The invention relates to thermoplastic filament for use in 3-D printers, in which the core of the filament is non-solid or mono-filament. The filament may contain a multitude of small micro-voids, (a micro foam), be a foam-core structure, be hollow, or have a monofilament core. The filament of the invention is especially useful with crystalline and semi-crystalline polymers, and reduces the negative effect of filament voids, formed by shrinkage during the cooling of the filament.