Investigation into mechanical and microstructural properties of polypropylene manufactured by selective laser sintering in comparison with injection molding counterparts

• A novel low-isotacticity polypropylene powder shows a good SLS processibility.
• SLS parts show high degree of crystallinity, existence of γ phase and coarse crystals.
• IM parts show low degree of crystallinity, absence of γ phase and fine crystals.
• SLS parts have higher strengths and moduli, but lower elongation and toughness than IM parts.

This work evaluated the processibility of a low-isotacticity polypropylene (PP) powder by selective laser sintering (SLS), and systematically analyzed and compared the melting and crystallization characteristics, crystalline structure, tensile properties and thermo-mechanical properties of the PP specimens fabricated by SLS and injection molding (IM). The results show that the PP powder has a nearly spherical shape, smooth surfaces, appropriate particle sizes, a wide sintering window and a low degree of crystallinity, consequently indicating good SLS processibility. In SLS, the molten PP continues to maintain at a high part bed temperature until the whole manufacturing process finished, thus demonstrating a low cooling rate. This gives rise to a high degree of crystallinity, formation of γ phase and coarse microstructure. On the contrary, in IM, the fully molten PP is rapidly cooled down to room temperature after injection, and thus show a higher cooling rate and rapid crystallization, leading to a lower degree of crystallinity, absence of γ phase and finer microstructure. Owing to these differences in crystallization characteristics and crystalline structure mentioned above, the SLS PP parts exhibit higher tensile strengths, tensile moduli and storage moduli, but lower elongation at break, toughness and glass transition temperatures, compared with the IM counterparts.

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