This effect is called “stair cut” and is common on all sheeters when multiple webs are pulled in through the draw roll.
Basically, what happens is the bottom web wraps the draw roll at a given angle and diameter. The second web wraps the draw roll at the same angle and diameter plus the thickness of the bottom web. Therefore, the distance the top web travels is greater and this causes the difference in length.
Two factors affecting stair cut are draw roll diameter and web thickness. The larger the draw roll diameter, the less stair cut – the thicker the material, the more stair cut.
