Pressure Decay Leak Testing for Die Cast Housings

A machined die cast housing fails an air leak test at the supplier but passes after it cools. Another housing passes with the test fixture clamped down and leaks after the production cover is installed. Both results can occur when the test conditions influence the reading. Before attributing a failure to casting porosity, establish which surfaces the fixture seals, how long the part stabilizes and what the test can detect.

A pressure decay test fills a defined cavity with air, isolates it from the supply and measures the change in pressure over a fixed time. A reliable setup separates pressure loss through the housing from pressure changes caused by the fixture, the air temperature or movement of the part. Uson guide to pressure decay testing

Mark the pressure boundary on the drawing

Identify the cavity that will be pressurized and every opening that the fixture must close. State whether the test covers the bare housing, a housing with permanent plugs installed or the finished assembly with its cover and gasket. These configurations answer different questions. A bare-housing test can screen the casting and machined walls. An assembly test can also detect leaks at installed seals and joints.

For a housing with an oil passage beside a coolant passage, define whether leakage between the two passages matters. If the fixture pressurizes both passages equally, there may be little pressure difference to drive flow between them. Testing one passage while controlling the condition of the other makes that potential leak path observable.

Set the inspection stage relative to drilling, tapping, finishing and any approved impregnation. Machining can expose a connected pore path. If the purchase specification requires acceptance after machining, use a test that covers the final machined surfaces. Keep the results from earlier screens tied to their own operation.

Set up the fill, stabilization and measurement steps

Pressure drop depends on the volume of gas under test. With the same test time and a similar small leak under comparable conditions, a larger cavity typically shows a smaller pressure drop. Include the connected fixture volume when developing the recipe. Recheck the setup when moving to another housing size, fixture or hose layout. Uson guidance on volume in leak testing

Check the fixture before investigating the casting

Run a stable reference part with known behavior and challenge the setup with an appropriate calibrated leak near the decision threshold. A reference part helps reveal drift or inconsistent sealing. A calibrated leak helps show whether the system can detect a leak of the size it needs to reject. Record which fixture, recipe and reference were used.

Inspect fixture seals where they meet machined ports and flanges. A worn seal can leak past the fixture. A soft seal that bridges a casting opening can also hide a path the test is supposed to detect. Check that the fixture loads the part consistently and that the sealing footprint leaves the area under inspection exposed to the pressure difference. Uson guidance on fixture effects and false leaks

If a result changes when the same housing is reseated, compare the seal marks, surface condition and loading sequence before adjusting the reject limit. If the reference part also fails, check the station before drawing conclusions about the casting lot. Define when a retest is permitted and retain the original result so an intermittent fault can be traced.

Worked example with a warm machined housing

Suppose borderline rejects cluster just after warm housings leave a wash station. The same parts give lower pressure drops after reaching the inspection area's temperature. To test that pattern, compare the pressure traces of the same dry housings at the normal incoming temperature and after conditioning. Keep the fixture, pressure and measurement interval the same, and record part temperature alongside the result.

Cooling of the trapped gas can lower pressure even without a leak. If the reference checks are stable and the difference tracks part temperature, trial a defined conditioning window or a revised stabilization step. Then challenge the revised recipe with the calibrated leak so a lower reject rate can be distinguished from lost sensitivity. If a housing still fails consistently after conditioning and fixture checks, use a suitable localization method to investigate where air escapes. Uson application note on temperature effects

Specify acceptance before the first production run

Provide the test medium, pressure, measurement interval and allowable pressure loss or leak rate. State the tested cavity, sealing points, inspection stage and temperature condition. If a leak rate is reported in standard cubic centimeters per minute, agree on the reference conditions and how the tester obtains that value. A pressure drop limit only has meaning with its defined volume, timing and setup.

For a casting quote, send the finished drawing, identify any adjacent passages that must stay isolated and state whether the supplier should test a bare housing or a finished assembly. Include the acceptance method and reporting requirements so the fixture and inspection step can be planned with the tooling and machining. Request a die casting project review from Aluprototype

Frequently asked questions

Can pressure decay tell me where a housing leaks?

It measures the net pressure change within the tested boundary. Use a separate localization method when you need to distinguish a leaking fixture seal, a machined joint or a path through the casting wall.

Does a passing leak test mean the casting is free of porosity?

The result shows that leakage stayed within the limit under the specified conditions. Isolated pores can remain inside the casting. Specify internal inspection separately if pore size or location matters for structural or machining requirements.