Flow marks in injection molding of polypropylene and ethylene–propylene elastomer blends: Analysis of morphology and rheology

Abstract

This paper reports an investigation of asynchronous flow marks on the surface of injection molded parts and short shots made from two different blends of polypropylene and ethylene-propylene random copolymer elastomers. Flow marks were observed on the surface with both blends; the spatial frequency of flow marks on the surface was greater in the blend B1, which also exhibited a greater contrast between the surface regions. The same blend was distinctly faster in the linear viscoelastic tests of shear creep recovery and shear viscosity growth. The degree of contrast between the flow-mark regions and the out-offlow-mark regions was examined with a detailed analysis of SEM micrographs of the surface regions as well as the near wall regions from short shots. This revealed that the dispersed phase was highly stretched to cylindrical strands in the glossy surface regions of both blends and retracted in the dull regions to different extents in the two cases. A comparison of the particle size distributions and aspect ratio distributions in different regions established that rapid retraction of the suspended elastomer phase was the dominant cause of changes in particle shape between surface regions. Nonlinear shear creep and creep recovery curves of the two elastomer components showed that at a time of 1 s, the fractional strain recovery of the elastomer in B1 was much higher than that of the elastomer in B2. Hence, the nonlinear elastic recovery of the elastomer phase at short times is an important factor in flow mark formation with blends of polypropylene and olefinic elastomers.

Injection molding-induced morphology of thermoplastic polymer blends

“…The dispersed phase was highly stretched to cylindrical strands in the glossy surface regions and retracted in the dull regions to some extent. This phenomenon was observed by Patham et al [76] in the injectionmolded blends of PP and EPR (ethylene-propylene random copolymer ) elastomers. Furthermore, the concentration of the dispersed phase in the flow mark region might be different from the out-of-flow mark region.…”

“…They attributed this to the preferential phase segregation mechanisms during injection molding as well as the surface energy difference between polymer components [69,70,77], The polymer component with low surface energy tended to segregate at the surface due to low polymer-air surface tension [78]. However, the result obtained by Patham et al [76] was not in agreement with the results mentioned above: the same concentration of the dispersed phase existed in both regions.…”

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

“…Brodil and Sehanodish, 12 examining thermoplastic polyolefins (TPO), showed that the rubber particles were elongated in the glossy region and were contracted in the dull ones. A similar observation was also reported by Patham et al 11 A corresponding difference in molecular orientation is also expected for a neat polymer. In Ref.…”

“…A fast relaxation of this stress difference would reduce the severity of the defects. However, as noted by Patham et al, 11 the relaxation characteristics must be carefully monitored. They observed that a too fast retardation of the elongated ethylene-propylene rubber particles in a PP-matrix, actually could enhance the difference in rubber deformation between the glossy and the dull regions, thus enhancing the flow marks.…”

Morphological analysis of the tiger stripe on injection molding of polypropylene/ethylene‐propylene rubber/talc blends dependent on based polypropylene design

“…Some models for the induction distance of the stripe have been proposed in previous studies 12, 14, 15. The authors note that the contact force of the molten blends to the mold wall is an important factor in estimating the induction mechanism.…”