one of claims 1 to 3 which includes the initial steps of placing an igniter composition into the tube and compacting the igniter composition by applying different pressures to different portions of an exposed surface of the igniter composition within the tube, prior to the steps of placing the doses of the particulate pyrotechnic timer composition into the bore of the tube.

5. A method according to any one of claims 1 to 4 which includes the step of performing the tube so that at one end which, during the placement of the pyrotechnic timer composition is a lower end, the bore is of a reduced cross section.

5 6. A rigid time delay element which includes a tube and at least a pyrotechnic timer composition in a bore of the tube, wherein the pyrotechnic timer composition comprises at least two segments with an interface surface between the segments with at least part of the interface surface lying in a plane which is not at a right angle to a longitudinal axis of the bore of the

0 tube.

7. A time delay element according to claim 6 wherein the interface surface includes first and second regions which are axially displaced within the bore and which are each substantially at a right angle to a longitudinal axis of the bore, and an intermediate region between the first and second regions.

5 8. A time delay element according to claim 6 or 7 wherein the tube has a diameter and bore thickness so that the tube acts as a heat sink and prevents combustion of the pyrotechnic timer composition at an inner wall surface whereby a core of the pyrotechnic timer composition, which is surrounded by a non-combusted portion of the pyrotechnic composition at

!0 the inner wall surface is able to combust effectively and reliably.

9. A time delay element according to claim 6, 7 or 8 wherein, at one end, the tube is shaped so that its bore is of a reduced cross section.

10. A time delay element according to any one of claims 6 to 9 which includes, at one end of the tube, an igniter composition inside the bore of the tube which abuts the pyrotechnic composition.

11. A rigid time delay element formed from a metallic tube with a reduced bore at a first end, an igniter composition inside the bore at the first end, a first segment of a pyrotechnic composition inside the bore, abutting the igniter composition, and a second segment of the pyrotechnic composition inside the bore abutting the first segment at an interface surface, and wherein the pyrotechnic composition adjacent a portion of the interface surface is compressed to a lesser extent than the remainder of the pyrotechnic composition.