IGES is older and often made of loose surfaces
IGES, the Initial Graphics Exchange Specification, was first published in 1980 as a U.S. national standard for passing CAD geometry between systems, and STEP was later developed as its successor. Many IGES files are a set of separate surfaces. Nothing in such a file says which side is the outside or that the surfaces close into a body.
That matters for printing. When surfaces arrive separately, some triangles can face inward after tessellation. On our test cube built from 6 separate surfaces this happened, and Edit → Repair turned every normal outward again.
How the units reach millimetres
Every IGES file declares a unit. The viewer shows it and scales the part to mm: our 1 inch test cube saved as IGES reads 25.4 mm on every side.
When a header and the numbers disagree, a part balloons. If the size runs past 2 m, the warning Check the units appears. The switch Numbers are millimetres undoes a false inch label; on the AS1 test assembly it turns 5080 back into 200 × 87 × 150 mm.
Converting IGES to STL, step by step
- Drop the .igs or .iges file, or press Open File.
- Wait for the one-time engine download on your first CAD file (7.6 MB, 3.1 MB gzip).
- Read the mesh check in Info. Flipped triangles or open edges mean a Repair pass.
- Press Download STL, or Other formats for 3MF or OBJ.
Triangle density for surface models
Coarse, Medium and Fine set how many triangles each surface gets. A rounded cube in our tests went from 48 to 84 to 172 triangles across the three. Freeform surfaces such as car body panels gain the most from Fine, and boxy brackets look the same at Coarse.
What IGES keeps and loses as STL
| Property | In the IGES | In the STL |
|---|---|---|
| Surfaces | exact, often trimmed one by one | triangles at Coarse, Medium or Fine |
| Unit | a flag in the global section | none; values written as millimetres |
| Face direction | may differ from surface to surface | outward after Repair |
| Closed body | implied, not guaranteed | watertight yes or no, shown in Info |
Troubleshooting an IGS file
If the file does not open at all, the message names the file and says it could not be read. Some programs write IGES with an .igs extension and others with .iges; both names work, so a failure usually means a damaged or truncated export. Ask for the file again or for a STEP version.
If it opens but looks striped light and dark, the normals are mixed. If the mesh check shows many open edges along the seams where surfaces meet, the surfaces do not quite touch. Repair welds vertices that sit at the same spot and closes the narrow gaps; wide gaps stay open and are listed in its message.
When Repair is not enough
Repair welds duplicate vertices, makes normals consistent and closes simple holes with a fan of triangles. A fan suits flat or small holes, such as narrow gaps between neighboring surfaces. A large curved gap will not get a smooth surface patch.
For those parts, ask for a STEP export from the source CAD, or open the IGES in the IGES viewer to see which patch is missing. The STL repair page lists everything the fixer does.
After IGS to STL: next steps
To know what the part will cost in filament, the 3D print cost calculator reads its volume. If you also receive STEP files, STEP to STL works the same way. There is no STL to IGES export; for a CAD exchange file from a mesh, STL to STEP writes a faceted solid.
Questions and answers
What is the difference between IGS and IGES?
None in content. IGS is the three-letter extension of the Initial Graphics Exchange Specification, and the picker here accepts both.
Why does my converted IGES look dark or patchy in places?
Some surfaces came in with their normals reversed. Run Edit → Repair; it makes all triangles face outward, as it did on our 6-surface test cube.
Can I convert IGES to STL online without sending the file?
Yes. OpenCascade tessellates the file inside your browser tab, and the IGES never goes to a server.
Is the STL watertight after conversion?
It depends on the IGES. Info shows watertight yes or no per body; Repair closes small gaps, while large missing surfaces need a better export.
Should I ask for STEP instead of IGES?
If the sender can export STEP, ask for it. STEP was developed as the successor to IGES and stores assemblies and the length unit. An IGES converts here too; expect to run Repair more often.
Why does the first IGES file take longer to open?
The OpenCascade engine loads on the first CAD file: 7.6 MB, or 3.1 MB compressed, from our own domain. After that the browser keeps it, and the next IGS or STEP file starts without the wait.