Grinding wood to produce a pulp for papermaking is termed mechanical pulping. To better understand and control this grinding process, a rheological model is postulated that accounts for the elastic and viscous properties of the wood in two orthogonal directions. From the model it is shown that these properties impart a phase lag between the normal and tangential grinding forces, and experimental measurements collected under five grinding conditions agree with the model prediction. Based on measured pulp properties, the grinding condition with the minimum phase lag yielded the best quality pulp, suggesting that the calculated phase lag could be used as a quality control tool to eliminate conventional pulp testing during production or as a feedback signal to control grinder operation in the mechanical pulping process.

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