In material extrusion (MatEx) additive manufacturing (AM), thermoplastic filament is extruded though a rastering nozzle onto previously deposited layers. The resulting strength of the build is limited by the strength of the weld between each layer. While numerous factors can affect the weld strength, the temperature of the interface between layers dictates the amount of interdiffusion and entanglement, and thus the weld strength. Experiment, theory, and simulation have found the weld strength in symmetric linear polymer systems scales with t^~0.25, where t is the isothermal welding time, before plateauing to the bulk strength. Determining this relationship for MatEx is the first step to understanding the polymer physics of weld formation in this unique process. However, the weld formation in MatEx is highly non-isothermal and requires calculating an equivalent isothermal welding time to make appropriate comparisons between printing conditions. Equivalent isothermal welding time was determined using a combination of in situ infrared thermography and horizontal shift factors from offline rheological measurements of the neat polymer. Weld strength was measured directly by mode III fracture using a simplified geometry limiting the measurement to a single weld. Since the processing conditions are known a priori this approach provides the data needed to estimate the final build strength at time of design. The resulting agreement between weld time and weld strength for a range of printing conditions and thermoplastics are discussed.
- Understand the polymer physics underlying weld formation in material extrusion additive manufacturing.
- Identify polymer properties and process conditions which will result in stronger welds.