Morphological Evolution Of Molecular Structure And Interface Behavior Of Plastic Products' Epidermis Affected By The Mold Temperature With Numerical Simulation

Injection molding is a complex physical process,which is the high temperature of non-newtonian polymer melt enter the lower cavity through the gating system under the packing pressure.Polymer fluid easily cause the melt solidification,shrinkage and crystallization behavior on account of high-speed shearing and the contact between the hot melt and the cold cavity.The appearance of the products and its mechanical property depend on the quality of plastic skin layer.Furthermore,weld lines is easily formed by flowing together of two streams,when the injection molding is in some condition,such as the way of the melt into the cavity,the product structure and the inhomogeneity of wall thickness.Moreover,the appearance and the mechanical properties of the product is affected by the existence of the weld lines.The complex flow phenomena observed in Macro scale was taken control of the microstructure of viscoelastic fluid itself during the filling stage.Therefore,it is the important and scientific significance thing that the morphological evolution of the melt microscopic molecular configurations was studied by computer simulation.The forming process of polymer melt was researched under different mold temperature based on the model between polymer and mold,Compass force field,3D periodic boundary,initial velocity of Boltzmann distribution and the summation method of Ewald.Firstly,the metal mould mode was constructed by a series of steps,such as importing Ni cell,cutting cell srystal,extend the cell and 3D boundary treatment.Then,a series of structural optimization were carried out by the Forcite module,for instance,geometry optimization,annealing treatment and dynamic relaxation.The metal mold was under different working temperature after the NVT dynamics simulation.Secondly,the polymer cell was constructed based on the lattice parameter of the metal mold,and the same optimization methods and computational method were used.In addition,the NHL method was selected as the temperature control based on high precision and higher rate of convergence,and the polymer mold was in the molten state based on the NVT ensemble.Then,polypropylene was chosen as the experimental material to research molecular mechanism of the product epidermis during the filling stage.Then,the model between polymer and mold was built based on the module of Build Layer.In order to avoid the crossing of polymer chains at the ends of the cell,3 nm vacuum layer was added in the model including 4 chains and the degree of polymerization is 100.Besides,the visualization of melt flow was obtained by molecular dynamics simulation based on two phase viscoelastic fluid model.Firstly,the polymer chains was composed of 100 monomer,and 6 chains was chosen as the experiment samples.The concentration model was built based on the cell processed by kinds of optimization technology.Take into consideration the complexity of molding process,the boundary conditions has been simplified by some assumptions.Then,the filling behavior of epidermis and the formation of weld lines were researched under different mold temperature.Furthermore,the glass transition temperature of PP was calculated by molecular dynamics method,and the glass transition temperature is 249.5 K through the calculation based on the situation changes between volume and temperature.The results are consistent with the simulation and experimental result of preliminary work,which proved its reliability that motion state and statistical regularity of polymer chains are solved by molecular dynamics method in injection molding process.The simulation results show that in the filling stage,the combination between polymer and mold is under the control of van der Walls.And the mean square displacement show that the mold temperature is beneficial to improve the activity of polymer chains.Density distribution,morphological evolution and filling rate of packing stage are show that diffusion and movement of polymer chains get promoted when the mold temperature increased.Meanwhile,it is in favour of the loose,disentanglement and transfer behavior of polymer.Interestingly,the filling of the cavity occur in the packing stage.The shape deformation and bending deformation of product is happened after the stripping stage,because of the internal stress generated by packing stage.The weld lines simulation experiment show that the interface with different melt varying form the clean interface to fuzzy interface,and the interface disappear until the last.In addition,the energy of the interface and the different melt thickness of the overlapping show that quality of welding get improved by the mold temperature.The volume shrinkage change is obviously happened under the coefficient between temperature and pressure in the packing stage,which is the feeding phenomenon on the macro performance.Afterwards,mean square displacement is change from nearly linear to the approximate index,which is show the motion process of the polymer chains from oscillatory motion,Einstein's movement to orientation deformation movement.Besides,the different melt thickness of the overlapping is exploited to characterize the intension of weld lines.Considering the motion activity of the chains after stripping stage,the weld lines can be improved and deformation of the product can be reduced by rising mold temperature,because of lower internal stress and better welding quality.