Bastech, Inc. Rapid Prototyping • Rapid Manufacturing

Tech Tips

What is a STL File?

Simply explained, a stl file is a format used by Stereolithography software to generate information needed to produce 3D models on Stereolithography machines.In fact, the extension "stl" is said to be derived from the word "Stereolithography."

A slightly more specific definition of a stl file is a triangular representation of a 3D object. The surface of an object is broken into a logical series of triangles (see illustration at right). Each triangle is uniquely defined by its normal and three points representing its vertices.

Surfaces are represented
by triangles.

The stl file is a complete listing of the xyz coordinates of the vertices and normals for the triangles that describe the 3D object.


What makes a "bad" STL file?

In short, a good stl file must conform to two rules. The first rule states that adjacent triangles must have two vertices in common. Second, the orientation of the triangles (what side of the triangle is in and what side is out) as specified by the vertices and normals must agree. If either of these two criteria are not met, problems exist in the stl file. Minor gaps and inconsistencies can usually be fixed by specialized STL-handling software. More significant problems usually require going back to the original CAD model for modifications.

Often a stl file can be termed "bad" because of translation issues. In many CAD systems, the number of triangles that represent the model can be defined by the user. If too many triangles are created, the stl file size can become unmanageable. If too few triangles are created, curved areas are not properly defined and a cylinder begins to look like a hexagon (see example below).

Faceted stl file translated
with coarse tolerance

File translated with
fine tolerance

When creating a stl file, the goal is to achieve a balance between unmanageable file size and a well-defined model with smooth curved geometries.


How do I create a STL file?

Most CAD software packages offer stl conversion add-ins.If you have access to conversion software, stl translation is relatively simple as long as you have a clean-surfaced 3D model and a high-end computer.Traditionally when converting to a stl file, the user is given several options for resolution (sometimes called chord height, triangle tolerance, etc.). Depending upon the size of the model, the geometry of small details, and the overall curvature of the part, the tolerance can typically be set to .001 inch for average models. Small parts or models with fine details may require a tighter tolerance.

If you do not have access to stl conversion software, Bastech is in most cases able to create stl files for you. We accept parasolid, surfaced igs, step, dxf, Pro-Engineer and Unigraphics part files, among others. (Additional CAD clean-up may be required if file translation is not clean between software formats.)

If you have any questions about this information or need assistance creating stl files, please feel free to contact Bastech and one of our engineers will be glad to assist you.