Additive Manufactured Thermoplastic Parts Strengthen by Remelting Assisted by Inert Powder Mold

High Resolution Image Download MS PowerPoint Slide Thermoplastic objects fabricated by material extrusion additive manufacturing suffer from lamellae defects that are artifacts of the layer-by-layer fabrication process. Eliminating these defects in post processing without losing printed morphology can improve the parts’ mechanical strengths. While solvent-vapor smoothening and abrasive grinding are effective postprocessing techniques for the surfaces, the interiors of printed parts are unaffected by these methods. On the other hand, thermal annealing is a postprocessing method that melts a thermoplastic object, and the melting process can affect the entirety of the object. Yet, molten thermoplastics would collapse and lose the printed morphology. To remedy the problem, one can use a temporary powder mold to stabilize thermoplastic objects during thermal annealing, so the object can liquidify into a viscous melt without losing morphology. This article details the use of inert powder as a temporary mold during a thermal annealing process that goes above the melting point, referred to as a powder-assisted remelting process. A printed object can be buried in an inert powder and heated beyond the thermoplastic’s melting temperature. As the thermoplastic melts, the surrounding inert powder hinders the flow of the polymer melt, which allows polymer molecules to entangle and preserves the morphology. Once cooled, the remelted part loses its lamellae defects. Poly(lactic acid) (PLA) samples and poly(ether ether ketone) (PEEK) samples were separately remelted, and each sample showed significantly improved mechanical properties. Through remelting, PLA samples improved their maximum stress as high as 170% and their maximum strain as high as 260% relative to their initial values. Similarly, PEEK samples showed improved maximum stress by as much as 500% and maximum strain by as much as 180% relative to their initial values. The remelting process combines a printed structure’s layers into a continuous isotropic bulk without obvious lamellae defects. The application of this thermal annealing method was demonstrated with a set of PEEK false teeth. The remelted structure displays continuous cross sections without lamellae defects. The powder-assisted remelting process is applicable for common plastic like PLA, and it is also applicable for engineering thermoplastic like PEEK. Due to its versatility, powder-assisted remelting should be applicable for a variety of thermoplastic materials made by additive manufacturing.