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Lesson 4.9DeformChapter 4 · Sheet and tube forming

Explosive forming

An underwater blast pushes sheet into one die

Explosive forming uses a small explosive charge fired under water to push a metal sheet into a one-sided die. The water carries the blast as a pressure wave, like a giant invisible hammer that hits the whole sheet at once. It shapes very large or very tough parts, such as rocket domes, without a press big enough to do the job.

An underwater blast pushes a sheet into a one-sided die in a split secondCross-section of an explosive-forming tank. The air under the sheet is pumped out before firing.waterdie2to vacuum pumpsheet blank5hold-down ring1firing cable3explosive chargestandoff4shock wavesheet (workpiece)tank, die and ringwatercharge and blastfinal shape
explosive forming · cross-section of the water tank, charge, sheet and dieCircled numbers match the steps below. The dashed line is the shape the sheet takes after the blast.

How it works

  1. Clamp the blank. A flat sheet (the blank) is clamped over a die by a hold-down ring (a heavy ring bolted around its edge). The die can be steel, ductile iron, concrete or even fiberglass for low pressures.

  2. Pump out the air. A vacuum pump removes the air between sheet and die. Trapped air would cushion the sheet and stop it from filling the die.

  3. Place the charge. The setup is lowered into a water tank or pool. A licensed blasting team hangs a measured explosive charge at a set standoff (distance) above the sheet.

  4. Fire. The detonation sends a shock wave (a sudden, very high pressure front) through the water. Water does not compress much, so it passes the pressure to the sheet very efficiently.

  5. The sheet hits the die. In about a millisecond the sheet slams into the die and copies its shape. The part is lifted out, checked, and usually trimmed; large parts may need several shots.

Key facts

Materials
Steels, stainless steels, titanium, aluminum and nickel alloys, often hard-to-form grades
Typical parts
Large domes and dished heads, cones, rocket and tank parts, complex panels
Part size
Up to several meters across, set by the pool rather than by a press
Precision
Good fit to the die, but springback and thickness vary; parts are often checked and trimmed afterward
Surface finish
Smooth die-side surface; no marks from a punch because there is none
Batch size it suits
Prototypes and very small series of large parts
Tooling and machine cost
Low compared with a press of the same power: no press, only a die, a tank and explosives handling
Speed
The forming itself takes about a millisecond; setting up each shot takes hours

Strengths and limits

Strengths

  • Huge forces without a huge press
  • Very large parts in one piece
  • Cheap one-sided dies
  • High speed lets tough metals stretch further

Limits

  • Explosives need licenses, a remote site and strict safety
  • Slow setup: few shots per day
  • Hard to automate, so poor for mass production
  • Springback and thickness are less predictable than in a press

Where it's used

  • Aerospace and spaceRocket tank domes, engine parts, aircraft skin sections
  • Pressure vesselsLarge dished heads and end caps for tanks
  • Architecture and artSculptures and decorative panels formed against molds
  • Research and prototypesOne-off parts in metals too strong for available presses

How to recognize it

  • Very large single-piece domes or panels with no weld seams across them.
  • Smooth, gently curved shapes that copy a one-sided die.
  • Usually found on rockets, tanks or art pieces rather than everyday products.

Compare with

Hydroforming4.5also liquid pushing sheet into a die, but the pressure comes slowly from a pump and a press.Electromagnetic forming4.8another high-speed method, using a magnetic pulse instead of explosives, for small parts.Explosion welding7.8the same explosives, used to bond two metal plates instead of shaping one.

Watch it

ONE POINT ACADEMY · A beginner explainer: charge, water tank, die and vacuum

Design Otherwise · How the process works and how it was used in history

GESPASA · A real industrial blast forming a large steel sphere

San Diego Air and Space Museum Archives · Archive film of aerospace parts made with high-energy forming

See real photos

Remember

  • Water carries the blast to the sheet; a vacuum lets the sheet fill the die.
  • Only one die is needed, and no press, so very large parts become possible.
  • It suits rare, big, tough parts, not mass production.

More in Sheet and tube forming

All 100 processes
4.1Sheet metal bendingA press brake folds sheet along straight linesOpen the lesson4.2ShortStamping and deep drawingMatched dies cut and press sheet into shape, often in a secondOpen the lesson4.3Metal spinningA roller presses a spinning disc over a formOpen the lesson4.4Flow formingRollers stretch a spinning tube thinner and longer over a mandrelOpen the lesson4.5HydroformingFluid pressure pushes tube or sheet into a dieOpen the lesson4.6Incremental sheet formingA round tool traces contours into clamped sheet, with no dieOpen the lesson4.7Superplastic formingHot gas blows a superplastic sheet into one dieOpen the lesson4.8Electromagnetic formingA magnetic pulse throws metal onto a fitting, without contactOpen the lesson4.10Laser peen formingLaser shots stretch one face so a plate curvesOpen the lesson

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