Dynamic thickness and temperature measurements during wet pressing and impulse drying.

Application: This study is a further step in the development of impulse drying. Understanding the physics of delamination and its prevention in the press nip may also help mills reduce delamination in the dryer section.

In this paper we summarize the results of dynamic sheet thickness and temperature measurements made while a sheet was undergoing wet pressing or impulse drying on an electrohydraulic platen press. We investigated the thermal and compression state of the sheet in order to gain insights into the dynamics of sheet heating and compression in an extended nip and to understand the physical mechanism inhibiting delamination in the decompression phase of a heated nip.

We used a newly devised experimental device to address these issues. The dynamic thickness measurement system was based on the impedance variation caused by eddy currents induced by thin copper perforated targets embedded in the sheet. Temperature profiles were measured by embedded fast-response thermocouples.

The main findings of the work are as follows:

* Impulse drying at elevated temperatures results in higher and nonuniform sheet compression as compared with wet pressing. The expansion of the hot sheet during nip opening was lower as compared with the sheet at conventional temperature unless the hot sheet delaminated.

* Analysis of the compression rate curves for both wet pressing and impulse drying indicated four compression intervals associated primarily with expression of air, expression of inter-fiber water, expression of intra-fiber water, and sheet expansion, respectively. Earlier fundamental work on transverse flow in the nip of a roll press intuitively predicted similar intervals in the press nip.

The temperature profiles in the sheet suggest that a heat pipe process progresses across the sheet during controlled decompression. This process occurs in a vaporization-condensation zone, which reduces with decompression time and tends to rapidly even out the sheet temperature across the sheet, thereby decreasing the likelihood of sheet delamination.

Rudman and Orloff are with the Institute of Paper Science and Technology, Atlanta, Georgia, USA. Email Orloff at [e-mail].