3D-numerical Simulation of the Laser Sintering processing of thermoplastic powders for the prediction of microstructural features and part warpage – 3D-SLS

This project deals with the selective laser sintering (SLS) process, in which plastic parts are manufactured layer by layer by melting thermoplastic polymer powder with a laser beam. Like the other additive manufacturing processes, SLS possesses unique flexibility and reactivity compared to conventional polymer processing technologies, which reduces drastically time-to-market. However, the lack of repeatability of the process and the poorly controlled quality of parts cause reticence from industry. Moreover, the range of polymers available is too restricted. The objective of this project is to contribute to wider acceptance of SLS for plastic part production by overcoming some major scientific and technological barriers.

The main purpose is to build modelling and numerical simulation of the whole process in order to predict final part microstructural features (crystalline morphology, residual porosity) and to compute thermally induced stresses and deformations (warpage). This is a real challenge since theoretical knowledge of this process is still quite limited, and consequently no such simulation code exists yet. Moreover, 3D numerical simulation is unavoidable, and multiphysics coupling is required to include the thermal history of the material and the temperature dependence of its properties while computing the transformations (melting, coalescence, densification, crystallization).

Experimental work is needed to document all the physical parameters, the kinetic and constitutive equations to be introduced in the code for the description of the material behavior throughout its transformation. This will be done partly by current laboratory techniques. But model experiments on different scales (i.e. from the macromolecular scale to that of a sintered object) will be also needed to determine specific properties in the most adequate manner. For example, the laser/polymer interaction must be characterized in relation with the thickness and density of the powder, and gas diffusion will be also investigated in details.
Well-designed PA6 powders from SOLVAY will be the subject of the study. Material parameters as particle size distribution, morphology and polymer molecular weight will be varied to better understand their impact on final part properties and to test the robustness of the simulation. Conversely, the project will demonstrate the feasibility of PA6 parts and help optimize the material characteristics to reach optimal processability. This will contribute to fulfill the expectations of possible new end-users for technical materials.

The numerical simulation will be performed at the mesoscopic scale, using the concept of homogeneous equivalent material. The heat equation with source term and phase change enthalpy will be resolved by the finite element method with COMSOL® software and MATLAB® interface, in order to introduce the necessary coupling laws. Avrami and Schneider models will allow computing the crystalline fraction and the size of the spherulitic entities. Upon cooling, the liquid/solid transition will be assumed with reference to a critical crystalline fraction, and stresses and strains will be supposed to develop in the sintered layers due to shrinkage of the solidifying regions. Stress relaxation and residual strains leading to warpage will be modelled in the framework of linear viscoelasticity.

Validation of the simulation results will be assessed on the one hand by implementing in situ thermal instrumentation for the monitoring of temperature history. On the other hand, parts will be manufactured by varying process conditions (laser power, scanning velocity, scan spacing). They will be characterized for crystalline microstructure, porosity and geometry to be compared to computations.
Correlations between polymer features, process conditions and part properties will be clarified thanks to numerical simulations. The latter will help to gain better control of the process and to improve quality of parts.