a band of bags of plastic material connected with each other by perforation lines . in the two end parts of each perforation line the number of perforations and / or the width of the perforations per unit of width is greater than in the central part of the perforation line . a device for manufacturing said band of bags comprises a perforating member with equidistantly spaced first perforating pins and second perforating pins are present on either side of a central area between the first perforating pins .