Warp control apparatus and method for sheet material

In a method for continuously fabricating substantially warp-free corrugated paperboard sheet comprising a top liner, a corrugated medium and a bottom liner, the sheet having a machine direction and a cross direction transverse of the machine direction, the steps comprising:

• forming the corrugated paperboard sheet from the top and bottom liners and corrugated medium;
• independently sensing the moisture content profiles of the top and bottom liners after the formation of the corrugated paperboard sheet;
• and
• controlling the cross direction moisture content of one liner relative to the other liner before the formation of the corrugated paperboard sheet in response to the sensed moisture content profiles to substantially equalize the moisture content profiles of the top and bottom liners.

In a method for continuously fabricating substantially warp-free corrugated paperboard sheet comprising a top liner, a corrugated medium and a bottom liner, the steps comprising:

• joining the top liner and the corrugated medium;
• applying controlled amounts of water to the top liner after it has been joined with the corrugated medium;
• applying controlled amounts of water to the bottom liner;
• bonding the bottom liner to the corrugated medium to form the corrugated paperboard sheet and heating the paperboard sheet as part of the bonding step;
• independently sensing the moisture contents of the top and bottom liners of the corrugated paperboard sheet after a sufficient lapse of time following heating of the paperboard sheet to permit the applied water to attain a substantially uniform concentration throughout the thickness dimensions of the top and bottom liners;
• and
• independently controlling the amounts of water applied to the top and bottom liners in response to the sensed moisture contents of said liners.

A system for producing non-warped corrugated paperboard sheet made from first and second opposing outer liners and a central corrugated medium, the system comprising:

• means for providing first and second liners;
• means for providing a corrugated medium;
• a bonding system operable to bond the first and second liners to opposite sides of the corrugated medium to form a unified corrugated paperboard sheet;
• a plurality of individually controllable water applicators disposed adjacent to the first and second liners and adapted to apply water onto the liners at a plurality of cross-directional locations before the corrugated paperboard sheet is formed by the bonding system;
• moisture sensors spaced from the water applicators, the sensors being adapted to measure the moisture content of each of the liners at the plurality of cross-directional locations after the application of water and after the corrugated paperboard sheet is formed by the bonding system;
• transport means for transporting the liners and corrugated medium from the bonding system and water applicators to the moisture sensors;
• and
• control means, operatively coupled to the moisture sensors and to the water applicators, for independently controlling the application of water by the applicators to the plural cross-directional locations of the first and second liners based upon the moisture measurements of the moisture sensors at the plural cross-directional locations.

In a system for fabricating corrugated paperboard sheet comprising first and second opposing outer liners and a central corrugated medium, apparatus for controlling the moisture content of said liners comprising:

• means for applying independently variable amounts of water to the first and second liners;
• means for sensing the moisture contents of the first and second liners after the water has been applied to the liners, said moisture content sensing means having outputs indicative of said moisture contents;
• means responsive to the outputs of the moisture content sensing means for controlling the amounts of water applied to the first and second liners;
• a paperboard sheet heating device for bonding the liners and the corrugated medium to form the corrugated paperboard sheet;
• and
• means for transporting the sheet from the heating device to the moisture content sensing means, said means for applying water being positioned upstream of the heating device so that water is applied to the liners before bonding, and said sensing means being disposed at a distance downstream from the heating device such that the time for transporting the sheet from the heating device to the sensing means is sufficient to permit water in the first and second liners to substantially equalize in concentration throughout the respective thickness dimensions of the liners.