Patent ID: 7691299

Claim:
A process for the production of an expanded polytetrafluoroethylene artificial blood vessel product including the following steps: 1) extruding a mixture, which comprises unsintered fine powder of polytetrafluoroethylene and a lubricant, into a predetermined shape by a ram extruder; 2) stretching the resulting extrusion product; and 3) sintering the thus-stretched product, wherein in said step 1, said extrusion is conducted using the ram extruder comprising an extrusion ram, a mandrel, a ram tip, a core pin attached to the mandrel, a barrel, a master die and a die orifice attached to the master die, wherein the extrusion ram, the ram tip, the mandrel, and the core pin are coaxially located within a space formed by the barrel, the master die and the die orifice, wherein the barrel and master die are combined together into a jointless, continuous unitary structure, and wherein the ram extruder is constructed to facilitate the mounting and dismounting of the jointless, continuous unitary structure on a main body of the ram extruder, in said step 1, the mixture which is preformed into a billet is loaded into the barrel portion of the jointless, continuous unitary structure through an opening thereof, and then, the jointless, continuous unitary structure is mounted on the main body of the ram extruder, in said step 1, said extrusion is a high speed extrusion and is conducted under such conditions that an extrusion speed determined from formula (a) becomes in a range of 19 m/min to 100 m/min, and said extrusion reduction ratio is set in a range of 250 to 700: extrusion speed(m/min)=[extrusion reduction ratio (dimensionless)×ram speed (mm/min)]/1,000 (formula (a)), in said step 2, said stretching is conducted by unidirectional stretching the resulting extrusion product at a stretch ratio of 4.5 to 15 times, and in said step 3, said sintering is conducted while stretching or subsequent to the stretching step, wherein the expanded polytetrafluoroethylene artificial blood vessel product has: i) an axial tear strength of not lower than 6,000 gf/mm as calculated in accordance with the following formula: L/[T×(V/100)]where L (gf) is an axial tear load, T (mm) is a wall thickness, and V (%) is a volume ratio of resin; ii) a fibril length of not shorter than 40 μm; iii) a porosity of not lower than 70% is obtained; and iv) a circumferential variation in thickness of not greater than 10% is obtained.