The adhesion performance and crack resistance of coatings on substrates are key indicators for evaluating their reliability in practical applications. Especially when the substrate is deformed, whether the coating can be deformed without cracking or peeling is directly related to the durability and appearance preservation of the product. As a classic method to simulate the forming process of materials, cupping test is widely used to evaluate the cracking resistance of coatings under progressive biaxial tensile deformation. This method is implemented through a cupping tester, which can provide quantitative and reproducible detection data, and provide an important basis for the research and development, quality control and performance comparison of coating systems.
Principle of cupping protrusion test
The core principle of the cupping test is to use a spherical punch of a specified diameter to push the back of the coated specimen at a constant speed and pressure, so that the front of the specimen (coated surface) gradually rises to form a "cup". In this process, the coating undergoes biaxial tensile deformation with the metal substrate. The test continues until the first observable cracking of the coating occurs, usually with a magnifying glass or microscope. The pressing depth of the punch at this time, i.e., cupping depth, is recorded as the main parameter to measure the cracking resistance of the coating. The larger the cupping depth value, the stronger the coating's ability to resist cracking under deformation.
The process can be described by the relationship between the cracking failure of the coating and the deformation of the substrate (ε) and the ductility of the coating itself (δc) and adhesion (Fa) is the result of a joint action. Cracking occurs when the local stress exceeds the cohesive strength or interfacial bonding strength of the coating.
Instrument composition
A standard cupping tester consists of the following core components: rigid frame, drive and load system, punch and die ring assembly, measuring system and specimen holding device. To ensure the accuracy and comparability of test results, each component must meet the requirements of relevant technical standards.
| Part name | Main technical requirements: |
| Punch | Spherical shape, high hardness, typical diameter 20mm |
| Mold ring | The inner hole diameter is usually 27mm or 33mm, and the edges need to be rounded |
| Load system | It can provide a stable and adjustable test force, with a common range of 1-10kN |
| Depth measurement | The resolution is at least 0.01mm, and the accuracy meets the standard regulations |
| Clamping force | It needs to be large enough to prevent specimen slippage, usually greater than 10kN |
The test should be conducted in a standard temperature and humidity environment, and the sample preparation (including substrate treatment, coating and curing) must strictly follow the corresponding product standards or agreed methods to ensure the consistency of the test basis.
Evaluation indicators
The direct output result of the cupping test is the cupping depth (mm) at the time of cracking. This value is a comprehensive reflection of the flexibility, adhesion and cohesive strength of the coating system. In R&D, coating systems are often optimized by comparing the cupping depths of coatings with different formulations, different thicknesses, or different curing processes. In quality control, cupping depth can be used as an acceptance indicator.
In addition to the final cracking depth, observing the cracking morphology is also analytical value. For example, whether cracking appears as a fine mesh crack (often related to coating cohesion) or as a large area of coating peeling (often associated with insufficient adhesion) can help pinpoint the main cause of coating failure.
| Observe the phenomenon | The main factors that may be pointed to |
| Fine mesh cracks | The ductility of the coating is insufficient, and the cohesive strength is limited |
| Local large areas peel off | The adhesion of the coating to the substrate interface is insufficient |
| Cracks in a specific direction | Substrate anisotropy or uneven coating application |
Applications:
Cupping tests are widely used in areas where coatings need to be formed or may be subjected to deformation. For example, in the coil coatings industry, it is used to evaluate the performance of pre-coated steel or aluminum sheet coatings and predict their performance during subsequent shearing, stamping, and bending processes. In the field of automotive coatings, it is used to evaluate the cracking resistance of body electrophoretic paints, middle coats, and topcoats when forming plates. In addition, this method is also commonly used in coating evaluation in industries such as home appliance shells and metal packaging containers.
A number of domestic and foreign standards have made detailed provisions on cupping test methods, providing a unified operating framework and judgment basis for testing. Operators should clarify the standard version on which they are based before conducting the test.
Conclusion
The cupping tester provides an efficient and intuitive means for evaluating the crack resistance of coatings under simulated biaxial tensile deformation. By measuring the critical cupping depth when the coating cracks, combined with the analysis of the crack topography, comprehensive information about coating flexibility, adhesion, and cohesion strength can be obtained. This method has significant practical value for industries that rely on coatings for metal forming processing, and is an important tool for coating material development, process optimization, and product quality control. To ensure the validity and comparability of the data, it is necessary to strictly follow relevant standards for instrument operation, sample preparation, and environmental control.
References
ISO 1520: Colours and varnishes – cupping test.
GB/T 9753: Colored paints and varnishes - cupping tests.
ASTM D2794: Standard Test Method for Rapid Deformation (Impact) Resistance of Organic Coatings (see Related Methods).
"Coil Coating and Coating Technology". Chemical Industry Press.