# Mechanical and Microstructural Response of FDM-Printed PETG and PETG+CF to Variable Infill Architecture and Lubricant Exposure

## TL;DR

This study examines how infill patterns and lubricant exposure affect the strength and structure of 3D-printed PETG materials.

## Contribution

The study quantifies the combined effects of infill architecture and lubricant exposure on the mechanical and microstructural properties of FDM-printed PETG and PETG+CF.

## Key findings

- Hexagonal infill at 30% density maximized tensile strength in PETG, while linear and triangular patterns showed higher stiffness.
- Increasing infill density improved strength in both PETG and PETG+CF, with PETG+CF showing higher strength at full density.
- Lubricant exposure reduced PETG strength but increased deformation to failure, with PETG+CF showing minimal changes.

## Abstract

Fused deposition modelling/fused filament fabrication (FDM/FFF) enables rapid manufacturing of functional polymer components; however, the reliability of printed parts remains strongly governed by internal architecture, process-induced porosity, and exposure to service fluids. This study quantifies the combined influence of (i) infill pattern (linear, triangular, hexagonal) at 30% density, (ii) infill density (30%, 60%, 100%) for linear infill, and (iii) short-term lubricant exposure on the tensile and microstructural response of FDM-printed polyethylene terephthalate glycol-modified (PETG) and short-carbon-fibre-reinforced PETG (PETG+CF). Specimens were printed following ISO 527-2 and tensile-tested at 5 mm/min. Microstructural analysis coupled quantitative porosity with mechanical response, Young’s Modulus, and strain-to-break. At 30% density, PETG with hexagonal infill achieved the highest tensile strength (18.54 ± 0.67 MPa), exceeding linear (16.99 ± 0.52 MPa) and triangular (14.15 ± 0.70 MPa) patterns, while triangular and linear patterns exhibited higher Young’s Modulus, indicating topology-driven decoupling of stiffness and strength. Increasing linear infill density raised strength to 31.35 ± 0.33 MPa (PETG) and 38.90 ± 0.28 MPa (PETG+CF) at 100%, consistent with reduced porosity. Seven-day immersion in SAE 15W-40 mineral engine oil reduced PETG strength by ~17% while increasing deformation to failure, whereas PETG+CF showed only minor changes. Overall, the results demonstrate that architecture-aware design, supported by quantitative porosity descriptors, is essential for ensuring the reliable mechanical performance of FDM/FFF-printed PETG-based components exposed to service fluids.