Simulation of warpage induced by non-isothermal crystallization of co-polypropylene during the SLS process

Abstract

Abstract. Polymer processing using Additive Manufacturing Technologies (AM) has experienced a remarkable growth during the last years. The application range has been expanding rapidly, particularly driven by the so-called consumer 3D printing sector. However, for applications demanding higher requirements in terms of thermo-mechanical properties and dimensional accuracy the long established AM technologies such as Selective Laser Sintering (SLS) do not depict a comparable development. The higher process complexity hinders the number of materials that can be currently processed and the interactions between the different physics involved have not been fully investigated. In case of thermoplastic materials the crystallization kinetics coupled to the shrinkage strain development strongly influences the stability of the process. Thus, the current investigation presents a transient Finite Element simulation of the warpage effect during the SLS process of a new developed polyolefin (co-polypropylene) coupling the thermal, mechanical and phase change equations that control the process. A thermal characterization of the material was performed by means of DSC, integrating the Nakamura model with the classical Hoffmann-Lauritzen theory. The viscoelastic behavior was measured using a plate-plate rheometer at different degrees of undercooling and a phase change-temperature superposition principle was implemented. Additionally, for validation porpoises the warpage development of the first sintered layers was captured employing an optical device. The simulation results depict a good agreement with experimental measurements of deformation, describing the high sensitivity of the geometrical accuracy of the sintered parts related to the processing conditions.

Materials perspective of polymers for additive manufacturing with selective laser sintering

“…Investigations on theoretical description of thermodynamic and kinetic process were started with modeling and simulation recently. 11 It could be shown that especially for PA12 powder, a model combining the Nakamura 12 with the Hoffmann-Lauritzen 13 theory, the determination of the crystallization rate presents a very good agreement with experimental results. Coupling the thermomechanical properties with the crystallization theory results in a better understanding of how the warpage or curling of parts develops during a SLS build in future.…”

Nucleation and impact modification of polypropylene laser sintered parts

“…The incidence of the laser beam just pushes the material over the melting point. Subsequently, the material remains in a molten, slightly undercooled state, where crystallization takes place only very slow [5]. The retarded crystallization ensures that dimensionally accurate parts can be fabricated.…”

“…To prevent shrinkage and deformations as an effect of rapid crystallization, the temperature of the built surface needs to be kept between the crystallization and melting temperature of the material [2][3][4]. The incidence of the laser beam just pushes the material over the melting point.…”

Robust Numerical Resolution of Nakamura Crystallization Kinetics

“…Thus, equation (2) should be integrated in a global multiphysical integration scheme. For such applications, the reader may refer to references [16,17,19,20,21,22].…”

“…The Nakamura function G is plotted over [0,1] in Figure 1. This differential form, a first-order non-linear ordinary differential equation, is more natural to implement and solve numerically, especially in a multiphysical framework [16,17,18,19,20,21,22,23]. The initial state of the material (amorphous or semi-crystalline) should be given as an initial condition for α.…”