GB2153053A

GB2153053A - Casting explosives

Info

Publication number: GB2153053A
Application number: GB08501190A
Authority: GB
Inventors: Klas Larsson
Original assignee: Bofors AB
Current assignee: Saab Bofors AB
Priority date: 1984-01-19
Filing date: 1985-01-17
Publication date: 1985-08-14
Also published as: IT8547554A0; SE8400238D0; ES8605748A1; IT1182118B; CA1306611C; NL8500071A; SE8400238L; SE451717B; IT8547554A1; GB8501190D0; FR2558463A1; GB2153053B; DE3501286A1; FR2558463B1; ES539665A0

Abstract

The present invention relates to a method and an apparatus for production of cavity-free cast explosive charges of meltable crystalline explosives. According to the invention the formation of solidification cavities in the explosive is prevented by keeping the upper part of the casting warm with the aid of the casting funnel (3) which is used during the casting and which contains a heat-retaining medium contained within a chamber formed in the casting funnel so as to be in heat- communicating relationship with the upper part of the mould into which the explosive is cast. <IMAGE>

Description

SPECIFICATION Method and apparatus for casting of explosives The present invention relates to a method for preventing the occurrence of air inclusions (cavities or pipes) in cast explosive bodies fabricated from meltable explosives which are crystalline in solidified condition, e.g. trinitrotoluol (TNT) and others. The invention also relates to an apparatus, a so-called casting funnel of special configuration, for carrying out the method.

The invention has particular application to the casting (filling) of the explosive charge in artillery shells and other warheads.

In artillery shells and other charges exposed to accelerations, any and every air inclusion comprises a potential safety risk, because the enclosed air, through adiabatic compression brought about by the acceleration of the shells upon the actual firing, can initiate an ignition of the charge while this remains in the barrel.

An ignition of this kind is known as a bore premature.

The demand for greater ranges for artillery has led to a transition to more slender shells and higher ejection velocities.

This has led to aggravation of the difficulties in casting shell entirely without air inclusions, and at the same time the risk of even minute cavities giving rise to bore prematures has increased.

In order to avoid the formation of air inclusions when casting explosives, efforts have previously been made to control the solidification of the molten explosive in such a manner that the air inclusions are counteracted. For this purpose use is commonly made today of a casting funnel provided with a built-in heating coil and a heating rod immersed in the centre of the shell into the molten explosive, the said heating rod being successively lifted up out of the shell in accordance with a throughly tested schedule. In many cases, moreover, the shell stands in a water bath, the temperature of which is also controlled in accordance with a specific schedule.This method thus calls for a thorough temperature control of both casting funnel and heating rod, and possible cooling, as well as positional control of the heating rod, all adapted to the specific shell type and explosive in question.

The temperature of the heating rod and heater coil in the casting funnel must be kept above the solidification point of the explosive in order for a cavity-free casting to be attained.

This temperature, however, must not be too high as too high a temperature can give rise to an undesirably coarse crystalline structure with low strength and poor initiability.

There are also serious problems with the apparatus used in the above-described casting method. For safety reasons it is necessary to use water-flushed heating coils in the casting funnel and heating rod. Since the heating rods can not be excessively thick, the heating coils within them must have small cross-sectional areas, so that there is a risk of clogging in particularly confined passages or where there are abrupt changes of direction.Consequently, operation of the method on a commercial scale means that one must not only have access to large solidification facilities where the newly-cast shells are given a possiblity of solidifying slowly in accordance with a predetermined and tested schedule, but also that one must have access to temperature regulating and control means which individual ally control the solidification process of each shell so that no shell in a series deviates from the others and contains larger or smaller cavities brought about by e.g. a faulty heating rod which in consequence of e.g. a partly clogged heating coil, has attained an excessively low temperature.

The relative complexity of the casting method now used has naturally led to attempts to arrive at simpler and cheaper solutions. One such solution is described in EP A81 200997.5, according to which there is proposed a method of avoiding solidifications cavities in shell castings by keeping the upper, front part of the newly-cast shell warm by means of an extra amount of molten explosive which communicates directly with the explosive in the shell and which is kept contained in a powerful heat-insulated casting funnel which surrounds the upper front part of the shell.It is asserted that with a sufficient extra amount of explosive in the casting funnel, which before the explosive is supplied has been heated to the melting temperature of the explosive, and with correctly chosen insulation around the casting funnel, it is possible to control the solidification of the explosive in the shell in such a manner that the explosive in the upper part of the shell, which also functions as a casting ingate, solidifies last, whereby solidification cavities can be avoided.

In the European application, two different embodiments of casting funnel are described.

One is provided with a space for the extra amount of explosive which, along a relatively thin partition wall, extends down along the foremost part of the shell so that the minimum cross-section of the shell at the muzzle is completely surrounded by explosive. In addition there is a roomy space for explosive above the shell muzzle. According to the second variant the molten explosive on the outside of the shell tip is replaced by particularly effective thermal insulation.

A disadvantage of the procedure and the apparatus according to the aforesaid European Application is the large amount of explosive which is consumed for keeping warm the upper part of the shell casting during solidification of the casting. This implies for pur poses of economy that large amounts of explosive continually have to be returned for remelting which is unfortunate because several consecutive remeltings of crystalline meltable explosives have a tendency to affect the properties of the explosive in a decidedly negetive manner. If the explosive used comprises a mixture of different explosive substances the necessary remeltings may also give rise to a change in the composition of the explosive.

According to the present invention, the heat-retaining function is provided, not by means of surplus explosive, but by a heatretaining medium built into the casting funnel but kept separate from the explosive. This heatretaining medium has a melting point which lies within i 304C from that of the explosive, and obviously it is desirable that it should have a high melting heat. It has proved to be especially suitable to choose heat-retaining media whose melting points lie within i 1 0 C from the melting point of the explosive.With regard to the melting heat of the heat-retaining medium, i.e. the amount of heat which is liberated upon its solidification, this desirably should exceed the melting heat for the explosive concerned which in the majority of cases is likely to consist of trinitrotoluol (TNT) or explosives containing trinitrotoleune.

In order to make the method of the invention more economic the heat-retaining medium is desirably of a type that can withstand a very large number of melting/ solidification cycles without being materially changed. The heat-retaining medium will then not be degraded upon heating to its molten state at the same time as the casting funnel or otherwise before casting is commenced.

The work devoted during recent years to development of heat storage media for, in particular, solar heating plants has meant that there are now available a number of meltable salts which can withstand a very large number of remelting/ solidification cycles and which have melting points that lie within the desired temperature interval. As an example of such a salt, mention can be made of magnesium dinitrate hexahydrate Mg (NO3)2 ~ 6 H2O. This salt has a solidification point of approx. 89"C which is very well adapted to the solidification point of trinitrotoluol which is approx. 80"C.

This makes the salt in question very suitable for use as the heat retaining medium in many instances, since trinitrotoluol is the most commonly used meltable explosive. The heat-retaining medium may consist of a salt or of a mixture of several different salts, or some other substance with high melting heat.

For performance of the method according to the invention, then, use is made of a casting funnel, which normally will be thermally insulated and which is elaborated as an ingate.

This casting funnel is positioned adjacent the upper part of the casting mould which it surrounds with a relatively small play. As pointed out heretofore the mould consists in the majority of cases of a shell body which is to be filled with explosive. The casting funnel contains a suitable heat retaining medium which is kept out of direct contact with the explosive but which is in heat-communicating relationship with the upper part of the casting mould. Advantageously, the heat-retaining medium is contained within an enclosed annular chamber which will completely surround the upper part of the casting mould when the casting funnel is in position.

A casting funnel in accordance with this invention, for the casting of an explosive of a defined melting point, is characterized in that it provides a first space which is adapted, when the casting funnel is mounted on top of the casting mould, to surround the upper part thereof and which at least partly is filled with a meltable heat-retaining medium having a melting point which lies within i 30 C of said defined melting point of the explosive, and in that said casting funnel is provided with a second, centrally-arranged, space elaborated as an ingate which extends at least partly above said first space with its heat retaining medium.

Preferably, the space for the heat-retaining medium forms an enclosed annular chamber, and both this space and the space defining the ingate of the funnel are enclosed by heat insulating material.

In the preferred embodiments of the casting funnel of the invention, the space for containing the heat-retaining medium is provided with a partition wall adapted to face the casting mould and configured to the form thereof. The partition wall consists at least in part of deformable material. The filling of the space is performed to advantage in such an amount that a certain positive pressure relative to the atmospheric pressure develops in the space when all heat-retaining medium has been melted down. In consequence, if the casting mould, the shell and the casting funnel are heated together, then the slightly flexible wall of the chamber will be forced into direct contact against the casting mould, to ensure good heat conduction between the heat-retaining medium and the casting mould and the explosive contained therein. There may however be occasions when it is pre ferred, for one reason or another, to heat the casting funnel and casting mould separately.

This may be appropriate, for example, if one wishes fo give them somewhat different starting temperatures, e.g. when the heat-retaining medium has a higher melting temperature than that of the explosive. In the latter case it may also be necessary to provide some insult tion between the chamber and the casting mould so that the latter does not acquire an excessively high temperature.

Above the first space of the casting funnel, and the heat-retaining medium or substance with a high melting heat contained therein, is a second space elaborated as a conventional ingate or casting funnel. Both of these spaces are suitably surrounded by a conventional thermal insulation. In addition, a thermally insulated cover, possibly provided with space of its own for a heat-retaining medium, can be applied above the inlet of the casting funnel to the second chamber after the casting mould has been filled with explosive.

It has been found that the casting funnel can be made advantageously of a suitable plastics, with sufficient heat resistance, e.g. a suitable polyester.

A casting funnel of the type described briefly heretofore must be tailor-made for each individual type of shell. This has become feasible now that computer programs have been made available with the aid of which it is possible to calculate how the explosive solidifies in a shell of a certain specific type and how it is influenced by a casting funnel of the type described heretofore. During the solidification the shell is permitted to stand freely or possibly with its lowermost part submerged in a water bath.Once the calculation values have been checked with practical trials and found to be correct there is no reason to carry out individual temperature measurements during the casting of each shell because the method according to the invention permits a uniform solidification process in all shells which are provided with the same type of casting funnel, which has been heated to the same temperature and supplied with the same amount of melted explosive, and permitted to cool in the same place. The extra amount of explosive in the ingate can then be made very small because the material in the casting funnel is that which solidifies last.

An embodiment of the apparatus according to the invention will now be described in somewhat greater detail and with reference to the accompanying, somewhat schematic, drawing.

The drawing shows a longitudinal section through an artillery shell (1) the lowermost, or rear, part of which rests in a water bath (2) and disposed above the upper, or front, of which is a casting funnel (3) in accordance with the invention. The casting funnel (3) provides a first space (4) closed towards all sides, which, via a relatively thin partition wall (5), delimits towards the outer side of the shell (1) and which is filled with a heatretaining medium, e.g. molten salt (6) with a solidification heat and in a total amount which have both been accurately adapted to the specific explosive (7), which, via a second space (8) of the casting funnel elaborated as an ingate or casting funnel proper, has been supplied to the interior (9) of the shell. The lower part of the casting funnel (3) is surrounded by an insulation (10), e.g. of polyeurethane, and after casting of the explosive its upper part is covered with a heat-insulating cover (11). For certain types of shell it may be appropriate to provide a closed chamber containing heat-retaining medium within the cover (11). It is then possible to use the same or different heat-retaining media in the cover (11) and the space (4).

Also shown in the drawing is a thread guard (12) which serves to prevent the explosive from penetrating up into the front fuse thread of the shell.

Claims

1. A method for preventing air inclusions in cast, normally crystalline, meltable explosives by slowing the cooling of the upper part of the casting so that it solidifies last, wherein there is used for the casting of the explosive a casting funnel which provides a space in heatcommunicating relationship with the upper part of the casting mould into which the explosive is cast, and which contains, out of direct contact with the explosive, a molten heat-retaining medium whose melting point is within + 30 C of that of the explosive.

2. A method according to Claim 1, wherein the heat-retaining medium is a meltable salt whose melting heat is not less than that of the explosive, and which is stable so that it can withstand a large number of melting-solidification cycles without being degraded.

3. A method according to Claim 1, wherein the heat-retaining medium is or comprises magnesium dinitrate hexahydrate.

4. A method according to any of the preceding claims wherein the heat retaining medium is contained within an enclosed chamber in the casting funnel, the interior of which chamber is so constructed, and the quantity of heat-retaining medium therein is such, that a positive pressure, relative the atmospheric pressure, develops in the chamber when the heat retaining medium is melted down, whereby the wall of said chamber facing the casting mould is forced into close contact with the outer surface of said mould.

5. A method according to any of the preceding claims, wherein the casting mould and the casting funnel are heated together before the casting of the explosive.

6. A method according to any of the preceding claims, wherein the casting funnel and the casting mould are heated to different temperatures before the casting of the explosive.

7. A casting funnel suitable for useas an ingate for the casting of a normally crystalline meltable explosive of defined melting point into a casting mould and being adapted to keep the upper part of the casting warm so that said casting solidifies from its bottom upwards in order to prevent air-inclusions in said casting, characterized in that said casting funnel (3) provides a first space (4) which is adapted, when the casting funnel is mounted on top of the casting mould (1), to surround the upper part thereof and which at least partly is filled with a meltable heatretaining medium (6) having a melting point which lies within i 30 C of said defined melting point of the explosive, and in that said casting funnel is provided with a second, centrallyarranged, space elaborated as an ingate which extends at least partly above said first space (4) with its heat retaining medium (6).

8. A casting funnel according to Claim 7, wherein said first space (4) forms an enclosed annular chamber, and wherein both said first space (4) and said second space are surrounded by heat insulation (10).

9. A casting funnel according to Claim 7 or Claim 8, wherein said first space (4) is provided with a partition wall (5) adapted to face the casting mould and configured to the form thereof, said partition wall (5) consisting of an at least partly deformable material; and said first space being at least partly filled with a sufficient amount of said heat retaining medium (6) and/or a gas filling the remainder of said space so that, when the casting funnel (3) is mounted on the casting mould (1) and heated to the melting point of said heat retaining medium, said partition wall (5) is forced into a close contact with the outer surface of the casting mould.

10. A casting funnel according to any one of Claims 7 to 9, formed from a sufficient heat-resisiting plastics, material such as a suitable polyester.

11. A casting funnel according to any one of Claims 7 to 10, which is provided with a separate cover (11) including a meltable heat retaining medium, said cover being adapted to be placed on top of the casting funnel (3) after the casting has been concluded.

12. A casting funnel according to any one of Claim 7 to 11, wherein said first space (4) contains one or more meltable salts as a heatretaining medium.

13. A casting funnel according to Claim 12, wherein said heat-retaining medium is or comprises magnesium dinitrate hexahydrate, Mg (NO3)2. 6H2O.

14. A method for preventing air inclusions in cast explosives, according to Claim 1 and substantially as hereinbefore described.

15. A casting funnel substantially as hereinbefore described with reference to the accompanying drawing.