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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 …