Proper coating of metal parts is an essential manufacturing step in the the automotive, aerospace, medical device industries. Paint, primer and organic coatings are applied to steel, galvanized steel, or aluminum strip to not only improve the appearance, strength, and durability of the metals but to help ensure that appropriate film thickness specifications are met so that the finished pieces function as intended and fit properly with other parts. Coil coating lines can benefit from online thickness coating gauges for applications including:
- Weldable primer coatings with zinc particles applied to chromated or galvanized steel
- Ultra-thin coatings
- Wet measurement directly after the coaters
- Dry measurement made after drying ovens
- Chromate-free coatings for corrosion protection
- Oil and anti-fingerprint coatings
There are two primary types of coating weight gauges; each measurement technique has special benefits and advantages but both improve quality assurance, material savings, coating uniformity and overall mill yield while reducing scrap.
Beta-backscattering is a well proven and precise method for the coating weight measurement of primer and/or finish paint. Usually one measuring unit will be installed for the base material (reference) measurement of the top and/or bottom side of the strip. This reference measurement compensates the influence caused by variations of a pre-coating and/or by alloy changes of the base material. Wet gauges measure the wet paint or wet primer for the top side of the strip behind the corresponding coaters. This allows a uniform coating quality to be achieved. Using wet measurement, the coating weight/thickness measuring value is available at once.
Dry gauges for primer and finishing paint are installed after the corresponding ovens. Sensors can be positioned to measure the top or bottom side of the strip or two sensors can be used to measure both sides. The influence of dynamic air temperatures at the sensor measuring point is compensated by real-time temperature measurement and compensation. During standardization and for scheduled maintenance, the gauge heads are driven to a stand-by position located just off the sheet.
The influence of dynamic air temperatures at the sensor measuring point is compensated by real-time temperature measurement and compensation. During standardization and for scheduled maintenance, the gauge heads are driven to a stand-by position located just off the sheet.
Coating thickness sensors for thin and ultrathin coatings on metal strip are available that use NIR full spectrum analysis. The sensor in these gauges is a true online scanning spectrometer that simultaneously captures the entire NIR spectral signature. With this technology, the system provides a long-term stable measurement and a high signal-to-noise ratio.
The sensor analyzes infrared absorption exhibited by hydrocarbon compounds in the material to be measured. The unique detector technology analyzes the full spectrum, instead of a few pre-selected wavelengths as in conventional IR sensor designs. Full spectral analysis, coupled with an extended infrared range, allows accurate measurements of coatings that were not previously available online. By simultaneously analyzing multiple wavelengths, the sensor can discriminate between different product components, as well as distinguish between components which exhibit similar but not identical IR absorption.
The measurement is designed to be insensitive to environmental effects. Proprietary techniques modulate the detector to the IR source, eliminating any interference from ambient light. Detectors are thermo-electrically cooled in order to maintain repeatable measurements even as ambient temperatures fluctuate. The sensor assembly’s distributed processing enables the full spectrum to be measured rapidly, providing a measurement which is insensitive to point-to-point formation variations in the coating. Conventional IR sensors, in contrast, exhibit up to a one second time delay between analysis points making them susceptible to errors due to coating and substrate variations.
Further reading: The Benefits of Hot Coating Weight Measurements to Coating Control