The Extrusion Process

Polymer Extrusion

The extrusion process is core to the FFF technology and relates to the continuous shaping of a fluidized polymer through an appropriately shaped tool, followed by the extrudate solidification. FFF polymers are typically extruded twice: initially the polymer is extruded in an industrial processing facility to generate the commercial FFF filament, subsequently the filament is extruded through the FFF system to generate the manufactured AM product.2 The fundamental extrusion process is technically similar for both stages, however the hardware complexity and sophistication between the industrial filament extrusion process and the in-situ extrusion during FFF processing are radically different. These differences introduce technical challenges that the designer must accommodate as well as DFAM research opportunities for FFF system developers.

Pellet extrusion typically consists of a motor-controlled screw system that transports pellets from a hopper and compresses these through a heated orifice. Temperature feedback control with multiple heating elements is used to precisely control the temperature profile through the system. The screw system is typically categorized by three zones: the transport zone, where solid pellets are loaded in the system; the transition zone, where mechanical working and heating fluidize the polymer; and the melt-conveying zone, where the fluid polymer is driven to the extrusion die. The extrusion die imposes the intended final geometry on the extruded material. An important challenge for extrusion systems is the phenomenon of die swell, which refers to the increase in cross-section area observed when the extrudate leaves the restriction of the die. For a given polymer choice, this effect is a function of flow resistance, which in turn is a function of melt temperature. Typical design measures used to stabilize the geometry of the extruded material include the manipulation of the extrusion-die geometry and associated material, as well as active cooling by forced air convection or water quenching. The specific layout of industrial extrusion technology exists in many variants; for example, a venting zone may exist to allow the exhaust of entrapped gasses; multiple counterrotating screws can be used to achieve more thorough mixing and to accommodate shear-sensitive materials; and dimensional feedback can be provided to any of the control inputs to increase dimensional stability and material yield.