DeFelsko manufactures hand-held, non-destructive coating thickness gauges that are ideal for measuring the dry film thickness of powder coating on a variety of substrates. 

Measurement Challenges

Non-destructive test methods are required to determine powder coating thickness on  both metallic and non-metallic substrates such as medium density fiberboard (MDF).  

Powder Coating inspectors often need a means to verify the thickness of parts as early as possible into the coating process.   This may require the immediate measurement of hot parts as they come out of the oven, or even the measurement of applied powder thickness prior to curing. 

Metallic Substrate Measurement Solutions

The PosiTector 6000 gages are ideal for non-destructive measurement of powder coating thickness on metallic substrates.  Various models of the PosiTector 6000 are available including an N series for non-magnetic substrates, an F series for magnetic substrates, and an FN series for measuring either application.  Higher accuracy F or N Microprobes, with a range up to 625 microns (25 mils), are available for measuring in smaller, hard to reach areas.  

PosiTector 6000

The PosiTest DFT gages are cost effective alternatives for non-destructive measurement of powder coating thickness on metallic substrates. Two models of the PosiTest DFT are available, each capable of measuring up to 1000 microns (40 mils). The DFT Ferrous model is recommended for magnetic substrates, while the DFT Combo model is ideal for measuring on both magnetic and non-magnetic substrates.

PosiTest mechanical gages are another cost effective solution for non-destructive measurement of powder coating thickness on magnetic substrates.  PosiTest models are available for thickness measurements of up to 2000 microns (80 mils). The rugged design of the PosiTest includes a wear resistant carbide probe making it ideal for use in production environments.

PosiTest

The non-destructive PosiPen mechanical gage has a smaller probe tip allowing it to be placed with pin-point accuracy on small parts, hard to reach areas and curved surfaces.  The PosiPen has a measurement range of 5 to 500 microns (.25 to 20 mils) and is ideal for quick quality checks.  The PosiPen Model B has a range of -100 to 230 ºC (-150 to 450 ºF) making it ideal for taking measurement on hot parts fresh out of the oven. 

PosiPen

An advantage of the PosiTector 6000 gage is the ability of the memory model versions to store and print readings. Since more and more customers of coated products are purchasing coating thickness gages to verify the quality of incoming products, it is becoming increasingly important that powder coaters have the ability to permanently record quality control data. Some powder coaters have even taken the next step and provide the customer with unsolicited reports showing the coating thickness readings as evidence of
their process quality.

Non-Metallic (MDF) Substrate Measurement Solutions 

The ultrasonic PosiTector 200 B Std gage is ideal for measuring the thickness of powder coating applied to non metallic substrates such as MDF.   The PosiTector 200 B has a range of 13 to 1000 μm (0.5 to 40 mils).  

PosiTector 200 B Std

Measurement Solution for Uncured Powder

The PosiTector PC Powder Checker, affordably measures uncured powder coatings using non-contact ultrasonic technology to automatically calculate and display a predicted cured thickness.

PosiTector PC Powder Checker

The Powder Comb may be used to physically verify the thickness of applied powder prior to cure.  Ideal for use in process setup, the convenient Powder Comb may be used on any substrate.  Various Powder Comb models are available allowing measurements from 75 to 1250 microns (3 to 50 mils).

Powder Comb

The PosiTector PC and Powder Comb conform to national and international standards including ASTM D7378-07.

Background on Powder Coating

What is the Application?

Powder Coating is an attractive paint like protective finish that is long lasting, and highly resistant to chips, scratches, and fading.  It provides a seamless coating from a virtually unlimited selection of colors, textures and finishes.

Powder coating is a cost effective one step process that doesn’t require successive coats and long curing times.  Powder particles used are a mixture of finely ground pigment and resin particles.  Charged powder particles are transferred onto an electrically grounded surface.  A variety of processes exist for powder application.  These applications range from electrostatic spraying for thinner coatings (0.001” – 0.010”) to dipping in a fluidized bed for thicker coatings (0.007” - 0.040”).

During the curing process the powder is fused into a smooth coating.  Dependent on the process, powders are either thermal cured (convection or infrared) or UV-cured.  The powders used may be either thermoplastic (same chemical composition after reflowing) or thermosetting (chemically cross-linked with themselves or other reactive components). 

Powder coating has been used on metal surfaces for over 40 years.  Recent advancements have expanded its use into ceramic, plastic (nylon and polycarbonate) and MDF (medium density fiberboard) applications.  Studies are ongoing to expand the use of powder coating to include substrates such as hardwoods, laminate floorings and particleboards.

UV-Curing

Unlike Thermal Cured powders, UV Curable powders separate the melting and film formation stages from the final curing stage. Short wave infrared and convection heat allows relative low temperatures to be used for the melting process. The result is lower temperature curing with improved flow. The UV wavelength and time required for curing is based on the color and expected film thickness. Coating thickness can range from 20 to 100 microns (1 to 4 mils). The most significant challenges of UV-curing are the current cost of the powders required, as wells as a need to position the UV light to ensure 100% coverage.

Since UV-curing does not require high temperatures to cure, a significant reduction can be attained in required space, conveyors and racking for the cooling process. Unlike thermal, UV provides instant curing which significantly increases throughput. Process savings are significant due to the decrease in energy and equipment required for heating. UV powders are also easy to clean and recycle, while providing a VOC-free process.

Available markets are expanded as lower process temperatures allow heat sensitive (e.g. medium density fiberboard and plastics) to be powder coated. The cost of powder coating large mass parts (e.g. engine blocks) that act as heat sinks for thermal curing is also greatly reduced using UV curing.

Why measure?

In addition to costs associated with applying excess powder, when too much powder coating is applied the cured coating may result in poor adhesion and tend to peel or chip from the substrate. 

When insufficient powder coating is applied it does not provide adequate coverage and protection.  For example, metals require a sufficient coating thickness for adequate protection from environmental effects such as corrosion (steel) or oxidation (aluminum).  In addition, inadequate powder coating thickness may result in poor surface finish and undesirable appearance or color.

Powder Coating thickness may also affect the application’s impact resistance, flexibility, hardness, edge coverage, chip resistance, weathering, resistance to salt spray, and ability to retain gloss.  Manufacturers provide a production specification sheet for powder coat materials.  Powder coaters attempt to apply an even coat meeting that specification.  Results of powder coating measurements enable the coater to adjust their coating process in accordance to the specifications.

DAVID BEAMISH is General Manager of DeFelsko Corporation, a New York-based manufacturer of hand-held coating test instruments sold worldwide. He has a degree in Civil Engineering and has more than 17 years experience in the design, manufacture, and marketing of these testing instruments in a variety of international industries including industrial painting, quality inspection, and manufacturing. He conducts training seminars and is an active member of various organizations including NACE, SSPC, ASTM and ISO.