Irregularities of cassettes, such as convex or concave face surface, occur when bending of sides is done not at 90°. Excessive bending leads to concavity, and bending at the angle over 90° results in convexity of the cassettes when it comes to installation and the sides are fastened perpendicularly to the surface of the façade.
Sometimes in order to have a side bending angle equal to 90°, they slightly over-bend it, and then, having noticed that it is over-bent, unbend it back. It is difficult to guess. Most frequently such kind of defect appears when processing ACP with rigid aluminum skins. As a result, the surface near the edges of the cassette is concave and the middle part is convex. To prevent this kind of defect, a bending machine should be used that allows to make bending at the intended angle, a little over 90°.
Similar defects occur when bending is performed without bending machine or special clamping fixture for bending. Non-simultaneous side bending manually, first one side and then the other one, almost always leads to irregularity of the cassette surface.

Wrong size of cassettes

Violation of perpendicularity of the sides