Research On Micro-foaming Injection Molding Process Of Polypropylene And Its Composites And Mechanical Properties Of Micro-foaming Plastic Parts

Lightweight structure is of great significance for resource conservation,energy saving and emission reduction.At present,using polymer composites to replace conventional metal has gradually become the main way to reduce the weight of components.Micro-foaming technology can introduce microporous structure into polymer composite components,which can not only achieve lighter products and reduce material consumption,but also help improve the specific strength,specific modulus,fatigue life and thermal stability of the components.At the same time,it gives the components heat insulation,sound absorption,shock absorption and other functional characteristics.Among many foaming processes,microcellular injection molding(MIM)is a flexible,efficient and environmentally friendly foaming technology that can directly form microcellular plastic foam products with complex shapes.However,conventional microcellular injection molding(CMIM),which is commonly used at present,generally suffers from the problems of reduced mechanical properties,limited weight reduction,and poor surface appearance of molded samples.The mold-opening microcellular injection molding(MOMIM)process adds the pressure holding and mold opening process on the basis of CMIM.Its technical feature is that after high holding pressure,the mold can move a certain distance in the mold opening direction which provides more space for polymer foaming and effectively improves the uniformity of cell distribution and size on the microstructure.Therefore,MOMIM can obtain plastic parts with a higher expansion ratio,which can achieve higher weight reduction goals.At the same time,the surface quality has also been significantly improved due to the existence of the high pressure holding process.Polypropylene(PP)has become one of the most widely used thermoplastics due to its excellent mechanical properties,low cost,and low density.However,due to the linear chain structure of PP,its melt strength is low and foaming ability is poor,and it is difficult to realize plastic parts with high expansion ratios.The introduction of glass fiber and calcium carbonate powder into the polypropylene matrix can effectively improve the melt strength and crystalline properties of the polypropylene,thereby achieving the goal of improving the foaming ability.Theoretically,the lightweight microporous foam material prepared by combining MOMIM with polypropylene shows a promising prospect in providing light and strong plastic products that can be used in automobiles,electronics,and appliances.This paper combines MIM with polypropylene composites,based on two polypropylene composite systems,by using CMIM and MOMIM processes,and using supercritical N2 as the physical blowing agent to prepare microcellular composite foam samples.The performance of each sample under the two processes and the influence of various process parameters in MOMIM on the cellular morphology and mechanical properties are discussed.The main conclusions are as follows:(1)The microstructure,crystallization properties,rheological properties,thermal stability,mechanical properties,and dynamic thermodynamic properties of polypropylene/calcium carbonate(PP/CaCO3)and polypropylene/glass fiber(PP/GF)composites are studied.The results show that the glass fiber and calcium carbonate powder can enhance the composite viscosity,thermal decomposition temperature,tensile modulus,and dynamic thermodynamic properties of the matrix,but will reduce the crystalline properties of the matrix.(2)Based on the polypropylene/calcium carbonate composite,the microscopic morphology,mechanical properties,and surface quality of MOMIM and CMIM were compared.The results show that MOMIM can uniform cell size and distribution,enhance the specific modulus of plastic parts,and reduce surface roughness.(3)Based on the controlled variable method,the influence trend of various process parameters on the microcellular structure of plastic parts in the MOMIM is studied.Among them,the increase in the injection rate and mold temperature reduces the uniformity of cell distribution and reduces the thickness of the skin layer.The cell structure is particularly sensitive to the pressure holding time.If the pressure holding time is too short,the cell size will be large,but if the time is too long,the cells will disappear.The mold opening distance is a key parameter for preparing samples with different expansion ratios.The larger mold opening distance,the more obvious the stretching of the cell wall along the mold opening direction.In terms of mechanical properties,the MOMIM samples show superior specific tensile and flexural modulus values compared to CMIM samples at the same magnification.(4)A lightweight and high modulus polypropy lene/glass fiber composite foam was prepared using MOMIM,which achieved a weight reduction rate of more than 60%,which was still higher than the flexural modulus value than un-foamed composites.The microscopic cell morphology and mechanical properties of each sample under different glass fiber content and mold opening distance were compared.It is concluded that the high content of glass fiber can refine the cell structure,uniform the cell structure in the thickness direction,and can significantly enhance the bending resistance and rigidity of the composite matrix.