Key Questions
The following key questions are answered in this module:
How does a paper or board machine's stock delivery system affect sheet formation?
The shear forces created by agitators, pumps, valves, cleaners, and screens in the stock system help keep the fibers dispersed in the dilute stock suspension and prevent flocs from forming before the stock reaches the headbox, which ultimately improves formation.
How does headbox design influence sheet formation?
Rectifier rolls, tube banks, converging channels, and other internal headbox elements are designed to create turbulence and shear. This helps keep the fibers dispersed until they reach the forming zone.
Why aren't all headboxes run at lower consistencies, since this helps keep the fibers separate and improves formation?
The headbox consistency can only be lowered until the drainage capacity of the former has been reached. If it is lowered further, the sheet will be too wet going into the press section.
Does the addition of starch affect sheet formation?
Starch is sometimes added to stock to increase sheet strength properties and improve drainage and retention. This can improve formation to a point, but over-use can lead to foam in the stock, which hurts formation and drainage.
How can the forming section influence sheet formation?
Breast roll shake mechanisms, formation showers, dandy rolls, foil box angles, and vacuum levels on the former can all affect sheet formation.