Development of material-adapted processing strategies for laser sintering of polyamide 12

Laser sintering of polymers (LS) is one of the most promising additive manufacturing technologies as it allows for the fabrication of complexly structured parts with high mechanical properties without requiring additional supporting structures. Semi-crystalline thermoplastics, which are preferably used in LS, need to be processed within a certain surface temperature range enabling the simultaneous presence of the material in both, the molten and solid state. In accordance with the most common processing models, these high temperatures are held throughout the entire building phase. In the state of the art, this leads to high cooling times and delayed component availability.

In this paper, process-adapted methods, in-situ experiments and numerical simulations were carried out in order to prove that this drawback can be overcome by material-adapted processing strategies based on a deepened model understanding. These strategies base on the fact, that the crystallization and solidification of polyamide 12 is initiated a few layers below the powder bed surface at high temperature and quasi-isothermic processing conditions. Therefore, isothermal crystallization and consolidation behaviour is analyzed by process-adapted material characterization. The influence of temperature fields during laser processing was analyzed in dependence of part cross-section, layer number and fabrication parameters and correlated to the resulting part properties. Furthermore, the possibility to homogenize the parts thermal history by controlling the part cooling is highlighted by a simulational approach. The authors show that the material-dependent solidification behavior must be taken into account as a function of the geometry- and layer-dependent temperature fields and demonstrate a major influence on the material and component properties. From these findings, new processing strategies for the laser exposure process as well as for the temperature control of the build chamber in z-direction arise, which allow for the acceleration of the LS process and earlier availability of components with more uniform part properties.