Selective laser sintering (SLS) is a promising additive manufacturing technique, where products ranging from gadgets to functional industrial parts are made out of locally heated layers of polymer powder. We developed a setup to study the sintering of two powder particles in a controlled environment and in-situ visualization of the sintering dynamics using optical microscopy and X-ray. Furthermore, we developed a computational model using an in-house code based on the finite element method to assess the mechanisms of viscoelastic flow and temperature dependency of two powder particles. Moreover, we investigated the isothermal crystallization of polyamide 12 (PA12) using in-situ synchrotron Wide Angle X-ray Diffraction (WAXD) during Flash-DSC measurements. With these results, we parameterized and validated a new numerical model to quantify the quiescent crystallization kinetics of the three important crystal structures of PA12.