If your ribbon is a flat strip of triangles, for example, it represents an object that can’t exist in reality. So we say it is non-manifold geometry. In the figure below, the leftmost is what I think is your ribbon. It is non-manifold as zero-thickness ribbons can’t exist. You either have to enclose the volume it occupies (middle) or make it a non-zero thickness (rightmost). Choose the one that suits your objective. There are VTK filters that can do both for you.
