Stainless Pressure Cup

The challenge

A pressure-rated cup needs a wall that is uniform all the way round, because any thin spot becomes the failure point under pressure. The part also has to stay round; out-of-round at the open end will not mate with the vessel body, and the ironed wall must be smooth to avoid stress raisers that start a crack.

We size the blank from the actual 304 limiting draw ratio rather than a catalogue cup formula, because 304 work-hardens quickly and the permitted reduction at 120 mm depth is narrower than a general stainless steel number suggests. Getting the draw reduction right on the first pass keeps the wall from thinning past the ironing reserve.

Downstream, the cup is welded or bolted to the vessel body, so the open-end diameter and the wall at the joint have to land where the mating part expects them. The draw and ironing are set so those features, not just the average wall, stay in tolerance.

How AUNOV draws it

The route is deliberately short — one draw and one ironing pass — because each extra stroke adds roundness error and handling marks. Tooling is built in-house, so the die radius and punch profile are tuned to this exact 304 coil rather than a generic set.

1. Blank sized from the measured 304 LDR (1.85–2.05) for the 120 mm depth
2. Single deep draw on a large press within the 5–500 T range
3. Ironing pass from 1.0 mm to 0.78 mm to set ±0.02 mm wall and improve roundness
4. Wall-thickness mapping and out-of-round check on the finished cup

Process control and inspection

During production the wall is checked by mapping thickness at four points around the cup rather than a single spot, because a pressure vessel fails at its thinnest point, not its average. We record those four readings per first article and keep them with the lot.

Roundness is measured at the open end where it meets the vessel body; a few hundredths of a millimetre of out-of-round is enough to break the seal, so the ironing pass is set to pull the diameter in uniformly rather than just thin the wall.

What this means for your part

If your pressure-vessel end is a deep stainless steel cup with a tight wall, the draw-plus-ironing route is what holds ±0.02 mm. Send the pressure rating and wall spec and we confirm the ironing allowance.

For a production run we also set up wall-thickness mapping so every cup is checked at the same four points, which is how we keep the pressure rating repeatable across a batch rather than only on a first article.

• Give us the pressure rating and the allowable wall range, not just a target wall, so we can size the ironing allowance.
• Keep the open-end diameter as a controlled feature; the vessel body it seals to sets the roundness budget.
• If the cup is welded to a flange, tell us the weld interface so wall and bottom radius are set to suit it.

Frequently asked questions