Double fold (paper testing)
Encyclopedia
A double fold is when a paper
sample is folded first backwards and then forwards about the same line, i.e. one complete oscillation. The number of double folds that is required to make a test piece break is used to determine the material's folding endurance
and fold number
.
The total folding angle (about the folding line) differs depending on which folding tester is used, for instance the Köhler–Molin instrument folds about 156° on each side of the vertical line, resulting in a complete oscillation
of about 2 × 312° for each double fold, while the MIT instrument employs a folding angle of about 135° on each side (that is, a complete oscillation of about 2 × 270° for each complete double fold).
Strictly speaking, the first double fold is more than one complete oscillation since the flap needs to be folded from vertical to its starting position before the counting begins.
Paper
Paper is a thin material mainly used for writing upon, printing upon, drawing or for packaging. It is produced by pressing together moist fibers, typically cellulose pulp derived from wood, rags or grasses, and drying them into flexible sheets....
sample is folded first backwards and then forwards about the same line, i.e. one complete oscillation. The number of double folds that is required to make a test piece break is used to determine the material's folding endurance
Folding endurance
In paper testing, folding endurance is defined as the logarithm of the number of double folds that are required to make a test piece break under standardized conditions:F = log10 d,...
and fold number
Fold number
Fold number refers to how many double folds that are required to cause rupture of a paper test piece under standardized conditions. Fold number is definied in ISO 5626:1993 as the antilogarithm of the mean folding endurance:...
.
The total folding angle (about the folding line) differs depending on which folding tester is used, for instance the Köhler–Molin instrument folds about 156° on each side of the vertical line, resulting in a complete oscillation
Oscillation
Oscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes...
of about 2 × 312° for each double fold, while the MIT instrument employs a folding angle of about 135° on each side (that is, a complete oscillation of about 2 × 270° for each complete double fold).
Strictly speaking, the first double fold is more than one complete oscillation since the flap needs to be folded from vertical to its starting position before the counting begins.