Effects of grammage on sheet properties in one-sided and two-sided roll forming

The influence of grammage on sheet properties in one-sided and two-sided roll forming was investigated in a pilot machine study with a softwood kraft pulp furnish over a range from 36 to 70 g/m(2). Up to 55-60 g/m(2), both the small-scale STFI formation and the (large-scale) Ambertec formation improved with increasing grammage. Up to about 50 g/m(2), one-sided roll forming showed better small-scale formation than two-sided roll forming. Both these effects can be attributed to an enhanced self-healing effect. Worse Ambertec formation for one-sided than for two-sided roll forming up to 60-65 g/m(2) may on the other hand be attributed to more pronounced floc generation in the prolonged dewatering phase of one-sided roll forming. With the density on a constant level, the tensile index increased with increasing grammage up to 50-60 g/m(2) where it leveled out. One-sided roll forming gave better tensile strength up to about 50 g/m(2), whereas the tensile strength leveled out on a similar level for both forming variants. The better small-scale formation is considered to be the explanation for the better tensile strength of one-sided than two-sided roll forming for the lower grammages. The positive effect of grammage on the tensile strength for both one-sided and two-sided roll forming is interpreted as at least partly being due to the improvement in formation with increasing grammage. Yet, a contribution from a reduced relative influence of surface layers with a lower bonding degree than the inner layers of the sheet cannot be excluded. In line with another investigation, significantly higher Z-strength for one-sided than for two-sided roll forming was noted over the entire grammage range investigated and above 40 g/m(2), no influence of grammage could be established.