Investigation of Adhesion and Flexural Behavior of FDM-Based PLA+-Wood Hybrid Joints for Furniture Applications

Abstract

In this study, the effects of different adhesive systems (isocyanate and polyurethane D4) and surface preparation processes (sanded/unsanded) on the adhesion strength and flexural performance of hybrid joints formed by fused deposition modelling (FDM) printed modified polylactic acid (PLA+) connectors and Scots pine (Pinus sylvestris L.) wood were experimentally investigated. Surface roughness (Ra) was measured with a profilometer prior to bonding, tensile tests were performed according to ASTM D3164-03, and three‑point bending tests were carried out according to ASTM D790-17. Flexural tests were applied in two different loading configurations, with the PLA+ layer located either in the tensile or compressive region to represent real-world usage conditions, and fracture surfaces were examined using an optical microscope. The obtained data were statistically evaluated using multifactor analysis of variance (ANOVA) followed by Tukey HSD tests. Tensile test results showed that the combination of isocyanate-based adhesive and sanded surface provided the highest adhesion strength (5.99 MPa) and maximum stress (11.99 MPa). PU D4-based adhesive generally exhibited lower performance, and the beneficial effect of sanding on maximum stress in the PU system was not statistically significant. In three‑point bending tests, adhesive type emerged as the primary factor, whereas the effect of surface treatment on bending stress was negligible. Fracture mode analyses revealed predominantly substrate failure for isocyanate-bonded joints and mainly cohesive failure within the PU D4 adhesive layer for PU D4-bonded joints. These findings indicate that isocyanate-based adhesives significantly improve the structural performance of FDM-based hybrid joints for furniture and customized production applications.