STL file: what it is and when it is enough for printing

What is inside an STL file

The STL format was created in 1987 for the first printers from 3D Systems. A model is built from thousands of small triangles that together make up its surface. The more triangles, the smoother the curves and the bigger the file.

STL knows nothing about colour, material or whether the numbers are in millimetres or inches. It comes in a text and a binary version, and binary is the usual one because it is smaller. The Library of Congress also has a detailed description of the format.

When STL is enough and when to send 3MF

For a single-colour part, a bracket, a small box or a spare part, STL is all you need. Upload it, pick a material and strength, and you see the price straight away.

If you want a print in several colours, send a 3MF instead. It remembers which part of the model has which colour, and the exact dimensions in millimetres too. With a plain STL we would have to assign the colours by hand.

What to watch out for with STL

• Units. Most programs save STL in millimetres. If your model looks like a grain of sand or huge after uploading, it was saved in inches or metres. You can scale it up or down in the calculator.
• Holes in the mesh. The model has to be closed, otherwise the printer cannot tell what is inside and what is outside. We repair small errors automatically and will write to you about bigger ones.
• An overly fine mesh. An STL with millions of triangles from a 3D scan takes longer to upload but prints just the same. We accept files up to 50 MB.

How an STL turns into a price

After you upload, we really do prepare the model for printing on our server, in a program called a slicer. It works out how many grams of material and how many minutes of printing the part needs, and how many copies fit on one build plate. The price per piece and the delivery date come from that, not from a guess based on volume.