The influence of fiber volume fraction and fiber length on the evolution of pore content and the paintability of sheet molding compounds

Abstract The porosity defects from components molded using sheet molding compounds is a well-recognized issue. It is the major source of paint defects in exterior applications, hindering the widespread use of these materials despite their inherent advantages over metals and thermoplastics. This study discusses the influence of fiber volume fraction and fiber length on the pore content and on the painted surface quality of class-A compounds. Compounds with varying glass fiber fraction and fiber length were compounded, compression molded and painted to analyze the influence of the aforementioned factors on the permeability of the fibrous network, the evolution of initial to final pore content and on the painted surface quality with regards to waviness and substrate-relevant paint defects. A new indicator, the pore transport efficiency factor is introduced to help quantify the related characterization feature. The results indicate that lower fiber fractions lead to decreased pore contents and thus, improved paintability.