Why 3D Print Files Get Rejected (and How to Fix Yours)

Every file we receive goes through a pre-print review before anything is sliced. Most of the files that go back to the customer fail for one of four reasons, and all four are fixable in your CAD software in a few minutes. Here is the checklist we run, so you can run it first.

01 The Mesh Is Not Watertight

A printable model has to be a closed, manifold solid: every edge shared by exactly two faces, no holes in the surface, no stray geometry floating inside the part. Sculpting and mesh-editing tools produce open meshes easily, and boolean operations in CAD can leave zero-thickness faces behind.

A slicer handed an open mesh either guesses at the interior or produces a hollow shell where you expected solid material. That is the single most common cause of a print that looks correct on screen and fails on the bed.

• In Fusion 360: use the mesh repair tools, or export from the solid body rather than from a mesh conversion
• In Blender: the 3D Print Toolbox add-on reports non-manifold edges and selects them for you
• Free check: Microsoft 3D Builder and Meshmixer both auto-repair most common mesh faults

02 The Units Are Wrong

STL files carry no unit information. They store numbers, and the slicer assumes millimetres. If you modelled in inches, a 4 inch bracket arrives as a 4 mm bracket, and a part designed in centimetres arrives ten times too small.

03 Walls Below the Minimum

Thin walls are the most common redesign request we send back. A wall that renders beautifully on screen can be physically thinner than the nozzle that has to produce it.

Below those minimums, walls either fail to form during printing or survive the print and snap during support removal. If a thin section is structural, add a rib rather than thickening every wall: it costs less material and prints faster. Our full design guidelines cover the rest of the geometry rules.

04 The Part Is Bigger Than the Bed

Our standard build volume is 256 × 256 × 256 mm. A part that exceeds it on any axis cannot be quoted instantly, but it is not a dead end: we routinely split larger models into interlocking sections, print them separately, and bond them into a single assembly. Send it through the RFQ form and we will scope the split.

05 Which Tool Takes Which File

The two ordering paths accept different things, and picking the wrong one accounts for a fair share of failed uploads.

For mechanical parts with tight fits, sending STEP through the RFQ path is worth the extra day. STEP preserves exact curved geometry, where STL approximates every curve with flat triangles.