Baggy paper webs
Baggy paper webs are a common cause for runnability problems in converting processes. The baggy paper can cause creasing when going through a nip, or it can have misregister problems in printing.
The objective of this project is to rank several assumed bagginess origins with respect to the amount of bagginess created. Suspected origins are the paper machine itself, converting (calendering), winding/unwinding, storage, or a combination of all.
Bagginess is a generic term for different types of runnability problems. All of them are caused by uneven paper length, whether the web has slack edges, pure camber, or slack and taut lanes. The length difference leading to runnability problem is of the order of 0.1%.
The hypotheses in this project are that baggy webs are caused by these mechanisms:
- Moisture streaks might exist in the dryer section of the paper machine. Those are stretched under different moisture content and are thus plasticized to different degrees. The moisture streaks may be present before the dryer, due to e.g. uneven dewatering through the wire, or they may be formed in the dryer section due to e.g. uneven temperature in drying cylinders.
- Calendering changes the paper dimensions, not only the thickness but also in-plane. Uneven paper and uneven calender rolls can thus cause uneven paper length.
- The paper roll may have ridges. Then the strain during storage is higher in the ridges than around them. Higher strain causes the paper to be longer in the ridge positions after storage.
The hypotheses are examined with laboratory studies, pilot trials and full-scale trials. This project ends in 2010, with the doctoral defence on Dec. 3, 09:15 in Rejmersalen, at Karlstad University, Sweden.
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Author: Cecilia Land
The page was updated on 2010-11-01 15:32