Coating Thickness Gauge: How to measure the dry film thickness of non-metallic substrates?

Film thickness is an important type of measurement in many manufacturing and research facilities. Variations in paint or coating thickness can affect many properties that affect the final product, including color, gloss, hardness, adhesion, scratch resistance and many others. In order to obtain the desired coating properties, the correct film thickness needs to be achieved. There are several ways to measure film thickness, both during the wet and dry stages of application. Wet Film Thickness Gauges, such as interchemical and comb gauges, can be used to measure the thickness of coatings before they cure. More often, however, research and quality control departments want to know the thickness of the coating after it has cured.

Instruments for measuring dry coating thickness can be divided into two categories; destructive and non-destructive. Destructive film thickness testing involves cutting the coating into the substrate, usually with the help of a specialized blade, and viewing the layers under a microscope to determine thickness. The disadvantage of this method is obvious: the product needs to be destroyed in order to take measurements. Additionally, destructive film thickness measurements are typically more time-consuming than other types of thickness measurements. It is generally preferred to use a non-destructive method commonly referred to as a dry film thickness or DFT meter. Most DFT gauges operate using one of two measurement principles that measure the thickness of thin films applied to metal substrates. The measuring principle used depends on whether the substrate is "ferrous", i.e. it contains iron and is usually magnetic, such as steel, or "non-ferrous", i.e. the substrate does not contain iron and is not magnetic, such as aluminium. Dry Film Thickness Gauges are usually selected based on whether the substrate is ferrous or non-ferrous, and there are many Thickness Gauges available that incorporate both measurement principles to perform measurements on any type of metallic substrate.

Even trickier is measuring dry film thickness on non-metallic substrates such as plastics. For non-destructive film thickness testing on these types of substrates, different types of gauges are required. The PosiTector 200 Ultrasonic Coating Thickness Gauge uses the principle of sound waves to measure dry film thickness. This is similar to sonar; sound waves are sent through a material, and the reflected sound waves are measured. Whenever a material of different density is encountered, the reflection changes, telling the Thickness Gauge that it has reached the substrate or a different type of coating. By using this measuring principle, the PosiTector 200 can measure film thickness on various non-metallic substrates, and unlike typical DFT gauges, it can even distinguish between different coating layers, measuring the thickness of up to three layers at a time.

PosiTector 200 for dry film thickness measurement of clear coats on plastic headlamp covers.

One such application involves automotive headlights. The clear plastic of the headlamp is coated with a hard protective clear coat to protect it from weathering and abrasion. The importance cannot be overemphasized that the clear plastic remains transparent so as not to obstruct the light beam. To achieve this, the varnish needs to be applied at the specified thickness; thick enough to maintain the protective properties of the coating, but thin enough so that the coating remains smooth and clear. Since the coating is applied to clear plastic rather than metal, typical DFT gauges are not suitable for this application. However, testing has shown that the PosiTector 200 is very effective at measuring the thickness of clear coats, reducing the need to destroy the product in order to measure.