Fusion bonding is understood to be the decisive mechanism for the interlayer strength in extrusion additive manufacturing. This study links the characteristic temperatures of semicrystalline thermoplastics, namely, PA6 with 40% carbon fibers, to the bond formation in respect to real-world processing conditions. Based on theoretical investigations, a process window is proposed for bonding to occur without polymer degradation. This range from the glass transition to the initial degradation temperature was determined through differential scanning calorimetry and thermogravimetric analysis. A second process window for optimal bonding is proposed from the extrapolated onset crystallization temperature, T eic , to the melt temperature, T m . The validation of these process windows was conducted by bending tests according to DIN EN 178. T m was confirmed as the upper limit, with the part's geometric integrity compromised at higher temperatures. T eic had to be refuted as lower limit as no discrete reduction in bond strength was determined in alignment with T eic . Authors suggest the lower limit is defined by the lowest substrate temperature to lead to interface temperatures above the onset of melt temperature. By utilizing thermal analyses, less time and resources are required to detemine a suitable process window for extrusion additive manufacturing.

Fusion bonding theory is applied to the additive manufacturing process to predict the strength developed across the interface between the deposited layers in material extrusion based large‐scale additive manufacturing. Relaxation times were determined through rheology investigations of the neat polymers and were extrapolated across the entire process temperature range to estimate the required welding times for optimal bond formation. For the calculation of bond formation during cooling from the melt, the semicrystalline polyaryletherketone is considered amorphous until the onset of crystallization which is determined by recreating the process thermal history in DSC measurements. A significant improvement in bond development during additive manufacturing is achieved through the use of specifically designed polymers with slower crystallization kinetics. Both in small‐scale additive manufacturing, achieving full bonding, and in large‐scale additive manufacturing, they achieve significantly higher interlaminar strength than the reference material, validated by three‐point bending. A very good match between the estimated degree of bonding and the tested strength of upright printed specimens in three‐point bending was found. While the fusion bonding model calculates a degree of bonding of 42.07%, mechanical testing showed 42.54% of the bulk flexural strength. The study outlines a procedure to evaluate materials for additive manufacturing by material extrusion based on small material samples to shorten and improve the process development. © 2020 The Authors. Polymer International published by John Wiley & Sons Ltd on behalf of Society of Industrial Chemistry