Sheet forming tool and a method for the manufacture of a corrugated sheet

A method for manufacture of a corrugated sheet (100), wherein a sheet (90) is advanced into a sheet forming tool (1), the sheet forming tool (1) having a lower tool element (40) and an upper tool element (50), each of the upper tool element and the lower tool element having a front side (23, 33, 41) and a rear side (25, 35, 42), wherein the lower tool element (40) comprises a first base element (2, 4, 6, 8) and a first finger element (3, 5, 7, 9) projecting from said first base element (2, 4, 6, 8), the first finger element (3, 5, 7, 9) forming a first ridge (22, 24, 26, 28) for forming a corrugation peak in the sheet, wherein the upper tool element (50) comprises a second base element (12, 14) and a second finger element (13, 15) projecting from said second base element (12, 14), the second finger element (13, 15) forming a second ridge (32, 34) for forming a corrugation trough in said sheet, with the first ridge (22, 24, 26, 28) being arranged opposite to the second ridge (32, 34) and the first ridge (22, 24, 26, 28) being offset from the second ridge (32, 34) so as to allow for an engagement of the first finger element (3, 5, 7, 9) and the second finger element (13, 15) in an engaged position the sheet forming tool (1) is closed by moving a first finger element (3, 5, 7, 9) towards a second finger element (13, 15) into an engagement position such that the sheet (90) is folded over the first ridge (22, 24, 26, 28) and the second ridge (32, 34) of the corresponding first finger element (3, 5, 7, 9) and second finger element (13, 15), then the sheet forming tool (1) is opened by moving the first finger element (3, 5, 7, 9) away from the second finger element (13, 15) and the corrugated sheet (100) is discharged from the sheet forming tool, and each of the first ridge (22, 24, 26, 28) and second ridge (32, 34) comprises a main portion (20) and an end portion (11, 21) and the angle β (36) between each of the first ridge (22, 24, 26, 28) and second ridge (32, 34) in the main portion (20) and the corresponding front side (23, 33, 41) being at least partly different from the angle between each of the first ridge (22, 24, 26, 28) and second ridge (32, 34) in the end portion (11, 21) and the corresponding front side and a space is provided between the first finger element (3, 5, 7, 9) and the neighbouring second finger element (13, 15) in the engaged position, wherein between the main portion (20) of the first ridge of the first finger element and the main portion (20) of the second ridge of the neighbouring second finger element, the sheet is not in contact with either finger element but is allowed to form freely in the space between the main portion (20) of the first ridge of the first finger element (3, 5, 7, 9) and main portion (20) of the second ridge of the neighbouring second finger element (13, 15) in the engaged position, and wherein the main portion (20) of the first ridge of the first finger element and the main portion (20) of the second ridge of the neighbouring second finger element interact with the sheet in the engaged position to form a linear portion of a corrugation in the sheet.