Identifying thermal effects during 3D printing by comparing in-layer infrared pre- and postheating of carbon fiber reinforced polyamide 6

Additive manufacturing technologies, particularly Material Extrusion (MEX), are rapidly evolving and gaining widespread adoption. However, parts produced by MEX often suffer from poor out-of-plane mechanical properties. Thus, to learn more about the fusion process of succeeding layers, this study investigates and compares the thermal and strength effects of in-layer infrared (IR) pre- and postheating during MEX 3D printing of carbon fiber reinforced polyamide 6 (PA6-CF). An experimental setup using a ~40W halogen bulb as an auxiliary heater was developed to compare no heating, preheating, and postheating configurations at print speeds ranging from 3-50mm/s to manufacture single-wall tensile samples with 0.8mm wall thickness and 0.3mm layer height. Layer temperature, TL, and maximum temperature resulting from the external heater, TH, were measured for all heating configurations using infrared imaging. Thermal increase, ΔTPre, and ΔTPost was calculated from these two values and compared at various print speeds. An experimental estimate of thermal increase, ΔTT, was also defined to aid in the explanation of the observed strength effects and how they relate to the thermal evolution of the layer fusion process. Tensile testing was conducted to evaluate interlayer strength and correlated to the thermal measurements. Results revealed thermal increase caused by IR heating at slower speeds and residual heat accumulation at higher speeds, both positively impacting strength. Postheating showcased a maximum TH of 272.9±5.2°C and a maximum UTS of 59.84MPa whereas preheating exhibited 219.2±2.8°C and 52.28MPa at a print speed of 3mm/s. In addition, postheating demonstrated tensile samples with fracture across several layers, indicating strong layer adhesion. Even though postheating was found to be more effective at lower speeds, preheating produced stronger tensile tests at higher speeds. In answer to this result, the study introduces the concept of Layer Fusion Evolution (LFE) to hypothesize on how the two top layers fuse together under the different IR heating configurations. Based on the insights from the presented hypothesis and the experimental estimate of thermal increase, a model is presented and compared to the strength results as a possible explanation of why preheating and postheating performs differently across various print speeds. This model exhibits the same trend as the strength measurements across different speeds, and due to the previously linked property of weld temperature and strength, supports the presented hypothesis. Furthermore the paper discusses future work and implementation of the findings and how they may aid in optimizing auxiliary heating strategies for MEX processes to improve out-of-plane mechanical properties and reduce the anisotropy in 3D printed components.