Fused filament fabrication (FFF) is one of the most commonly used techniques in additive manufacturing (AM) due to its operational flexibility, easy maintenance, low energy demands, low material wastage, and economic viability. In spite of notable developments in the FFF process, there are currently only a handful of materials that can be printed consistently using this technique. Polypropylene (PP) has been used in a wide range of applications due to its attractive physical, mechanical, chemical, and optical properties. The crystallization behavior of PP is relatively fast which enables it to be processed effectively using extrusion and injection molding processes. However, it is of paramount importance to control the crystallization process and resulting morphology in order to successfully 3D print parts using PP since it crystallizes relatively rapidly and traps residual stresses in the additively manufactured parts, which in turn results in part warpage and poor dimensional stability.<br/>The goal of our work is to enable FFF of PP with mechanical properties and quality approaching injection molded PP parts. We present the effects of low molecular weight miscible additives on the thermal behavior, crystallization, and morphology of PP blends. We are able to lower the crystallization and melting temperatures of the blends compared to neat PP which enables additional diffusion during solidification in FFF. Polarized optical microscopy highlights the differences in crystalline nucleation and growth, which particularly impact the structure at the interlayer boundaries between deposited roads. The combined modifications in crystallization rates, times, and morphology significantly impact the z-axis adhesion and residual stress state, which directly affect mechanical properties and part warpage. Posting printing the parts were annealed that significantly improved polymer chain diffusion and interlayer adhesion. As a result, the tensile modulus and strength of the printed blends approaches over 90% of injection-molded PP parts. We can leverage these results to modify processing conditions to optimize mechanical properties of the thermoplastics generated using FFF.

3D printing with polypropylene blends

ACS Fall 2020 Virtual Meeting

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