Abstract: An apparatus for forming an object by an additive process, commonly referred to as 3-D printing, is provided. The apparatus includes a filament comprising a series of irregularities along the length of the filament, a nozzle depositing a layer of melted material onto a surface and a feeder feeding the filament to the nozzle. A heating element melts the filament and a controller controls the position of the nozzle in a plane to control the configuration of the layer of melted material deposited by the nozzle. Additionally, a method for forming polymer filament used in 3-D printing devices is also provided. The method includes the steps of melting a polymer, selecting an extrusion characteristic calculated to induce a polymer flow instability and extruding the melted polymer using the selected extrusion characteristic to produce an extrusion having distortions created by the polymer flow instability.