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GLB to STL output has inverted normals — prints come out hollow

Liam O'Brien

81d ago222 Views2 Replies

I downloaded a model from the marketplace and converted it from GLB to STL for 3D printing using the converter tool. The STL file previews fine in the 3D viewer but when I slice it in Cura, half the faces are inverted and the slicer thinks the model is hollow inside.

I've tried the same model in PrusaSlicer and same issue. The original GLB looks perfect in the viewer. Is the converter flipping normals during conversion?

Anyone else had this problem? Any workaround?

Replies (2)

Best Answer

Marcus Rivera

81d ago

This usually happens when the original GLB model has double-sided materials. In glTF/GLB, double-sided rendering hides the fact that some faces have inconsistent winding order. But STL doesn't have a "double-sided" concept — it relies purely on face normals for inside/outside.

Before converting, try opening the GLB in Blender:

Select the mesh
Go to Edit Mode
Select All (A)
Mesh → Normals → Recalculate Outside
Export as GLB again

Then convert that fixed GLB to STL.

Liam O'Brien80d ago

That fixed it perfectly. Recalculated the normals in Blender and the STL now slices correctly. The print came out solid. Thanks Marcus!

Aisha Mohammed

80d ago

I hit the same issue last month. Another option if you don't want to open Blender every time: the remesh tool on iLove3D can sometimes fix this automatically because it regenerates the entire mesh topology. Worth trying if the model isn't too detailed.

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GLB to STL output has inverted normals — prints come out hollow

Replies (2)