Overview
The flexibility of colored paint is a key performance indicator to evaluate the ability of its coating to resist cracking or peeling when the substrate is deformed. In industrial applications, coatings are often subjected to various stresses, such as bending or shaping metal components. The cylindrical shaft bend tester provides a reliable method for quantitatively evaluating flexibility by simulating the bending deformation of coatings under standard conditions. This testing method widely adheres to domestic and international standards, such as GB/T 1731, ISO 1519, and ASTM D522, ensuring the comparability and accuracy of test results.
Detection principle
The core principle of the cylindrical shaft bending tester is to bend a test plate coated with paint film on a cylindrical shaft of a specified diameter, and determine its flexibility by observing whether cracks or peeling appear on the surface of the paint film. The instrument is mainly composed of a set of cylindrical shafts of different diameters, bending devices and fixed fixtures. During the test, the painted side of the test plate is bent evenly 180 degrees in a short period of time around the cylindrical axis with the painted side facing outward, and then the paint film is immediately checked with a visual or magnifying glass. The smaller the bending diameter, the higher the requirements for the flexibility of the paint film. The process can be roughly described by the coating strain formula:
ε = t / (2R + t) × 100%
where ε is the maximum strain of the coating, t is the thickness of the coating, and R is the radius of the cylindrical shaft. This formula reveals the influence of coating thickness and bending radius on strain, which provides a theoretical basis for understanding the test mechanism.
Test process and key parameters
The standard test process includes test plate preparation, condition adjustment, bending operation, and result evaluation. The test plate is usually made of tinplate or thin steel plate, and the size is specified according to the standard. The coating should be applied evenly and thoroughly dried. Before testing, the test plate should be adjusted in a standard temperature and humidity environment. The bending operation should be smooth and fast to avoid impact. Key parameters include cylindrical shaft diameter series, bending speed, ambient temperature and humidity, and result criteria. These parameters must be strictly followed to ensure test repeatability.
Influencing factors
The results are determined based on the degree of cracking or peeling of the paint film. Typically, flexibility is characterized by a minimal cylindrical shaft diameter that does not cause film failure, with smaller diameters indicating better flexibility. The main factors affecting the test results include: paint film composition and thickness, substrate properties, drying conditions, and test environment. For example, a paint film that is too thick or not dried sufficiently can lead to a decrease in flexibility. In addition, consistency in operating techniques is also crucial for the reliability of results.
Standard control
Cylindrical shaft bending test is widely used in coating, automotive, marine, construction and furniture industries for quality control and product development. Different standards have slight differences in test details, and the following table provides a brief comparison of the core requirements of common standards:
| Standard name | Commonly used cylindrical shaft diameter range |
| GB/T 1731 | 1 mm, 2 mm, 3 mm, 4 mm, 5 mm, 10 mm |
| ISO 1519 | 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 8 mm, 10 mm |
| ASTM D522 | 3.2 mm, 6.4 mm, 9.5 mm, 12.7 mm, 19.1 mm |
In practical applications, it is necessary to select appropriate standards and parameters according to product specifications.
Technical highlights
In order to ensure the effectiveness of the test, the following points should be noted: the cylindrical shaft of the instrument should be kept smooth and undamaged; The edge of the test plate should be smooth and burr-free; Bending operation should be carried out under standard temperature and humidity; For multi-coating systems, individual layers or overall performance can be tested separately. In case of disputes in testing, it is recommended to add parallel tests or refer to a broader standard method for validation. Regular calibration of instruments and training of operators are also important parts of ensuring data accuracy.
Epilogue
As a classic detection method, the cylindrical shaft bend tester provides an intuitive and standardized method for evaluating the flexibility of color paints. By strictly following standard processes and understanding the influencing factors, this test can effectively guide the optimization and quality control of coating formulations in production and R&D. As material technology advances, so do testing methods, but their core principles will continue to serve industry needs.
References
GB/T 1731-2020, Determination of paint film flexibility
ISO 1519:2011, Paints and varnishes — Bend test (cylindrical mandrel)
ASTM D522-2021, Standard Test Methods for Mandrel Bend Test of Attached Organic Coatings