Powder coating thickness is arguably the most important measurement during the application and inspection of protective coatings. Powder coatings are designed to achieve their intended function when applied within the thickness range specified by the manufacturer. Many physical and cosmetic properties of the finished coating are directly affected by dry film thickness (DFT). DFT can affect the coating's color, gloss, surface profile, adhesion, flexibility, impact resistance, and hardness. If the film thickness is not within tolerance, the installation of the coated assembly can also be affected. Let's learn more about this.
1. Complying with Standards
Measuring powder coating thickness requires different methods depending on whether the test is performed before or after the powder cures. The American Society for Testing and Materials (ASTM) maintains a series of standards describing these techniques.
Test Method D 4138 describes destructive thickness measurements on solid substrates using a sectioning instrument.
Practice D 7091 describes nondestructive thickness measurement methods for metallic substrates using magnetic thickness gages and
eddy current thickness gages. Test Method D 6132 describes nondestructive thickness measurement methods for nonmetallic substrates using ultrasonic thickness gages.
Standard D7378 describes three methods for measuring the thickness of pre-cured powder coatings to estimate the cured thickness.
2. Film Thickness Measurement
Film thickness measurements can be performed before or after curing and crosslinking. The type of substrate, the desired coating thickness range, the size and shape of the coating, and the economics of the job determine the measurement method used.
For uncured powder coatings, height measurements can be made with powder combs and electronic gauges using specialized powder probes. Since the thickness of the powder coating decreases during the curing process, the reduction factor must be determined to predict the cured DFT. Alternatively, ultrasonic instruments measure uncured powders without contacting the surface and can automatically predict the cured thickness.
After curing, various handheld devices can be used to perform DFT measurements directly on the coated section. These non-destructive thickness gauges use magnetic induction, eddy current, or ultrasonic principles, depending on the substrate type. Less common methods include micrometer measurement, destructive dry film methods such as cross-sectioning, and gravimetric (mass) measurement.
2.1 Standard Units of Measurement
The normal standard unit used in powder thickness measurement in the United States is the mil; 1.0 mil equals one thousandth of an inch (1/1000 inch). If the manufacturer specifies a thickness of 2.0 to 5.0 mils, the final cured thickness of the powder should be between 0.002 inches and 0.005 inches. The metric unit of measurement is called the micron (micrometer); 25.4 microns equals 1.0 mil.
Applicators must apply powder coatings uniformly and according to the product specification sheet. This provides the greatest benefit from specific powder specifications. Most thickness measurement specifications apply to the cured thickness of the powder, so we are seeing the emergence of different thickness measurement techniques.
2.2 Cured Film Thickness Measurement
The micrometer is one of the original instruments used to measure DFT and is still used today. It has the advantage of measuring any coating/substrate combination, but has the disadvantage of requiring the bare substrate thickness to be measured simultaneously. Two measurements must be taken: one with the coating and one without. The difference between the two readings, and the height change, is the coating thickness.
Two destructive techniques are also available. One involves microscopically observing the coated portion in a cut cross-section and measuring the film thickness. The other involves using a zoom microscope to view a geometric cut through the cured coating. This method is used when inexpensive, non-destructive methods are unavailable or when non-destructive results need to be confirmed.
The most common method for measuring cured powder thickness is with an electronic DFT gauge. They are handheld, easy to operate, and relatively low-cost. They utilize magnetic induction, eddy current, or ultrasonic principles, depending on the material type.
When the part is made of steel, a mechanical gauge can be used. It employs a permanent magnet and a calibrated spring. The device measures the force required to pull the magnet away from the coated steel surface. Magnetic pull-off gauges are rugged, simple to operate, inexpensive, portable, and generally require no calibration or adjustment. They are a suitable and cost-effective alternative in production applications where only a few readings are required.
Due to their simplicity, versatility, accuracy, and record-keeping capabilities, electronic DFT gauges are a popular choice for both large and small powder operations. They use magnetic induction for steel substrates and eddy current for other metals. Sometimes both principles are combined in a single instrument. Test results are displayed directly on an easy-to-read liquid crystal display (LCD). A wide selection of probes is available for measuring irregular shapes or accurately measuring very thin or very thick coating systems.
Measuring non-metallic substrates such as coated plastics or wood requires the use of ultrasonic pulse technology. This provides a cost-effective way to perform non-destructive quality control that was previously unavailable to the industry. One benefit of this measurement technique is the ability to measure individual layers in a multilayer coating system.
2.3 Pre-cured Film Thickness Measurement
The measurement methods discussed so far have been used to measure the thickness of partially cured powders. It is also possible, and in some cases even preferable, to measure the coating immediately after application to predict the thickness of the cured powder coating.
If a coating is improperly applied after it has dried or chemically cured, correcting it requires expensive additional labor time, can lead to film contamination, and can introduce problems with adhesion and coating system integrity. Measuring film thickness during application can determine whether the applicator requires immediate correction or adjustment.
2.4 Dry Powder Measurement
While most powder coating specifications specify a target cured thickness, this can be used to determine whether the applied powder meets the thickness specification before final curing and crosslinking.
There are good reasons for the need for an accurate cured DFT prediction, especially on moving lines. Depending on the length of the oven, the number of parts being cured, the time required for the curing process, and the time required to manually measure the DFT value after curing, there can be a considerable delay before the operator can intervene in the application process to make necessary corrections.
If coating defects are discovered, large coated parts may have to be reworked in a trimming loop or, if the cost of rework is too high, they may even have to be scrapped. For some operations, these drawbacks are unacceptable to meet the demands of modern processing procedures.
Measuring powder in the pre-cured, pre-gel state ensures the correct cured film thickness. This allows for setup and fine-tuning of the application system before curing. This, in turn, reduces scrap and overspray. Accurate predictions can avoid peeling and recoating, which can lead to adhesion and coating integrity issues.
ASTM D 7378 describes three procedures for measuring applied powder coatings:
A. Hard Metal Notch (Comb) Gage
B. Electronic Coating Measurement Instrument with a Dedicated Powder Probe
C. Non-Contact Ultrasonic Instrument
Metal Notch Gage. This instrument measures thickness manually by dragging the hand across the applied powder. Similar to a wet film thickness gage, the instrument determines the powder height between the highest-numbered tooth with a mark and powder adhered to it and the second-highest tooth without a mark and no powder adhered to it. These simple instruments are inexpensive but only accurate to within a few mils. Measurements can be made on a suitable rigid substrate, but the mark will be left in the powder that is not covered as the powder flows during the curing process.
Electronic Gage. This instrument uses a dedicated powder probe to measure the thickness of applied powder. A microneedle built into the probe penetrates the powder coating and the substrate. The probe is then manually pressed against the surface of the powder layer to measure the thickness. This method is only suitable for flat metal substrates and may leave marks on the finished product.
The above two methods are only for measuring the height of uncured powder coatings. However, as mentioned earlier, most thickness specifications often refer to the cured powder thickness. Since powder coatings often lose up to 50% of their thickness during the curing process, these two steps require establishing a reduction factor for each specific coating powder to predict the cured film thickness. The reduction factor is determined by measuring the cured powder coating thickness at the same location as the uncured powder height and then subtracting the two measurements.
Non-contact ultrasonic gauges. ASTM D 7378. Method C, describes a relatively new instrument that has quickly become a popular solution for dry powder thickness measurement. It is an ultrasonic instrument that can non-destructively measure the uncured powder to predict the final DFT value, leaving no traces that could affect the finished product.
These instruments are handheld and battery-powered, and are ready to use for most powders right out of the box. Their simple operation and electronic design make them quick and efficient for line operators.
The non-contact nature of coating thickness gauges is a crucial advantage. This means that after measurement, the component can be reintroduced into the ongoing process.
3. Accuracy of Film Thickness Measurements
These instruments are simple to operate, but a prudent user should regularly verify their operation, especially when following procedures specified in international ISO standards. These three steps ensure optimal accuracy.
3.1 Calibration
Calibration of coating thickness gauges is typically a documented process performed by the equipment manufacturer in a controlled environment. A calibration certificate showing traceability to a national metrology institute is issued. Recalibration does not have a standard interval, nor is it an absolute requirement, but a standard interval can be established based on experience and the operating environment. A one-year calibration interval is a typical frequency recommended by many instrument manufacturers.
3.2 Verification
This is an accuracy check performed by the user against a known reference standard. This quick check ensures that the instrument is measuring properly and that the user is operating it correctly. For many gauges, accuracy can be verified by measuring a plastic shim or epoxy coating standard with an assigned value traceable to a national metrology institute.
3.3 Adjustment
Adjustment, or calibration adjustment, is the act of calibrating the gauge's thickness reading to match a known reference sample in order to improve the gauge's accuracy for a specific coating within a specific portion of its measuring range. This operation is rarely necessary in the powder coating industry because the acoustic properties of the powder coating material do not vary much.
4. Summary
The above is information regarding powder coating thickness. If you have further questions, please feel free to contact us.
