A coating on a sheet should neither crack nor flake off when the sheet is bent. A mandrel bend test device is used to test how well a lacquer or other coating withstands such bending tension. This standard test is used to investigate practically all coatings before they are launched on the market.
The mandrel bend test is not the only important tensile test for sheet coatings. The Erichsen cupping test is also often used. The tests differ from one another: in the mandrel bend test sheets are bent rapidly in one dimension, in the Erichsen cupping test slowly in two dimensions.
As of 1992 the results of mandrel bend tests have been comparable via the ISO 6860 standard of the International Organization for Standardization (ISO). The German version of the standard is DIN EN ISO 6860 – mandrel bend test (with conical mandrel). This describes a procedure for achieving a standard assessment of how resistant lacquers, paints or other coatings are to cracking or flaking from a metallic surface.
In the mandrel bend test sheets coated on one or both sides are bent around a cone. The coating at the thin end of the cone is stretched or compressed the most. The size of the bend radius in which no cracks or flaking is to be seen is an indication of how well a coating adheres or the extent to which it can be stretched or compressed.
Amongst other things, ISO standard 6860 states the conditions under which mandrel bend tests are to be conducted. Some examples
- The mandrel is positioned horizontally. It is 203 mm long. Its largest diameter is 38 mm, its smallest 3.2 mm.
- The test sheets are approximately 180 mm long, 100 mm wide and up to 0.8 mm thick. They need to be even and have no visible ridges or cracks. They are usually made of steel, tin or aluminium.
- Three test sheets are sufficient for a test series. The tests are conducted at 23 °C (+/–2 °C) and 50% humidity (+/–5 %).
- After the test the coatings are immediately checked with the naked eye for cracks or flaking. It is also possible to use a magnifying glass with 10x magnification.
- The length of the cracks is measured from the thinner end of the mandrel along the test surface and stated in cm. An average value is taken from three measurements
In practice, companies mostly use the conical mandrel bend test as a yes/no test. This involves sheets being coated on one or both sides. If a lacquer stretches well, it will not be evident from the coating that the sheet has been bent. However, if cracking or flaking occurs during stretching or compression, the respective coating has not passed this test.
There are also test devices with cylindrical mandrels. These are used to test tensile strength with a sole bend radius. As cylinders of varying diameter can be fitted to these devices, test series with declining cylinder diameter can be carried out to determine the diameter at which damage to the coating first occurs.