a moving length of a thermoplastic web is subjected to edge knurling along the margins of its two opposite edges by passing each of such margins through a knurling zone including a backing wheel in supporting engagement with the reverse surface of the margin and in engagement with the obverse margin surface a freely rotatable knurling wheel carrying a plurality of raised minute surface projections thereon as an exterior knurling pattern , while heaving a portion of the rim periphery to a temperature sufficiently high to soften a surface stratum of the web margin by conduction from the heated rim , and simultaneously adjusting the force exerted by the two wheels on the web margin to cause said knurling projections to enter and depress localized areas of the heat softened surface stratum while the regions of the same surface stratum adjacent such depressed areas become upset and raised above the original surface without causing deformation of the reverse web surface as a consequence of the entry of the knurling projections into the obverse web surface stratum .