Abstract: An improved valve assembly having a valve housing of cobalt-nickel or cobalt-chromium-tungsten based alloy or of titanium/titanium alloys, a sewing ring component of polyester held to the outer surface of the valve housing a disc accruder tiltably held on suitable supports within the housing, the valve disc being made of modified ultra high molecular weight polyethylene. The manufacture of the valve disc and modification of the UHMW-PE takes place simultaneously and comprises cryo machining of a blank of UHMW-PE, solid state compaction, annealing of the compact followed by further cryo machining, annealing and cleaning steps.