The patterns observed indicate uneven corrugating roll nip pressure, with the center being low, causing variable convertibility quality across the sheet of the combined board. This issue can lead to excessive wear in the center and affect the quality of the final product.

The possible causes of this problem include incorrect crowning, excessive wear in the center, and incorrect end load pressure application. If the roll has incorrect crowning, it may be necessary to replace it with the correct crowns. In case of excessive wear in the center, renewing the roll may be necessary. If the issue is related to incorrect end load pressure application, it is recommended to check and correct the end load pressure.

It is important to address this issue promptly to ensure the quality of the final product.