Die Casting Leak Testing: Air Decay & Helium

Leak testing is production verification, not quality inspection. It does not improve part quality — it separates conforming parts (which will seal in service) from non-conforming parts (which will not) before they reach the customer.

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Methods Compared

Method	Medium	Sensitivity	Speed	Cost	Best Application
Air decay	Dry compressed air	~1 cm3/min	Fast (15-30 s)	Low	General pneumatic, low-pressure hydraulic
Nitrogen decay	Dry nitrogen	~1 cm3/min	Fast	Low	Hydraulic (cleanliness), coolant circuits
Hydrostatic	Water	Visible leak	Moderate	Low	Burst pressure qualification
Helium tracer (hood)	Helium	<10^-8 mbar·L/s	Moderate	High	Refrigerant, sealed electronics, precision
Hydrogen tracer	H2/N2 mix	~10^-7 mbar·L/s	Moderate	Medium	Alternative to helium

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Air Decay — Production Standard

Air decay is the standard 100% production test for hydraulic and pneumatic die cast components. The part is pressurized to the test pressure, valved off, and monitored for pressure decay.

Test sequence:

1. Part clamped in fixture with test ports sealed
2. Pressurize to test pressure (typically 1.25-1.5x rated working pressure)
3. Valve off and allow 3-5 s stabilization (thermal equilibration)
4. Monitor pressure decay for hold time (15-20 s)
5. Compare actual decay to limit — pass or fail

Key sensitivities:

• Temperature variation of +/-1°C changes air pressure by ~0.3% — temperature-stabilized test rooms or temperature compensation essential for tight limits
• Part volume affects sensitivity — smaller part volume = more pressure drop for same leak rate = easier to detect small leaks

Typical acceptance criteria:

Application	Test Pressure	Hold Time	Decay Limit
Pneumatic valve body (10 bar)	15 bar	30 s	<0.5 cm3/min
Hydraulic manifold (100 bar)	150 bar	45 s	<2 cm3/min
Cooling circuit (5 bar)	8 bar	30 s	<0.3 cm3/min

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Helium Mass Spectrometer Testing

Helium testing detects leak rates orders of magnitude smaller than air decay — down to 10^-8-10^-10 mbar·L/s. This sensitivity is necessary when:

• The fluid being sealed is expensive (refrigerant)
• Leakage creates regulatory or safety problems (oxygen systems, toxic gas)
• Service life requires leak-free integrity over years (hermetically sealed electronics)
• Standard air decay passes but field failures occur — indicating very small leak rates just below air decay detection

Hood (bomb) method: Part is pressurized internally with helium, placed inside a sealed chamber connected to the spectrometer. Accumulated helium concentration in the hood is measured after a dwell time. Best for production 100% testing of small to medium parts.

Sniffer probe method: Part is pressurized with helium; a probe is passed over external surfaces. Localizes the leak position. Best for qualification, troubleshooting, and root cause investigation.

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How to Write a Pressure Test Specification

Vague specifications ("pressure tight," "no leaks") create supplier-customer disagreement. State the test method, medium, pressure, hold time, and acceptance criterion explicitly:

Good examples:

"100% air decay test. Test pressure: 150 bar. Stabilization: 5 s. Hold time: 45 s. Maximum decay: 2 cm3/min."

"100% helium leak test (hood method). Test pressure: 8 bar helium. Dwell time: 60 s. Maximum leak rate: 10^-6 mbar·L/s."

Common specification errors:

• Specifying "1.5x working pressure" without stating the working pressure
• Specifying decay rate without specifying part volume or test pressure
• Specifying "helium leak test" without specifying sensitivity limit or test method

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KastMfg Pressure Testing Capability

Equipment	Range	Sensitivity
Air decay test stations (4 units)	Up to 200 bar	<0.1 cm3/min
Nitrogen high-pressure station	Up to 350 bar	Pressure decay
Helium mass spectrometer (hood)	Any pressure	<10^-8 mbar·L/s

All test records archived: part serial number, test date, test pressure, hold time, actual decay or leak rate, pass/fail. Available to customers on request for 5 years minimum.

Specifying a leak test on a new part? Include it in the RFQ from the start — test pressure, medium, and acceptance limit quoted together with the casting are the difference between a spec the line can hold and one that gets renegotiated after PPAP.

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Relationship Between Alloy, VADC, and Test Performance

Alloy + Process	Typical Air Decay (100 bar, A size part)	Helium Leak Rate
A380 standard HPDC	70-75% pass rate	10^-4-10^-5 mbar·L/s typical
A413 standard HPDC	90-97% pass rate	10^-4-10^-5 mbar·L/s
A413 + VADC	99-100% pass rate	<10^-6 mbar·L/s
A380 + VADC	98-100% pass rate	<10^-6 mbar·L/s

FAQ

Q: What is the leak rate spec conversion between air decay and helium testing? A: These methods measure different things, so there is no direct conversion. Air decay reports a pressure drop over time (often quoted in cm³/min); helium reports a tracer gas flow (mbar·L/s). The two numbers only line up if you fix test pressure, gas, and temperature — many drawings quote "cc/min at X bar", which is only comparable to a measurement when all three are stated. A practical rule of thumb: a tight air-decay spec (<0.5 cm³/min) corresponds to helium rates around 10⁻⁴–10⁻⁵ mbar·L/s on the same part, and specs tighter than roughly 10⁻⁵ mbar·L/s generally require tracer gas rather than pressure decay.

Q: Air decay vs helium leak testing — which one for die cast parts? A: Air decay for 100% production screening of hydraulic and pneumatic components — it is fast (15–30 s cycle), cheap, and robust enough for the shop floor. Helium for parts with tighter specs (refrigerant circuits, sealed electronics) or for root-cause work when an air-decay failure needs localizing. Many programs run both: air decay at every machine, helium sampling or containment qualification in the lab.

Q: Can an air decay test detect the porosity that matters? A: It detects connected porosity — the leak paths that run wall-to-wall. It cannot see enclosed shrinkage voids, which are a cosmetic or X-ray concern but not a leak path. That is why vacuum-assisted die casting (VADC) shows such different pass rates: fewer connected paths means a tighter distribution, not just a better average.

Q: Why did my part pass air decay but fail in the field? A: Usually one of three causes: the test pressure was below field conditions (test at 1.25–1.5× rated working pressure, not below it), the hold time was too short to catch a marginal leak at part volume, or the leak only opens under thermal or pressure cycling rather than static pressure. Cycling qualification is a separate test from production screening.

Q: What pressure should a die casting leak test specification call out? A: State four things explicitly: test pressure (typically 1.25–1.5× rated working pressure), medium (dry air, nitrogen, or tracer gas), hold time, and the decay limit in absolute units with the volume basis. A spec that says only "leak test per customer standard" cannot be validated with calibrated reference leaks — and that validation, not the datasheet, defines what the line can actually hold.

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Related

• helium leak test vs pressure decay - choosing the method by leak spec
• die casting defects

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Pressure test specification support: casting@kastdiecast.com | +86 138 1403 4409