The Sensitive Touch

Coating thickness gauges that use ultrasonic measurement techniques support or replace destructive methods for measuring the thickness of coatings over many substrates such as wood products, composites, plastics and concrete.

Why Measure Thickness?

Coatings are designed to perform their intended function best when applied within a tight thickness range as specified by the manufacturer. For example, conversion varnishes are harder than other coatings and should not be used in excess of 5 mils dry thickness to prevent cracking or other finish failures. Nitrocellulose lacquer should usually be kept lower than 3 mils. A consistent mil thickness is paramount when applying lacquer base coats and crack coats to achieve a desired crackle finishing effect.

On medium density fibreboard (MDF), powder coating thickness typically ranges between 3 to 9 mils. Usually the thicker the mil coverage on MDF, the more durable the finish. Factory specifications often call for a stated ±1 mil tolerance. This level of quality cannot be determined just by looking at it.

There are other benefits to precisely measuring finish thickness. When companies fail to check and verify coating quality of incoming material, they waste money reworking product. By checking spray operators’ technique they ensure the coating is being applied in compliance with the manufacturers’ recommendations. Besides, applying excessive film thickness can reduce overall efficiency. Regular testing can also reduce the number of internal reworks and customer returns due to finishing defects.

How Best to Test?

Over metal, testing the thickness of coatings is commonplace for quality control and inspection purposes. When the base metal is carbon steel, a magnetic method is used. Eddy current devices are used for the other metals such as copper and aluminum.

These instruments can't measure the thickness of finishes over non-metal substrates such as composites and wood, so alternate techniques include:

• optical cross-sectioning—cutting the coated part and viewing the cut microscopically
  
• height measurement—measuring before and after with a micrometer
  
• gravimetric—measuring the mass and area of the coating to calculate thickness
  
• dipping wet film thickness gauges into wet paint and calculating dry-film thickness using the per cent of solids by volume
  
• substitution—placing a steel coupon alongside the wood part and coating it at the same time.

These tests are time-consuming, difficult to perform, and are subject to operator interpretation and other measurement errors. Applicators find destructive methods impractical. To get a statistically representative sample, several products from a lot might need to be scrapped as part of the destructive testing process.

With the arrival of ultrasonic instruments, many finishers have switched to non-destructive inspection.

Ultrasonic Breakthrough

Quality professionals are already familiar with various aspects of ultrasonic testing wherein high-frequency sound energy is used to conduct examinations and make measurements. Ultrasonic testing can detect and evaluate flaws in metal, measure dimensions, ascertain material characterization and more.

Wall-thickness measurement is perhaps the most common and simple of ultrasonic tests. Precision ultrasonic wall-thickness gauges permit quick thickness measurement of objects without requiring access to both sides. For coating measurement, however, these gauges are not ideal. They do not have sufficient sensitivity to measure the thickness of acrylic fillers, factory primers, lacquers, UV finishes, powder coatings and other materials used over wood and other substrates.

A Sound Measurement Technique

Ultrasonic testing works by sending an ultrasonic vibration into a coating using a probe (transducer) with the assistance of a couplant applied to the surface. The vibration travels through the coating until it encounters a material with different mechanical properties—typically the substrate but perhaps a different coating layer. The vibration, partially reflected at this interface, travels back to the transducer. Meanwhile, a portion of the transmitted vibration continues to travel beyond that interface and experiences further reflections at any material interfaces it encounters.

Because a potentially large number of echoes could occur, the gauge is designed to select the maximum or “loudest” echo from which to calculate a thickness measurement. Instruments that measure individual layers in a multi-layer application also favour the loudest echoes. The user simply enters the number of layers to measure, say three, and the gauge measures the three loudest echoes. The gauge ignores softer echoes from coating imperfections and substrate layers.

Measurement Accuracy

The accuracy of any ultrasonic measurement directly corresponds to the sound velocity of the finish being measured. Because ultrasonic instruments measure the transit time of an ultrasonic pulse, they must be calibrated for the “speed of sound” in that particular material.

From a practical standpoint, sound velocity values do not vary greatly among the coating materials used in the wood industry. Therefore, ultrasonic coating thickness gauges usually require no adjustment to factory calibration settings.

Where the Coating Meets the Substrate

A factor influencing the accuracy and repeatability of ultrasonic measurement is how these coatings interface with non-metal substrates.

Ultrasonic gauges are designed to average small irregularities to produce a meaningful result.

On particularly rough surfaces or substrates where individual readings may not seem repeatable, comparing a series of averaged results often provides acceptable repeatability.

A Final Echo

Ultrasonic coating thickness measurement is now an accepted and reliable testing routine used in many non-metal industries. The standard test method is described in ASTM D6132-04; “Standard Test Method for Nondestructive Measurement of Dry Film Thickness of Applied Organic Coatings Using an Ultrasonic Gauge” (2004, ASTM). To verify gauge calibration, epoxy coated thickness standards are available with certification traceable to national standards organizations.

Quick, non-destructive thickness measurements can now be taken on materials that previously required destructive testing or lab analysis. This new technology improves consistency and throughput in the finishing room.

Potential cost reductions include: minimizing waste from over coating by controlling the thickness of the coating being applied; minimizing rework and repair through direct feedback to the operator and improved process control; eliminating the need to destroy or repair objects by taking destructive coating thickness measurements.

Today, these instruments are simple to operate, affordable and reliable.