Basis weight variability
Changes in the basis weight of paper over time can be due to problems with the process control system, with consistency control, with the stock valve, or changes related to wet-end chemicals. Among the mechanical factors to consider are (a) a variable level of stock in the machine chest, especially if there is a run-out at the end of the production of a particular grade that requires a certain furnish or dye addition to the thick stock, (b) a malfunction of a dilution valve in the consistency control area, and (c) variations in the vacuum applied at the flat boxes or couch roll. Many more possibility are listed in the CPPA manual titled "Operating Difficulties on Fine, Kraft, and Specialty Paper Machines." Since many of the possible causes of basis weight variations lie in the thick-stock area of the machine, and since most modern paper machines rely on basis weight measurements at the reel, there is a process control delay, and there is a danger of overshooting if the gain constants are set too high or if the stock valve adjustments are too coarse.
A variable efficiency of the retention aid program may cause changes in basis weight, though these will tend to be short-term if the control system is able to correct for them. Things to check include (a) the concentration, viscosity, and flow rate of retention aids, (b) an variations in pressure of dilution water that carries retention aid to the paper machine, and (c) the possibility that variations in cationic demand of the system are sufficient to affect the efficiency of retention aids. Variations due to retention efficiency can be minimized by such practices as aiming for a higher level of first-pass retention (reducing the fines content of the white water), online control of white-water solids, and online control of cationic demand.
Beck, M. W., "The Importance of Paper Machine Process Control and Wet End Stability," Proc. TAPPI 1997 Engineering Papermakers Conf., 593 (1997).
Gess, J. M., and Wilson, P. H., Troubleshooting the Papermaking Process, TAPPI Press, Atlanta, 2001, ISBN 1-930657-58-7, TP R298, www.tappi.org.
Rantala, T., Nokelainen, J., and Artama, M., "Wet End Management by Controlling Consistencies and Charge," TAPPI 99 Proc., 1189 (1999).
PLEASE NOTE: The information in this Guide is provided as a public service by Dr. Martin A. Hubbe of the Department of Wood and Paper Science at North Carolina State University (firstname.lastname@example.org). Users of the information contained on these pages assume complete responsibility to make sure that their practices are safe and do not infringe upon an existing patent. There has been no attempt here to give full safety instructions or to make note of all relevant patents governing the use of additives. Please send corrections if you find errors or points that need better clarification. Go to top of this page.