Plastic plays a major role in the medical device industry, where it has been used for more than 40 years. Advances in technology, growing populations, and a rise in infectious diseases have contributed to the increased use of pre-sterilised, disposable plastic medical products. Yet inspecting these plastic products is challenging. Here, we’ll share some tips on how to improve the inspection process for plastic tubing and parts using ultrasonic and Hall effect thickness gaging technology.

The medical industry employs strict quality controls on manufactured plastic products, including extruded disposable medical tubing. Extruded tubing requires precise measurements for small diameter, thickness and concentricity tolerances. These products are used in minimally invasive surgery and examinations using miniaturised optics. Catheter tubes also have tight tolerances for wall thickness.

In the past, manufacturers cut samples to inspect tubes and measured wall thickness using calipers. Now Ultrasonic Thickness (UT) gaging provides a fast, nondestructive alternative to inspect small-diameter tubing.

In ultrasonic testing, a transducer transmits sound energy into a test material and detects sound energy reflected from the part’s back wall or an internal defect. An UT gage measures time of flight and calculates the part thickness using a known sound velocity for the test material.

The appropriate transducer type, size, and frequency vary based on the application. For tubing with a diameter of less than 0.125 inches (3 mm), we recommend a 20 MHz focused immersion transducer, but depending on the material this may vary.

To efficiently transmit sound energy into a small-diameter part, you must focus the sound into a narrow beam. Beam focusing combined with an immersion setup provides good coupling on radiuses too small to measure with contact transducers. Focused immersion transducers use a contoured acoustic lens with a column of water to focus the sound beam for increased sensitivity. A desktop immersion-tank and probe fixture can also help ensure repeatable measurements. The tank and probe fixture create a steady column of water and help keep the test piece centered in the sound beam.

To create a consistent, low-flow water column, use an immersion tank with a bubbler nozzle which matches the application. A V-notch bubbler simplifies tubing concentricity checks, enabling you to rotate the tubing to quickly measure thickness around the circumference and slide the tubing over the probe to measure thickness along the part length.

The transducer and instrument setup for a tubing product are selected after initial testing of product samples. Using a transducer and a fixturing device, such as a bubbler, set the thickness gage to mode two. 

Measurements are made between an interface echo that represents the test piece’s near surface and the first backwall echo, using a delay line or immersion transducer. This method is often used for measurements on sharp concave or convex radiuses in confined spaces with delay line or immersion transducers, as well as for in-line measurements of moving material with immersion transducers. Depending on the tube material, the minimum measurable wall thickness is typically 0.1 mm (0.004 inches) for diameters as small as 1.5 mm (0.060 inches).

Figure 1 shows a typical measurement of plastic tubing with an Olympus 38DL Plus gage. The first echo from the left represents the interface of the water and the tube, and the second echo represents the backwall, tube inner wall. The gain and blanking parameters should be adjusted for optimum echo detection.

Figure 1: Plastic tubing, 2 mm (0.080 in.) diameter, 0.37 mm (0.014 inches) wall thickness

While you can simplify small-diameter tubing inspections by using proper equipment and calibrated instruments, measurement errors may still occur. Here are three tips to avoid common issues:

When using an immersion setup, sometimes the interface echo is too small to reliably detect. This can happen when measuring plastics where the acoustic impedance is close to that of water. Increase the initial instrument gain to improve your chances of reliably detecting the interface echo.

2. Calibrate at the measurement temperature

In-line immersion thickness measurements of tubing are often performed in a factory shortly after the tube has been extruded. However, sound velocity of plastic drops rapidly as temperature increases. So, you must calibrate your thickness gage to a sample at the measurement temperature. 

3. Make final gain adjustments at the measurement temperature

Attenuation of sound energy increases with temperature, especially for thicker samples. To account for changing signal amplitude, adjust instrument gain on a sample at the measurement temperature.

Blow moulded and thermoformed plastic parts are commonly used for medical packaging and trays. Previous quality control methods involving cutting the plastic to measure the thickness with calipers introduced problems. This is because firstly, a burr is often left at the cut edge. If the operator measures over the burr, it’s not a true wall measurement. Assuming the operator carefully avoids burrs, they are limited in where they can measure with mechanical devices due to poor access to tight corners. Operator variation is another problem, and calipers can cause errors when you hold them at an angle to the part. Thickness readings will also vary between operators when calipers are used on materials that can be compressed.

Two nondestructive inspection methods can reduce or eliminate these problems:

One advantage of an UT gage is that you only need access to one side of the material, enabling measurements of closed containers, large sheets, and other objects with limited access to both sides.

Measurement accuracy depends on material sound velocity accuracy, so inaccuracies can occur if material sound velocity changes unpredictably. Changes, such as substantial temperature shifts or density variations can also affect velocity. To avoid these errors, calibrate and measure at ambient temperature if possible. Otherwise, calibrate and measure at a known, constant position during manufacturing.

2. Use a Hall effect thickness gage

The Hall effect uses a magnetic field applied at right angles to a current-carrying conductor. If a ferromagnetic target, such as a steel ball of known mass is placed in the magnetic field the induced voltage changes. As the target moves away from the magnet, the magnetic field and the induced voltage change predictably. Plotted voltage changes can compare induced voltage to the distance of the target from the probe.

To measure, place a Hall probe on one side of the product and place a ferromagnetic target on the other. The gage displays the distance between the target and the probe (wall thickness). This method has its advantages such as no couplant is used, measurement accuracy is independent of material velocity, and wall thickness in tight areas and thin samples can be measured. Additionally, it’s also easy to scan the probe around a part to verify thickness at several points or look for minimum thickness in an area. A limitation of this method, however, is that you must place a target ball inside the part. This therefore prevents its use on closed containers.

A Hall effect thickness gage can measure up to about 25 mm (1 inch). It can measure compressible materials, as long as you use the smallest ball possible to avoid compressing it. This gage is often in a production area and used by moulding equipment operators. Many operators prefer using Hall effect gages like the Olympus Magna-Mike 8600 instrument when measuring small, thin-wall (less than 0.100 inches or 2.5 mm) parts with tight corners, and most blow moulders favour Hall effect gages since they must inspect parts with complex shapes, relatively thin and flexible walls and corners which are difficult to measure with other gages.

The optimal gaging method ultimately depends on the plastic part you’re measuring. It’s possible to quickly calibrate both gages in a few steps to produce accurate, repeatable results, and users often find operator technique is less of a factor with these methods than with mechanical gaging. Calibration data is stored with logged readings to provide a record of operator work. Both gages provide datalogging capabilities, therefore eliminating transcription errors.

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