Exporting for print

You've finished modelling. Now the part has to leave Kapy as a file your slicer can chew on. There are two formats worth knowing, and they answer two different questions. STL is what you send to a printer: a triangle mesh, the universal print format. STEP is what you send to another engineer: a real solid that other CAD packages can keep editing. Pick the wrong one and you'll either lose all your parametric work or hand someone a faceted blob they can't open. Pick the right one and export is a non-event — which is how it should be.

STL for printing, STEP for handing off

STL describes your part as a skin of flat triangles. It throws away the history, the parameters, the smooth math of your curves — all of it. What's left is geometry a slicer understands directly, which is exactly why every slicer on earth eats STL. Kapy also exports 3MF where your slicer supports it; it's a tidier modern container, but the mental model is the same triangle mesh inside.

STEP is the opposite. It's a B-Rep solid — boundary representation, faces and edges defined by true equations, not triangles. A cylinder stays a cylinder, not a 64-sided prism. Send STEP to someone running another CAD tool and they can keep working with it. Send STEP to a slicer and most will refuse or quietly convert it badly.

The .gltf Kapy generates is not on this list. That's the format behind the in-app thumbnails and preview — it is not a print file.

Watertight and manifold, or it won't print

A mesh has to describe a sealed solid before a slicer can fill it. Two words cover that:

• Watertight — no holes in the skin. Every triangle edge is shared with exactly one neighbour, so there's no gap where "inside" leaks to "outside."
• Manifold — the mesh is a valid solid surface: no flipped normals (a triangle facing the wrong way), no two faces meeting along a single edge, no self-intersections where the surface passes through itself.

Hand a slicer a mesh that fails either test and you get the classic symptoms: missing top surfaces, a part sliced inside-out, walls in the wrong place, or a flat refusal to slice.

Why a clean solid exports clean

Here's the good news, and it's the reason parametric CAD is worth the trouble: a clean solid always exports a clean mesh. If your part is one well-formed B-Rep body, Kapy tessellates it into watertight, manifold triangles every time. You don't babysit the export.

The trouble starts upstream, in the booleans. When you union or subtract bodies that share a coincident face, or that overlap by a sliver, the boolean can produce a sloppy result — and a sloppy solid tessellates into a sloppy mesh with self-intersections or zero-area triangles. Kapy's mesh booleans use manifold-3d precisely to keep STL output watertight, but the cleaner your inputs, the less it has to fix. The fix lives in the model: make overlapping bodies overlap by a real amount (say 0.1 mm) instead of kissing exactly, and the boolean comes out clean.

Mesh resolution: faceted vs enormous

Tessellation has a dial: how closely the triangles hug the true surface, usually set by a chord deviation (how far a triangle's flat is allowed to stray from the real curve). It's a trade.

• Too coarse — a cylinder turns into a visible polygon. Holes look hexagonal, fillets look like steps. The file is tiny but the print is ugly.
• Too fine — every curve is razor-smooth, but the STL balloons to tens of megabytes, slices slowly, and the printer can't resolve detail finer than its 0.4 mm nozzle anyway.

Aim for the middle: smooth enough that you can't see facets at print scale, no finer. A deviation around 0.01–0.05 mm covers almost everything you'll print.

Get these right and the file you drop into The built-in slicer is one the printer can trust.