Flexibility of Elastic Coatings Tested by Cylindrical Mandrel Bend Tester

Test Principle:

The cylindrical shaft bend tester is used to evaluate the flexibility of elastomeric coatings under substrate bending conditions. During the test, the test plate coated with elastic coating is bent to 180 degrees around a cylindrical axis of different diameters to observe whether cracks, peeling or peeling occur on the surface of the coating. This method simulates the bending stress of coatings in practical applications, and is commonly used in industrial protection, architectural decoration, and other fields. The test is based on domestic and foreign standards (such as GB/T 1731, ISO 1519) and quantifies the flexibility level by changing the shaft diameter.

Instruments and materials

The equipment required for the test includes the cylindrical shaft bending tester itself (equipped with a set of cylindrical shafts with diameters of 1mm, 2mm, 3mm, 4mm, 5mm, etc.), a bending fixture, and a standard size test plate (usually 120mm×50mm×0.2mm soft metal sheet or steel plate). The elastomeric coating sample should be prepared according to the product manual and coated on a single side of the test plate, and the coating thickness should be controlled within a certain range (e.g., 40–60μm). The test can only be carried out after the coating has completely dried (left at room temperature for 24 hours or dried according to specifications).

Trial steps

The operation process is as follows: first, insert the test plate into the bending fixture, ensuring that the coating surface is facing outward; Then select the smallest diameter cylindrical shaft (such as 1mm), and smoothly drive the fixture to bend the test plate to 180 degrees to fit the cylindrical axis, and the process should be continuous and uniform to avoid impact; Remove the test plate immediately after bending and inspect the coating surface for cracks, peeling or other defects with the naked eye or under a magnifying glass. If no defects are found, the test is repeated with a larger diameter cylindrical shaft (such as 2mm, 3mm, etc.) until the coating is damaged. Record the maximum shaft diameter that is ultimately non-fracturing as an indicator of flexibility.

Result evaluation

Flexibility ratings are typically expressed in terms of the minimum bending shaft diameter that can be tolerated, with smaller shaft diameters indicating better flexibility. The test results can be summarized in the following table to facilitate the comparison of the properties of different coatings or formulations:

In the example above, the paint remains intact at 2mm shaft diameter and cracks at 3mm, resulting in a flexibility rating of 2mm. In practice, the test should be repeated at least three times for each shaft diameter to obtain the most consistent results.

Influencing factors

The test results are affected by a variety of parameters: insufficient dryness of the coating will reduce the flexibility; Ambient temperature and humidity may alter the mechanical properties of the coating; Substrate thickness and material can also affect bending stress distribution. Additionally, bending too fast can create instantaneous stress concentrations, leading to unrealistic cracks. It is recommended that the test be carried out in a standard environment (temperature 23°C±2°C, relative humidity 50%±5%), and the coating thickness and drying time are strictly controlled in accordance with the standard specifications.

Scope of application:

This test method is suitable for evaluating a variety of elastic coatings, such as acrylates, polyurethanes, fluorocarbons, etc., and is commonly used for quality control of protective coatings for steel structures, wooden components, and plastic parts. Regular testing optimizes coating formulations and improves product durability in dynamic bending environments.

Notes:

Gloves should be worn during operation to avoid hand oil contact with the coating surface; The instrument should be kept clean, and the cylindrical shaft should be wiped with a cleaning cloth before each test; The edges of the test plate should be free of sharp burrs to prevent additional stress concentrations during testing. When recording data, it is recommended to attach curved images or videos for subsequent comparative analysis.

Cited source

1. GB/T 1731-2019 "Method for Determining the Flexibility of Coating Films" Relevant technical provisions.

2. ISO 1519:2011 "Colored paints and varnishes - Bending tests (cylindrical shafts)" Test principle part.

3. Research on the evaluation method of flexibility of elastic coatings in domestic laboratory testing technology.