Man measure powder coating thickness with ultrasonic testing

Measure powder coating thickness to obtain optimal results

Powder coatings are often considered one of the most efficient and durable industrial coatings. However to guarantee durability and optimal functioning it is crucial to measure powder coating thickness before and after cure. Coatings are namely designed for a purpose; the best product performance is obtained by applying the coating in the thickness specified by the powder manufacturer.

The powder coating film thickness affects appearance and physical properties. It can namely have an influence on the colour, gloss, surface profile (texture), adhesion, flexibility, hardness and impact resistance. Assembly may be influenced, parts might not fit. Therefore coatings are given a minimum and maximum dry film thickness (DFT).

Estimate DFT by measuring powder coating thickness before cure

Powder coating dry film thickness is the one specified by powder manufacturers and coating specifiers. However, measuring the film thickness already before cure helps estimate the eventual DFT. The advantage of measuring powder coating thickness before cure is that, if it seems like the DFT will not meet the specification, the thickness can be modified easily without making additional costs. Once the coating is cured, film thickness corrections may require stripping and recoating.

There are three tools that can be used for measuring powder coating thickness before cure, or for estimating dry film thickness.

Powder thickness measuring toolMeasurementSubstrtaesNote
Powder CombHeight of powder layer (to be combined with reduction factor* to estimate DFT)Any substrateMay affect appearance of cured coating
Magnetic GageHeight of powder layer (to be combined with reduction factor* to estimate DFT)MetalsMay affect appearance of cured coating
Ultrasonic Thickness CheckerAutomatically estimates DFTNon-metalsDoes not touch the powder, will not affect the cured coating

*as powder coating shrinks when it cures, the reduction factor is used to describe the percentage of shrinkage.

Our coating experts are happy to provide you with product suggestions and additional information on measuring powder coating thickness before cure.

Monitoring film thickness during the curing process

The film thickness and formation can also be monitored while the powder coated piece is in the curing oven. This is usually done with the help of oven recorders which allow for maintaining ideal curing environment. The ideal circumstances also allow for accuracy in estimating dry film thickness.

Powder coating Dry Film Thickness measurement tools

Once the powder coating has been cured, the actual dry film thickness can be measured. The suitable tools are defined by the substrate type and the conductivity of the coating. The following dry film thickness measurement instruments are suitable for most coatings.

Steel: Magnetic DFT measuring

Magnetic gages exist as mechanical and electronic hand held devices which determine the thickness of cured powder coating film. The most common type of magnetic gages is a pull-off gage which defines the coating thickness by measuring the force needed to pull of the permanent magnet off the coated steel surface. The portable pull-off magnetic gages exist as roll-back and pencil models.

The electronic devices that measure powder coating thickness on steel also rely on magnetic technology.

Metals other than steel: Eddy Current

To measure powder coating film thickness on other metals than steel, electrical eddy current instruments are used. These are suitable for example on measuring DFT on aluminium. The main advantage of electrical DFT instruments is that they do not damage the cured coating layer.

Non metals: Ultrasonic dry film thickness measurement

Powder coated non-metal substrates such as wood and plastic require an ultrasonic pulse-echo technique to define the cured film thickness. The ultrasonic testing relies on ultrasonic vibration and a gel temporarily applied on the surface. This method is rather new on the coating inspection sector, and it allows non-destructive DFT measurement. It makes it also possible to determine the thickness of a single coating layer in a multi layer system.

Other methods: Micrometre, cross-sectioning and gravimetric measurement

In addition to the substrate-specific film thickness measurement tools, there is also equipment that is suitable regardless the substrate material. These include destructive and non-destructive methods.

  • Micrometres
    Micrometres can be used on any substrate, but they require access to bare substrate before coating is applied. Measurements need to be taken before and after coating to define the DFT.
  • Cross-sectioning
    Cross-sectioning is a destructive method where the cure coating layer is cut into sections and inspected microscopically.
  • Gravimetric measurement
    In gravimetric mass measurement a V-groove is cut into the coating layer and a scale microscope used to view geometric incision through the cured coating.

The destructive methods to measure powder coating thickness are used when non-destructive methods are not an option or to confirm the results of a non-destructive measurement.