Dry Film Thickness Measurement On Non-Metal Substrates Reply

Film thickness is an important type of measurement for many manufacturing and research facilities. Variations in the thickness of a paint or coating can influence a multitude of properties affecting the final product including color, gloss, hardness, adhesion, scratch resistance, and a host of others. In order to attain the desired properties of a coating, the correct film thickness must be achieved. There are several ways to measure film thickness, both in the wet and dry phase of application. Wet film thickness gages such as interchemical and comb type gages can be used to measure the thickness of a coating before it has been cured. More often however, research and quality control departments want to know the thickness of a coating after it has cured.

Instruments for measuring the thickness of a dry coating can be split into two categories; destructive and non-destructive. Destructive film thickness tests involve cutting through the coating down to the substrate, often with the help of a specialized blade, and then looking at the layers under a microscope to determine the thickness. The drawback of this method is obvious: the product must be destroyed in order to take the measurement. In addition, destructive film thickness measurements are usually more time consuming than other types of thickness measurements. Generally preferred is a non-destructive method using what is typically called a dry film thickness or DFT gage. Most DFT gages operate using one of two measurement principles that can measure the thickness of a film applied to a metal substrate. The measurement principle used depends on whether the substrate is “ferrous”, meaning it contains iron and is typically magnetic, like steel, or “non-ferrous” meaning the substrate does not contain iron and is not magnetic, like aluminum. A dry film thickness gage is generally selected based on whether the substrate is ferrous or non-ferrous, and there are many gages available that contain both measurement principles for measuring on any type of metal substrate.

Much trickier is measuring dry film thickness on a non-metal substrate such as plastic. For non-destructive film thickness tests on these types of substrates, a different type of gage is needed. The PosiTector 200 uses a sonic principle to measure dry film thickness. This operates similar to sonar; sound waves are sent through the material, and the reflected sound waves are measured. Whenever a material of a different density is encountered the reflection will change, telling the gage it has reached the substrate or a different type of coating. By using this measurement principle the PosiTector 200 can measure film thickness on a wide range of non-metal substrates, and unlike typical DFT gages it can even differentiate between different layers of coatings, measuring the thickness of up to three layers at once.

Dry film thickness measurement of a clear coat on
plastic headlamp covers using the PosiTector 200.

One such application involves automotive headlamps. A hard protective clear coat is applied to the clear plastic of the headlamp in order to protect it from weathering and abrasion. It is crucial that the clear plastic remains clear so as to not obstruct the light beams. In order to achieve this, the clear coat must be applied at a specified thickness; thick enough that it retains the protective qualities of the coating, but thin enough that the coating remains smooth and clear. Since this coating is applied on clear plastic rather than metal, a typical DFT gage will not work for this application. However, tests have shown that the PosiTector 200 is very effective at measuring the thickness of the clear coat, alleviating the necessity of destroying the product in order to measure it. This instrument can save not only time by taking quicker measurements, but money as well by not wasting product. If your company has a need to measure dry film thickness on a non-metal substrate, be sure to talk with your BYK-Gardner representative about free sample testing today.

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