Accelerated Non-Isothermal Powder Bed Fusion of Polypropylene Using Superposed Fractal Exposure Strategies

Laser-based powder bed fusion of polymers (PBF-LB/P) is predominantly based on the quasi-isothermal processing of semi-crystalline polymers. To overcome existing limitations in the range of polymers suitable for PBF-LB/P, the non-isothermal, support-free powder bed fusion of polymers by means of superposed fractal, phase-shifted exposure strategies is proposed. Using polypropylene as a model material, thermographic monitoring allows for observing cooling rates exceeding 50 K s−1, leading to temporary supercooling of the polymer melt. By varying the applied laser power, an interdependence of structural boundary conditions and applied exposure parameters on emerging temperature fields and corresponding crystallization temperatures can be derived. Resulting thermal process properties, assessed in situ, are characterized by the exposure-dependent crystallization of the polymer melt. Formed temperature fields implicitly influence the layer formation and formed crystalline modifications, yielding the exposure-dependent formation of α/β-PP. The presented results indicate the suitability of the proposed discretized exposure strategies for significantly increasing the build rate in support-free, non-isothermal powder bed fusion of polymers while significantly reducing the thermal exposure of applied materials compared to existing additive manufacturing processes. Allowing for the targeted modification of part properties, the proposed processing strategy represents the methodological foundation for the additive manufacturing of thermo-sensitive, functionalized multi-material systems and the integration of pharmaceuticals.