Revisiting intimate contact development in material extrusion additive manufacturing

Interlayer bonding quality in polymer-based material extrusion additive manufacturing (MEAM) is widely recognized as poor, with previous studies characterizing interlayer bonding through two primary mechanisms: degree of intimate contact (Dic), which focuses on the width of the contacting area, and healing, which describes the interdiffusion of polymer chains within the contact zone. This study provides direct evidence of microvoids at layer interfaces in printed polyethylene terephthalate glycol (PETG) parts, demonstrating that apparent contact areas may not achieve complete interfacial contact. Intimate contact quality was investigated by varying layer height, print speed, and temperature. Microvoids were quantified using polarized optical microscopy and micro-computed tomography. A novel method was developed to measure contact pressure during printing, revealing an inverse relationship between contact pressure and intimate contact quality. Print parameters influenced both pressure and contact quality in an interdependent manner, with print speed emerging as the most influential factor. Experimental validation of a conventional composite intimate contact model showed that it fails to predict interfacial microvoid formation in polymer extrusion processes. These findings refine current bonding theories for MEAM, highlight the importance of microvoid quantification for predicting mechanical performance, and provide new insights for optimizing printing parameters to produce stronger, more reliable components.