Year 1999
Erika Rajala

Measuring paper smoothness and predicting printability using on-line Surface Sensor

The new quality control system was installed on the paper machine 6 at the Stora Enso’s Paper Mill in Imatra containing an on-line smoothness measurement. The PrecisionPLUS Surface Sensor measures paper smoothness optically during the paper making process by scanning over the moving web in cross direction. The aim of this thesis was to verify the Surface Sensor measurement by calibrating it to the PPS-roughness values and to find the possible relations with paper printability.

In the literature part of this study, mechanical printing methods are introduced as well as the requirements for smoothness of different printing papers. The surface smoothness of paper and paperboard is presented. Different methods for measuring smoothness both in a laboratory and on-line during the paper making process are discussed and compared. The PrecisionPLUS Surface Sensor is introduced. Also the relation between paper printability and smoothness is discussed. Different printability measurements are presented.

The experimental part of this study began with the application study of the Surface Sensors installed all over the world. Then the repeatability test of the L&W Autoline at the laboratory of the Stora Enso’s Paper Mill in Imatra was performed. The PrecisionPLUS Surface Sensor installed on the fine paper machine at the Imatra Paper Mill was then calibrated to show the PPS-roughness values measured by the Autoline. The calibration was carried out by changing the nip pressures on the calender and measuring the surface smoothness continuously with the Surface Sensor and with the Autoline PPS. A good linear correlation between the results of the Surface Sensor and the Autoline PPS was found and the slope and intercept of the line were used as the calibration coefficients. The Surface Sensor also correlated well with the Hunter-gloss of the paper. The Surface Sensor responded logically to the changes made on the calender.

The test on paper printability was carried out after calibration. Different roughness levels were formed on the paper surface. In the first trial run the calender pressure was changed. In the other trial run the surface temperature of the calender was raised using the induction-heating instrument, Calcoil. The paper roughness was measured on-line with the Surface Sensor and with the Autoline PPS. Finally, the paper samples from each trial run were printed using the offset sheet fed test press of the Imatra Paper Mill. The properties of the print quality were measured and compared with the results received with the Surface Sensor. A linear correlation between the gloss of the printed paper and the Surface Sensor-measurement was found. The number of missing dots had a good linear correlation with the Surface Sensor also. During all the tests the PrecisonPLUS Surface Sensor seemed to have a linear correlation with the changes of calender pressure and calender temperature. Therefore the Surface Sensor could be used as a tool to show the quality changes during the paper making process.

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