In this work, the relation of injection molding parameters to the mechanical properties of various polypropylene grades is investigated to find the optimal processing parameters to minimize weld‐line effects like reduced mechanical and/or optical performance. Injection molded test specimens of five polypropylene grades with different melt flow rates were characterized for their mechanical behavior and compared with equal‐shaped specimens with a colliding weld line in the middle of the specimen. For the production of these weld‐line specimens, a special mold was used and injection molding parameters were systematically varied, tensile (ISO 527‐2) and impact (ISO 179‐1/1eU) properties were measured, and statistical analyses were performed to gain insight on the correlation between melt flow rate and weld‐line behavior. It showed that mechanical properties of specimens with weld lines can be influenced by the processing parameters, but the effect is limited. Positive correlations were found between tool temperature and tensile modulus and strength. Polypropylene grades with low melt flow indexes seem to be more susceptible to weld‐line‐induced property reductions. In a second test series similar to the pure PP investigations, glass‐fiber‐ and talc‐filled PP were used to gain insight into the weld‐line behavior of PP composites. In addition to mechanical characterization, optical and scanning electron micrographs were taken of the weld‐line areas. It was found that the reinforcing particles align along the melt flow front in the weld‐line area. As a result, the mechanical performance of weld‐line specimens is poor. Injection molding parameters were found to have only a small effect.
The interactive effects between additives and weld lines, which are frequent injection-moulding defects, were studied in high-density polyethylene (HDPE) and compared to weld-line-free reference samples. These materials were formulated around a D- and I-optimal experimental design, based on a quadratic Scheffé polynomial model, with up to 60 wt% calcium carbonate, masterbatched carbon black and a stabiliser package. Where reasonable and appropriate, the behaviours of the systems were modelled using statistical techniques, for a better understanding of the underlying trends. The characterisations were performed through the use of conventional tensile testing, digital image correlation (DIC) and scanning electron microscopy (SEM). A range of complex interactive effects were found during conventional tensile testing, with DIC used to better understand and explain these effects. SEM is used to better understand the failure mechanics of some of these systems through fractography, particularly regarding particle effects. A measure is introduced to quantify the deviation of the pre-yield deformation curve from the ideal elastic case. Novel analysis of DIC results is proposed, through the use of combined time-series plots and measures quantifying the extent and localisation of peak deformation. Through this, it could be found that strong shifts in the deformation mechanisms occur as a function of formulation and the presence/absence of weld lines. Primarily, changes are noted in the onset of continuous inter- and intralamellar slip and cavitation/fibrillation, seen through the onset of localised deformation and stress-whitening.
In this work, the effects of weld lines, additives and the degree of QUV weathering on the tensile behavior of a range of high‐density polyethylene composites with calcium carbonate, stabilizers and a carbon black/SEBS masterbatch are studied. The degree of weathering is characterized using FTIR‐derived carbonyl, double‐bond and carbonate indexes based on curve fitting, to allow for the fairer comparison of specimens with and without calcium carbonate. Weld‐line specimens exhibited more rapid degradation than that seen in the reference specimens, while the exposed surfaces of the specimens degraded more quickly than the unexposed surfaces. ISO G154 Cycle 1 and Cycle 6 weathering protocols were compared. The additives were found to be effective at decreasing oxidative degradation, albeit with reduced effects at higher loadings and in mixed systems. These findings were mirrored in the mechanical properties of the specimens, with the modified specimens even exhibiting broadly improved properties with increasing aging. Elongation at break was most sensitive to weathering, with increasing degradation with increasing weathering across almost all specimens.
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