Weld lines in injection moulded polypropylene parts. Mechanical properties morphology relationships

Abstract

The morphology of polypropylene plaques containing a weld line, has been investigated by x-ray wide and small angle scattering, light scattering and polarization microscopy. The results are correlated to mechanical values obtained from tensile tests. While the lamellar morphology is of neglectable influence, it is found, that the mechanical properties of the samples are strongly influenced by the spherulitic structure. Clustering of &type spherulites and strong morphological inhomogeneities in the vicinity of the weld line are the primary reason for the mechanical weakening of the plaques. The temperature of the mould has a strong effect on the spherulitic structure and thus on the tensile properties of the samples. ZUSAMMENFASSUNGDie Morphologie von Polypropylen-SpritzguBteilen mit Bindenaht wurde mit R6ntgenweit-und Kleinwinkelstreuung, Lichtstreuung und Polarisationsmikroskopie untersucht. Die erhaltenen morphologischen Daten wurden mit mechanischen Werten aus Zugversuchen korreliert. Es wird gefunden, daJ3 die mechanischen Eigenschaften der untersuchten Proben im wesentlichen von der Sphtirolithstruktur beeinflat werden. Anhaufungen von Spharolithen mit Kristallen der hexagonalen $-Modifikation und eine starke Inhomogeniat der Spharolithstruktur senkrecht zur Bindenaht sind der wesentliche Grund fur die verschlechterten mechanischen Eigenschaften. Diese Spharolithstruktur wird durch die Temperatur des SpritzguBwerkzeugs stark beein-fluDt. Die Lamellen-Uberstruktur hat dagegen einen vernachlassigbar kleinen EinfluD auf das mechanische Verhalten.

The unique flow of polypropylene at the weld line behind an obstacle in injection molding

“…A skin layer was homogeneously formed at the weld as well as at at other positions, but the amount of β‐form spherulites at the weld line was smaller than those in the other positions. A decrease in the number of β‐form spherulites was detected at the cold weld line [13], and the generation of β‐form spherulites increased at the mold temperature of 100°C as compared with that at 40°C [14]. At the lower mold temperature, α‐form crystals form because the crystallization rate of α‐form crystals is faster than β‐form crystals at temperatures below 90°C and above 133°C [15].…”

Computer simulation of weld lines in injection molded poly(methyl methacrylate)

“…Some researchers have reported improvement in weld-line strength with increasing mold temperature ( 1 1, 2 1 ) whereas others have presented data showing the opposite (24). Yet others have found that the weldline strength first increases but then decreases with increasing mold temperature (25). The third processing parameter influencing the weld-line severity is the injection time (injection speed, injection volumetric flow rate, injection pressure).…”