| The development of electronic and electrical equipment has brought great convenience to people’s lives,and the innovation of 5G communication technology has become a“booster”for the revolution of the national science and technology and also the comprehensive national strength.However,the problem of electromagnetic radiation and interference generated by the information transmission and interaction of electronic equipments is becoming increasingly serious,which not only would seriously affect the normal operation of the equipment and cause information interference and leakage,but also would do harm to human health.As a result,the preparation of the corresponding electromagnetic shielding materials has become an urgent problem with the rapid development of communication technology.At present,the preparation of electromagnetic shielding materials and parts is mainly based on traditionally melt processing technologies,e.g.extrusion,injection molding,hot pressing and casting,which have difficulties in processing and shaping the desired electromagnetic shielding parts with diversified and complicated structures.Hence,it is hard to meet the urgent requirements of high-speed development of the information elctronics for the high-performance electromagnetic shielding parts.In order to overcome such a challenge,it is urgent to establish and develop some new methods and new technologies to fabricate the electromagnetic shielding parts with complicated,diversified and personalized structures.The“additive manufacturing”,also called as 3D printing technology,has shown the unique advantages in the design and fabrication of the complicated models,which is expected to provide a new way for the preparation of the multi-structure and multi-size target shielding parts.But there are still some problems need to be solved,e.g.the printing theory is not complete,the printable materials is deficient and especially there is lack of the functional materials with high-efficiency electromagnetic shielding property.With regard to the above challenges,this study aimed to develop the fused deposition modeling（FDM）3D printing technology by using the finite element simulation（FES）to deeply analyze the flow behaviors of polymer melt during FDM printing process and further establishing a FDM printing flow theory.Under the above theoretical guidance,a novel strategy based on GNPs local enrichment was proposed,and the high-flowability and multi-functional polylactic acid/graphene nanoplates（PLA/GNPs）and polylactic acid/graphene nanoplates/multi-walled carbon nanotubes（PLA/GNPs/MWCNTs）printing filaments were thus prepared,providing a novel method and new idea for fabrication of functional materials,which are suitable for FDM 3D printing.Finally,a series of PLA-based electromagnetic interference shielding parts with the complicated/diversified structures and the desired shielding property were designed and further fabricated by using the above novel functional filaments.This is the further development and application of FDM 3D printing technology in electromagnetic shielding fields.In this paper,the innovative work can be illustrated from the following aspects:1.The complicated flow behaviors of polymer melt during FDM 3D printing process were investigated by using numerical simulation,and the distribution characteristics of various melt parameters under the effect of external field（temperature-field and stress-field）were also quantified.The simulation results showed that in FDM 3D printing process,the velocities of melt were in the range of 4.8×10-4～2.2×10-1 m/s,the shear rate was in range of 6.5×10-1～2.5×103s-1,and the pressure was in the range of 1.6×106～8.7×106Pa,respectively.Meanwhile,the laminar characteristics of the melt were revealed by the Reynolds number(1.1×10-6～9.8×10-4),which was theoretically calculated out in FDM printing.Also,the Weissenberg parameters of melt in FDM printing process were quantified to evaluate the effect of the shear action on orientation structure of the molecular chains.2.Based on the modified-Frenkel bonding theory,the interlayer bonding mechanism of filaments were theoretically analyzed and revealed.The growth process of thin necks and the bonding efficiency between PLA filaments at different temperatures were accordingly investigated.The temperature variation factor for melt surface tension in bonding process was obtained by fitting（Δ（38）/ΔT=0.29 m N/m·K）.Further,under the guidance of the above bonding mechanism,the effect of interfacial bonding efficiency on the interlayer bonding strength of printed parts was investigated and the printing parameters for preparation of parts with strong interlayer bonding were accordingly optimized.Moreover,this study also built a theoretical basis for high-efficiency preparation of printed parts.3.A novel local enrichment technology for GNPs was proposed for preparation of the high-flowability and multi-functional PLA/GNPs printing filaments,and the corresponding FDM 3D printing was successfully realized.By comprehensively comparing the conventional PLA/GNPs printing filaments prepared by the melt-compounding method and the novel filaments prepared by the local enrichment strategy,the former exhibited the much better performance and functionality（including higher electrical,thermal and electromagnetic shielding properties）than the conventional one.The result showed that the rheological percolation threshold was decreased from 3.48 wt%for the conventional method to 1.59 wt%for the novel method,the electrical conductive percolation threshold was decreased from3.97 wt%to 1.70 wt%,and the thermal conductivity of the optimal sample was increased to 0.52 W/m·K,which was 108%higher than that of pure PLA sample.The total electromagnetic interference shielding efficiency was increased up to 34.4d B（8.2～12.4 GHz）,which was 52%higher than that of the conventional one.This novel technology could provide a novel method and new idea for preparation of functional materials suitable for FDM 3D printing.4.In order to solve the problem about the deterioration in mechanical properties caused by the excessive addition of single component filler of GNPs and achieve the high-efficiency electromagnetic shielding of composites and parts,the carbon-based hybrid fillers（GNPs/MWCNTs）,with the strong synergistic effect,were introduced into the PLA matrix.The mechanical property test results showed that the yield strength and Young’s modulus of printed parts were increased to 55.0MPa and 3095 MPa,which were 18.2%and 32.0%higher than those of the pure sample,respectively.The evaluation of the electromagnetic interference shielding revealed that the electromagnetic shielding efficiency could be up to 36.8 d B,which could fully meet the standards of the shielding material applications.In addition,the electromagnetic shielding efficiency of the printed porous electromagnetic shielding parts was found to reach～36 d B（8.2～12.4 GHz）,which has exceeded the standard of electromagnetic shielding for commercial apllication（20 d B）.Furthermore,the effects of cell structure and cell size of printed porous parts on the electromagnetic interference shielding performance and the shielding mechanism were also systematically investigated.The critical cell size of the shielding parts in a specific band is proposed for the first time（in X-band frequency,the critical value of PLA-based porous parts is 4.24 mm）,which laid a theoretical foundation for design and printing of multi-structure and multi-size shielding parts.In this way,under the guidance of shielding theory,a series of PLA-based electromagnetic interference shielding parts with diversified structures and desired shielding property were successfully printed.These shielding parts mainly include the porous electromagnetic shielding ventilation plate,the hollow electromagnetic shielding ring and the electromagnetic shielding radiator.The related investigations could further promote the development and application of FDM 3D printing technology in electromagnetic shielding fields.In conclusion,in this innovative study,we have successfully established a FDM 3D printing flow theory suitable for analysis of the flow behaviors of polylactic acid-based melt,developed a novel method and technology for preparation of the functional filaments,and finally designed and printed the commercial electromagnetic shielding parts with complicated and diversified structures.This could lay a theoretical and experimental foundation for the successful application of the obtained novel functional materials and the corresponding FDM 3D printed parts in electromagnetic shielding fields. |
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