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Coating thickness gauges that use ultrasonic measurement techniques have become popular because they support or replace destructive methods for measuring the thickness of coatings over wood and wood products.
Coatings serve a variety of functions. Some are designed to restore, protect, waterproof and beautify wood structures. Others are specifically formulated to seal and fill pores and to provide an aesthetically pleasing surface texture. Penetrating finishes are absorbed into the wood and harden to create a strong protective barrier that will not flake off.
Why measure thickness?
Coatings are designed to perform their intended function best when applied within a tight thickness range. For example, conversion varnishes are harder than other coatings and should not be used in excess of 5 mil dry thickness in order to prevent cracking or other finish failures. Nitrocellulose lacquer should usually be kept lower than 3 mil.
On MDF, powder-coating thickness typically ranges between 3 to 9 mil. Usually the thicker the mil coverage, 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 measuring finish thickness precisely. 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. Finally, regular testing can 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.
Since these instruments can' t measure the thickness of finishes over wood, alternate techniques have been used including:
- 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 percent 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 wood 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 nondestructive 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.
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 don't have sufficient sensitivity to measure the thickness of acrylic fillers, factory primers, lacquers, UV finishes, powder coatings and other materials used over wood.
The first hand-held instrument designed specifically for coating thickness measurement appeared on the market 14 years ago and is now into its fourth generation. It uses a single-element transducer and advanced numerical techniques to filter and enhance digitized echoes. Today's hand-held ultrasonic coating thickness gauges are simple to operate, affordable and reliable (Fig. 1).
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 (Fig. 2).
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 multilayer application also favor 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 the wood substrate. Fig. 3 shows two examples of coated wood. These photos, taken at higher resolution than most field destructive tests are capable of, clearly show the boundary between the finish and the wood. The finish coating may look smooth on top, but thickness may be inconsistent. Wood substrates are often grainy with varying degrees of surface roughness and primer penetration. Such porosity and roughness may promote adhesion, but they increase the difficulty of attaining repeatable thickness measurements.
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. (Fig. 4)
The LCD displays three "peaks" representing three material interfaces.
Ensuring the right sound
Ultrasonic testing brings distinct benefits to the wood industry. Furniture, flooring and musical instrument manufacturers often apply several layers of lacquer or similar finishing materials. Some processes require the ability to identify the thickness of individual layers or series of layers. When applied at the wrong thickness, the coating layer that beautifies and protects a high-quality guitar, for example, can easily detract from its sound. Too much coating can dampen the guitar's acoustic resonance; too little can have the reverse effect.
Musical instrument manufacturers now use ultrasonic gauges to accurately and nondestructively measure the lacquer on their valuable products. As a result of using this new technology, they have not only decreased their lacquer usage, but they are able to take meaningful measurements without excessively disrupting their production process. There's no need to scrap product to measure a coating thickness, and the thickness over the entire surface can easily be measured to ensure a smooth, even coating. Rework is minimized due to an increased ability to control the coating process.
A final echo
Quick, nondestructive 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 overcoating 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.
Ed. Note: David Beamish is general manager of DeFelsko Corp., a New York-based manufacturer of hand-held coating test instruments. He has a degree in civil engineering and has more than 20 years' experience in the design, manufacture and marketing of these testing instruments.
author: By David Beamish