An iris valve for controlling the flow of bulk solid materials includes a flexible cylindrical sleeve, a control ring, and a polymeric isolator disc, all sandwiched between upper and lower body members. Rotating the control ring twists the flexible sleeve in an iris pattern to close an opening through the valve. Clamp bands attached to the upper body member and control ring include a machined helical pattern to securely grip the ends of the flexible sleeve, and a threaded handle locks the control ring in place against the body members.