Print Version November 2008
DeFelsko manufactures hand-held, non-destructive ultrasonic coating thickness gages that are ideal for non-destructively measuring the dry film thickness of wood finishes and other coatings on wood and wood products. Many industries now use this nondestructive technology in their quality programs.
Two models are ideal for wood coatings.
Application #1: Measuring total thickness
Some wood finishes are applied in a number of layers to achieve their desired objective. Our PosiTector 200 B1 is the ideal solution when applicators only need to know the final, total thickness of the coating.
The PosiTector 200 is ready to measure most wood finish applications right-out-of-the-box. It has a measuring range of 13 to 1000 microns (0.5 to 40 mils) and is ideal for measuring lacquers, primers, UV cured coatings, acrylics, powder coatings and other polymer coatings. It requires no calibration adjustment for most applications, it is mils/microns switchable, and has a large, thick impact resistant Lexan display.
Application #2: Measuring lacquer on guitars
A good example of how our PosiTector 200 B1 measures the total thickness of a multi-layer application can be found in the manufacturing process of musical instruments.
Lacquer is applied to a guitar substrate (typically wood or graphite) to attain a glass-like sheen while showing the beauty and luster of the wood. The tough durable finish of the lacquer protects the wood from moisture, chemicals and marring, while optimizing the acoustic resonance of the instrument.
The method and benefits of ultrasonic thickness testing for guitars is discussed in a separate article.
Application #3: Measuring on veneers and laminated composites
Thickness measurement is not limited to natural wood. Some of today’s modern materials are a combination of synthetic and natural materials.
In woodworking, veneer refers to thin slices of wood that are usually glued and pressed onto core panels (typically, wood, particle board or medium density fiberboard) to produce flat panels such as doors, tops and side panels for cabinets, parquet floors and parts of furniture. Composite materials with a wood veneer surface are increasingly used in aerospace, underwater, and automotive structures.
The wood or wood product surface is often finished with a polymer coating, the total thickness of which can easily be measured with a PosiTector 200 B1.
Application #4: Measuring on Rough Coating Surfaces
Coatings with rough surfaces challenge any measurement method, and ultrasonic testing is no exception. The PosiTector 200 is equipped to handle these situations.
At a microscopic level, thickness can vary (see Fig.1). Meaningful thickness measurements are best obtained by taking several measurements in the same general location and averaging the results.
On rough surfaces the PosiTector 200 typically identifies the thickness from the top of the coating peaks down to the substrate. This is represented by distance #1 in the Fig.2. Couplant fills the voids between the probe and the coating to assist the ultrasonic pulse enter the coating.
Severe roughness can cause the gage to display low thickness values (distance #2). This happens because echoes from the couplant/coating interface are stronger than the coating/substrate interface. The PosiTector 200 has a unique user-adjustable SET RANGE feature to ignore roughness echoes.
To obtain meaningful thickness measurements on rough surfaces, the memory mode of the PosiTector 200 (Fig.3) is used to calculate a running average.
Application #5: Measuring powder coating on MDF
Medium density fiberboard, or MDF, is an engineered wood product made of particles of wood mixed with a synthetic resin. Powder coating is a dry finishing process, using finely ground particles of pigment and resin, which are electrostatically charged and sprayed onto the MDF. The charged particles adhere until melted and fused into a tough, even coating through the application of heat and energy.
MDF is suitable for powder coating because of its low porosity and homogeneous surface. MDF products include office furniture, kitchen and bath cabinets, doors, store fixtures and displays, and ready-to-assemble furniture for the office and home.
These single coat applications typically range in thickness from 3 to 9 mils and are easily measured with a PosiTector 200 B1. For textured surfaces shown above, PosiTector 200 has a unique user-adjustable SET RANGE feature to ignore roughness echoes. And the instrument’s memory mode is used to display the average of a series of measurement results. The large LCD also displays the number of readings taken, the standard deviation, and the highest and lowest readings.
Fig. 4In this example, 10 measurements have been taken. The last measurement of 18.2 mils is displayed along with standard deviation and max/min values of all 10 readings. The average value is 18.26 mils.
Application #6: Thick, Polyester-based polymers
Polymer coatings are applied to wood based materials such as MDF (medium density fiberboard), plywood, chipboard and particleboard to provide an aesthetic durable finish. Entire industries have been developed around the use of polymer coatings. For example, the polyester based Granicoat® (Fig.5) is sprayed on to wood surfaces such as portable buffet tables, kitchen countertops, bath vanities, as well as desk and bar tops.
Fig.5Measuring Granicoat® thickness.
For these thick coating applications we recommend our PosiTector 200 C3. It has a measuring range of 50 to 3800 microns (2 to 150 mils) and is ideal for thicker coatings. Its graphics mode is useful in determining the thickness of thin primers and other hard-to-distinguish layers.
Fig.7: Using the cursor.
Figure 6 shows what would appear on the instrument’s LCD when a measurement is taken when graphics mode is turned on. Both the thickness values and a graphical representation of echo “spikes”, "peaks" are displayed. Interestingly, a smaller peak can also be seen. Although it is too faint to measure automatically, a green cursor can be positioned over top for manual measurement.
The green cursor in figure 7 tells us the indicated peak is 4.4 mils thick.
Application #7: Dry Adhesive
There are many types of wood adhesives. Wet adhesives cannot be measured ultrasonically because the probe of the PosiTector 200 must touch the surface of the material being measured. However, dry adhesive is much like any other single layer coating and can be measured by the PosiTector 200 B1.
Application #8: Measuring the Individual Layer Thicknesses in a Multi-layer Application
The PosiTector 200 B3 is capable of measuring BOTH the total coating system thickness AND up to 3 individual layer thicknesses in a multi-layer system. It also features a graphic readout for detailed analysis of the coating system.
In the above example, layer 1 is 1.5 mils thick. Layer 2 is 1.5 mils thick. Total thickness is 3.0 mils. The graphical LCD displays two “peaks” representing two material interfaces. A two-step process adjusts the instrument for multi-layer applications.
Application #9: Other applications ideally suited for ultrasonic coating thickness testing
UV cured coating on hickory
Water borne acrylic primer on plywood
Factory primer on untextured molded hardboard
UV cured coating on unfilled maple
Roll and curtain coated acrylic filler/base/top coats on hardboard
Sprayed lacquer on oak
Application A: Coatings with large, solid particles
The probe of the PosiTector 200 contains an ultrasonic transducer that sends an ultrasonic vibration into the coating. This vibration travels through the coating until it encounters a material with different mechanical properties - typically the substrate. The vibration is partially reflected at this interface and propagates back to the transducer.
For best accuracy, the ultrasonic vibration must proceed unimpeded through the coating. Large solid particles such as sand deflect the vibration and prevent a clear echo from returning to the probe. Therefore the PosiTector 200 is unable to repeatably measure these applications.
Application B: Wood Stains
A wood stain is a sub-category of paint. It consists of a pigment suspended in a "vehicle" of solvent and binding agent (alkyd, linseed oil, acrylic, polyurethane, lacquer, or resin). It is formulated to impart or transport the pigment into the pores of the surface rather than creating a film on top of the surface.
The PosiTector 200 is designed to measure the thickness of films built-up on the surface of wood. Stains, which penetrate deep into wood fibers to seal and protect wood, cannot be measured ultrasonically.
How to Measure
Ultrasonic testing measurement works by sending an ultrasonic vibration into a coating using a probe (i.e. a transducer) with the assistance of a couplant applied to the surface. A 4 oz bottle of a common water-based glycol gel is included with every instrument. Alternatively, a drop of water can serve as couplant on smooth, horizontal surfaces.
Fig.10Taking a measurement.
After the drop of couplant has been applied to the surface of the coated part, the probe is placed flat on the surface. Pressing down initiates a measurement (see Fig.10). Lifting the probe when a double beep is heard holds the last measurement on the LCD. A second reading may be taken at the same spot by continuing to hold the probe down on the surface. When finished, wipe the probe and the surface clean with a tissue or soft cloth.
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 gages usually require no adjustment to factory calibration settings.
Graphics Mode (PosiTector 200 - Advanced models only)
The right hand side of the PosiTector 200’s screen can be used to display a graphical representation of the ultrasonic pulse as it passes through the coating system. This powerful tool enables the user to better understand what the gage “sees” below the surface of the coating.
Fig.11PosiTector 200 B3 with Memory Mode ON Left: PosiTector 200 B3 with Graphics Mode ONRight: PosiTector 200 B3 with Graphics Mode OFF
As the probe is depressed and the ultrasonic pulse travels through the coating system, the pulse encounters changes in density at the interfaces between coating layers and between the coating and the substrate.
A “peak” depicts these interfaces. The greater the change in density, the higher the peak. The more gradual the change in density, the greater the width of the peak. For example, two coatings layers made of essentially the same material and "blended" would result in a low, wide peak. Two materials of very different density and a well-defined interface would result in a high, narrow peak.
The PosiTector 200 B3 chooses the highest of peaks when trying to determine coating layer thickness. For example, if the number of layers is set to 3, the 3 highest peaks between the Lo & Hi Set Range are selected as the interfaces between these layers. The peaks that the Gage selected are indicated by red triangle arrows (see Fig.12).
In Fig. 12, the left (Lo - 1.9mils) and right (Hi = 11.0 mils) Range values are displayed as two numbers at the top left and top right of the graphics display screen. Lo (the minimum limit) is at the left. Hi (the maximum limit) is at the right. Echoes or peaks (thickness values) outside these ranges are ignored. Range values are set and modified using the Set Range menu option.
This Graphics display can be manipulated with the Set Range menu option. In addition to being able to adjust the Lo & Hi Range values, a green Cursor can be positioned anywhere between the set range values to investigate other peaks.
Fig.13A green cursor is shown.
Other Measurement Methods
Conventional magnetic and eddy-current gages only work on metals. Since these instruments can' t measure the thickness of finishes over wood, alternate techniques have been used including:
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.
Fig.14High-powered magnification reveals surface roughness of both the coating and the wood substrate.
A typical destructive technique requires cutting the coated part in a cross section and measuring the film thickness by microscopically viewing the cut. Another cross sectioning technique uses a scaled microscope to view a geometric incision through the dry-film coating. To do this, a special cutting tool makes a small, precise V-groove through the coating and into the substrate (see Fig.15). Gages are available that come complete with cutting tips and illuminated scaled magnifiers. A detailed description of this test method is provided in ASTM D4138-07a, “Standard Practice for Measurement of Dry Film Thickness of Protective Coating Systems by Destructive, Cross-Sectioning Means”.
Although this method' s principles are easy to understand, opportunities abound for introducing errors. It takes skill to prepare the sample and interpret the results. Also, adjusting the measurement reticule to a jagged or indistinct interface can generate inaccuracy, particularly between different operators. This method is used when inexpensive, nondestructive methods aren't possible, or as a means of confirming nondestructive test results.
With the arrival of ultrasonic instruments, many finishers have switched to non-destructive inspection.
Background on Wood Coatings
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 in order 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 fiberboard (MDF), powder coating thickness typically ranges between 3 to 9 mils. 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 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 drastically reduce overall efficiency. Finally, regular testing can reduce the number of internal reworks and customer returns due to finishing defects.
Fig.16The PosiTector 200 B3 can calculate the average and standard deviationof a series of measurements for each layer in a multi-layer system.
Why measure with Ultrasonics?
Manufacturers and applicators alike have long believed that there is no simple and reliable means for non-destructively measuring coatings on wood and wood products. Their common solution was to place metal coupons next to the part and then measure the thickness applied to the coupon with either a mechanical or electronic gage -- magnetic or eddy current. This labor intensive solution is based on the assumption that a flat coupon placed in the general coating area receives the same paint profile as the wood part in question. An ultrasonic solution enables the user to measure the total coating thickness of the actual part. Dependent on the ultrasonic gage utilized and the coating application process, an added advantage is the ability to identify multiple distinct layers.
Ultrasonic coating thickness measurement is now an accepted and reliable testing routine used in wood industries. The standard test method is described in ASTM D6132-08. “Standard Test Method for Nondestructive Measurement of Dry Film Thickness of Applied Organic Coatings Using an Ultrasonic Gage” (2008, ASTM). To verify gage 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:
Today, these instruments are simple to operate, affordable and reliable.
Couplant is required to propagate ultrasound into the coating. Water is a good couplant for smooth coatings. Use the supplied glycol gel for rougher coatings. While it is unlikely that the couplant will damage the finish or leave a stain on the surface, we suggest testing the surface by using the couplant on a sample. If testing indicates that staining has occurred, a small amount of water can be used instead of couplant. Consult the Material Safety Data Sheet available on our website and your coating supplier if you suspect the couplant may damage the coating. Other liquids such as liquid soap may also be used.
The PosiTector 200 Standard models can record 250 measurements. PosiTector 200 Advanced models can store 100,000 measurements in up to 1000 batches for on-screen statistical purposes, for printing to an optional Bluetooth Wireless Printer, or for downloading to a personal computer using the provided USB Cable and one of the PosiSoft Solutions.