A,B,C's of Leak Testing Metal Cast Parts
Leak testing is critical in the manufacture of metal cast Parts to assure that porosity or leaks are detected.
Testing of the completed Part, or testing of the casting is done to ensure that porosity or leaks do not get extended in the machining processes to allow “through” porosity or leaks. In
most metal cast parts, InterTech’s Mass Flow Leak Test Technology — capable of measuring leaks equivalent to as small as a 0.5 micron hole size — have proven superior to error-prone Pressure Decay testing methods.
The superiority of InterTech’s Mass Flow leak test technology for metal cast parts applications has been well- established. The ability to achieve low Gage R&R for casting leakage in a range of 1 sccm to 20 scem is due to not only InterTech’s superior patented Mass Flow sensors, but also a wide range of testing features that optimize leak testing
systems.
An InterTech Design Report
There are several reasons why automotive designs rely on so many metal cast parts — for engine blocks, cylinder heads, transmission housings, suspension components, steering systems, and various brackets. The primary reason is the superlative strength to weight ratios that can be achieved with magnesium and aluminum castings.
Both the clean air regulations that mandated ever more stringent fuel efficiency standards, and the economic driver of consumers seeking lower fuel costs, have created the longtime rationale for automotive designers to use near-net-shape metal castings. Then with a constant focus on improving manufacturing methods for high volume mass production. Aluminum and magnesium are relatively lightweight, enabling automotive design teams to reduce overall vehicle weight to improve fuel efficiency. Metal castings also allow for complex 3D geometries such as one finds in transmissions and engines. Metal cast parts are also very durable, adding to the overall ability of automotive designs to withstand wear and tear, especially in heat conditions.
Though metal castings are superlative solutions for many automotive parts, the metallurgical and process related issues that cause some cast parts to be leakers must always be anticipated and managed. The casting process can sometimes cause defects, small holes, that will occasionally combine to create pathways for fluids or gases escape.
Even well-cast parts can become leakers post-casting — through improper handling, machining, or thermal stresses. For these reasons, leak testing aluminum, magnesium and some steel castings are almost always required.
Il — Benefits of Mass Flow Leak
Testing over Pressure Decay Testing
A cost-conscious decision maker might ask —
Why not simply use lower-cost Pressure
Decay test methods?
Although Pressure Decay leak testers might have lower upfront costs, it is rarely the case when overall costs of manufacture are considered. In most leak test applications for parts of all sizes — large included — it is the real world costs of the entire system and process that need to be considered. The cost of the leak test instrument when considered in the lifetime terms is actually negligible.
Pressure-Decay leak testing measurements are highly vulnerable to changes in testing conditions such as drafts or temperature. For more complex
design metal castings such as those with multiple chambers, there are often difficulties in determining the volume of the test parts and test circuits, which must be known in order to calculate results with the Pressure Decay test methods. Large volume parts might have small leaks that significantly impact Part function, requiring a more precise leak detection system.
Pressure testing methods require relatively more stabilization times due to adiabatic heating when parts are filled with compressed air. Thin wall castings are also very susceptible to part expansion at higher pressures, which must be anticipated and managed, a task that InterTech Mass Flow leak testers with sophisticated control and data acquisition software is geared to handle. Mass flow leak testing is more accurate, especially when InterTech’s patented Mass Flow sensors, that are tuned to application
requirements, are used. Also, a 1.0 sccm standard is not reliably achieved by Pressure Decay testing
due to the inherent error sources of any two- measurement test system compared to the one
direct measurement of leak rates by Mass Flow sensors. The longer the interval between the two measurements the higher the probability for measurement error. The larger the internal volume of the part being tested the higher the probability
for errors. This is because the larger the part volume the smaller the change in pressure for a given leak rate. When you increase test volumes longer test times are required.
Ill - Beyond Sensors — InterTech Technology to Optimize Leak Testing
A) Temperature Compensation
Most large cast parts that need to be leak tested — engines, oil pans, transfer cases, etc. — are usually machined with processes that can open up leaks. That is why leak testing is usually done after these steps. Their materials — magnesium or aluminum
—make them susceptible to temperature changes as well. Parts that have been machined typically have gone through a parts washer prior to testing. All of these factors mandate that the leak testers’
operating software have sophisticated algorithms to accurately account for temperature effects.
When using a Mass Flow type leak detector to measure air leaks, the resulting measurement is composed of: The decrease in the number of air molecules (the leak) in the test volume, and; an apparent, or virtual leak, determined by the instantaneous rate of change of temperature of the testair (dT/dt). Changes in part temperature by upstream machining on castings is only one of several factors that can effect measurements.
These temperature compensation calculations are automatically generated by InterTech’s Mass Flow leak testers’ software such that the leak rates measured by the test instruments are reliable.
B) Ex-Heat®
By reducing stabilization time — the slowest part of the leak test — on large open test volumes — manufacturers of large cast parts can reduce
testing times by as much as 25%. InterTech’s Ex-Heat® process reduces stabilization time and variation by extracting the warm unstable air from
the test part, replacing it with stable air, at the correct test pressure. In this way, Ex-Heat speeds the system stabilization required to accurately
measure the leak rate value.
InterTech’s Ex-Heat[1.1] works by adding preliminary purge phase to the beginning of all test cycles. During this phase an even larger
volume of gas or liquid is quickly added to the part while a valve rapidly ushers the adiabatic heat out of the test system. This results in a more stable pressure environment for testing. Ex-Heat system precisely controls the timing of this process, namely the opening and closing of valves
to begin and end the newly added purge phase of testing. It does so when thresholds are met measured by time, volume, pressure or other pre-set parameters.
Ex-Heat allows for further savings by eliminating the use of filler blocks. Filler blocks are a cumbersome way to attempt to stabilize test conditions. They are used as a way to make the internal volume of the test part smaller. That becomes a problem by making changeovers more difficult. In some cases, filler blocks preclude successful integration with robots.
C) Fast Fill®
Fast Fill, a companion technology to Ex-Heat, is also used to shorten the stabilization phase of testing by introducing air quickly into the part being tested without creating turbulence. Fast Fill test circuit designs basically bypass any turbulence generated noise that might otherwise
interfere with measurement accuracy.
D) LeanTest®
InterTech’s LeanTest® takes the advantage of single measurement to determine leak rate a step further. By determining early in the test cycle that a Part is clearly in or out of specifications so it can quickly pass or fail. LeanTest is built on InterTech’s superior patented Mass Flow sensors that give direct measurements of leaks in the 0-20 sccm range combined with dynamic test parameters for various stages of testing. When setting up an InterTech LeanTest system, one defines the pass delay, pass factor and pass dwell.
Pass Delay is the minimum time delay, after the stabilization time has started, before the Pass Factor leak limit is read. Pass Factor is the percentage of the Hi Reject value that the Part must be equal or below when the early pass leak value is read. And, Pass Dwell is the time the early pass leak value must be equal or below the Pass Factor limit after the Pass Delay time. Instead of waiting for the entire test cycle.
(Fill-Stabilize-Test-Sort) to finish, InterTech’s Mass Flow leak detector monitors the leak rate in real-time. If the leakage value drops below the accepted threshold during the stabilization phase, the Part is passed immediately.
For example, if the Stabilize time is set to 5 seconds, with the Pass Delay set to 3 seconds, and the Pass Dwell set to 1 second, the early pass
leak value will read after the 3 second Pass Delay. It must be equal or below the Pass Factor for 1 second before the early pass will be triggered as an Accept. This shortens the total test time by 1 second. In this way, LeanTest can achieve a test cycle time reduction by as much as 50%, with 30% reduction in test cycle times not uncommon.
Early fails quickly remove large out-of-range leaks detected during stabilization times, allowing the test system to quickly move to testing the next part. Because direct measurement of leak rates are used, the initial leak rate measured during the test cycle will become lower as the test cycle proceeds. That means that test parts that immediately register leaks in an application’s acceptable range are also rapidly passed and the test system again moves on to testing the next part. Late Passes are reserved for those parts that test in the marginal range.
These parts require longer test times if and when the initial leak measurements are within a prescribed window. The bottom line is that LeanTest optimizes leak testing in multiple ways: using only time actually needed to pass or fail parts; adjusting test cycle times based on each part’s test characteristics instead of static test parameters; and using a Gage R&R set up that significantly reduces false rejects or false positives.
IV — Leak Testing Aluminum and Magnesium Large Paris — Case Studies
A) Retainer and Transfer Case Housings
Cast aluminum is the material of choice for retainer housings. For example, durable aluminum cast thermostat housings are designed to ensure that engines do not lose coolant and overheat. Similarly, transmission bearing/seals are designed to prevent the loss of transmission fluid and resulting difficulties or failure in shifting gears.
Camshatt retainer plates are key to th automobile’s internal oiling system, and so on. The complex geometries of transfer cases, and the need to handle multiple sealed joints, pose particular leak testing challenges. Because transfer cases are large-volume components, detecting small leaks via traditional Pressure Decay methods is difficult, as the change in pressure for a given
leak rate is minimal, leading to potential inaccuracies. Additionally, large parts under test pressures are prone to stretching or similar deformations that can mask leaks by reducing internal pressures and causing false test failures.
- Retainer housings are leak tested to a 3.0 sccm accept limit at 3.5-4.0 psig in a total test [2.1]cycle time of 32 seconds.
- Transfer case rear housings are leak tested to a 3.0 sccm accept limit at 24 kPa (3.5 psig) to 28 kPa (4.0 psig) in 25 second test cycle time.
- Transfer case front housings are leak tested to a 3.0 sccm accept limit at 24 kPa (3.5 psig) to 38 kPa(5.5 psig) in 35 second test cycle time.
- Family of magnesium transfer case covers are robot loaded, automatically tested, marked and unloaded at rate of 106 parts per hour. Four models of castings are tested to a 9.1 sccm limit at a pressure of 52 kPa (7.5 psig) in 18 seconds test time. Fast-Fill speeds testing time.
B) Oil Pans
Qil pans, critical to engine function, are prime examples of why Mass Flow leak testing is superior to pressure testing for several reasons: 1) oil pans are large volume castings; 2) oil pans typically have large, complex, and sometimes non-machined mating surfaces, challenging ability to adequately seal parts for testing; and 3) in real-world manufacturing environments where fast production speeds are required, one must manage possibly part shrinking or expansion when there are thermal variations and stability issues in recently cast, welded or washed parts.
InterTech manages these constraints to achieve 10% Gage R&R while testing a family of oil pans with internal volumes ranging from 11 to 24 liters.
- A4 4 liter engine oil pan assembly with manual clamping and sealing is tested to a 10.0 scem limit.
- The 10 liter volume oil pan body is tested at 0.5 bar with an optimized test time of 34 seconds.
- The mini sump, a 2.2 liter volume part, is tested at 1 bar with an optimized test time of 10 seconds.
- A 4.4 liter engine oil pan body casting (18.7 liter volume[3.1]) is tested at 0.5 bar to a10.0 sccm leak limit, in 30 seconds test time.
- An 11 liter volume oil pan casting is tested at 5 psig to a 5.0 scem leak limit at rate of 120 parts per hour.
- A 6.4 liter engine oil pan is tested sequentially: the 24 liter body is tested at 1.0 bar to 2 10.0 sccm leak limit in 35 seconds test time; the 2.2 liter coolant circuit is tested at 1.5 barto a 1.5 sccm leak limit.
- An 8 liter oil pan is tested to a 10 sccm limit at pressure of 100 kPa (14.5 psig) in 19 seconds test time with 20% Gage R&R.
C) Transmission Castings
Similar to other large cast automotive parts, transmission castings are large with complex geometries that make both stabilizing pressures throughout the casting and pinpointing leak locations difficult.
InterTech’s solution for leak testing a complex design transmission casting met specifications for a10 sccm leak rate at 1 bar (14.5 psig), to fully test at a rate of 120 pph. The testing including accommodating six variations in the geometry of the same part. A unique test fixture was used to test all four chambers of the test part and to ensure there were no blockages in vent passageways prior to testing.
The difficult internal sealing challenge in this application — both in clearance and alignment problems — was achieved by using a unique arrangement of compression expanded ring seals. Sensors able to “read” surface grooves machined
into test parts enable parts identification, and to halt testing if the wrong part variation was inserted.
V — Additional Testing Features of InterTech Leak Testing Solutions
For these and all InterTech automotive parts testing, ISO 16494 defect prevention requirements of less than 10% Gage R&R is met or exceeded. Proven, reliable InterTech leak test instrumentation and fixture designs ensure production efficiency.
InterTech leak test technology is thoughtfully designed to optimize all steps of manufacturing by integrating testing with other requirements for quality, efficiency and user-friendliness.
- Full Integration with robotic systems, with flexibility to handle families of parts and quick changeover times.
- Power Clamps™: developed to provide consistent sealing force. They are also used to simulate mounting conditions of the parts being tested. InterTech’s Power Clamps also retract after testing to allow either robotic arms or human operators access and clearance for parts loading and unloading.
- Sensor and vision inspection systems for part verification and gaging.
- When required by applications, InterTech test instruments can also test for blockage or specified threading as part of the test cycle.
- EtherNet/IP capability for seamless test cell integration into quality control and manufacturing systems, to ensure traceability and compliance with ISO requirements. Part programs and related test parameters (pressure, leak limits) can be selected either manually or remotely (Profinet, Ethernet/IP). Automated Gage R&R mode for audit traceability to verify system performance.
- Real-time display of test data.
- SPC Capabilities: test results are stored for future analysis and test curves are displayed for each test instrument.
VI - Summary: Mass Flow Leak Testing Large Automotive Castings Means Cost-Savings
InterTech’s patented Mass Flow sensing technology creates many avenues for cost-savings for manufacturers of large aluminum and magnesium castings in automotive applications.
1) Eliminating the inevitable error-prone two measurement Pressure Decay testing methods means that there are fewer defective parts getting the relative expensive costs of impregnation with resins at later steps of production.
2) The relatively long stabilization times otherwise required for leak testing large volume parts are minimized by InterTech’s Ex-Heat and Fast-Fill patented[4.1] technologies.
3) Dynamic test cycles, the hallmark of InterTech’s LeanTest, speeds testing up to 50% by facilitating quick pass and quick fail parts identification.
4) InterTech PowerClamps and other unique test fixtures make testing the relatively uneven surfaces and often complex designs of cast parts relatively straightforward — speeding production lines without sacrificing quality.
5) Both real-time test data display and data handling facilitate process improvement quality systems to optimize production.
InterTech’s deep bench of engineering talent provides no-cost in-depth consultations for your unique testing challenges. These evaluations will determine the ROl of replacing non-optimal Pressure Decay and differential pressure decay with Mass Flow testing technology and related testing innovations. Contact InterTech Development Company to schedule an in-depth leak testing application analysis.