A process for post treatment of an ultrasonically welded seamed flexible imaging member belt including providing an elongated support member having a smooth flat supporting surface, providing a flexible belt having parallel edges and a welded seam extending from one edge to the other edge, the belt seam including a seam region including an overlap and two adjacent splashings, thermoplastic polymer material having a glass transition temperature and an inner and outer surface, supporting the inner surface of seam on the smooth flat supporting surface with the seam region of the belt held down by vacuum against and conforming to the flat supporting surface of the support member, contacting the seam with a heated surface, the contacting heated surface has a profile that is substantially parallel to the smooth flat supporting surface of the support member, heating the seam region with the heated surface to raise the temperature in the seam region to a temperature of from about 2null C. to 20null C. above the Tg of the thermoplastic polymer material, and compressing the seam with the heated surface with sufficient compression pressure to smooth out the seam. Apparatus for carrying out the process is also disclosed.