rheology in, printable swirl out
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Color only — it never changes the shape. GLB and USDZ carry it; STL has nowhere to put color, so it downloads the same file for every flavor.
STL for slicing and printing. GLB for everything else — Blender, Android, three.js, most online viewers. USDZ opens straight into AR Quick Look on iPhone/iPad.
Millimeter units — it prints at the size shown. Watertight and manifold, but it rests on the rounded underside of the bottom coil rather than a flat facet, so give it a brim; open corkscrews will want supports.