Explosive forming is a metalworking technique in which an explosive charge is used instead of a punch or press. It can be used on materials for which a press setup would be prohibitively large or require an unreasonably high pressure, and is generally much cheaper than building a large enough and sufficiently high-pressure press; on the other hand, it is unavoidably a batch process, producing one product at a time and with a long setup time.
There are various approaches; one is to place metal plate over a die, with the intervening space evacuated by a vacuum pump, place the whole assembly underwater and detonate a charge at an appropriate height above the plate. For complicated shapes, a segmented die can be used to produce in a single operation a shape that would require many manufacturing steps, or to be manufactured in parts and welded together with concomitant loss of strength at the welds. There is often some degree of work hardening from the explosive-forming process, particularly in mild steel.
Tooling can be made out of fiberglass for short-run applications, out of concrete for large parts at medium pressures, or out of ductile iron for high-pressure work; ideally the tooling should have higher yield strength than the material that is being formed, which is a problem since the technique is usually only considered for material which is itself very hard to work.
Explosive forming was used in the 1960s for aerospace applications, such as the chine plates of the SR-71 reconnaissance plane and various Soviet rocket parts; it continued to be developed in Russia, and the organising committees of such events as EPNM tend to contain many members from the former Soviet Union. It proved particularly useful for making high-strength corrugated parts which would otherwise have to be milled out of ingots much larger than the finished product.
Other uses of explosives for manufacturing take advantage of the shaped charge effect, putting the explosive directly in contact with the metal to be worked; this was used for engraving of thick iron plates as early as the 1890s. See also explosively formed projectiles for a variety of military applications of the same kind of technology.
An example would be a yacht constructor who produced boat hulls by making a concrete "swimming pool" into which sheet-metal was placed, and when water filled and explosively fired, produced a complete hull-form.
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