| Polypropylene plastics have strong corrosion resistance,high low-temperature strength,easy processing,and light weight,and are widely used in industrial production.Taking the automotive industry as an example,each car has approximately 200 plastic components,most of which are made of polypropylene.With the increasing attention and urgent demand for energy-saving and emission reduction technologies in the economy and society,lightweight materials and structural components have become important development directions in the manufacturing industry.Injection molding is an important processing method for polymer plastic parts,and microcellular injection molding(MIM)is a very promising technology for achieving lightweight polymer plastic parts.Although MIM plastic parts have excellent properties such as high specific strength,sound insulation,shock absorption,and energy absorption.However,there are still many technical bottlenecks in this technology,such as low product foaming rate,uneven foam cell structure,and low mechanical properties,which limit the promotion and application of MIM technology.There is an urgent need to study the control process method of microcellular foam injection molding and its mold design technology.First of all,in order to enhance the foaming rate and improve the cellular structure of polypropylene foam plastic parts in the MIM process.The MIM molding process mechanism of polypropylene blend modified materials(hard)was studied.Secondly,in response to the problems of low melt strength and poor uniformity of plastic foam pores in the MIM process polypropylene/elastomer physical blend material(soft).Studied the MIM molding process of polypropylene/elastomer physical blend modified materials(soft).Then,in order to pursue high foaming rate,better cell morphology,and mechanical properties,the binary(PP/PTFE)and ternary(PP/PTFE/Talc)modified polypropylene composite materials MIM and MOMIM(Mold Opening microcell injection molding)molding processes were studied,respectively.The methods for improving foaming rate and mechanical properties were elaborated.Finally,applied research was conducted.A MOMIM mold design method was proposed,and problems such as easy jamming of the mold sealing frame,easy bulging of product edges,and poor surface quality of microporous foam products were analyzed and explored.The feasibility of the newly developed mold design method has been verified through the successful development of multiple application cases.The main research work and results of this article are as follows:(1)In order to enhance the foaming rate and improve the cellular structure of polypropylene foam plastic parts in MIM process,a method of modifying PP by blending polypropylene and polyethylene was proposed.Three composite materials were prepared:block copolymer polypropylene(BCPP),BCPP/random copolymer polypropylene(RCPP),and BCPP/high-density polyethylene(HDPE).Studied and revealed the relationship between the nucleation patterns of foam cells and mechanical properties of three types of PP composite material with MIM molding processes.Research has shown that with the increase of injection speed,melt or mold temperature,the thickness of the solid skin layer gradually decreases;There is no significant correlation between injection speed and cells density;The cell density has little effect on the elongation at break,while the thickness of the solid layer has a great effect on the elongation at break;The foam BCPP/RCPP sample has the best tensile properties,and its tensile toughness is higher than that of the solid sample.The impact performance of BCPP/HDPE foam sample is the best,and its overall impact strength is slightly higher than that of BCPP/RCPP foam sample;The tensile toughness of foamed BCPP/RCPP samples is the best,but the impact toughness is the worst,indicating that their impact performance is relatively sensitive to deformation rate.(2)There is relatively little research on the physical blend material(soft plastic)of polypropylene/elastomer in MIM process,which has low melt strength and poor uniformity of foam cells in microcellular foam plastic parts.A method was proposed to manufacture TPO through physical blending of PP and elastomer,and to improve the dispersion of talc powder by adding Talc and coupling agent,thereby improving the melt strength of TPO composite materials.Through the MIM process experiment of TPO/Talc composite materials,it was revealed that the addition of Talc can refine the foam structure of plastic parts,increase the density and uniformity of foam cells.The tensile properties of the four prepared TPO/Talc composite MIM process plastic parts were tested,and the results showed that their tensile strength and modulus were both improved to a certain level.With the increase of talc powder content,the tensile strength and modulus showed a trend of synchronous improvement.Compared with pure TPO plastic parts,the tensile strength and modulus of TPO composite with a talc content of 10wt%increased by 9.3%and 45.9%,respectively.This is mainly attributed to the refinement of the matrix crystal structure and bubble structure,as well as the reinforcement effect of talc powder.(3)A microcellular foam plastic component designed to achieve higher foaming rate,better cell morphology,and mechanical properties.A fiber reinforced PP composite material was prepared by blending PTFE and PP,and its crystallization behavior and rheological properties were studied.PP and PP/PTFE microcellular foam parts were prepared using MIM and MOMIM processes.The results indicate that compared with spherical PTFE materials,in-situ fibrillar PTFE materials are more conducive to accelerating the crystallization of PP,refining crystallization,and enhancing its viscoelasticity.Compared with PP raw material foam plastic parts,the tensile strength and impact strength of PP/fibrillar PTFE(MOMIM)foam plastic parts can be increased by 21%and 133%,respectively.Meanwhile,compared with PP raw material foam plastic parts,the thermal conductivity of PP/fibrillar PTFE foam plastic parts can be reduced by 32%.In addition,compared to plastic parts manufactured by the MIM process,plastic parts manufactured by the MOMIM process have better surface quality.(4)In order to further improve the foam density and mechanical properties of plastic parts,a study was conducted on the process of ternary modified polypropylene(PP/PTFE/Talc)material.Due to the incorporation of in-situ fibrilized PTFE nanofibers and talc powder,the melt strength and crystallinity of PP matrix are significantly improved,thereby improving the foaming ability of MIM and MOMIM processes,resulting in a refined cellular structure.The experiment shows that the ternary blend PP foam samples prepared by conventional MIM and MOMIM process exhibit good cells structure and mechanical properties,including an increase of 341%in tensile toughness,408%in notch impact strength,and 121%in bending strength.(5)The author of this article is an engineering doctoral student who combines daily work with academic research through the resources of the entrusted training unitQingdao Hisense Mold Co.,Ltd.A set of MOMIM process Injection mold construction was designed and manufactured,and an industrial experimental production line was established.The design method of Injection mold construction for MOMIM process was studied,and the design method of product edge,the sealing design method of Injection mold construction and the design method of air inlet and exhaust system of Injection mold construction for MOMIM process were proposed.The problems such as the sealing frame of open mold microcellular foaming Injection mold construction is easy to jam,the product edge is easy to bulge,and the product surface quality is poor were analyzed and explored.Finally,based on the research and development technology,dozens of industrial injection molding products were developed with MIM processes and molds,verifying the practicality of the MOMIM process,mold design methods,and supporting technical guidance documents developed. |
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