A correct paint- or coating thickness is an important parameter in the surface treatment industry. The performance of a coating system is based upon correct application of this system following the paint manufacturers specification sheet. Each layer from primer to topcoat and individual intermediate layers are exactly specified with a minimum and maximum coating thickness. Often both a dry and wet film thickness is specified.
Although the dry film is the leading parameter often the wet film is measured during application in order to give the operator an indication of the applied wet film thickness. Measuring wet film thickness during application identifies the need for immediate correction and adjustment by the applicator. The relationship between dry and wet film thickness depends on the volume of solids in the coating. The applied wet film thickness depends on many variables such as the type of coating, application method (air pray, brush, roller, airless spray, electrostatic, tribo gun, etc.), equipment quality, operator, substrate material, the size and shape of the part etc.. Incorrect coating thickness could lead to technical problems or excessive costs.
Too low dry film thickness can lead to:
- Insufficient corrosion protection
- Hiding power problems
- Rust Spotting
Too high film thickness causes problems like:
- Extra material costs
- Sagging (Sags), Runs
- Mud Cracking
Multiple layer systems are often measured layer by layer. After cure of each layer a large number of measurements are made and stored so statistics of each following paint layer can be compared with the previous layer to define the thickness of each individual paint film. .
Techniques and systems
There are many different techniques and systems to measure coating thickness. The choice of coating thickness gauge type depends on type of application, substrate properties, budget and other requirements. The following type of instruments are common in today’s coating and surface treatment industry:
Magnetic pull-off gauges are suitable for ferrous metal substrates only and use a permanent magnet combined with a calibrated spring which is linked to a graduated scale. The magnetic force between the magnet and ferrous steel substrate pulls the two together. The coating thickness separating the two makes it easier to pull the magnet away. The more coating between magnet and steel the easier the pull. The actual coating thickness is determined by measuring this pull-off force. Testing with magnetic gauges is sensitive to surface roughness, curvature, substrate thickness, and the make up of the metal alloy. Magnetic pull-off gauges are rugged, simple, inexpensive, portable, and usually do not require any calibration adjustment. They are a good, low-cost alternative in situations where electronic gauges are not allowed due to explosion risk or when quality goals require only a few readings during production.
Eddy current techniques are used to non-destructively measure the thickness of non-conductive coatings on nonferrous metal substrates. A coil of fine wire conducting a high-frequency alternating current (above 1 MHz) generates an alternating magnetic field at the surface of the instrument’s probe. When the probe is near to a conductive surface, the alternating magnetic field will set up eddy currents on the surface. The characteristics of the substrate and the distance of the probe from the substrate (the coating/layer thickness) affect the level of the eddy currents. The eddy currents create their own opposing electromagnetic field that is sensed by the exciting coil or by a second, adjacent coil. Modern dry film thickness gauges often combine the magnetic induction and eddy current methods in one instrument. These instruments are basically suitable to measure each non-magnetic layer on a magnetic surface or each non-conductive layer on a conductive metal surface such a aluminum , copper, brass, stainless steel….
Magnetic and Electromagnetic Induction
These gauges are used to measure non-magnetic layers on magnetic (ferrous) surfaces. Magnetic induction instruments use a permanent magnet to generate a magnetic field. A magneto-resistor or Hall-effect generator is used to sense the magnetic flux density at a pole of the magnet. Electromagnetic induction instruments use an alternating magnetic field. A soft, ferromagnetic rod wound with a coil of fine wire is used to produce a magnetic field. A second coil of wire is used to detect changes in magnetic flux. These advanced electronic instruments measure the change in magnetic flux density at the surface of a magnetic probe as it comes close to a steel surface. The level of the flux density at the probe surface is directly related to the distance from the steel substrate. By measuring flux density the coating thickness or layer thickness can be determined.
Ultrasound or Ultrasonic
The ultrasonic pulse-echo technique is used to measure the thickness of coatings on nonmetal substrates (plastic, wood, concrete etc.) without damaging the coating. The probe of the instrument contains an ultrasonic transducer that sends a pulse through the coating. The pulse reflects back from the substrate to the transducer and is converted into a high frequency electrical signal. The echo waveform is digitized and analyzed to determine coating thickness. In some circumstances, individual layers in a multi-layer system can be measured.
PIG or Destructive measuring
This method is often used on reference panels or in forensic inspections with failing coating systems. With this system a precise incision is made with a special cutting tool perpendicular to the surface. An illuminated measuring microscope is used to measure the width of the cut. Depending on the type of cutting tool the width is multiplied with a factor which leads to the coating thickness. This system is ideal for those circumstances where electronic gauges cannot be used. The thickness of individual layers can be determined as well with this system.
Wet film thickness measuring
Wet-film is most often measured with a wet film comb or wheel. The wet-film comb is a flat precision plate with calibrated notches on the edge of each face. They are made of aluminum, plastic, or stainless steel. The gauges are placed squarely and firmly onto the surface to be measured immediately after coating application and then removed. The wet-film thickness lies between the highest notch that touched the coating and the next uncoated notch. Wet film thickness (WFT) combs measurements are neither accurate nor sensitive, but they are useful in determining approximate wet-film thickness in an early stage of application of coatings on articles where size and shape prohibit the use of more precise methods. A wet film thickness wheel is a high precision, accurate and easy to use instrument which consists of a set of three wheels. The central wheel is of a smaller diameter and is eccentric relative to the two outer wheels. The wheel is rolled through the wet coating and the interface of where the inner wheel starts touching the paint indicates the wet film thickness. Better wet film thickness wheels are equipped with roller bearings for smooth rolling and knurled outer wheels to allow measurements to be taken on slippery coatings or on fast moving substrates such as coil coating.
An air-ultrasonic device can be used non-destructively / non-contact on uncured powder coatings on smooth metal surfaces to predict the thickness of the cured film before the powder coating is cured in the oven.
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