By Michael Beamish
When applied correctly, powder coating is a versatile and durable finish that offers many benefits such as its ability to withstand abrasion, scratches, and marring. This article will review common tools and test standards that can help ensure tight quality controls for consistent and efficient coating—saving time and money.
Dry Film Thickness Measurement—Post Cure
Test Standards: ASTM D7378/D7091/D4138
Film thickness is arguably the single most important measurement made during the application and inspection of protective coatings. Powder coatings are designed to perform their intended function when applied within a thickness range specified by the manufacturer. Physical and appearance properties such as color, gloss, surface profile, adhesion, flexibility, impact resistance, and hardness of the coating are directly affected by the dry film thickness (DFT).
Digital Coating Thickness Gauges: ASTM D0791
A variety of type 2 digital coating thickness gauges are available for measuring post-cure powder coating thickness on metal parts. They use a magnetic principle when measuring on magnetic steel and an eddy-current principle when measuring on non-magnetic metals. Measurement results are displayed on easy-to-read screens with typical tolerances between ±1% and ±3%.
Coating thickness probes are available in a wide variety of shapes and sizes. Microprobes are available for precision placement on unique geometries and in areas with low headroom. ‘Thick’ probes have the ability to measure up to 2.5 inches 63 mm) of coating.
Electronic coating thickness gauges often feature fast measurement speeds, internal memory, and onscreen statistics. More advanced features include powerful scan modes to quickly analyze large surfaces and USB, Bluetooth, and WiFi connectivity options.
Ultrasonic Digital Coating Thickness Gauges: ASTM D6132
For non-destructive measurements on non-metal substrates, ultrasonic coating thickness gauges using pulse-echo techniques are recommended. Ultrasonic testing works by sending an ultrasonic pulse into the coating using a probe (transducer) with the assistance of a gel (or drop of water) temporarily applied to the surface.
Advanced features include the ability to measure individual layer thicknesses in a multi-layer system. Other features include graphics modes, useful statistical and measuring modes, and software for analyzing measurements.
Destructive Paint Inspection Gauges: ASTM D4138
Destructive practices involve cutting, grinding, or drilling into the coating system to the substrate and measuring the coating thickness using an illuminated microscope with measuring reticle. These tools are ideal for measuring DFT on scrap parts or test samples.
Powder Thickness Measurement—Pre-cure
While most powder coating specifications give cured thickness targets, it is possible to determine if applied powder is within thickness specifications before the finality of curing and crosslinking. Measuring powder in the pre-cured, pre-gelled state helps ensure correct cured film thickness. It enables the application system to be set up and fine-tuned prior to curing. In turn, this will reduce the amount of scrap and overspray. Accurate predictions help avoid stripping and recoating which can cause problems with adhesion and coating integrity.
Non-contact Ultrasonic Powder Thickness Gauges: ASTM D7378 Method C
Non-contact ultrasonic powder thickness gauges measure uncured powder coatings and automatically display the predicted cured thickness without having to contact the dry powder. Some models feature advanced functions such as fast measurement speeds, internal memory, and on-screen averaging.
They also have the advantage of being non-destructive. This means that after measurement, the measured components can seamlessly be reintroduced into the process.
While uncured powder thickness gauges have been around for many years, recent improvements have made them more affordable and much easier to use. With their increased measurement speed and ability to measure on moving lines and swaying parts, these instruments can be introduced before parts pass through the oven without significantly changing the production process. Their enhanced technologies provide better measurement results on awkward geometries and diameters as small as 1 inch, all at a cost significantly less than years past.
Powder Combs: ASTM D7378 Method A
Powder combs are inexpensive notched gauges which work similarly to traditional wet film thickness gauges. The powder comb is held perpendicular to the coated surface and dragged through the uncured powder. Powder height is considered to be a ranged value between the highest numbered tooth that made a mark and has powder clinging to it, and the next highest tooth that left no mark and has no powder.
Oven Profile Logging
After powder coating has been applied to a part, it must be held at elevated temperatures for a predetermined length of time to reach full cure. Oven temperature logging is essential to ensure consistent quality and sufficient cure over the entire surface of the part.
Oven temperature loggers are designed to measure and record air and surface temperatures while being subjected to extreme oven environments. Most feature a data logger with multiple thermocouple channels (ports) that accept a variety of k-type thermocouples and a high temperature barrier box to withstand long run times at high temperatures.
Some temperature loggers feature the ability to create a cure index. Measuring only the elapsed time that the part is above a selected temperature can misrepresent curing. Using supplier curing specifications, the cure index accounts for the curing that happens when the part is above and below the target temperature but still above the minimum cure temperature. When the cure index is at or above 100%, the part or location is fully cured.
Gloss Measurement
Test Standards: ASTM C584/D523/D4039
Measuring gloss can improve the production process by identifying process issues, maximizing consistency, reducing waste, and improving overall quality.
Reflected light is responsible for how the object is perceived. On a smooth, polished surface like a mirror, light is reflected opposite to the angle at which it arrives; this is referred to as specular reflection. On a rough surface, light is reflected at all angles and a relatively small amount of light reflects at the specular angle. This is called diffuse reflection.
Digital gloss meters measure gloss using a calibrated light source and light sensor at opposite, specular angles to each other. Usually, a gloss meter will feature one or more geometries at which the light is sent and received including 20°, 60°, and 85°. Percent (%) reflectance, gloss units (GU), and haze are just some of the common parameters measured using gloss meters.
Advanced gloss meter features include scanning and statistics modes, live graphing, and software for analyzing measurement data.
Cross Hatch Adhesion Testing
Test Standards: ASTM D3359 Method B
Powder coatings adhere strongly to substrates with bond strengths beyond the limits of traditional pull-off adhesion testers. Instead, the adhesion strength of powder coatings is typically determined by cross hatch adhesion tests. These tests are fast, simple, and relatively inexpensive to perform but rely on subjective rating systems to describe a coating’s adhesion strength.
Cross hatch adhesion tests, as defined by ASTM D3359 and ISO 2409, determine a coating’s ability to resist separation from a substrate when a lattice pattern is cut into the coating. A special tape is applied and swiftly removed to reveal the amount of coating removed by the tape. The resultant lattice pattern is then evaluated and rated in accordance with the descriptions and illustrations found in the classification chart shown in the appropriate standard.
Poor adhesion may indicate improper pretreatment or an under-cured film.
Pencil Hardness Test
Test Standards: ASTM D3363
A pencil hardness test (also known as the Wolff-Wilborn test) is useful for developing quality assurance programs and ascertaining coating attributes in relation to degree of cure and proper application conditions. Pencil (coating) hardness is a film’s ability to resist deformation such as marking, gouging, or scratching by pushing a pencil’s graphite tip across the coated surface.
Pencil hardness tests determine the relative hardness of paint and coatings by use of pencils containing various hardnesses of graphite. Pencils are held at a 45° angle, with or without the use of an optional pencil cart, and pushed across the surface with enough force to either mar the surface or crumble the lead. ASTM D3363 specifies to begin testing with the hardest pencil and work down the pencil hardness scale (6H — 6B) until a pencil will not cut through the coating to the substrate (gouge hardness) or will not scratch the film (scratch hardness).
As the use of powder coatings continues to grow and diversify, so too does the need to implement strict quality control practices in your job shop. As outlined, these measures ensure longevity of the coating and are a primary contributor to meeting cost and performance expectations.
Michael Beamish is vice president and general manager of DeFelsko Corporation.