Mechanisms causing instability and change are commonplace in nature. Similar phenomena exacerbate the tasks of devising, building and maintaining systems for efficiently manufacturing paper to a tight specification and high standard of uniformity.
In the case of a machine calender stack, CD control can impart stability to an otherwise unstable system. Instability of the headbox or approach flow system can however seriously affect the MD control of basis weight. Some types of instability lie outside the reach of control, and must if possible be eliminated through improved equipment design. For this reason a good understanding of instability is necessary.
This paper considers some specific examples of unstable behaviour, including eddy and vortex formation, waves and other amplification mechanisms on Fourdrinier wires, corrugation growth, uneven wear, self-excited vibration, and thermal deformation affecting calender stacks.