Maya Ellison
Measuring the thickness of a hardcoat on plastic is an important step in quality control and assurance. There are a variety of methods and tools available to measure the thickness of coatings on plastic, each with its own advantages and disadvantages. Some common methods include the use of coating thickness gauges, microscopy, X-ray fluorescence analyzers, and the coulometric method. The choice of method depends on factors such as the type of coating, substrate material, thickness range, and cost. Coating thickness gauges, for example, can be used to measure coatings on both ferrous and non-ferrous metals without damaging the coating, while X-ray fluorescence analyzers are widely used in plating shops to measure metallic coatings.
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What You'll Learn
Magnetic induction method
The magnetic induction method is a widely used technique for measuring the thickness of coatings on ferromagnetic substrates. This method is particularly useful for non-destructive thickness measurement of magnetisable materials. It is commonly employed in industries such as automotive, aerospace, and architecture.
The magnetic induction method utilises a probe with an iron core surrounded by an exciter coil. A low-frequency alternating current flows through this coil, generating an alternating magnetic field around the iron core's poles. As the probe approaches a magnetised object, the iron core amplifies the magnetic field. This amplification is registered as a voltage by a measuring coil, and the voltage difference corresponds to the distance between the probe and the object. When measuring coated parts, this distance directly represents the coating thickness.
Magnetic induction paint meters use a permanent magnet to create a magnetic field, while electromagnetic induction meters employ an alternating magnetic field. The change in magnetic flux density at the probe's surface, as it nears the object, is measured to determine the coating thickness.
It is important to note that magnetic permeability varies among materials, so recalibration of the measuring device is necessary when switching between different materials to ensure accurate measurements. This is particularly crucial when dealing with curved surfaces, as the portion of the magnetic field passing through the air changes, affecting the readings.
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Microscopy
Another optical microscopy technique involves slicing the plastic substrate with a microtome and measuring the sample under a microscope with a graduated stage. Alternatively, a section of the substrate can be covered with tape, which is then pulled off, and the difference in thickness measured with a micrometer.
Atomic Force Microscopy (AFM) is a similar technique to nanoindentation, which involves dragging a cantilever or indenter across the surface of the material. As the cantilever rises and falls with the peaks and valleys of the surface, a laser aimed at the back is deflected, providing a displacement value that corresponds to the thickness of the coating.
Confocal microscopy is a variation used for very thick films, which measures the thickness by determining the step height of a scratch in the film.
A different type of microscopy, electron microscopy, can also be used. This technique involves fracturing the sample, ion-milling it, and placing it under a Scanning Electron Microscope (SEM) or Transmission Electron Microscope (TEM).
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Ultrasonic pulse-echo technique
The ultrasonic pulse-echo technique is a non-destructive method of measuring the thickness of coatings on plastic. It involves sending an ultrasonic vibration into the coating using a probe (transducer) with the assistance of a couplant applied to the surface. The couplant fills the voids between the probe and the coating to assist the ultrasonic pulse in entering the coating. The vibration travels through the coating until it encounters a material with different mechanical properties, typically the substrate, but it could be a different coating layer. The vibration is then partially reflected at this interface and travels back to the transducer.
Ultrasonic instruments measure the transit time of an ultrasonic pulse, so they must be calibrated for the "speed of sound" in the material being measured. The accuracy of the measurement directly corresponds to the sound velocity of the finish. However, sound velocity values do not vary greatly among the coating materials used in the plastics industry, so ultrasonic coating thickness gauges usually require no adjustment to factory calibration settings.
The PosiTector 200 is a hand-held ultrasonic coating thickness gauge that can be used to measure the thickness of coatings on plastic. It has a unique user-adjustable SET RANGE feature to ignore roughness echoes. With memory turned on, the PosiTector 200 calculates and displays the number of readings taken, the average of those readings, the standard deviation, and the highest and lowest readings.
Ultrasonic testing can be used to detect and evaluate flaws in metal, measure dimensions, ascertain material characterization, and more. It is a widely used technique for basic shapes and products of many materials, precision machinations of parts, and determining the thickness of equipment walls caused by corrosion and erosion.
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Couloscope
The Couloscope is a measuring instrument widely used in different industries for measuring the thickness of coatings. The device can measure the thickness of virtually any metallic coating, including multi-layer coatings, on any substrate material. The Couloscope CMS2 STEP version is used for quality control of nickel multi-layers.
The Couloscope works according to the coulometric method by anodic dissolution (DIN EN ISO 2177). This involves dissolving a metal coating off its metallic or non-metallic substrate by means of an electrolyte under a controlled electric current, i.e., reversing the electroplating process (de-plating). The current required for de-plating is directly proportional to the mass of the metal to be removed. De-plating takes place in a measuring cell (miniature electrolytic bath). The measuring area is defined by a plastic gasket attached to the measuring cell.
The Couloscope CMS range measures coating thicknesses by means of de-plating layers through electrolysis. The coulometric method can be used to precisely determine multi-layers on any kind of substrate. The Couloscope CMS has a measuring range of 0.05 to 40 µm and is integrated with a graphical LCD that supports the operation and displays the coating results.
The Couloscope is a versatile instrument that can be used in a variety of industries, including automotive, aerospace, and architectural. It is a fast, accurate, and easy-to-use technique for measuring coating thickness.
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X-ray fluorescence analyzers
X-ray fluorescence (XRF) spectrometry is a technique used to measure the thickness of coatings on different materials, including metals, plastics, glass, and wood. It involves irradiating a specimen with X-rays and measuring the emitted X-rays characteristic of the elements present in the coating layers and the substrate. This method can be used to determine the thickness of single or multiple layers of coatings, making it versatile for various applications.
Handheld XRF analyzers, such as the Vanta™ analyzer, are commonly used for measuring coating thickness. These analyzers emit X-rays that strike the sample, causing it to fluoresce. The returning X-rays are then detected and used to calculate the thickness of the coating. This technology is advantageous as it is fast, efficient, non-destructive, and can be used on large samples without the need for cutting. Additionally, it can provide accurate thickness measurements, helping manufacturers control costs by ensuring coatings are not excessively thick.
The Vanta handheld XRF analyzer can measure coating thickness from 0.00 to approximately 60.00 microns, depending on the material. It can be used to determine the material chemistry of alloys, metals, and other materials. The thickness of coatings can be measured on various substrates, including plastics. This makes it a valuable tool for quality control in industries such as automotive and aerospace, where coatings play a crucial role in corrosion resistance, decoration, and drag reduction.
While XRF is a powerful technique, it may not be suitable for measuring the thickness of non-metal coatings like paint. In such cases, magnetic or eddy current gauges are more commonly used. However, XRF remains a well-established technology for metal and alloy identification and thickness measurement, offering precision and speed in nano-order level coating thickness measurements.
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Frequently asked questions
There are several tools that can be used to measure hardcoat thickness on plastic. These include the F10-ARc anti-reflection coating system, the F10-AR, and the F3-sX. Other tools include the couloscope, coating thickness gauges, and the PosiTector 200.
The F10-ARc anti-reflection coating system uses an optical contact probe that reduces the effect of backside reflections. The surface of the acrylic sheet is placed on the contact probe, and spectral data is collected and analyzed by FILMeasure to determine the coating thickness.
The PosiTector 200 is an ultrasonic paint thickness gauge that uses the ultrasonic pulse-echo technique to measure the thickness of coatings on non-metal substrates (plastic, wood, etc.) without damaging the coating.
