Simulation and experiment validation of temperature control during cooldown phase of selective laser sintering (SLS)

Thermal stresses, induced by inhomogeneous temperature distribution inside a part during the cooldown phase of selective laser sintering, can be a major cause of part rejection for geometric deviation from its as-built specification. A validated cooldown simulation can provide predictions of temperature distribution in both parts and part cake which might enable alternative cooling profiles to reduce the likelihood of such rejections. This work describes experiments and comparative simulations developed to validate using predictive tools to assist in developing cool down control profiles for an SLS machine. In the experiments, thermocouples were inserted inside the part cake to monitor temperature at preselected locations during cooldown. The results from initial experiments and simulations were compared at these locations, to obtain improved estimates of uncertain powder conductivity and convective heat transfer parameters. The resulting simulation were then compared with independent experiments to evaluate the utility of such simulations. Though diffusion time in the part cake prevents active closed loop control in cooldown based on thermal measurements at the part, the simulation can be used to determine an open loop control profile of the build box heaters based on temperature gradient and resultant stresses inside the part