Selective laser sintering: A definition of the process and an empirical sintering model

Selective Laser Sintering{dollar}sp{lcub}rm TM{rcub}{dollar} (SLS) is a method of rapid prototyping three-dimensional objects from a computer image. The computer image is a CAD drawing or the result of a computer imaging process. The three-dimensional computer image is numerically sliced into thin layers, and the SLS machine then reproduces these slices, with a laser, by drawing the two-dimensional patterns onto thin layers of powder. Each new powder layer is placed above the previous one, building the three-dimensional object from the bottom up. The laser selectively adds heat to sinter the powder particles of that layer in a two-dimensional pattern, and to bond the surface layer to the underlying layer.; Determining the optimal set of operating parameters when forming a three-dimensional SLS object is important to produce objects which have uniform density and sharp surface features. This research includes the numerical modeling and predictive control of the SLS process. The sintering model is used to predict final part densities and ultimately to control the SLS machine.; Both finite-difference and finite-elements sintering models have been developed based on the conduction-diffusion equation. The model input includes SLS operating parameters, and the material specific information; such as thermal conductivity, specific heat, and sintering rate. The viscous sintering rate of amorphous polymer powders is measured in an isothermal experiment designed for this purpose. The model output includes void fraction and temperature profiles within a sintered layer as a function of time. Verification of the sintering model is done by comparing the sintering depths measured from samples made under varied conditions in the SLS machine, to those predicted by the model at the same conditions. Modeling of crystalline materials which involves the solution of a moving boundary problem is under current investigation.; The model can be used to optimize the SLS operating parameters for new amorphous materials after initial measurements of the material's sintering rate and solid thermal conductivity. The model also provides insight into a control strategy for the dynamic control of the laser power and scan speed to provide a more homogenous energy distribution when scanning a series of vectors that have variable lengths.