Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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method for this mea- surement system. GRAVIMETRIC Fig. 5. Typical coating thickness calibration standards. ing equation: T = (m x 10)/(A x d) where T is the thickness in micrometers, m is the mass of the coating in milligrams, A is the area tested in square centimeters, and d is the density in grams per cu- bic centimeter. It is difficult to relate the mass of the coating to thickness when the substrate is rough or the coating uneven. Laboratories are best equipped to handle this time- consuming and often destructive method. WET-FILM MEASUREMENT Wet-film thickness gauges help determine how much material to apply wet to achieve a specified dry-film thickness provided that the percent of solids by vol- ume is known. They measure all types of wet organic coatings, such as paint, varnish, and lacquer, on flat or curved smooth surfaces. Measuring wet-film thickness during application identifies the need for im- mediate correction and adjustment by the applicator. Correction of the film af- ter it has dried or chemically cured requires costly extra labor time, may lead to contamination of the film, and may introduce problems of adhesion and in- tegrity of the coating system. The equations for determining the correct wet-film thickness (WFT), both with and without thinner, are as follows. Without thinner: WFT = desired dry film thickness % solids volume With thinner: WFT = desired dry-film thickness of solids by volume/[100%] + % of thinner added] There are four types of gauges: notch, lens, eccentric roller, and needle mi- crometer. Each has its own unique operating procedure. Notch gauges, also called step or comb gauges, are most common. They are inexpensive field gauges, 309 By measuring the mass and area of the coating, thickness can be determined. The simplest method is to weigh the part before and after coat- ing. Once the mass and area have been determined, the thickness is calculat- ed using the follow-

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