Abstract: An improved method of warm folding polymeric sheet material and apparatus for accomplishing the method. The method involves applying conductive heat to both sides of the material prior to folding and is accomplished in steps including: (a) positioning the sheet material; (b) positioning a mandrel plate so as to define a fold line for the sheet material; (c) positioning an exterior heater against the material along the fold line; (d) positioning an interior heater element against the material opposite the exterior heater; (e) and (f) applying preselected amounts of heat to the fold line area through the heaters; (g) retracting the interior heater; (h) mechanically folding the sheet material along a fold line; and (i) annealing the fold by applying additional heat through the exterior heater. The process may also be augmented by adding additional steps.The primary use of the invention is in folding polyvinyl chloride jackets for use in enclosing floppy disks.