3D measurements on extrusion marks in plastic bags.
The surface of different types of garbage bags was scanned using non-contact laser profilometry. Manufacturing defects known from visual inspection of the bags (fish eyes, arrowheads, and surface scratches) appear as differences in the height profile. Extrusion lines could only be detected in low-density polyethylene bags. Their correlation length is a couple of centimeters, whereas visually they continue over meters. This can be used to link together the end of one garbage bag with the beginning of the subsequent bag. Extrusion lines are of the order of 0.1 microm for high-density polyethylene garbage bags.