Patent ID: 7128861

Claim:
A method for producing hollow fiber membranes having a round cross section, which comprises steps of: melt kneading a mixture comprising polyvinylidene fluoride and an organic liquid or a mixture comprising polyvinylidene fluoride, an organic liquid and an inorganic fine powder; extruding the kneaded mixture to form hollow fibers; and extracting the organic liquid or the organic liquid and the inorganic fine powder from the hollow fibers, wherein the method includes the steps of drawing the hollow fibers before termination of the extraction or after termination of the extraction and then shrinking the fibers, wherein the drawing is carried out by take-off machines each comprising a pair of opposed infinite track type belts of which each outer surface is made of an elastic material, one of said machine provided on an upstream side of the drawing and the other provided on a downstream side of the drawing, and wherein the hollow fibers are interposed between said opposed infinite track type belts and both the belts are moved at the same speed in the same direction to straightly carry the fibers in each of the take-off machines, and fiber carrying speed in the take-off machine on the downstream side is faster than the fiber carrying speed in the take-off machine on the upstream side, and wherein the shrinking is carried out by take-off machines each comprising a pair of opposed infinite track type belts of which each outer surface is made of an elastic material, one of said machine provided on an upstream side of the shrinking and the other provided on a downstream side of the shrinking, wherein the hollow fibers are interposed between said opposed infinite track type belts and both the belts are moved at the same speed in the same direction to straightly carry the fibers in each of the take-off machines, and fiber carrying speed in the take-off machine on the downstream side is slower than the fiber carrying speed in the take-off machine on the upstream side.