STL and extensions

Abstract Three approaches to improve the free form fabrication process are discussed: (i) the use of approximation control parameters (ii) the development and use of standard transform language (STL) extensions and (iii) methods to handle exact geometrical models at the rapid prototyping (RP) level. The first two of these are concerned with improving the quality of the data exchanged, the third with improving the quality of the process itself. It is shown that the use of various STL approximation control parameters can improve the quality but does not always result in a good approximation. To improve this situation we propose two versions of an extended STL format with the following advantages: (i) smaller final models are obtained that allow faster geometry processing, (ii) the reconstruction of exact object geometry is possible which permits avoidance of volume distortion for consistent control of slice and layer approximations and (iii) a mechanism is provided that allows attachment of manufacturing information. We describe in pseudo-code and develop an ACIS implementation for the writing of the extended STL format, for the reconstruction of a solid from the extended STL file and for the consistent facetting of an exact model. Slice timing results show significant time reductions for the exact and consistent facetted models over the STL-based facetted models. The main conclusion, though, is that STL should be replaced with a STEP-based data exchange standard.