a method of sealing a tube of flexible thermoplastics material by welding in which opposite sides of the tube are pressed flat between two jaws . the flattened tube thus has two opposed flat walls which have inner faces in contact , and these flat walls are connected at two opposite edges by folds caused by the flattening of the tube . the jaws are maintained a fixed distance apart , and one is rapidly reciprocated along its longitudinal direction by a vibrator , relatively to the other jaw . this causes the contacting flat walls to rub and become so heated by friction that the contacting plastics faces melt . these molten areas unite in a weld when reciprocation of the jaw is stopped and the plastics cools .