Additive Manufactured\nThermoplastic Parts Strengthen\nby Remelting Assisted by Inert Powder Mold

Thermoplastic objects fabricated by material extrusion\nadditive\nmanufacturing suffer from lamellae defects that are artifacts of the\nlayer-by-layer fabrication process. Eliminating these defects in post\nprocessing without losing printed morphology can improve the parts’\nmechanical strengths. While solvent-vapor smoothening and abrasive\ngrinding are effective postprocessing techniques for the surfaces,\nthe interiors of printed parts are unaffected by these methods. On\nthe other hand, thermal annealing is a postprocessing method that\nmelts a thermoplastic object, and the melting process can affect the\nentirety of the object. Yet, molten thermoplastics would collapse\nand lose the printed morphology. To remedy the problem, one can use\na temporary powder mold to stabilize thermoplastic objects during\nthermal annealing, so the object can liquidify into a viscous melt\nwithout losing morphology. This article details the use of inert powder\nas a temporary mold during a thermal annealing process that goes above\nthe melting point, referred to as a powder-assisted remelting process.\nA printed object can be buried in an inert powder and heated beyond\nthe thermoplastic’s melting temperature. As the thermoplastic\nmelts, the surrounding inert powder hinders the flow of the polymer\nmelt, which allows polymer molecules to entangle and preserves the\nmorphology. Once cooled, the remelted part loses its lamellae defects.\nPoly(lactic acid) (PLA) samples and poly(ether ether ketone) (PEEK)\nsamples were separately remelted, and each sample showed significantly\nimproved mechanical properties. Through remelting, PLA samples improved\ntheir maximum stress as high as 170% and their maximum strain as high\nas 260% relative to their initial values. Similarly, PEEK samples\nshowed improved maximum stress by as much as 500% and maximum strain\nby as much as 180% relative to their initial values. The remelting\nprocess combines a printed structure’s layers into a continuous\nisotropic bulk without obvious lamellae defects. The application of\nthis thermal annealing method was demonstrated with a set of PEEK\nfalse teeth. The remelted structure displays continuous cross sections\nwithout lamellae defects. The powder-assisted remelting process is\napplicable for common plastic like PLA, and it is also applicable\nfor engineering thermoplastic like PEEK. Due to its versatility, powder-assisted\nremelting should be applicable for a variety of thermoplastic materials\nmade by additive manufacturing.