Research Of Roll-to-roll Powder Hot Embossing Forming Process For Fabrication Of Micro-structures On Polymeric Film

In recent years,new energy and energy-saving technology,new media and information technology has achieved rapid development,and optical film with the surface of the fine structure attracted widespread attention.As the typical microstructures,micro-pyramid arrays structure having a high optical performance and the advantages of large specific surface area,is the ideal structure of the diamond grade reflective film solar trap,bioreactors and the like.However,the apex angle,dihedral angle and height of the micro-pyramidal structure have a significant impact on the condenser performance of the micro-pyramid arrays.Roll-to-roll hot embossing is considered as an ideal method for a large area,continuous and efficient manufacture of the polymer film with micro-pyramid arrays structure.However,the application of a continuous roll-to-roll hot rolling process on the different kind of polymer films with embossed micro-pyramid arrays structure was observed four typical forming defect modes in terms of the platform-like defect,the collapse-like defect,the high-low defect,and the bubble-like defect,which severely restricts the efficient application of this process.Combined the advantages of roll-to-roll hot embossing,continuous efficiency and high precision structure of powder metallurgy technology,we proposed a new process to manufacture the polymer microstructure,roll-to-roll powder hot embossing,which makes the fabrication of the micro-pyramid arrays structure on polymeric film possible.It uses polymeric micro powder to substitute polymeric film in the conventional roll-to-roll hot embossing process.Due to the combination of the two processes advantages,this process seems to be a promising technology to achieve different industrial applications at lower cost.In addition,some process defects of roll to roll hot embossing and powder hot embossing,such as spring back and continuity,seem to be effectively addressed.To further demonstrate the feasibility of roll-to-roll powder hot embossing,the process was systematic experimental studied in this paper,and the main work and conclusions are as bellow:1)Build test platform of roll-to-roll powder hot embossing andfeasibility experimental analysisBy adding the polymer powder poured device in the laboratory conventional roll-to-roll film hot embossing apparatus,experimental test platform of roll-to-roll powder hot embossing was built,and the micro-pyramid arrays structure mold was selected to validate process feasibility.Reduce the adhesion effect between the polymer powder material and the mold surface and forming defects in roll-to-roll powder hot embossing,and enhance process efficiency through surface treatment of the micro-pyramid arrays structure mold.Based on the micro-pyramid structure characteristics,evaluation indexes of the roll-to-roll powder hot embossing feasibility experiments,such as micro-pyramid shaped height and inclination,were confirmed.Using a laser scanning confocal microscope to analyze the evaluation indexes of the feasibility experiments,and then verify the feasibility of roll-to-roll powder hot embossing and experimental test platform.Moreover,clear "stratification phenomenon" in the cross section of a forming microstructures sample was observed.2)Finite element simulation analysis of powder hot embossingThe Melting point,thermal conductivity and specific heat capacity of the powder material as the significant thermodynamic parameters were also measured.It also measured the average particle size and size distribution of the powder particle.They are all essential material parameters to a systematic study of relationship between the powder material properties and the roll-to-roll powder hot embossing process.The finite element simulation model of roll-to-roll powder hot embossing was established according to the characteristics of the process and the measured properties of the polyethylene powder.The finite element simulation method was used to analyze the results of the powder particles deformation and the heat transfer in the process,which cannot easily obtain through the experiments.The reason of the "stratification" phenomenon in the cross section of the sample is studied,and the reference range of the experimental parameter is provided for the subsequent roll-to-roll powder hot embossing.1)Process parameters and forming process window experimental analysisBased on the feature of roll-to-roll powder hot embossing,analyze heat transfer process of the polymer powder melting and solidification forming,which obtain the maximum of the powder mass without "stratification phenomenon" after different process parameters are inputted,and roll-to-roll powder hot embossing process is further optimized.Process parameters studies have been made by considering one variable at a time.Out of the several controllable parameters available on the control panel of the roll-to-roll hot embossing machine,mold temperature,feeding speed and applied force were chosen as the input parameters to investigate the effect of process parameters on micro-pyramid arrays structure forming quality.By measuring the height and angle between edge and the bottom surface of the forming pyramid structure,assess the influence of mold temperature,feeding speed and applied force on the forming quality of roll-to-roll powder hot embossing.In addition,use response surface analysis to obtain forming process window of polyethylene,which provide a reference for the applications of roll-to-roll powder hot embossing.