Dry air leak testing offers foundries improved QC data.
In the foundry industry, leak testing traditionally has been used simply to ensure that castings that leak do not reach customers. But economics, competition and improved technology are leading to increasing use of volumetric leak testing as a means to satisfy quality standards. Today's leak detection systems can actually help deliver a better product while holding down costs.
Expanded Role in Quality
Leak testing technology has helped expand its role as a quality control tool. Depending on the end-use application of the casting, customers may require 100% leak testing to verify integrity, particularly when the casting is part of a liquid or gas container. It may also be required as input to SPC or other quality programs, helping to ensure that final assemblies comply with common published standards.
Specific leak readings may be used to evaluate such attributes as casting porosity, seals, assembly deficiencies, fit and function problems and fastening joining integrity. For example, in some cases, a specific leak rate may indicate that a part will not leak liquid, but could indicate excessive porosity that might result in premature part failure in service.
Dry Air Testing
Commonly, production leak testing in the foundry industry has consisted of some form of bubble testing, often carried out in a hostile environment of oil mist, dust and varying temperatures. In bubble testing, an operator pressurizes a casting, forcing in air and sealing the opening. It is then submerged in a water bath and the operator watches for a stream of escaping bubbles to signal a leak.
Though this method can detect minute leaks and is useful as a go/no-go indicator, it is time-consuming and cannot provide an exact measure of the leakage rate. It is best used where production rates are low and skilled personnel are available.
The need for shorter test times has traditionally tended to conflict with the reliability and precision of leak readings and their ability to detect very small leaks. But with the choice of the proper leak sensor for an application, it is now possible to gain faster test cycles and apply tighter leakage specs without sacrificing accuracy.
Dry air leak testing meets the above specifications, and is a viable option for the foundry industry. It can provide accurate quantitative leak readings without creating a bottleneck in the production process. Two basic methods are available. One measures the rate of pressure change, while the other directly measures leakage mass-flow.
Rate of Change Testing
In the rate-of-change method, the test part is pressurized to a level measured by a gauge and then isolated from the pressure source. Any change in the part's gauge pressure over time must be convened by calculations into a measure of leakage.
A faster and more accurate version of this method pressurizes a reference volume along with the test part. A transducer reads the pressure differential between the nonleaking reference and the test item. Again, calculations convert this reading into a measure of leakage.
The problem with pressure change systems is that they require measurement of test part pressure at two different times in order to calculate the change of pressure over time. An error in either measurement results in an equivalent or larger error in the required difference calculations. Other variables - such as ambient temperature changes, drafts, test part deformity or seal creep - increase the probability of error when using longer measurement intervals.
The Mass-Flow Method
In the past, direct leakage mass-flow measurement was viewed as slower and less reliable than pressure change detection. However, recently improved mass-flow sensor technology, coupled with the use of microprocessor-based electronics and control reservoirs, has dramatically raised the performance of these leakage measurement systems.
In the mass-flow method, a reference volume called a control reservoir is pressurized along with the test part. The test part and the reservoir are connected, and a mass-flow sensor is stationed between them. When the pressure source is closed off, any leakage in the test part is naturally compensated for by air flowing into it from the control reservoir. In the process, that air flows across the sensor, which measures its amount directly in standard cubic centimeters per minute (SCCM).
Use of the control reservoir greatly enhances the accuracy of leak testing, because it is more stable than conventional pressure regulators. Automated mass-flow testing stations using this technique can now provide accurate, reliable and fast leak detection in challenging production-line installations.
The tests are also accurate over a much wider range of leak/volume ratios and testing conditions than pressurized systems. They are particularly suited to rapidly detecting small leaks or leaks in large castings. And it is practical to test large volumes of castings in this way. Typically, 20-liter passages in a casting can be tested in less than 60 seconds. With computerized control, it is possible to conduct simultaneous tests on several different passages of the same casting.
Unlike pressure change testing, the mass-flow method uses a single point measurement, which is generally more accurate and completed in much less time (typically under a second), minimizing the impact of uncontrollable variables. For example, mass-flow is less affected by temperature variations than pressure change. Where conditions are severe, a special computer system for temperature compensation can be employed to counter fluctuations.
Testing Line Integration
One of the benefits of mass-flow leak testing is that it is easily adaptable to high-volume production applications. Testing stations, ready with quick-change custom tooling for various parts, can be customized to accommodate the work flow. The following sample applications were successfully tested in foundry environments:
Transmission housings - Here, two different types of the casting were tested to 3 SCCM at a rate of 200 pans per hour. A two-station semiautomatic test stand with interchangeable tooling fixtures was employed. The computer stored all test data, which could later be displayed, printed or downloaded to a remote computer. Acceptable parts were stamped as such immediately, while rejects were unloaded to a reject container.
Machined diecast housings - In this application, 23 models of four basic types of machined diecast clutch housings were produced in a sequence determined by the plant's central computer. The production rate was 400 parts per hour to be tested at a level of 1 SCCM.
To accommodate this quantity, the system was designed with four independent test stations controlled by one leak test computer. The units used mass-flow leak sensors and noncontact temperature compensation. The stations were fitted with four quick-change interchangeable tooling fixtures, each with multiple quick-change inserts for the various casting types.
Other features of the system included noncontact housing position sensors, casting-out-of-position alarm, seal leak alarm, and bidirectional communication with the plant's main computer to receive model-change signals and send test data. Rejected parts required manual data input from the operator.
Four-liter intake manifold - These castings were produced at a rate of 100 parts per hour and tested to a level of 6 SCCM. A single-station test stand was employed with semi-automatic operation, manual load/unload, mass-flow leak detection, and multiple pneumatically operated seals and clamps.
From these applications, it can be seen that modern leak testing systems are flexible enough to be adapted to a variety of production rates and part varieties, without slowing down foundry operations. With or without computerization, they can be integrated with other quality control tests, providing another measurement of the integrity of a foundry's castings.
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|Title Annotation:||quality control|
|Date:||Jan 1, 1995|
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