| The properties of foamed polymers are closely related to cellular quality.Thus,improving cellular quality has been a popular research direction of researchers all over the world.Foaming behavior under the flow field in the molding process of polymer foaming has been studied and made certain achievements by some scholars,but the factors affecting foaming behavior in the injection molding process are very complex.It is important to regulate the flow field of injection molding through molding methods,material intrinsic characteristics,and technological factors,to improve the foaming quality.In this paper,a kind of polypropylene composite material with stable intrinsic properties was prepared,and the influences of foaming behavior under the flow field of chemical foaming injection molding were explored.The following results had been achieved:(1)A polypropylene composite with stable intrinsic properties was prepared by material composition design.It was found that PH50 with HMSPP content of 50 wt%had the most stable intrinsic properties and maintained consistent rheological properties at different cooling rates.Its crystallization behavior and melt strength were ideal.This is beneficial to obtain excellent foaming quality and stable bubble structure in chemical foam injection molding.At the mold temperature of 120℃,the average cell diameter and cell density of the foaming products were 19.03μm and 1.9×10~7cells/cm~3,respectively.(2)Compared with FT0 and FAMP,FPH50 not only had an ideal foaming quality but also a very stable surface quality.FPH50 had a much smoother surface,and its surface quality has a strong correlation with the cooling rate.At 120℃mold temperature,foaming products with a surface glossiness of 53.7 GU could be obtained,and almost no surface defects can hardly be observed.(3)The visualization system was used to study the foaming behavior of polypropylene composite at the mold-filling stage of short-shot foaming injection molding.It was found that increasing the injection pressure had an effect on the flow velocity field at the melt front.Under the premise of keeping the same weight reduction,the quality of short-shot foam can be improved by reducing the pressure drop.The final cell structure of 47.98μm and 1.2×10~6cells/cm~3can be obtained by the nozzle with the 5mm aperture.The size was reduced by 56.6%and the density was increased by 8.23times.(4)The study of the flow field on foaming behavior in the mold-filling stage of core-back foaming injection molding showed that the increase of the injection pressure and the injection speed had an inhibitory effect on the nucleation of the bubble in the mold-filling stage,while the injection amount and the injection time had little effect.Through the study of foaming behavior under the flow field of different diameters nozzle,we found that the number of cells in the filling process is three to four orders of magnitude smaller than that in the final state.(5)The flow field study on foaming behavior in the core-back stage of core-back foaming injection molding showed that increasing core-back pressure and core-back speed can both reduce the size and increase the density of cells,but the increase of core-back pressure would lead to the deformation of cells and worsen the cellular quality.With 90%core-back pressure,the cell size and density were 16.35μm and3.74×10~7cells/cm~3,respectively.With 90%core-back rate,excellent cellular structure with cell size and density of 18.90μm and 3.36×10~7cells/cm~3was obtained. |
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