product is discharged from the extruder preferably the isocyanate prepolymer is added in an amount of from about 0 5 to about 15 0 by weight of the preformed thermoplastic polyurethane preferably the isocyanate prepolymer of the invention comprises a prepolymer having free nco in the range of from about 10 to about 30 by weight of the prepolymer more preferably from about 15 to about 25 by weight of the prepolymer nco levels above the top of this range tend to also have high functionalities and result in prepolymers with poor flow characteristics similarly nco levels below the bottom of the range tend to bunch up and flow poorly the previously prepared thermoplastic polyurethanes that may be used according to the invention include in particular the known thermoplastic polyurethanes that may be obtained for example in accordance with u s pat no 3 963 679 and u s pat no 4 035 213 the disclosures of which are incorporated herein by reference using the starting materials described therein the polyester polyols used in forming the isocyanate prepolymer advantageously have an average functionality of from about 2 0 to about 3 0 their average hydroxyl number values generally fall within a range of from about 56 up to about 1200 suitable polyester polyols can be produced for example from organic dicarboxylic acids with 2 to 12 carbons preferably aliphatic dicarboxylic acids with 4 to 6 carbons and multivalent alcohols preferably diols with 2 to 12 carbons preferably 2.