Dynamical Curl of Paper in Copying

We studied the curl of paper in copying by carrying out experiments with a special device where roller speed and temperature can be controlled. In addition to curl, we measured the moisture loss of paper in the hot nip. The moisture loss and nip dwell time were two important parameters setting the curl level. Stable curl values were obtained only immediately after the nip or when the copied paper has been air-conditioned back to an equilibrium with the surrounding relative humidity. In between these two limits, transient curl values varied among parallel experiments and were very sensitive to asymmetry in paper structure. We observed relaxation of the curl when paper was kept flat in storage for a long time. However, a large part of the relaxed curl recovered back when paper was again let freely deform.