Reliability challenges in additive manufacturing of continuous fiber-reinforced sandwich structures

• Mechanical strength and stiffness of additively manufacturing CFRC degrade as batch production extends over long periods. • Interlayer debonding is the dominant failure mode after long waiting periods in batch production. • The variability of flexural strength and stiffness is larger in fiber-reinforced samples than in non-reinforced ones. • Exposure to humidity weakens the bonding strength of reinforcement fibers. • Humidity control and fiber storage protocols are crucial for the reliable production of additively manufactured CFRC. Additive manufacturing of continuous fiber-reinforced polymer composites faces challenges in achieving consistent flexural strength and stiffness. Additively manufactured sandwich structures with continuous fiber reinforcement were produced in different batches and subjected to flexural tests. The production replicated real-world conditions, including filament spool changes, fiber aging, and time gaps between batches. The mechanical properties were consistent in early batches, but variability in flexural strength and stiffness increased from one batch to the next, reaching deviations up to 60% for glass fiber and 70% for carbon fiber in later batches. Although the dual-head additive manufacturing system protects the polymer filament from humidity during the sequential fiber deposition process and waiting periods, similar provisions are also necessary for the reinforcement filament to minimize or eliminate polymer-fiber interlayer debonding.