This Converter reads and writes stereolithography models in the file format defined by 3D Systems for their fabrication machines.

It supports both the binary and ASCII formats. It adds two entries in the conversion lists:

Stereolithography ASCII .stl
Stereolithography binary .stl

Stereolithography model files often use the ".STL" file name extension, although some programs use ".SLA".

Geometry
This Converter reads both binary and ASCII stereolithography files. Attribute bytes in binary stereolithography files are skipped.

In stereolithography files, all objects are composed of triangles. Orientation of triangles is important to stereolithography. Each triangle's normal is used to determine the inside and outside of the real-world model.

If you are using stereolithography files as a medium of exchange between programs, this may not be a concern. If you intend to create real-world models, please consult with your service bureau. They will describe the limits of the model-making process.

Material attributes
Stereolithography files do not any materials, so all triangles are imported with a white material.

Forcing recognition
Interchange's automatic recognition of files is foiled by the free-form nature of stereolithography files. They do not contain a fixed, recognizable set of bytes at the beginning of the file.

Interchange will attempt to read any stereolithography file that begins with the word "solid", or whose header contains the words "facet", ".STL", or "outer".

If Interchange cannot recognize your stereolithography files, consider using a text editor or a binary editor to change the description at the start of the file to contain these words. Be sure to operate on a copy of the file. Or, see the Interchange Options dialog description for more information on forcing recognition of stubborn objects.

Binary stereolithography files must contain 200,000 triangles or less. When creating binary STL files, this Converter does not enforce the 65,535 polygon limit defined by 3D Systems.

Options dialog

Geometry constraints
Stereolithography is an exacting process. If you are building a plastic prototype, you will most likely give the resulting stereolithography file to a service bureau. Their system will perform a series of checks and verifications of your geometry.

This Converter does not attempt to perform any of these validity checks. You may need to edit your original model in order to make a stereolithography file that will result in a good translation to real-world materials. Keep in mind the following common constraints:

Move your model to the positive quadrant so all coordinates are positive. Build thin vertical walls to support unwieldy parts of the model that might sag during construction. Remember to include drains in hollow regions if your service bureau is using a liquid polymer process. Instead of a lone polygon hanging into space, you need a thin rectangular box in the shape of the polygon.

Your model must describe a topologically unambiguous region in space. The normals of each triangle must point outward. Points in adjacent triangles must be at exactly the same location, otherwise there is a gap.

Ask your service bureau about the resolution of the laser beam. Don't bother to make details smaller than the minimum size.

Background
The stereolithography format was defined by:
3-D Systems
26081 Avenue Hall
Valencia, CA 91355
(805) 295-5600
(805) 257-1200 FAX
www.3dsystems.com for their prototyping hardware. It is also used in Parametric Technologies Pro/ENGINEER software and Hewlett-Packard's Solid Designer, so it can be a useful exchange format when IGES or DXF is not in common between the source and destination 3D programs.

One service bureau that produces stereolithography models is:
Alliant Techsystems
MN11-1885
600 Second Street Northeast
Hopkins, MN 55343
(612) 931-5373
www.atk.com