a method of making a catheter balloon , and a balloon formed thereby , in which a polymeric tube is radially expanded in a mold having a wall with an outer surface and an inner surface defining a chamber , and having at least a section with one or more channels in the wall . the channels result in improved transfer of heat to the polymeric tube within the chamber of the mold during blow molding of the tube to form a balloon , so that the tube is heated more quickly and evenly . one aspect of the invention is directed to a balloon having sections with improved thin walls .