Flow-induced Surface Defects and Ageing Behaviour of Injection-moulded Polymers

Doctoral thesis, 2012

Injection moulding is one of the most commonly used manufacturing methods for the production of polymer-based components such as car bumpers and other automotive details. The surface appearance of these products is of great importance since it is directly connected to the perceived quality. Injection-moulded thermoplastics can exhibit a number of surface defects which lower their quality impression. To a large extent, this work deals with characterisation of flow-induced surface defects called tiger stripes or flow marks. They consist in alternating glossy and dull bands which appear on the surface of injection-moulded polymers, especially when long flow lengths are involved. The flow marks are approximately perpendicular to the flow direction and they are often opposite in phase on the two sides of the moulded object. The materials typically used for injection moulding of car components, and studied in the present work, are elastomer-modified polypropylene grades containing fillers. Two main theories behind the formation of the flow marks are discussed here: a slip-stick phenomenon and a snake-like flow which can initiate an unstable flow front during the filling of the mould. Several techniques were used in order to characterise the banded regions. Stereographic scanning electron microscopy (SEM), as well as gloss evaluation, revealed that the dull regions were rougher and less glossy than the other type of bands. SEM micrographs of the cross section of the stripes indicated differences in the filler distribution between the different bands; the dull bands seemed to contain a lower amount of filler in the proximity of their surface. A course of events leading to this difference is suggested. Rheological studies of the polymer melts suggested that a higher melt elasticity is beneficial since it reduces the propensity for the defects generation. The effect of ageing on the severity of the flow marks is discussed in terms of surface roughness, surface hardness and thermal stability. Acrylonitrile-butadiene-styrene (ABS) copolymer is another engineering polymer widely used to produce vehicle interior components as well as domestic appliances, toys, etc. ABS is known to be sensitive towards ageing especially when exposed to UV-light. The effect of ageing on the appearance of injection-moulded ABS samples was studied here, in particular colour changes were evaluated. Natural coloured and grey coloured ABS specimens, containing the same type and the same amount of stabilisers, were exposed to a heat ageing treatment and to artificial weathering. In general, the natural coloured ABS suffered the most severe discolouration and the artificial weathering produced the strongest effect. Interestingly, an imposed surface texture appeared to affect the colour change induced by ageing. Chemical and mechanical changes on the surface layers of the investigated samples were reported.