The purpose of this study is to describe a technique for increasing the mechanical strength and toughness of three-dimensional (3D) printed thermoplastic parts by enclosing them within a conformal, dissolvable support shell. The mechanical property gains are achieved by subjecting the 3D printed parts to a post-print thermal annealing process at a temperature above the Tg of the core polymer. The shell mechanically supports the part during the annealing process so that the as-printed part geometry is maintained; after annealing, the support shell is dissolved to leave behind a high-strength, annealed thermoplastic part.
Parts are printed using acrylonitrile butadiene styrene (ABS) and shelled with polyvinyl alcohol (PVA) as part of the printing process. After printing, parts are annealed at a temperature (135°C) above the glass transition temperature (Tg) of ABS for 72 h, and then immersed in water to dissolve the support shell.
Coupon testing demonstrates more than 2× improvement in Izod impact strength compared to conventionally printed ABS. Additional demonstration parts subject to complex loading conditions show increases in failure load of 2–3× via the shell-annealing approach. These improved failure properties are due to wetting, reptation and entanglement processes that have been shown to occur under annealing conditions above Tg. Surprisingly, the PVA shell provides mechanical support during annealing, despite exhibiting an as-printed Tg below the annealing temperature. DMA and FTIR demonstrate that the PVATg increases during annealing due to crosslinking and crystallization.
This shell-annealing approach can be executed using off-the-shelf filaments and printers, making it easily accessible to the 3D printing community.
This study shares a new approach for printing and post-treating additively manufactured thermoplastic parts to achieve mechanical properties that are 2–3× higher than conventionally printed parts.
