In the coatings, inks and coatings industry, dry film thickness refers to the physical thickness of the solid film formed by the coating material after it has been completely cured on the surface of the substrate. It is a physical parameter with a decisive influence in the coating system, which directly correlates and quantifies the final physical barrier strength, optical properties, functional performance and durability life of the coating. Unlike wet film thickness, which is a process control parameter, dry film thickness is the final state indicator on which product specifications, technical protocols, and performance tests are based, and its precise measurement and tight control are important to ensure that the coating achieves its design function.
This paper will systematically elaborate on dry film thickness from four aspects: definition and core value, measurement method standards, influencing factors and industry applications.
1. Definition and core values
Dry film thickness is the actual thickness of the coating that is stable on the substrate after all drying or curing processes such as solvent volatilization and chemical reactions. The value of this parameter runs through multiple aspects of product development, quality control, and engineering acceptance.
1. Core determinants of performance
Most of the properties of coatings are directly or indirectly related to the thickness of the dry film. For example, in the field of protective coatings, the anti-corrosion life of the coating is usually positively correlated with its dry film thickness, and reaching a specific thickness is a technical prerequisite to ensure the protection life. For decorative coatings, the thickness of the dry film affects its covering power, color saturation and uniformity of surface gloss. In the printing industry, the dry film thickness of ink directly affects the color density, wear resistance, and suitability of subsequent processing.
2. Cost and process calibration basis
Dry film thickness is the core basis for calculating coating material consumption and evaluating construction efficiency. Optimizing and accurately controlling the thickness of the dry film while meeting the performance requirements is an effective means to control material costs and avoid waste. At the same time, it is also an objective yardstick to verify whether the construction process parameters (such as spraying pressure, scraper clearance, roll speed, etc.) are reasonable and stable.
There is a definite conversion relationship between the dry film thickness and the wet film thickness controlled during construction, and the formula is -
Dry film thickness = wet film thickness × the percentage of solids by volume of the coating
This relationship is the bridge between process control and the final result.
2. Measurement method of dry film thickness
In order to obtain accurate, comparable and reliable dry film thickness data, systematic measurement standards have been established at home and abroad. Mainstream standards include the international standard ISO 2808, ASTM D7091, ASTM D6132, and the national standard GB/T 13452.2 harmonized with it. According to the measurement principle and applicable scenarios, dry film thickness measurement methods are mainly divided into two categories - non-destructive thickness measurement method and destructive thickness measurement method.
1. Non-destructive thickness measurement
This method does not damage the coating, is suitable for on-site quality control and finished product inspection, and is a widely used method.
Magnetic thickness measurement- Suitable for the measurement of non-magnetic coatings (such as paints, plastics, enamels) on magnetic substrates such as steel. The principle is to convert the coating thickness using the change in magnetic flux or magnetic attraction between the probe and the magnetic substrate.
Eddy current thickness measurement method- Suitable for the measurement of insulating coatings (e.g. anodized layers, paints, ceramics) on non-ferromagnetic metal substrates (e.g. aluminum, copper, stainless steel). The principle is to use a probe to generate eddy currents in a conductive substrate, and the coating thickness is determined by inducing changes in impedance.
Ultrasonic thickness measurement- Suitable for coating measurement on non-metallic substrates (e.g. plastic, wood, concrete). The principle is to calculate the thickness by the reflection time of ultrasonic waves at the interface between the coating and the substrate.
2. Destructive thickness measurement
These methods locally damage the coating and are often used in the laboratory or when arbitration is required, and the results are considered benchmarks.
Microscopy- According to standards such as ISO 1463 or ASTM B487, an observation bevel is made on the coating specimen to directly observe and measure the coating thickness on the cross-section using a metallographic microscope or electron microscope. The method is intuitive and accurate.
Indicative scale method- The gauge with a precision probe is fixed on the surface of the coating and the thickness value is read by mechanically contacting the substrate.
| Measurement method type | Main features and applicable scenarios |
| Magnetic/eddy current method (non-destructive) | The operation is fast and convenient for large-scale census on site. It should be selected according to the properties of the substrate and calibrated with a standard sheet. |
| Microscopy (destructive) | The results are accurate and reliable, and are considered arbitration methods. Sample preparation is required, and the equipment and operation requirements are high. |
3. Key factors affecting the accuracy and consistency of dry film thickness
The formation of dry film thickness is not simply determined by the instantaneous construction time, but is influenced by the combination of materials, processes, environment and measurement.
1. Materials and construction factors
The inherent properties of the coating, such as volumetric solids, construction viscosity, leveling and thixotropy, determine the shrinkage rate and uniformity from wet film to dry film. Construction process parameters, such as the speed and angle of the spraying gun, the knife pressure and clearance of scraping, and the roll speed and pressure of the roller coating, are the direct external variables that control the thickness distribution.
2. Substrate and geometry factors
The surface roughness of the substrate can significantly affect the measured readings of dry film thickness and the actual protection effect. On rough surfaces, the coating material fills the troughs, making the effective shielding thickness at the crest possible to be lower than the measured average. In addition, complex geometric parts such as workpiece edges, edges, and welds are prone to insufficient film thickness due to paint sagging or shrinkage, requiring special construction skills and measurement attention.
3. Measure operating factors
The calibration status of the measuring instrument, the choice of probe, the electromagnetic properties of the substrate (for magnetic/eddy current methods), the pressure and angle at the time of measurement, and the selection and number of measurement points all directly affect the accuracy of the reading. Usually the standard specifies statistical sampling rules such as the "ten-point measurement method" to obtain a representative average thickness.
4. Application of dry film thickness in different industrial fields
There are differences in the focus and control accuracy of dry film thickness in different industries.
In the field of heavy anti-corrosion coatings (e.g. bridges, ships, storage tanks) - dry film thickness is the core guarantee of the life of the anti-corrosion design. Relevant standards such as ISO 12944 have clear minimum requirements for the thickness of the total dry film of coating systems in different corrosive environments. During construction, strict "process-result" dual control is required, that is, the wet film thickness and the final dry film thickness are monitored at the same time to ensure that each coating reaches the specified range.
Automotive and industrial coatings – high requirements for uniformity and appearance of dry film thickness. The thickness of each layer of electrophoretic primer, middle coat, color paint and varnish is finely controlled to ensure overall appearance, stone impact resistance and durability. Automated non-destructive thickness measurement systems are often integrated into the production line.
Printing and functional coatings – The dry film thickness of inks or functional coatings (such as conductive inks, optical coatings) directly determines the key properties of the product, such as resistance value, light transmittance, barrier properties, etc. In this field, the control accuracy of film thickness is often required to reach the micron or sub-micron level, and the microscopy method is a commonly used precision measurement method.
Summary
All in all, dry film thickness is a core quality parameter that closely integrates material science, process engineering, and performance requirements. From macroscopic anti-corrosion engineering to microscopic functional coatings, their precise design, process control, and final verification form the cornerstone of modern coating technology quality assurance systems. A deep understanding of their measurement principles, influencing factors, and industry application specifications is essential for predictable, reproducible, and reliable coating performance.