An accurate steady-state approach for characterizing the thermal conductivity of Additively manufactured polymer composites

This study investigates the accuracy of the steady-state meter bar implementation of ASTM D5470, popularly used to measure the thermal conductivity of 3D-printed polymers and polymer composites manufactured using fused filament fabrication (FFF). We show that a meter bar approach can result in significant error in thermal conductivity measurement, depending on the meter bar design and the thermal conductivity and thickness of the tested samples. This is because a fraction of the thermal power measured by the meter bar bypasses the sample through the surrounding insulation. To address this issue of heat bypass, a modified version of the steady-state guarded hot plate method (ASTM C177) is proposed; this version features a one-sided, cup-shaped guard heater that greatly simplifies the standard implementations of ASTM C177 and C1044. This approach also does not require the calibrated heat flux meter bars described by popular implementations of ASTM D5470 or E1225. Simulations of this approach for typical FFF composite samples shows less than 2% measurement error. A detailed design of this apparatus is presented and employed to measure the thermal conductivity of several 3D-printed PLA polymer composites which showed a significant degree of anisotropy.