There are two cases of "forward" and "reverse" warping at different positions on each side of the same board.
the reason
Unevenly on double sided paper or single sided paper on paperboard
Single-sided paper, single-sided paper, or double-sided paper are heated unevenly during preheating. There may be dry or wet lines on single-sided or double-sided paper.
Measures Check the amount of glue applied to the double-sided and single-sided machines, find and correct any lateral misalignment, and make the face paper or web evenly in contact with the preheater and heat it on single-sided or double-sided paper. Using a segmented sprayer If a segmented sprayer does not solve the problem, consider using a different supplier's face paper