Standard Tessellation Language (STL) is a file format native to the Stereolithography software created by 3D Systems. This file format is supported by many CAD software packages and is widely used for rapid prototyping and computer-aided manufacturing (CAM).STL is a facet-based representation that approximates surface and solid entities only with triangles (i.e., STL files describe only the surface geometry of a three dimensional object). Entities such as points, lines, curves, and attributes such as layer and color, in the CAD systems will be ignored during the output process.An STL file consists of a list of facet data. Each facet is uniquely identified by a unit normal (a line perpendicular to the triangle and with a length of 1.0) and by three vertices (corners). The normal and each vertex are specified by three coordinates each, so there is a total of 12 numbers stored for each facet. This data is used by a slicing algorithm to determine the cross sections of the three-dimensional shape to be built. This format approximates the surfaces of a solid model with triangles. With creating STL files, there are some common errors that will lead to poor part build or delayed leadtimes because the files are unusable. Some of the errors are:
Manifold (Leaks)
Although it is not explicitly specified in the STL data standard, all facets in a STL data file should construct one or more closed volume entities. If a file is not ‘water-tight’, it is said to contain ‘leaks’. When a ‘leaky’ STL file is processed by slicing algorithms, the algorithms may not correctly detect the error, and as a result produce slice boundaries that are not fully closed. When the erroneous slices are used in the rapid prototyping process, the laser beam, cutter or whatever tools that generate the slice will ‘escape’ from the openings of the boundaries. Some recent pre-processing software such as 3D LightYear by 3DSystems, will try to correct the error by adding extra segments to link up broken boundaries.Degenerated Triangles Degenerated triangles are very thin triangular facets in the tessellated solid whose three defining vertices almost lie in a single line. The sides of degenerated triangles could create extra vectors in a given layer. These are usually the result of the design application’s attempt to tessellate a complex geometry.
Narrow or Wide Gaps
Violations of the Gapless Exterior Rule, narrow or wide gaps are empty spaces between defined triangular facets in the tessellated solid. These are also typically the result of the design application’s attempt to tessellate a complex geometry.
Inverted Triangles (Incorrect Normals)
Inverted triangles are a violation of the Right-Hand Rule for the formation of valid STL files. These are triangular facets in the tessellated solid whose normal vector (based on the order in which the vertices of the triangle are listed in the STL file) appears to be oriented in the opposite direction of the normal vectors of adjacent triangles. In other words, when examining the continuity of a region’s exterior surface, these triangles (based on the respective directions of their normals) appear to be oriented such that their exterior surfaces are facing inward.
Unmatched Triangle Sides
Unmatched triangle sides are a violation of the Vertex-to-Vertex Rule for the formation of valid STL files. These are triangles whose vertices touch the edges rather than the vertices of adjacent triangles.These are just some of the issues that arise from the creation of STL files. In the end, it is best to consult an expert during the file creation process if you experience any issues. Most issues can be solved pretty quickly…you just need to know what to look for.
