Mechanical Reinforcement and Warpage Mitigation of FGF 3D ‐Printed High‐Density Polyethylene Composites by Incorporating Vinyl‐Functionalized Carbon Fiber

High‐density polyethylene (HDPE) composites face severe warpage in fused granular fabrication (FGF) 3D printing due to crystallization shrinkage and poor fiber‐matrix compatibility. Herein, vinyl‐functionalized carbon fiber (mCF) was prepared via desizing, oxidation, and silane grafting to reinforce HDPE/POE blends. The modification enhanced interfacial adhesion, reduced porosity, and promoted formation of a percolated filler network, as confirmed by SEM, FTIR, and rheology. Compared to raw CF composites, mCF systems exhibited higher storage modulus, lower tanδ , and improved stress transfer, leading to superior mechanical strength and thermal resistance. Notably, warpage was significantly suppressed at moderate loadings, while high mCF content caused viscosity‐driven porosity increase. The 12.4 wt% mCF composite achieved balanced reinforcement and dimensional stability. This work demonstrates that interfacial engineering via silane coupling is an effective strategy to mitigate warpage and enhance performance of polyolefin‐based 3D‐printed components.