A statistical analysis of data from the production line at the Munksund paper mill

Halfdan Grage

Centre for Mathematical Sciences
Mathematical Statistics
Lund Institute of Technology,
Lund University,

ISSN 1403-9338
A multivariate statistical analysis of paper strength and production process data from the SCA Munksund paper mill is presented. The analysis is divided into five parts: (i) paper quality variables, (ii) process variables from the paper machine, (iii) process variables from digester and refiner, (iv) a combination of all process variables, and (v) a further analysis of the z-strength.

Analysing the correlations between the paper strength variables, two groups of variables could be discerned: one group comprising the variables tensile strength in MD, tensile stiffness in MD, elongation in CD, and bursting strength, and another group containing the variables SCT in CD, tensile strength in CD, and tensile stiffness in CD. The strongest correlations between paper machine process variables and paper strength variables were found for the strength variables in the first of the above mentioned groups. The same strength variables were negatively correlated with the wet end process variables conductivity, base line paper machine flow, and the amount of return fibre, while they were positively correlated with the concentration at the base line refiner. Combining wet end and paper machine process variables in a regression analysis for the paper strength variables yields a limited increase in the amount of variance explained, compared with the case where only wet end process variables are used as predictors, due to the correlations between the process variables. A more detailed study of the time series of z-strength measurements showed that
this was non-stationary, which makes it hard to fit a single over-all linear regression model which will give good predictions over the whole year -or even from one production run to another.