An apparatus for forming an inflatable closing plug has a winding head rotatable around a path defining a winding plane. The winding head is arranged to dispense one or more continuous lines from a supply of line via a guide arrangement. A preform is rotationally supported on an axis at a shallow angle with respect to the winding plane such than the winding plane intersects the preform. By providing a spreading arrangement to locate the windings on the preform over a band having a width perpendicular to the winding plane, a smoother distribution of windings can be achieved.