By Steven Soderberg, Lorin Industries Inc.

Anodized sheet products are often chosen for the aesthetics and lasting durability of the surface finish, but one issue that should not be overlooked in purchasing anodized aluminum sheets is the flatness. Why is flatness important, how is it defined and measured, what causes a lack of flatness and how can it be prevented or corrected?

Why is flatness important?

Lack of flatness results in problems with dimensionality and aesthetics. For architectural applications, it is important that metal wall panels are straight and level, thus you need to start with a level sheet. For consumer goods and transportation products, flatness is critical to ensure that the final formed or stamped pieces fit properly.

How is flatness defined?

Lack of flatness is defined as a distortion of the surface of a sheet such as a bulge or a wave, usually transverse to the direction of rolling and it is often described by location across width, such as edge buckle, quarter buckle, or center buckle.

The Aluminum Association Standards measure flatness as a certain rise (height) over a given distance (cycle, or length of the wave). Flatness quality is calculated in I-Units. Maximum acceptable flatness is 1% of wavelength or 27 I-Units or less. Measurements are taken to the nearest 1/32” with rulers having (at minimum) a 1/64” graduation.

What causes flatness problems?

Thickness variation is one of the primary causes of flatness issues. When the metal is rolled, the mill tries to control the process so that the metal has uniform thickness across the width of the coil. As the thickness is decreased, the metal becomes longer.

If the metal is thinner in some parts of the coil, then that can create buckle in the sheet.  If the edges of the coil are thinner than the middle of the width, it can create edge wave.  If the middle of the width is rolled thinner than the edges, you can create center buckle in the sheet.

Coiled aluminum can also have coil set, where the metal takes on the shape of the wind when the coil is unwound. Coil set is defined as a longitudinal bow in an unwound coil in the same direction as the curvature of the wound coil.

Another problem can occur when a flat sheet is cut into smaller pieces. The larger sheet may be flat because of stresses in the sheet. When cut, the stresses may be released and cause the metal to no longer be flat.

Avoiding Flatness Issues

Most flatness problems can be corrected by leveling the sheet, and Lorin utilizes a Bradbury Leveler to produce anodized aluminum sheets to guarantee flatness according to well-established industry standards.