Preparing a file for 3D printing: formats and fixes

Preparing a file for 3D printing means exporting a closed, correctly scaled model in a format the printing software can read. At 3Descu that is an STL, OBJ, 3MF, STEP or STP file of up to 50 MB, drawn in millimetres, with a watertight mesh. Our instant quote calculator checks the format, the size, the scale and the mesh as soon as a file is dropped in, and it tells you what it found before it shows a price.

Get a free quote to see those checks run on your own file.

File formats the 3Descu calculator accepts

Format	What it stores	Where it usually comes from
STL	A surface as a mesh of triangles, and nothing else: no units, colour or material	Almost every CAD and 3D program
OBJ	A mesh of the surface	A common mesh exchange format
3MF	A mesh inside a compressed package	Default export of Bambu Studio and OrcaSlicer
STEP / STP	True curved CAD surfaces, not a triangle approximation	SolidWorks, Fusion 360, CATIA and most CAD tools

Format	Our advice
STL	Fine for most parts. Export in millimetres at a fine enough resolution
OBJ	Treat it like an STL
3MF	Good, with one part per file
STEP / STP	The best file to send when you have it. We convert it to a mesh automatically on upload

Native CAD files and other formats are not read directly. Export them as one of the five above first.

STL vs STEP: which file to send

STEP is the better file to send when it exists, because nothing has been thrown away yet. It stores the curved surfaces you designed. An STL stores an approximation made of flat triangles: export it too coarsely and a cylinder becomes a many-sided prism, which both prints faceted and genuinely measures less volume than the part you drew.

A STEP file is also usually far smaller than an STL of the same part, which matters when a dense mesh runs into the size limit. The glossary defines both formats, along with watertight, build volume and the other terms used below.

Units: always export in millimetres

An STL carries numbers, not units, and those numbers are read as millimetres. A model drawn in another unit therefore arrives the wrong size by a fixed factor: 10 for centimetres, 25.4 for inches, 1000 for metres. Volume grows with the cube of that factor, so a unit error never produces a price that is only slightly off.

The calculator raises a scale warning in three cases:

• The model is under 1 mm in every dimension. It was very likely exported in metres.

• The model is over 1000 mm in one dimension. It was likely exported in inches or centimetres.

• The largest side is narrower than one 0.42 mm bead of the nozzle. Nothing that small is printable, so the units are almost certainly wrong.

The fix is to set the document units to millimetres before exporting, rather than scaling afterwards. You can also send the file as it is and say so in the notes; we confirm the size before printing either way. Whatever the file, read the dimensions the calculator reports back before you read the price. You know roughly how big the part is, and the file may not.

Watertight meshes and open edges

A watertight mesh has no holes in its surface, so it encloses a definite volume. A mesh that is not watertight has no unambiguous volume, which means no unambiguous weight and no unambiguous price. An open edge is an edge that belongs to only one triangle, which leaves a gap. A non-manifold edge is shared by more than two faces, usually a boolean operation that did not resolve or a shell with an internal wall.

The calculator counts open edges against the number of triangles and responds in proportion:

What the calculator finds	What it means	What to do
Open edges on under 1% of triangles	Minor imperfections, repaired automatically in the quote and before printing	Nothing
Open edges on 1% to 10% of triangles	Volume estimated within about 5%; we repair before printing	Proceed
Open edges on 10% or more of triangles	Volume estimate may vary by about 15%	Re-export from CAD with "watertight" or "solid body" checked, or proceed and we repair it
Closed mesh with non-manifold edges	An unresolved boolean or an internal wall	Proceed. If the internal structure is deliberate, say so in the notes so we keep it

An open edge warning is information, not a rejection. We repair every mesh before printing and confirm the weight from the repaired mesh before you are invoiced. The how it works page covers what happens between the quote and the invoice.

File size limits

Limit	Value
Size per file	50 MB
Files per quote	20
Total size per quote	200 MB

The instant quote analyses the mesh in your browser, which is why it caps each file at 50 MB. A larger file is not too big to print, only too big to quote instantly. Export it as STEP, which is usually far smaller for the same part, or use the "Email this file to us" button that appears on the upload screen and a person quotes it by hand at the same pricing. More than 20 files, or more than 200 MB in total, go in a second quote.

Assemblies: one part per file

A single STL, 3MF or STEP file can hold several bodies. Measured as one mesh, they price as a single merged solid, which reads high, and the bounding box spans the whole assembly rather than any one part. Upload each part as its own file instead. One quote still covers the whole basket, and each part gets its own quantity. If an assembly arrives in one file anyway, we correct it in the human review, at the cost of a round trip.

What to do when the calculator flags a problem

Message on screen	Cause	Fix
Unsupported file type	The file is not STL, OBJ, 3MF, STEP or STP	Re-export as STL, 3MF or STEP
Over the 50 MB instant-quote limit	The mesh is too dense to analyse in the browser	Export as STEP, or email it from the upload screen
Does not contain a readable mesh	No solid body was exported	Re-export as STL or 3MF with the solid body selected
Appears to be damaged or incomplete	The export or download did not finish	Export again, or send a different format
Check the scale of this file	Units other than millimetres	Re-export in millimetres, or note it and we confirm the size
Open edges or non-manifold edges	The mesh is not watertight	See the table above; usually no action
Exceeds build volume	Larger than 500 x 500 x 500 mm	Split the part, or send it for manual review

Most design problems are cheaper to fix in CAD than on the machine. Wall thickness, overhangs and orientation are covered on the design rules page, and the process itself on the FDM 3D printing page.

What happens to your file after upload

Files are encrypted in transit and at rest in EU storage, and deleted automatically 90 days after upload. If your organisation needs an NDA before sharing a design, we sign it fast once the text is agreed.

Get a free quote by uploading the corrected file. Re-quoting costs nothing, and every quote is reviewed by a person before an invoice is issued.