Headbox induced sheet variability, PaperCon2017

The properties of the web in a paper machine exhibit a large degree of variability in both MD and CD, on length scales ranging from millimetres to kilometres and above. This has far reaching negative consequences on e.g. the runnability of the papermachine, the fibre material resource efficiency and product quality. In this study we investigate the potential interplay between dilution flows injected into the headbox flow, and design parameters like the headbox contraction ratio and the strong local acceleration caused by a slice bar, with the objective to see if sheet disturbances and variability may result. Trials were conducted on a pilot machine in which dilution water flows were injected into the main nozzle flow in two different headboxes with different nozzle contraction ratios. Configurations with and without a slice bar protruding into the flow at the nozzle exit were trialed. It was found that both the CD-width of the low basis weight area resulting from the dilution flow, as well as the formation, was influenced by the slice bar. Also the nozzle contraction appears to influence these properties.  

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