Extruded Acetal has internal stresses built up inside of the sheet. The extrusion process at the factory results in different parts of the sheets solidifying at different times as the sheet cools, possibly causing some parts of the sheet to tighten from the cooling before other hotter sections have yet to finish cooling. When a sheet is cut, it relieves that stress. As a result, the piece that is cut, especially for thin sheets come out slightly warped.

 

All shapes (i.e. rod and plate) have a balanced stress state with respect to the shape centerline. If during machining, Cutting more material is removed from one side of the centerline than the other, the material will “dish up” towards the side with more material removed. The amount of residual stress locked into the material determines how much the material will move once the outer material is removed.
The same condition exists in metals. However, the lower stiffness of plastics makes the magnitude of this more significant.