Eddy Current Testing Ensures Top Quality for Medical Device Manufacturer

Medical device manufacturers must adhere to the strictest quality requirements.  A major hypodermic needle manufacturer needed to perform inspection on 100% of their products.  Their testing criteria was to detect through-wall cracks down to 0.100 inches (2.5 mm) long x 0.004 inches (0.1 mm) wide in needles down to 0.011 inches (0.28 mm) in diameter.  The needle material is medical grade stainless steel.  Other non-destructive testing methods such as leak testing or visual inspection were not able to meet the inspection thresholds or keep up with the production rate.  The hypodermic needle manufacturer worked closely with Criterion NDT to develop an in-line testing solution.

In-Line Hypodermic Needle Testing Solution

To efficiently perform 100% inspection of the hypodermic needles, the eddy current inspection was performed just prior to the needle cutting process.  This ensured that defective materials could be easily cut out in-line, in real-time.  The test is designed to find defects in both the raw material and weld areas.  The testing solution consists of an eddy current encircling coil and eddy current test instrument interfaced to the production line cutting/sorting mechanism.

Eddy Current Testing Instrument

The eddy current testing instrument induces an electromagnetic signal into the hypodermic needle material through the encircling coil.  See Figure 1.  Eddy currents are generated within the material and then the same encircling coil measures the eddy current flow and sends the information back to the instrument.  The eddy current instrument compares the signal with a “known good” response and identifies whether a defect is present.  Figure 2 shows a typical flaw response beyond the detection threshold seen as a red circle.  If a defect is present, the eddy current instrument signals a PLC in the material handling system which automatically removes the defective material.  Criterion NDT’s eddy current instruments are capable of testing tubing at various rates depending on flaw size.  In addition to providing a flaw detection process, the data can be used to monitor the production line as part of a continuous process improvement program.

Eddy Current Coil Design

Running in continuous operation required that the eddy current coil be able to withstand extreme wear conditions.  On the coils for this inspection, Criterion NDT uses TTZ (Trans-Toughened Zirconia) ceramic inserts to provide maximum coil life.  The hardness of the TTZ protects the electrical windings from wear and damage caused by snags and burrs in the tubing.  A single coil can accommodate a number of different tubing diameters.  See Figure 3.

Benefits of Eddy Current Testing

Real-time eddy current inspection allows manufacturers to perform 100% in line inspection at production line rates.  It also helps manufacturers reduce scrap and warranty costs, and improves overall manufacturing processes.  Eddy current testing is a green technology and unlike other testing methods does not require the use of chemicals or dyes.

Figure 1 – Eddy Current Testing Instrument and Coil

Figure 2 – Eddy Current Flaw Indication

Figure 3 – Eddy Current Probe/Coil with Medical Tubing Sample