| At present,with the breakthrough of micro-nano processing technology bottleneck,the use of a micro-nano injection molding process to prepare polymer parts containing micro-nano structure has become a hot topic of current research.Polymer injection molding is a non-isothermal and non-steady state flow molding process.It is different from the macro injection molding process.Due to the effect of micro-scale effects,the wall slip at the micro-nano scale and the heat conduction at the melt-mold interface have a great influence on the polymer molding quality.However,due to the complexity of the micro-flow of the polymer in the molten state,it is inconvenient to carry out experiments,and it is difficult to accurately quantitatively analyze and obtain the physical quantity at the micro-scale of the fluid,which brings great difficulties to the research.At present,the mechanism of wall slippage and the heat conduction law of the melt-mold interface under micro-nano-scale flow conditions have not been fully understood.This paper uses molecular dynamics calculation methods,which can make up for the shortcomings of theoretical methods and macro experiments in exploring the micro-nano-scale flow phenomena of polymer polymers,and explore the relationship between the wall slip formation mechanism and the thermal boundary at a deep level.The impact will ultimately provide a theoretical basis and scientific guidance for the improvement of plastic product quality and the development of new micro-nano injection molding processes.Aiming at the mechanism of the wall slip of the polymer melt,this paper takes the molten polymer Poiseuille flow between parallel plates as the research object,and polymethyl methacrylate(PMMA)as the research object Materials,according to Navier’s slip law,the use of Materials Studio software to establish the molecular dynamics flow atomic analysis model of polymer macromolecular wall slip.Firstly,the open source calculation program LAMMPS was used for dynamic calculation,and the influence mechanism of the wall wettability,external driving force and shear rate on the wall sliding behavior of polymer macromolecules was studied,and the near wall surface under different shearing conditions Spatial morphology evolution process of entanglement-untangling of polymer chains nearby.Analyze both the velocity slip state and the structural characteristics of the molten polymer during the micro-nano channel;then,the thermal conductivity changes under different external load conditions are discussed,and the boundary slip and thermal conductivity are also analyzed.The interaction between the two;finally,the micro-injection heat transfer experiment was used to study the change of thermal conductivity under different external loads.The purpose was to analyze whether the influence of the macro and micro scale sliding on the thermal resistance of the interface is regularly consistent.At the same time,moldflow numerical simulation software was used to simulate the injection filling length of the important boundary condition interface heat transfer coefficient values obtained under different conditions(molecular dynamics simulation,heat transfer experiment,moldflow default value),and then compared with the actual filling experiment.The validity of the boundary condition parameters in the simulation of injection molding filling at the micro-nano scale is determined.Research results:(a)When the mold wall property is strongly hydrophobic,the polymer melt exhibits significant boundary slip velocity at both sides of the wall boundary.When it is strongly hydrophilic,the boundary slip velocity of the polymer is approximately zero;(b)As the external driving force increases,the boundary slip velocity increases accordingly,but after exceeding a threshold force,the growth rate of the boundary slip velocity gradually becomes smaller.The slip length increases continuously as the driving force increases,but after exceeding a certain value,the slip length growth rate begins to gradually decrease;(c)PMMA molecular chains can maintain the original random winding state in the early stage of shearing or when the shear rate value is low.However,as the shear rate increases or the shearing time becomes longer,the molecular chain groups gradually start to separate,and the single molecular chain unwraps from the entangled grid,or detaches from the wall surface.The mean square displacement curve and the self-diffusion coefficient prove that the molecular chain breaks the kinetic energy barrier and the winding between the polymer chain and the chain,the main reason is the shear rate conversion;(d)The existence of speed slip will reduce the interface thermal resistance between the mold and the polymer melt,enhance the interface thermal conductivity,and is not conducive to filling the polymer melt at the micro-nano scale;(e)Under both micro and macro conditions,the existence of speed slip causes the reduction of interface thermal resistance.The difference is that slip at the micro-nano scale is not conducive to mold filling,but it is not affected by the size effect under macro conditions,and the effect of wall slip on heat conduction is not significant,Under the action of strong shear,the viscosity of the polymer melt will become smaller,which is conducive to mold filling flow.At the same time,it is shown that the boundary parameter interfaces thermal resistance value calculated by molecular dynamics can more accurately reflect the heat conduction law of the melt-mold interface at the micro-nano scale. |
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