Web tension required to drag the newly-formed corrugated board can be as large as 3000 pounds. The main reason for warp is often irregular moisture profiles in the liner webs. Complementary profile dryers can balance out the moisture profiles in the liner webs. Moisture migration from the outside liner to the inside liner and imbalance causes warp. Lifting of the weight rolls in response to machine speed is important to control warp.
Excess starch increases the overall moisture content of one side of the board, increases warp.
Pre-heaters designed to use both high pressure and low pressure superheated steam. Friction and web tension are not a concern as the paper is heated by the hot steam film. The steam aspects of the Pre-heater provide moister that improve warp control. In order to create adequate heat transfer and bonding, a beltless double facer's fixed weight blanket needs to apply substantial downward force. United's process studies show that the overall web tension required to drag the newly-formed corrugated board out of the heating section can be as large as 3000 pounds.