The thickness of a dry paint film is important to the performance of the coating. Thin films may appear to be the wrong color, or they may not provide the necessary level of performance. Water can more easily penetrate to the substrate if the film is too thin. Thick films can be a problem too. Thicker films are more likely to crack in use or they may create problems when two mating parts must be fitted together.
Wet-film thickness is measured for process control. It can be measured with a handheld immersion gauge that has a series of teeth along the edge of different lengths. The gauge is placed on the freshly painted surface, and the edge of the gauge is supported by two teeth of the same length for measurement control. All the other teeth are different in length and shorter than the control teeth. The wet thickness is measured as the last tooth that leaves a mark in the paint film. Therefore, if the wet gauge reads 3 mil and the coating is 50% solids, then the dry-film will be about 1.5 mil. This is a rough method of predicting the final dry-film thickness, but it does give the painter an indication of whether or not he/she is applying the correct amount of paint without waiting until parts come out of the oven. This method of measuring leaves marks in the paint, requiring the part to be reworked.
The dry film thickness of paint on iron or steel surfaces is easily determined with a pull-off gauge. Pull-off gauges reflect the force required to pull a magnet suspended on a spring inside a metal case away from the painted surface. The force will decrease as the paint thickness increases, because the paint keeps the magnet further away from the metal. Pull-off gauges may be graduated in microns, mil (thousandths of an inch), or arbitrary units. They are direct measurement devices.
Dry-film thickness can be measured much more accurately with an electronic instrument. If the base material is metallic, a device is used that measures the strength of eddy currents induced in the metal by a probe containing a conducting coil. The strength of the eddy currents decreases as the paint thickness increases. The read-out meter for eddy current strength can be graduated in any desired thickness units.
If the dry paint is on a nonmetallic surface, film thickness can be measured with a beta ray back scattering gauge. This instrument emits low-energy radiation in the form of beta rays (electrons) that pass through the paint and are reflected by the more dense material beneath it. The quantity of beta rays that are reflected back to the gauge decreases as the film thickness increases. These gauges also work when the painted surface is metallic.
Dry-film thickness on nonmetallic surfaces is sometimes determined by cutting a “V” shaped trough through the paint and measuring the width of the cut at the top of the trough. A thicker film will produce a wider “V” at the top. A low-power magnifying glass with internal calibrations is often used to make this measurement. It may be calibrated directly in thickness units.
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