Reduced weld strength due to secondary cell formation in vibration weld region of microcellular glass fiber reinforced nylon-6 shells

Abstract

Microcellular plastic parts reduce weight and increase dimensional stability, but a significant decrease in weld strength is observed when the weld region reaches the cell area. In this study, 30 wt.% glass fiber reinforced nylon-6 shells with weight reductions of 0%, 7%, and 10% were fabricated by microcellular injection molding with nitrogen gas followed by vibration welding. Although the weld depth of the vibration weld was much less than the thickness of the cell-free surface layer, microstructural analysis of fracture surfaces by scanning electron microscopy and optical microscopy confirmed the presence of cells at the weld region that lowered burst pressures by 17% and 22% for shells with weight reductions of 7% and 10%, respectively, when compared with the burst pressure of 1.16 MPa for solid shells. The irregular sizes and elongated shapes of these cells suggest that they were generated in the molten polymer by secondary nucleation of residual nitrogen gas during the vibration welding process. This assumption is corroborated by the fact that no cells are formed in the weld area of solid shells with 0% weight reduction and is consistent with recently reported similar findings for glass fiber reinforced polypropylene.