Study On The Compressive Properties Of Polypropylene And Its Composites Foamed Injection Products With Multiple Penings

As a lightweight material,microporous plastics are widely used due to their low cost,low density and good insulation properties.Microcellular foams are usually prepared by stripping the polymer matrix with gas at high pressure and then imposing sudden thermodynamic instability during the foaming process.Foam parts made by foam injection molding(FIM)show great differences in mechanical properties compared to solid parts made by conventional injection molding(CIM),due to differences in pore structure.Foam hole is easy to merge and collapse in the process of refoaming,resulting in poor foaming effect and poor performance of parts.Therefore,it is necessary to study the effects of bubble density,bubble size and bubble orientation on the foaming performance.This paper takes polypropylene(PP)and its blends with low density polyethylene(LDPE)as raw materials,blends according to the set proportion,and adding a certain proportion of chemical foaming agent made by the laboratory,adjusts the injection molding process parameters to prepare the corresponding PP and PP/LDPE microporous foaming materials.During this period,The process of gas dissolution and desorption is controlled by reciprocating compression and expansion of the gas-containing melt,and the orientation of the bubbles of the sample is controlled by closing the mold in the mold after the sample is cooled.A section of PP and composite injection molding samples obtained by injection molding was cut off,and the mechanical properties of different injection molding samples were obtained by compression test using an electronic universal testing machine,and the influence of bubble structure was determined.Crystallization properties of different injection samples were tested by differential scanning calorimetry(DSC).Finally,different injection samples were soaked and quenched in liquid nitrogen,and then the microscopic morphology and pore structure of the materials were observed by scanning electron microscopy(SEM),and the results were recorded.Subsequently,the pore morphology of each sample was analyzed by software to determine the influence of various processes.The compression strength and Young's modulus of PP will change under different mold opening and closing sequence.The compression strength and Young's modulus of the sample will increase when the mold is closed for the last time.At the same time,PP microporous foaming material has a better bubble structure and the diameter of the bubble cavity decreases obviously in the mold closing state.The pore structure also affects the crystallization performance of PP.The smaller diameter of the pore makes the molecular chain move more easily,and the resistance of regular arrangement becomes smaller,which improves the crystallization performance.The compression strength and Young's modulus of PP/LDPE will change under different mold opening and closing sequence,and the changing trend is similar to that of PP material.The compression strength and Young's modulus of the sample will increase in the case of the last clamping,but the variation range is smaller than that of PP material.At the same time,PP/LDPE microporous foaming material has a better cellular structure and the cellular density increases significantly,which is up to 5times of that of ordinary injection molding sample.At the same time,the addition of LDPE can increase the melt strength and improve the bubble structure of PP material.The pore structure also affects the crystallization performance of PP/LDPE.The smaller diameter of the pore makes the molecular chain move more easily,and the resistance of regular arrangement becomes smaller,which improves the crystallization performance.The compression strength and Young's modulus of the injection foamed samples will change under the influence of the orientation of the foamed pores caused by the opening and closing of the mold.When the sample thickness is the same,the sample with the bubble orientations parallel to the compression direction has higher compression strength and Young's modulus.After moving the opening and closing mold for several times,the bubble structure of the microcellular foaming material was further improved,the diameter of the bubble cavity became smaller and the density of the bubble cavity became larger.The crystallization temperature and crystallinity of plastics are increased when the orientation of the bubble is parallel to the direction of compression.Along with the above processing work,PP and PP/LDPE microporous foaming materials with different cellular structures were prepared.The results show that the cavity density and the cavity orientation can be adjusted by using the appropriate mold opening and closing operations,resulting in the final mechanical properties improved.In particular,samples with high density bubble holes oriented in the compression test direction are manufactured by a controlled die opening and closing operation,which contributes to the increase of compressive strength.The work in this paper is helpful to various experiments and practices in improving the mechanical properties of PP products and improving the microporous structure of PP.The research in this paper also has a better exploration significance for promoting the understanding of the relationship between the bubble density,the bubble orientation and the macroscopic properties of PP microporous materials.