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. Unfortunately, the mechanical properties of sintered parts are not as good as those of products fabricated by conventional methods. Therefore, we need to find the weakest link in the relation between the SLS process and the mechanical properties. For this, we developed a setup to study the sintering of two powder particles in a controlled environment. With this setup, we are able to precisely locate the two particles, subject them to the main features of a SLS device, and at the same time allow 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. In this model, the material and process parameters can easily be varied to study the effect on the sintering. 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.