Fused deposition modeling (FDM) is based on the principle of material extrusion where a thermoplastic filament is drawn through a nozzle, heated to its melting (or glass transition) point and then deposited layer by layer. Once deposited the thermoplastic layer fuses with the layer below, cools and hardens. This process is repeated until the 3D structure is complete. The most common polymer used for FDM is acrylonitrile butadiene styrene (ABS) but polycarbonate (PC) and polyetherimide (PEI) are also used. As the applications for 3D printing and fused deposition modeling increase, there is a necessity to develop novel polymers with unique mechanical properties that are also printable. Material characterisation is critical to this development process for assessing the melt characteristics and structure of the polymers, as well as the mechanical properties of the printed part.
Polymer properties and processing conditions play a major role in determining the final mechanical properties of the finished part with processing conditions often dependent on polymer properties. The temperature dependent rheological behavior of the polymer melt is therefore important for describing and controlling the behavior of the polymer in the liquefier (where polymer is melted), during the extrusion process, and the fusing behavior once the polymer is extruded.
Rheometry is a key tool for characterizing such behavior with rotational rheometry and capillary rheometry both useful tools for understanding melt properties. Capillary rheometry is based on the principles of capillary or pipe flow so is directly relevant for assessing melt flow behavior under process relevant conditions. It is also possible to determine die swell behavior using this technique, which can impact on print resolution.
The rheological behavior of the melt and mechanical properties of the finished part are related to the molecular structure and molecular weight distribution of the feedstock material. Hence it can be important to validate these properties to ensure a consistent supply of raw material or to assist in the development of novel materials. Gel Permeation Chromatography (GPC) is a useful tool in this regard since it can determine molecular weight distribution but also structural parameters such as molecular density and degree of branching.