Definition
The paint film conical shaft bending tester is a special inspection equipment used to evaluate the cracking and peeling resistance of coatings when bending around a specific conical shaft under standard conditions. The instrument provides key quantitative data for the flexibility, adhesion, and comprehensive mechanical properties of coatings by simulating the stresses caused by substrate deformation in real-world applications. Its test methods are clearly stipulated in many domestic and foreign industrial standards, and are an important tool for quality control and research and development in the fields of coatings, automobiles, ships, light industry and anti-corrosion engineering.
Principle
The core working principle of the instrument is based on mechanical bending deformation. The instrument is equipped with a set of standardized conical shafts with different diameters that continuously decrease in diameter from large to small end to form a taper surface. During the test, the test plate coated with the paint film is close to the surface of the cone shaft, and it is bent evenly under the action of controllable external force, so that the paint film will undergo tensile deformation with the substrate. Due to the different radii of curvature at each position of the conical shaft, the tensile stress of the paint film on different parts of the shaft is also different. By observing at which diameter area the paint film begins to crack or peel off the substrate, its flexibility and adhesion limits can be determined. The maximum tensile strain ε on the surface of the paint film is correlated with the bending radius of curvature r and the thickness d of the paint film, and the relationship can be expressed as:
ε ∝ d / r
The radius of curvature r varies continuously along the conical axis, thus achieving a continuous strain gradient test in a single test.
Measurement method
Standard measurement processes usually follow specifications such as GB/T 11185, ISO 6860, ASTM D522, etc. First, prepare the flat test plate of the specified size, and complete the coating, drying and conditioning according to the product standard requirements. Place the test plate with the painted side facing outward between the base of the instrument and the conical shaft. Operate the handle or drive at an even and slow speed to bend the test plate 180 degrees around the conical axis, usually in 1 to 2 seconds. Immediately after bending, the paint film is observed with the naked eye or with a magnifying glass under standard lighting conditions. The result is usually expressed as the minimum conical shaft diameter (or corresponding shaft number) where the paint film does not crack or peel off. Some standards require checking whether the paint film is peeling off from the substrate. The test should be performed in a standard temperature and humidity environment, and the environmental conditions should be recorded.
Influencing factors
The accuracy and repeatability of test results are influenced by various factors. The properties of the paint film itself are the primary factors, including the glass transition temperature of the resin system, the volume concentration of the pigment, the cohesion, and the adhesion between the coating and the substrate. The thickness, material, and surface treatment of the substrate also directly affect the uniformity of stress transfer during bending. Environmental conditions, especially temperature and relative humidity, can affect the physical state of the paint film, and too low a temperature can lead to brittleness. The test operation process, such as the uniformity of the bending speed, whether the direction of the applied force is perpendicular to the test plate, and the human factor in observing and judging are all regulated through rigorous training. In addition, the quality of the preparation of the test plate, such as the uniformity of the thickness of the paint film, is also a factor that cannot be ignored.
Applications
The tester is widely used. In the field of industrial coatings, it is used to evaluate the ability of steel structure anti-corrosion coatings, marine coatings, container coatings, etc. to withstand deformation during component processing or use. In automotive manufacturing, it is used to test the flexibility of body e-coats, midcoats, and topcoats during stamping or assembly. In the light industry such as coil coatings, furniture coatings and electrical enclosure coatings, this test is a key part of predicting whether the coating will crack in subsequent processing. In addition, in the research and development stage of coating products, it is also an effective evaluation method for screening resin formulations and optimizing the amount of additives to improve the mechanical properties of coatings.
Selection considerations
When choosing a suitable paint film conical shaft bending tester, multiple technical parameters need to be comprehensively considered. First, the range of conical shaft diameters covered by the instrument must meet the requirements of the relevant product standards, usually covering series from large to small diameters. Secondly, the manufacturing accuracy of the instrument is crucial, and the taper accuracy of the conical shaft, the surface finish, and the smoothness of the drive mechanism directly determine the accuracy of the test gradient and the repeatability of the results. The material of the instrument should be robust and durable, resistant to corrosion and maintain dimensional stability over time. The user-friendly design of operation, such as whether it is labor-saving and whether it is convenient for the placement and removal of the test board, also affects the efficiency of daily testing. Finally, it is necessary to confirm that the instrument meets specific national or international standards that are mandatory for the target market or industry.