Rheological properties of elastomer‐modified polypropylene and their influence on the formation of flow marks

Abstract

A premium appearance is essential for many polymeric products and for this reason surface defects associated with the manufacturing process, e.g. injection molding, are detrimental. In this study, the interest is focussed on defects arising during injection molding of such elastomer‐modified polymers that are often used in the automotive sector to produce interior and exterior components. In particular, defects denoted as “flow marks” or “tiger stripes” were investigated. These defects appear on the surface of the injection‐molded components, especially if long flow lengths are involved and consist of alternating glossy and dull bands. Here an attempt was made to elucidate to what extent some important rheological properties of the polymer melts influence the generation of such flow marks. Hence, the flow properties, mainly in shear; of three different grades of elastomer‐modified polypropylene containing mineral fillers were correlated to their propensity for defect generation. In summary, it was noted that a higher melt elasticity, as reflected in pressure losses during flow through a capillary, the degree of die swell and to some extent the dynamic‐mechanical behavior, leads to less severe flow marks or retards the formation of such defects. Elongational draw‐down experiments also indicated a more stable flow in the case of the melt that exhibited the highest elasticity. Furthermore, subjecting the measured flow behavior to a Mooney analysis, the results obtained pointed to that wall‐slip in itself is not a primary cause for the appearance of the type of flow marks studied here. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Ageing of polymeric surfaces exhibiting flow marks—Effects on surface characteristics

“…The corresponding data for Grade C were 1000 kg/m 3 and 18 g/10 min, respectively. Grade C has been reported to exhibit a lower melt viscosity than Grade B and also higher melt elasticity 7. Moldings of Grade B exhibited flow marks, whereas those of Grade C did not.…”

“…Two grades of commercial elastomer‐modified PP containing around 15 wt % talc as filler were used. They were denoted as B and C following the notation in the previous study in which these materials were employed 7. Grade B had a density of 990 kg/m 3 and a melt flow index (230°C/2.16 kg) of 16 g/10 min.…”

“…The rheological properties of the polymer melt may also play a role in this context; for example, a more pronounced elasticity of the melt has been reported to reduce the problem, cf. Refs 6–8. and also the recovery behavior of the melt can be of importance 9…”

Visualization analysis of tiger-striped flow mark generation phenomena in injection molding

“…As a result, we have discovered that these flow marks occur due to asymmetrical fountain flow phenomenon in the flow process [1]- [3] . On the other hand, we have also reviewed the causes of unstable flow front of melt in the mold, conditions for the numerical analysis of these causes, conditions of the rheological characteristics of materials [4]- [8] . However, we have not yet verified asymmetrical fountain flow and clarify the details of the generation mechanism.…”

Ghost marks—gloss‐related defects in injection‐molded plastics

“…. In general, very little is reported on gloss‐related defects other than flow marks such as the striped patterns, also known as tiger stripes, formed by alternating dull and glossy regions approximately perpendicular to the flow direction [2–4].…”