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Sophisticated leak testing ensures reliability of complex medical parts.

An automatic leak tester and a customized testing process proved essential to the production of a complex medical component by American Technical Molding, Inc. (ATM) of Upland, Calif. The part, intended for Kendall Healthcare Products Co., had to meet a very low leakage-rate specification for each of five different internal air passages. ATM knew that conventional underwater bubble testing would not suffice. The solution turned out to be a unique test apparatus that mimics the way the part functions in the end use.

ATM, an ISO 9002-certified precision molder, was asked by Kendall to injection mold a manifold that distributes air pressure through four circuits of a pneumatic leg sleeve designed to prevent clots in leg veins during and after surgery. ATM selected InterTech Development Co. (IDC) of Skokie, Ill., to provide a custom test system that would verify that each manifold would restrain leakage to less than 5 sccm (standard cc/min). The task was made tougher by the complexity of the manifold.

Slightly bigger than a beeper, the manifold mates five separate air passages - an air-supply plenum and four distribution circuits - on one side with four solenoid valves on the opposite side. The circuit passages terminate in a row of four ports at one end of the manifold, forming a receptacle that mates with a coupling fitted onto four flexible tubes that guide the air pressure from each manifold circuit to a corresponding section of the leg sleeve.

Molding the manifold of polycarbonate/ABS blend with [+ or -]0.002-in. tolerances and a very tight spec for flatness around the valve seats was a challenge in itself, according to ATM engineering manager Bill Maywald. Enclosing the manifold is a flat cover molded of the same material that is ultrasonically welded over the plenum-and-channel side. The cover is molded with continuous grooves that mate with "energy director" fins molded into the manifold body on top of each cavity.

ATM ships the welded manifolds to the supplier of the solenoid valves for further assembly, followed by leak testing. "Failure to pass that post-assembly test due to faulty molding would be very disruptive and costly," says Maywald. So ATM wanted a test that would ensure reliability of the manifolds before they left its plant. "We've done simple leak/no-leak bubble testing, but we had no experience with testing to a given low-level leakage rate," Maywald notes. "We needed a test-equipment supplier with strong service and support to guide us through it."

FIVE-STEP LEAK TEST

IDC customized ATM's leak-testing system around its microprocessor-controlled M-1045 pressure-decay instrument, chosen for its simplicity of design and operation. In a five-step test sequence, the instrument fills the test cavities to a specified pressure, then senses any pressure drop that would indicate leakage. A display reports fill pressure and simultaneously converts pressure-drop data into sccm to show the actual leakage rate. At the cycle's end, the display reports "accept" or "reject" and signals with a green or red panel light.

All test-parameter values - e.g., times, pressures, and failure setpoints - are entered via the front-panel touchpad. Calibration is simple and automatic, initiated by a single keystroke and insertion of a plug-in calibration master.

Intertech's customization included development of a sequencer that steps the test automatically through the five air passages in less than a minute. Even more critical was the customization of the test fixture. It simulates the closure of the solenoid valves against the molded valve seats. During the test, four pneumatic cylinders move rubber seals (of the same durometer as the actual valve seals) forward to contact the valve seats under controlled pressure. Simultaneously, a fifth cylinder advances the instrument's air-supply connection to the plenum inlet, and a sixth cylinder blocks off an open port intended to supply a pressure sensor in the finished unit. The plenum is then pressurized to 3 psig for 6 sec, after which each of the four ports is pressurized separately for 10 sec apiece.

The IDC tester automatically stores data for the last 100 tests and can perform statistical calculations to show the mean, standard deviation, and [+ or -]3-sigma values. Data stored in this MS/DOS-based unit can be recalled for display, printed out, or downloaded to standard spreadsheets.

TEST MIMICS USE CONDITIONS

The IDC tester connects to the manifold's four air-circuit passages through the same tubes and coupling that the finished Kendall system uses. Maywald explains, "Using production parts as test fittings brings the test environment closer to real operating conditions and actually tests the design integrity of the coupling as well."

In early production runs, ATM encountered some assembly test failures, the result of overlooking the fact that the solenoid valves close under spring pressure of only 8 oz. The test fixture was holding the simulated seals closed with 20 lb of force, and the seals would deform enough to compensate for any knit lines or uneven flatness in the valve seats, allowing a bad seat to pass the test. IDC then refitted the fixture with the same plungers and springs used in the solenoid valves so as to apply the same pressure as is applied in use.

IDC's M-1045 sells in the $5000-6000 range, and a full customized package starts at $15,000. Other recent applications include leak testing of waterproof cameras, automotive fluid reservoirs, a blood-gathering system that separates different blood components, and a breathing apparatus with 12 passages.
COPYRIGHT 1996 Gardner Publications, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1996, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Author:Sherman, Lilli Manolis
Publication:Plastics Technology
Date:Sep 1, 1996
Words:893
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