PURPOSE: To improve jointability, by bending a sectorial or trapezoidal and band-shaped filter medium so as to form its cross section in the longitudinal direction into a continuously wavy shape and further curving it to joint both ends to each other, in forming both side edges into rugged shapes.
CONSTITUTION: A band-shaped filter medium 1 has a sector shape basically; folded lines 2 are set to it along concentrically circular arcs (b)-(b'), (c)-(c'),... arranged at equal intervals between a short circular arc (a)-(a') and a long circular arc (k)-(k'), and rugged parts corresponding to each other are provided to both side edges. Said band-shaped filter medium 1 is bent along the folded lines 2 so as to form its cross section in the longitudinal direction into a continuously wavy shape. Next, the medium 1 bent in such a manner is curved by putting the circular arc (a)-(a') to the circumferential side to joint together the parts 3 equivalent to both side edges, respectively. At this time, the jointability is improved because the contacting surface is increased by the rugged parts provided to the side edges of the filter medium.
AKATSUCHI HAJIME
FUKUDA TOSHIAKI
MIYAGAWA SUSUMU
