Temperature‐dependent influence of molecular orientation and internal stresses on the deformation of injection‐molded polypropylene parts

Abstract

Local orientation measures by infrared dichroism and thermal stress measures by dilatometric analysis of image (DAI) have made it possible to quantify the evolution of the structure versus the thickness of the injected plastic parts. The stiffness local properties on the one hand, and thermal expansion on the other hand, have then been established. Deformation measures made in temperature have pointed out its deformation induced by thermal stresses and that attributed to the gradient of molecular orientation and cristallinity of the polymer. A calculation of plastic part deformation constituted of elementary layers whose Young's modulus and coefficients of thermal expansion values have been previously evaluated, has enabled us to corroborate experimental results.

Processing-Induced Morphology: Its Relationship with Tensile-Impact Behaviour in Injection-Moulded Polypropylene

“…There was no preferential orientation in the fl ow direction (<P2cos θ> below 0.7). The orientation profi les were nevertheless similar to those observed by other authors, with four characteristic zones (and thus seven layers) 11,26,28 . The zones were as follows:…”

“…Furthermore, it was also interesting to quantify the effective molecular orientation state through the thickness by infrared dichroism. This analysis led to a fi ner assessment of the structure developed during each step of the injection process, as described elsewhere 11,22,26,27 . Therefore the "skin layer" previously designated by I (Figure 11) can be divided into two sub-layers (designated by 1 and 2; Figure 12).…”

“…In the case of injection-moulded parts, the molecular orientations due to the melt fl ow and the cooling rate through-the-thickness induce morphological gradients. This involves a non-homogeneous structure that can be compared to a laminated composite where each elementary layer brings its contribution and infl uences the mechanical behaviour of the parts 11 . Many authors [12][13][14] have studied these skin-core structures.…”

Subsequent but Independent Cavitation Processes in Isotactic Polypropylene during Stretching at Small- and Large-Strain Regimes

“…Isotactic polypropylene (iPP) has been variously used in commercial products such as automobiles, packaging, and electric appliances because of its excellent solvent resistance, unique mechanical properties, and low cost. − Understanding the mechanical properties of iPP therefore deeply attracts attention from both industry and academia. One of the most important subjects is the mechanical fracture of iPP during deformation, which normally depends on the stretching temperature; − load rate; , and material properties, including crystallinity, − molecular weight, − and some other parameters . In general, two principal fractures, classified as brittle fractures and ductile ones, have been proposed for a solid polymer under load .…”

Physical Phenomena of Thermoplastic Articles in the Vicinity of Their Glass Transition Temperatures

“…Understandably, some of the greatest challenges in controlling dimensional stability over time and articles' structural integrity/durability are related to the development of internal stresses during processing and their manifestation in the manners of bowing, warping and shrinking. While analytical models for the prediction of the bowing and the warping due to temperature gradients have already been developed [1, 2] and many studies have tackled these physical phenomena in great detail [3][4][5], we seek to understand how periodic temperature gradients affect the dimensional stability of thermoplastic articles, especially when temperature reaches the vicinity of their glass transitions.…”