Plain Detonators

Metal cups are sometimes inserted in detonators to provide extra confinement for the composition. It is often claimed that such cups, by increasing the mechanical strength, also increase the safety of handling of the detonator. In the case of plain detonators this is true to a limited extent, but the difference is not of practical importance. Of much greater importance is the ensurance of the absence of grit or hard particles, the presence of which can cause dangerous sensitiveness in the detonator. 

Detonators must be capable of storage for long periods under various types of climate. After such storage the detonators should still be in safe condition and also perform normally. 

The original initiating explosive used by Nobel and all manufacturers for many years was mercury fulminate. This had the disadvantage of decomposing slowly in hot climates, particularly under moist conditions. For this reason mercury fulminate is no longer widely used. In most countries it has been replaced by a mixture of dextrinated lead azide and lead styphnate. In the U.S.A. some detonators are made containing diazodinitrophenol. 

The introduction of lead azide led to a difficulty in the choice of metal for the detonator tube. Under moist conditions, lead azide and copper can react to form cuprous azide on the inner wall of the tube and thus in a particularly dangerous position. Therefore with plain detonators, which cannot be sealed, copper cannot be used when lead azide is employed. Such detonators are usually made from aluminium tubes, or occasionally zinc. 

The initiating explosive used must ignite with certainty from the spit of a safety fuse. It must be remembered that the intensity of the spit can be reduced if the safety fuse is not cut squarely and also that the fuse may in practice not always be fully inserted into the detonator. Lead azide by itself is not sufficiently easily ignited to give a satisfactory plain detonator and it is therefore used in admixture with lead styphnate, which is very readily ignited by flame. The proportions of such mixtures vary from 25 to 50% of lead styphnate. Mercury fulminate and diazodinitrophenol are sufficiently sensitive to flame not to require such additives. 

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Many other methods of making lead azide in a safe form have been described, but the only one to have found commercial importance consists of replacing the dextrine by a small proportion of gelatine. When properly made this form of lead azide is as safe to handle as the dextrinated form and has improved sensitiveness to flame. It can therefore be used by itself in electric and delay detonators, but not in plain detonators as it is not ignited with certainty by safety fuse. 

Although the requirement for flame sensitiveness is the main consideration for initiating explosives for plain detonators, others are important in manufacture. The explosive must be capable of compression into a coherent mass and at the same time leave the equipment free from adhesions. Lead azide can be somewhat deficient in cohesion, and to improve this a small proportion of tetryl is sometimes added to the... 

The general construction of a British plain detonator is shown in Fig. 9.1 other countries in general adopt similar designs. 

The normal initiating charge in a British detonator is lead azide modified with gelatine. In the case of plain detonators a small proportion of lead styphnate is added to the azide to ensure satisfactory ignition from safety fuse. 

The actual filling of brass or aluminium delay elements is similar in principle to the filling of plain detonators described above. The loading is, however, carried out in a number of stages so as to ensure uniformity of density of the column throughout its length. 

Assembly commences with a plain detonator containing its base charge and initiating explosive. These detonators are held in plates and the processes are insertion of suitable delay elements, followed by pressing in the normal way. They are then checked by X-ray examination to ensure... 

Plain detonator. An open detonator with no means of ignition attached. 

See also Ref 36, p 54 and Ref 38, p 52-3). Although each of rhese detonators contain two charges priming or upper (ASA) and secondary or base (Tetryl or PETN), they are still called in Gt Brit the "plain detonators". Corresponding devices are known in US as compound blasting caps (See Ref 44, p 186-L and Fig), but the ASA detonators have no. reinforcing cup", as do the US caps... 

In the book of Fordham (Ref 51, p 108-09) are described Brit "plain detonators" such as No 6, which contains as a base chge 0.24g PETN and No 8 Star, which contains 0.8g PETN. As a priming charge, in both cases, a 2/1 mixt of dextrinated LA/LSt is used with ca 3% A1 powder and, sometimes, 3% Tetryl added... 

McAdam Westwater(1958), 9-10 (Detonators, historical) Chapter 5, "Blasting Accessories" pp 51-2 (Safety fuse) 52-3 (Plain detonators, including ASA-Tetryl and ASA-PETN) 52-9 (Electric detonators) 54 (Fusehead) 59 (Electric powder fuses) 59-60 (Detonating fuses) 60 (Detonating... 

Fordham (1966), pp 113-14 (Description of Esop s, lead plate and nail tests used for plain detonators. The first two tests measure only the end blow of detonators, while the 3rd test can give a rough indication of its side blow. These tests are more fully described in Vol 1 of Encycl, listed here as Ref 30)... 

Fig. 64. Detonator manufacture—the plating section of the automatic plain detonator unit. Courtesy Imperial Chemical Industries Ltd., Nobel Division.

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