being so adjusted as to penetrate the foam of the mandrel to an extent a little beyond the eye 45 of the needle fig5 a 5b 5c the thread clamp is then released fig5 d and the needle rises by gradually releasing the thread fig5 e through the layers and thus forming a small unclosed loop 53 which is retained solely by the action of the foam material and friction in the layers just below the interface between the foam material and the first layer it will be understood that the elastic pressure exerted by the foam material in closing onto the loop after the withdrawal of the needle on one hand and the friction and gripping action of the threads in the superimposed layers on the other hand upon withdrawal of the needle are sufficient to retain the thread 50 which freely travels along the groove 44 of the needle in the course of this withdrawal after having moved out of the layers of thread the needle is raised above the surface of the layers a distance equal to the total thickness of the superimposed layers on the mandrel plus the stitching pitch i e the desired spacing between two stitches fig5 f the thread clamp is then actuated for stopping the thread in the needle the latter is advanced and the cycle is recommenced to form continuously a large number of loops 53 with the same thread 50 the pins implanted in the mandril may be removed as the work progresses so as to avoid hindering this progression when the operation for introducing threads through the layers is terminated the assembly is impregnated either by leaving the reinforcement element on the mandrel or by previously removing the mandrel in the conventional manner in this respect the.