On the Characterization and Generation of Flow Marks on Polymeric Surfaces

Licentiate thesis, 2009

Polymer-based materials are widely used in the automotive industry for producing vehicle parts, for instance car bumpers, dash boards etc. A common manufacturing process for such components is injection moulding. Injection-moulded thermoplastics can however exhibit a number of surface defects such as flow marks (or tiger stripes), weld lines, air traps and sink marks. All these imperfections lower the perceived quality of the final products. The attention is here focused on the surface defect commonly denoted “flow marks” or “tiger stripes”. Tiger stripes are alternating glossy and dull bands on the polymer surface. They are approximately perpendicular to the flow directions and often opposite in phase. In other words, if one side of the moulded part shows a dull area, the corresponding region on the opposite side will be glossy. The reasons for the appearance of these stripes are not entirely clear; however a slip-stick phenomenon and/or an unstable flow front during the mould filling are two main hypotheses which might account for their formation. Materials typically used for injection moulding of car components are often based on elastomer-modified polypropylene (PP) containing talc as a filler. The PP-grades in this work here contained between 10 and 15 wt% talc and a very low amount of elastomeric particles. By means of stereographic scanning electron microscopy, it was observed that the surface of the dull bands was rougher than that of the glossy bands. Furthermore, the surface regions of the dull bands contained less filler particles than the corresponding regions of the glossy bands. In order to further understand the relation between the surface topography and the gloss of textured polymeric surfaces, the same SEM technique, combined with a filtering procedure, was used to characterize the surface structure of a number of injection-moulded specimens. The results indicated that small differences in gloss in the low gloss region could be associated with small-scale variations in the surface topography. It is to be expected that the rheological properties of the polymer melts can influence the generation of the flow marks. Hence, the flow properties, mainly in shear, of three different grades of elastomer-modified PP containing mineral fillers were correlated with their propensity for defect generation. It was noted that a higher melt elasticity, as reflected in the pressure losses during flow through a capillary, the degree of die swell and to some extent the dynamic-mechanical behaviour, led to less severe flow marks or retarded the formation of such defects. Elongational draw-down experiments also indicated a more stable behaviour in the case of the melt exhibiting the highest elasticity.