Experimental Study on the Influence of Inter-Layer Ironing Parameters on the Surface Quality of MEX-Fabricated PLA-LW and PLA Parts

The current trend in both academic research and industrial applications is to expand the use of additive manufacturing processes across an increasing number of functional domains. This has led to substantial efforts aimed at improving the overall performance of additively manufactured components, particularly those produced by Material Extrusion (MEX). One of the main limitations of MEX-fabricated parts is the presence of air voids formed between deposited lines and layers, which reduce mechanical strength and structural uniformity. In this context, the present study investigates the influence of inter-layer ironing process parameters, an approach intended to modify near-surface morphology and improve the surface quality and dimensional accuracy of two commonly used materials, namely lightweight polylactic acid (PLA-LW) and standard. The experimental setup involved varying the ironing direction, nozzle diameter, and ironing spacing, while keeping all other manufacturing parameters constant. A complete 4 × 3 × 3 factorial experimental design was employed, considering three process parameters: ironing direction at four levels, nozzle diameter at three levels, and ironing spacing at three levels, resulting in 36 parameter combinations applied to each material separately. Performance evaluation included surface roughness (Sa), Shore D hardness, waviness (Wa), microscopic morphology analysis, and qualitative ultrasonic inspection used to observe internal void-related features, whereas the quantitative analysis focused on Sa, Wa, and hardness variations. The results were compared with those obtained for specimens produced using final-layer-only ironing and no ironing. Inter-layer ironing generally improved surface quality compared with non-ironed specimens, reducing Sa by up to 85.22% and increasing Shore D hardness by up to 10.40% for individual PLA-LW specimens manufactured using the 0.4 mm nozzle. The waviness response was strongly material-dependent; increasing the ironing spacing from 0.1 to 0.3 mm reduced Wa by 47.71% for PLA-LW and 13.39% for standard PLA, while the same spacing increase reduced Sa by 25.61% for PLA-LW but increased Sa by 10.87% for standard PLA. While PLA-LW specimens exhibited localized surface defects possibly associated with the compaction or collapse of near-surface voids, standard PLA specimens showed more pronounced waviness and material accumulation, highlighting the different responses of compact and foamed polymer structures to repeated thermo-mechanical ironing actions.