Effect of the skin-core morphology on the mechanical properties of injection-moulded parts

Abstract

The presented study deals with the effects of different processing parameters during injection moulding on the morphological structure through the thickness of the injection-moulded samples and, consequently, on their mechanical properties. In this work, tensile bars of an isotactic polypropylene were injected under different conditions such as the flow rate, the melt temperature and the mould temperature. The morphological structure of the samples was investigated with polarized light microscopy using thin cross-sections cut perpendicularly to the flow direction. The fountain flow in the mould cavity influenced the crystallization kinetics and the presence of three distinct crystalline zones was observed; namely, the highly oriented non-spherulitic skin, the shear-nucleated spherulitic intermediate layer and the inner core composed of spherulites with a low orientation. The results showed that the flow rate has the highest influence on the thickness of the oriented skin layer. The mechanical properties of the tensile samples demonstrated that the larger thickness of the two outer skins provides the higher tensile strength. The same effect was also confirmed with a microhardness test where the skin layer was harder than the inner spherulitic core. Keywords: skin-core structure, polypropylene, morphology, tensile strength, microhardness Predstavljena {tudija obravnava vpliv razli~nih procesnih parametrov pri vbrizgavanju na morfologijo strukture, zaradi debeline vbrizganih vzorcev, in posledi~no na njihove mehanske lastnosti. V tem delu so bile natezne palice izotakti~nega propilena vbrizgane pri razli~nih pogojih, kot so: hitrost te~enja, temperatura snovi in temperatura kokile. Morfolo{ka struktura vzorcev je bila preiskovana z mikroskopijo s polarizirano svetlobo, z uporabo tankih rezin, odrezanih pravokotno na smer toka. Lijakasto litje v votlini orodja je vplivalo na kinetiko kristalizacije in opa`ena je bila prisotnost treh razli~nih kristalnih podro~ij, in sicer: mo~no orientirana ne-sferulitna skorja, s stri`enjem nukleirana sferulitna vmesna plast in notranje jedro, sestavljeno iz sferulitov, brez orientacije. Rezultati so pokazali, da ima hitrost toka najve~ji vpliv na debelino skorje. Mehanske lastnosti nateznih preizku{ancev so pokazale, da ve~ja debelina dveh zunanjih skorij zagotavlja vi{jo natezno trdnost. Enak u~inek je bil potrjen tudi s preizkusom mikrotrdote, kjer je bila plast skorje bolj trda kot pri notranjem sferulitnem jedru.

PBT plasticity loss induced by oxidative and hydrolysis ageing

“…In the technical literature, one can find some examples of unexpected PBT brittleness [6] where the skin-core morphology induced by injection molding is reported to be a potential failure, since this latter is well known to impact polymer properties [7][8][9][10] . The influence of morphology, especially spherulites size, on resistance and toughness has mainly been studied on PP [11][12][13] .…”

Adjusting process parameters in zamak-8 prototype tooling to obtain functional prototypes in PP

“…Other factors that could influence the mechanical properties of the material are thickness of the lamellae [18], the multi-layer morphology that make up the molding [15], the molecular orientation [19] and internal stress [20]. In this work, the increase in T m may have reduced skin thickness, as observed in other works [15,21], resulting in a more homogeneous material (morphology and crystallinity) throughout the molding. This, in turn, might have decreased the internal stresses and improved the mechanical properties.…”

Insight into the Surface Properties of Wood Fiber-Polymer Composites

“…The skin layer increases. At the highest injection rate, the mold and melt temperatures have a negligible effect on the skin thickness [ 46 ].…”