Flowability and fiber content homogeneity of natural fiber polypropylene composites in injection molding

Abstract

As an index of viscosity, flowability of natural fiber polypropylene composites is assessed by spiral mold test using injection molding machine. The effect of the processing parameters like the injection temperature at 180-220°C, injection pressure at 500-1000 bar and the mold temperature of room temperature and 80°C on flowability is studied. The investigated parameters are the fiber shape (Non branched straight cellulose versus low flexural stiffness and branched hemp fibers), fiber length (0.5 versus 1.5 mm chopped cellulose fibers) and fiber content Wf (10 versus 30 wt%). The results show that the most significant parameters in a descending order are the pressure, temperature and the mold temperature. Another aspect of the product quality is the homogeneity of the fiber content through the whole injected spirals. Therefore samples are taken out at even distances along the spiral. The cut samples are dissolved to extract fibers and calculate the fiber content. Fiber content definition along the injected spirals proved that it is not constant and the deviation in its measurements can reach to almost 30% from the nominal value. The results prove the separation of fibers before being injected specially in cases of low pressure and low injection temperatures. Secondly, the fiber content varies in a cyclic way. The number of cycles increases with the increase of temperature and pressure but with smaller amplitude of variation. The variation of the fiber content in cellulose fluctuates around an almost constant value which is not the case with hemp variation. Thirdly, the fiber content shows in most of the cases a parabolic trend with a minimum point just shortly after the input injection point. Fourth, the change in fiber length behaves more or less like change in fiber content especially at both low pressure and temperature. Fifth, the variation in fiber content (Wfmax – Wfmin) along the spiral is clear in composites with hemp fibers at high processing temperature / pressure. These results about variation in fiber content are studied in terms of the measured fiber dimensions. Fiber length is defined by the dynamic image analysis system QICPIC. Fiber length proves that the longer the fiber length, most likely the higher the fiber content. However, these results are still in need to be deeply analyzed and better understood in order to open the door in future for robust simulation.