Microinjection des polymères semi-cristallins : Microstructures et textures des matériaux

Abstract : Due to the development of microsystems, it is compelling to find techniques that allow the production of increasingly small pieces. Among these techniques lies the microinjection, which is the injection of thermoplastic materials in micro-molds. This method is considered as the prime choice and is being established in the industrial field. However, technical and scientific obstacles prevent its development on a larger scale. Fully aware of this problem, the Centre For Material Forming (CEMEF) in Mines ParisTech, the Process and Engineering in Mechanics and Materials laboratory (PIMM) in Arts et Métiers ParisTech, along with GETELEC company submitted to Mat&Pro in the National Agency for Research (ANR) a project entitled Micronnect. This project aimed at elaborating a new concept for a microinjection machine, studying the rheology at high deformation rates, and for us in the PIMM laboratory, understanding and determining the influence of high deformation speeds on the microstructures and the properties induced in the semi-crystalline polymers, the large majority of polymers used today in microinjection. Studies were carried out on one high-density polyethylene (HDPE) and one polyamide 12 (PA12) that were injected in low thickness plates (200, 300 and 500 µm). The optical microscope provided analysis showing that the " skin-core " morphology is only formed by two visible layers: a transparent skin layer of a constant thickness, and a quite uniform core layer. Combined analysis of diffusion and diffraction of X-radiations (SAXS and WAKS) with a synchroton microbeam allowed us to determine the microstructure induced in the thickness of the pieces. Unlike the higher-thickness injection, the skin layer is formed of shish-kebabs and turns out to be the most oriented layer. Thickness plays a preponderant role with microstructures in shish-kebabs that reach the core once the thickness decreases to 0.3 or 0.2 mm. The flow length, the injection time, as well as the mold's temperature are also significantly influential. We will seek to reach the fastest possible injection times, a few hundredths of a second, in order to allow, simultaneously, a good filling of the molded cavities, and a decrease of crystalline orientations, leading, thus, to a better relaxation of the polymers chains after the filling and before the crystallization. Therefore, one may understand that classic injection technologies ought to evolve.
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Submitted on : Wednesday, December 12, 2012 - 12:52:20 PM
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Nada Bou Malhab. Microinjection des polymères semi-cristallins : Microstructures et textures des matériaux. Polymères. Arts et Métiers ParisTech, 2012. Français. ⟨NNT : 2012ENAM0045⟩. ⟨pastel-00764106⟩

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