Selective laser sintering is a 3D printing technology with the potential to fabricate lightweight products with complex design. During the manufacturing process polymer particles are locally heated with a laser beam and thereby melt and sinter together. The final material macrostructure and its porosity as well as the crystal microstructure depend on the processing conditions. Whereas most studies focus on ex-situ characterizations of the printed part, little is known about the time-dependent structure evolution during printing. Recently, we have developed a setup that allows in-situ visualization of the sintering process on the level of a particle pair, both by optical microscopy as well as X-ray characterization [1,2]. From the recorded optical images, the growth of the neck radius formed between both particles is analyzed as a function of time. Moreover, the time-resolved 2D-WAXD images allow to analyze the crystallization kinetics and crystal type formation. In the present work, we study the laser sintering of PA12 particles, by combining our in-house developed setup with high flux synchrotron radiation. The effects of the bed chamber temperature and particle size on the sintering dynamics and crystallization kinetics are studied. Infrared imaging of the particles during sintering shows that both parameters affect the temperature profile in the particles and studies of the neck growth process show the effects on the flow profile during sintering. As a result, both the crystallization kinetics as well as the crystalline morphologies depend on these processing conditions. Our approach of combining a full material characterization with sintering experiments under well-defined conditions has resulted in a general understanding of the effects of material and process parameters on laser sintering, which provides a major step towards rational design of laser sintering processes. [1] Hejmady et al. Soft Matter 2019, 1373 [2] Hejmady et al. Rev. Sci. Instr. 2019, 083905