Characterization of Flow-Induced Surface Defects in Injection Moulded Components - Case Studies

Injection-moulded parts involving long flow lengths can exhibit visible flow defects characterized by alternating glossy and dull bands on the polymer surface, approximately perpendicular to the flow direction. These flow defects are sometimes referred to as “flow marks” or “tiger stripes” and are considered to have strong negative effect on the appearance of the component and thus on the perceived quality. The reasons for the appearance of these stripes are not entirely clear; two main hypotheses relate to a slip-stick phenomenon or to an unstable flow front during the mould filling (which however may be interrelated). The present work deals with an experimental characterization of the banded regions of automotive components manufactured in mineral-filled polypropylene-based materials. It is shown using a stereographic scanning electron microscopy technique that the dull band regions were more surface rough than the other type of region. Furthermore, it was found that the surface regions of the dull bands contained less filler than the corresponding glossy regions. Possible causes for this are discussed. No significant difference in chemical composition between the surfaces of dull and glossy bands was detected.