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This paper characterizes and analyzes the microstructures of injection-molded polypropylene parts reinforced with 20 wt% of hemp fibers in order to understand the process induced variations in thermomechanical properties. In-thickness fiber orientation and fiber content were determined by X-ray tomography along the flow. The fiber content along the flow path was also determined by direct fiber content measurements after matrix dissolution, showing an increase of 2%/100 mm for a 2.2 mm-thick plate due to fiber migration during the filling stage. A typical shell/core structure for fiber orientation in injection molding was observed, but with a very clear transition between the layer solidified under high shear rates and the core in which the fiber content was reduced by more than 50%. The orientation of hemp fibers is lower than the one of glass fibers, especially in thickness direction. However, the overall fiber orientation in the injection direction induces significant anisotropic thermomechanical properties, which cannot be explained by simple micromechanical models that consider isotropic mechanical properties for hemp fibers. These phenomena must be taken into account in process simulation codes for injection molding to better predict thermomechanical properties as well as part shrinkage and warpage to design molds.

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An original methodology using Digital Image Correlation (DIC) has been designed to precisely measure full-field shrinkages of injection molded polymer plates and then to give the opportunity to compare quantitatively extensive numerical simulations to experiments. The principle of the methodology is based on the full-field strain determination between a reference image of the mold and that of injection-molded parts, which are 275 × 100 × 2.2 mm3 plates. To allow for DIC calculation, 50 µm-depth engravings were machined by electro-discharge process at the surface of the mold. The result of the analysis is a 2D full-field shrinkage map over the whole plate surface (i.e. flow and transverse), with a standard deviation of 0.03%. The marking density has been shown to have a roughly linear influence on the precision of shrinkage measurement. This methodology allows the quantification of the effect of several injection parameters on in-plane shrinkage fields: holding pressure, injection flow rate and direction, geometry of injection gates, or geometrical constraints. Once the best set of parameters of material constitutive laws is identified for the simulation of polymer plates, the simulation procedure is ready to be applied on more complex 3D geometries.