Temperature Dependent Hygromechanical Behavior of Additively Manufactured Nylon (polyamide 6) and ULTEM (polyetherimide) Continuous Carbon Fiber Composites

Fused deposition modeling (FDM) is used extensively for rapid prototyping with 3D printed thermoplastic materials. Nylon (polyamide 6) is a popular filament material; however, the mechanical properties are known to degrade in the presence of moisture and/or elevated temperature. In contrast, ULTEM (polyetherimide) has higher strength/stiffness and resistance to moisture/temperature but is more expensive. This work compares the impact of hygrothermal conditioning on the flexural strength and modulus of Markforged Onyx (Nylon + chopped carbon fiber) and ULTEM 9085 samples with and without the addition of continuous carbon fibers printed using a Markforged FX20 3D printer. Samples for bend testing were conditioned at 90% RH at 22°C for 26 days, and mass/dimension increase was recorded. 3-point bend testing according to ASTM D7264 was conducted while varying temperature from 22°C (room temperature) to 50°C using an Instron load frame with an environmental testing chamber. The rate of flexural strength/modulus decrease with increasing temperature was compared for the four different materials (Onyx, Onyx + continuous carbon fiber, ULTEM, ULTEM + continuous carbon fiber) in the as-printed and conditioned state. Ductile vs. brittle failure mechanisms are discussed considering the polymer moisture content and microstructure. Fiber-polymer interfacial bonding is also discussed. The results will help inform end users of the optimal material to use given the operating load, moisture, and temperature conditions for their specific application.