STL is the everyday file format for 3D printing. Here is what it stores, why slicers love it, how it compares to OBJ and STEP, and how Formo builds a valid printable STL from a text description.
An STL file describes the outside of a 3D object as many flat triangles stuck together. Your slicer loads that shell, checks that it is a solid, then slices it into layers your printer can deposit or cure. The name comes from early stereolithography workflows; today the same extension is used for FDM, resin, and most hobby print pipelines.
STL does not remember how the part was designed — no sketches, no features, no color. It is a snapshot of the surface geometry. That simplicity is why it travels well between tools: CAD exports it, AI generators emit it, and slicers import it without needing the original app.
Print software needs a closed surface it can turn into toolpaths. Triangle meshes are a common way to approximate any solid shape. Because STL caught on early and stayed simple, printer makers, slicer authors, and file-sharing sites all speak it. If someone says "send me the STL," they usually mean "send a printable solid of this part."
Newer formats such as 3MF can carry more print metadata (colors, multiple objects, unit hints). Many workflows still start from STL because support is universal. Formo can provide STL alongside other formats when you need them — the core idea is the same: a solid you can slice.
STL— triangle mesh for printing. No built-in color or CAD history. Best default when the goal is "slice and print."
OBJ — also a mesh, often used in graphics and rendering. It can store more appearance data, but for printing you still need a solid, closed shape. Many people convert OBJ to STL (or import OBJ into a slicer) when the mesh is already printable.
STEP — precise CAD geometry (curves, assemblies, editable features in CAD software). Ideal when an engineer will open the part in SolidWorks, Fusion, or similar. Home slicers typically expect a mesh (STL/3MF) rather than a raw STEP as the main print file.
You do not have to draw every triangle. Formo takes a plain-language description — size, features, how parts should fit — and builds a mesh that is checked for watertightness and dimensional consistency before download. The result is an STL (and related formats) meant for your slicer, not a render-only sculpture.
If you are new to printing, start with a clear size in millimeters and open the file in any common slicer to confirm scale. If you already print, treat Formo like a fast way to get a solid when you do not want to open CAD for a one-off part.
STL (often called Stereolithography) is a 3D file format that stores the outer surface of a solid as a list of triangles. Slicers read that mesh, cut it into layers, and turn those layers into printer instructions. It does not store color, materials, or editable CAD history — only the shape.
Almost every consumer slicer and print service accepts STL. The format is simple, old, and widely supported, so a single download works across FDM and resin workflows. Other formats (3MF, OBJ, STEP) are useful too, but STL remains the common interchange file when you just need to print a solid part.
STL is a triangle mesh aimed at printing. OBJ is also a mesh and can carry more appearance data (like UVs or materials) but is still not a full CAD model. STEP is a precise CAD exchange format with exact curves and features — better for engineering edits, not what most home slicers expect as the primary print input. Formo focuses on printable meshes (STL/3MF) and can also provide STEP when you need a CAD-friendly solid.
For slicing, the mesh should be a closed solid (watertight) without open holes or non-manifold edges. Units and scale should match what you expect in millimeters. Even a valid STL can still be hard to print if walls are too thin or overhangs are extreme — that is design for printing, not just file format.
Yes. Formo turns a plain-language part description into a watertight mesh and lets you download STL (and related formats) after validation. You describe dimensions and intent in chat instead of drawing the triangles by hand.