Process for extruding a thermoplastic sheath in the form of a tubular film provided with perforations and device for carrying out the process

Device for extrusion of a sheath of thermoplastic material in the form of a tubular film comprising longitudinally extending areas provided with perforations alternating with longitudinally extending areas of continuous surface, said device comprising:

• a downwardly directed extrusion head adapted to extrude a downwardly drawn tubular film;
• a truncated skirt forming a shaper for expanding said tubular film and extending at the periphery of said extrusion head;
• a source of pressurized air;
• a chamber supplied with pressurized air, said chamber being contained inside said skirt;
• communication means for conducting pressurized air from said source to said chamber;
• first and second channel means passing through the wall of said skirt and communicating with said chamber and opening out on the outer wall of said skirt immediately downstream of the extrusion head;
• a rotating distributor located inside said chamber and adapted to obturate said channel means intermittently;
• and wherein the width of said first channel means are greater than the width of the second channel means and said first channel means, forming perforation channels, are so adapted as to blow an air jet strong enough to perforate the film, whilst the second channel means, forming air-cushion channels, are adapted to blow an air jet of mechanical characteristics less than those of the first channel means and thus are unable to perforate the film, the air blown by said second channel means forming an air-cushion between the film and the wall of the skirt facilitating the flow of the film on the skirt.