Abstract: A banded cylindrical piston for use in fluid cylinders is produced by hot-forming a relatively thin, flat washer made of polytetrafluoroethylene (PTFE) onto a grooved piston. A hot-forming sizing tube has a beveled entrance end on which the washer is initially preheated, a first section of a greater diameter than the end diameter of a finished unit for preliminarily elongating the preheated washer as it assumes sleeve shape and a final hot-forming second section in which the PTFE material is forced to flow under pressure into the piston grooves. The cross-sectional configuration of the groove or grooves is such as to enhance flowability of the material during hot-forming toward and into contact with a side of the groove nearest the head end of the piston to improve the mechanical gripping effect of band to piston. The side of the groove nearest the head end of the piston is preferably at an acute angle relative to an adjacent land on the periphery of the piston.

Abstract: A piston for use in a hydraulic cylinder which is subject to sudden and extremely high shock load pressures and high temperature operating conditions is provided with a plurality of interlocking axially-spaced circumferential grooves. A relatively thin, flat washer of heat-formable fluoropolymer is snapped over a retaining ring integral with the piston and the piston is then moved axially through a heated hot-forming cylinder, causing the washer to conform to the cylindrical shape of the piston. At least one circumferential groove has a side wall which is undercut relative to its cylindrical surface to provide a locking angle. Preheating the washer prior to hot-forming the band and forming a sloping ramp in opposition to the undercut side wall result in improved flow of the fluoropolymer toward the undercut side wall. An additional advantage of the preheating is the formation of an improved band skirt with an improved, sharply-defined sealing edge.