TC won't unfold a circle-to-square loft, although it's possibly the most common duct blend next to circular tube-to-tube entries. The most likely reason I can think of is because TC lofts this shape with compound curvature. In the past we got around this by exploding and manually laying out the facets, but with the unfold tool, if the face is constrained to single curvature, unfold will always work. Analysing the circle-to-square blend, there are four iscoceles triangular planes from the sides to the quadrants of the circle. The shoulders between them describe offset cones, which TC has the tools to build. Use a 90 degree arc as a quadrant of the circle, check the 2D base and offset cone boxes. Make the cone from quadrant arc to vertex of the square and the resultant surface is constrained to the single curvature parameter and will unfold.
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