unfolded original orientation upon exiting the heating station the bristles are cool and will stay in their new substantially parallel orientation either by the stress relief imparted to the bent bristles or by the additional bonding which occurs along the sides of the base string or both more details of the preferred apparatus 50 are shown in fig9 in particular the anvil 82 is mounted on a support plate 98 which is adjustably mounted on a backing plate 100 that is mounted under a opening 102 formed in the base plate 84 the backing plate 100 is provided with parallel slots 104 which facilitate longitudinal adjustment of the anvil 82 along the length of the belt path which transports the bristle string lateral adjustments of the anvil 82 can be accomplished by using shims to mount the anvil 82 to the support plate or by adjusting the position of the backing plate laterally inwardly or outwardly similarly the ultrasonic stack assembly 105 is mounted on a support plate 106 which is adjustably mounted on the backing plate 100 the stack assembly 105 is pressed into the bristles base string and anvil 82 by a pneumatic cylinder 107 the horn 80 is driven by a transducer 108 with enough energy to deliver sufficient thermal energy to the bristles to cause partial melting the transducer 108 horn 80 and anvil 82 are commercially available and their operation is well known using the preferred monofilament materials which are basically nylons and polyesters the ultrasonic energy preferably rapidly heats only the surfaces of the materials to their melting points before the energy is removed by the forward.