Leak Testing Die Castings -Methods, Sensitivity, and Specification

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 (15-20 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 1.25-2.5x rated working pressure)
3. Valve off and allow 3- 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.

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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

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Pressure test specification support: yaoqingpu1983@gmail.com | +86 138 1403 4409