Measuring the mechanical properties of paper during manufacturing is difficult but essential. One such property is bending stiffness, which describes the paper’s resistance to bending forces. Everyday examples of the importance of bending stiffness in paper and board products can be found in milk cartons and cardboard boxes. If the bending stiffness is too low, the carton will buckle when you grab it, or the box will collapse under the weight of other boxes when stacked.
Mikko Salo conducted his doctoral dissertation at Doctoral School of Industry Innovations (DSII) of Tampere University in collaboration with Valmet Automation. His research was based on Valmet Automation's prototype and the associated methodology for estimating paper bending stiffness.
Surface shape measurement enables estimation of paper bending stiffness during manufacturing
One common way to measure the bending stiffness of paper is by bending a strip of paper and measuring the force required to produce a certain angle. According to Salo, various similar destructive tests are generally used by the paper industry to measure the bending stiffness of paper post-manufacturing for quality assurance. He says it is clear such tests can’t be conducted during manufacturing on a long and wide paper web that moves fast.
Valmet Automation has studied methods for measuring the bending stiffness of paper online. Their latest attempt is based on deflecting the moving paper web using a vacuum load, measuring the shape of the deflected surface using computer imaging, and finally estimating the bending stiffness by affiliating the measured load and deflection with the bending stiffness using known plate equations. Valmet Automation had earlier developed a prototype for testing the method using their own existing commercial sensors.
In his doctoral research, Mikko Salo studied Valmet’s prototype and the associated methodology and tested it using data measured from a paper mill during manufacturing as well as from a test facility.
“My findings suggest that the method has difficulty estimating the variables derived from the required measurements for the plate equations. I also identified some unexpected anomalies that contradicted the assumptions of the proposed method,” he says.
He then studied the method extensively using simulated data, and suggested reasons for the anomalies found.
“The simulated data revealed that the method works in theory, though a different measurement method may be needed in practice,” he concludes.
Mikko Salo, who has a background in automation engineering and mathematics, focused his research more on structural mechanics and plate theories. His initial goal was to develop feedback control for the paper's bending stiffness.
“However, my thesis has little to offer to those interested only in automation engineering. I hope to continue developing new measurement technologies in academia or industry in the future,” he says.
Currently, Mikko Salo works at Tampere University as a doctoral researcher.
Public defence on Friday 12 December
The doctoral dissertation of MSc (Tech) Mikko Salo in the field of automation engineering titled Paper bending stiffness estimation from online deflection measurement will be publicly examined at the Faculty of Engineering and Natural Sciences at Tampere University 12 o’clock on Friday 12 December 2025 at Hervanta campus, Konetalo, Room K1702 (Korkeakoulunkatu 6, Tampere).
The Opponents will be Professor Jaan-Willen Simon from the University of Wuppertal and Professor Jari Hämäläinen from the LUT University. The Custos will be Professor Mikko Kanerva from Tampere University.