The properties of coating systems measured by the Mandrel Bend Test are rapidly increasing in importance due to the gain in popularity of coil coating where the coated base material is subjected to a forming operation. Most any base material that will later be subjected to a forming operation can be accommodated.

The Pentagon Mandrel Bend Test Apparatus is constructed of black anodized aluminum. There are two side plates, each of pentagon shape. Each of the five sides are 6.5 inches long, resulting in a 10.0 inch high assembly. One of the side plates is 1/8 inch thick and has one of the sides rounded to form a high-strength mandrel of this thickness. The other side plate is identical except it has a thickness of 1/4 inch and provides the 1/4 inch mandrel.

The theoretical determination of the relative length change of the two surfaces of the base as it is bent around the mandrel is a complex matter. The compression of the surface in contact with the mandrel usually is not as great as the expansion of the opposite surface. Neither is this compression - expansion ratio constant from material to material or with different thicknesses of the same material.

The relationship between coating elongation at the interface with the base as a function of mandrel diameter is shown in the following graph which has been prepared from data contained in ASTM Method D 522-88. This graph is representative of a thin coating, one mil or less in thickness, applied to Cold Rolled Steel, SAE No. 1010, dead soft, finish “A” with a thickness of 1/32 inch.

Within limits and with most Base - Coating systems, the percent elongation at the interface is proportional to the thickness of the base and can be calculated with a fair degree of accuracy with data from the graph below for base thickness other than 1/32 inch.

The preparation of the test specimens must be made with considerable care if good comparative and reproducible results are to be obtained. Improper base cleaning, resulting in poor adhesion, will normally result in a lower tolerated elongation. Coating thickness and uniformity is of paramount importance, not only due to the usual relationship of thickness with most physical characteristics but also due to the direct effect in the mandrel bend test.

The test panels should be prepared in a manner and with a coating thickness as near as possible to that which will be encountered by the base - coating system process. Compensate for the direct effect of coating thickness in the Pentagon Mandrel Bend Test by adding the appropriate correction factor from the graph at left for each mil of coating thickness to the base - coating interface elongation as determined from the earlier graph. This graph is also prepared from data contained in ASTM Method D 522-88.