Research On The Molecular Morphology Effect Of Shear Rate And Cooling Temperature On Warpage Of Injection Products

Polymer injection molding is a non-isothermal unsteady flow molding process in which the melt is filled into the mold cavity by the nozzle and the plastic product is finally formed.The melt is subjected to the shear field and the pressure field at the same time in the mold filling flow and pressure keeping cooling period.As a result,a series of complicated changes will occur in the morphology and structure of the microscopic aggregates,which will ultimately affect the macroscopic physical properties such as the mechanical properties and dimensional stability of the molded articles.Therefore,an in-depth analysis of the micro-aggregation morphology and molecular evolution mechanism of the polymer molecular chains in the injection molding process will help deepen the exploration of the macro defect formation mechanism of polymer injection molding and provide product quality improvement and new process development.Theoretical basis and scientific guidance.In this paper,isotactic polypropylene(PP)was used as the research material to construct 20 all-atom molecular lattice models with a degree of polymerization of 100.After the cell model was optimized and balanced,the stable initial equilibrium conformation was obtained.According to the polymer flow filling model points,the equilibrium of the PP molecular chain under different shear rate filling flow simulation.After the end of the shear flow,the final conformation of shear pressure cooling was used as the initial conformation of the shear pressure cooling to simulate the cooling and cooling of the PP unit cell under constant pressure.Based on the simulation results,the micro-mechanism of the macroscopic warping deformation of the product was investigated from the perspectives of molecular chain morphology evolution,energy response and molecular structure change.The results show that during the filling process,as the shear rate increases or the shearing time prolongs,the PP molecular chain entanglement network is gradually destroyed,and the molecular chain disentangles and separates from the entanglement network.Through the internal rotation of the C-C bond on the molecular chain,the flow orientation in the flow direction is gradually formed.The larger the shear rate in the flow process,the greater the degree of orientation of the molecular chain,the rapid increase in the energy of the system and the sharp fluctuations,so the molecular chain will be frozen in an extremely unstable orientation after cooling;With the decrease of temperature,the molecular chain migration and activity of PP molecules are significantly reduced,and the potential energy and kinetic energy of the system are continuously reduced.In the morphological state,the molecular chains will elastically shrink due to elasticity,and tend to re-aggregate to form a new tangled network.However,since the molecular chains in the cooled and oriented state cannot fully recover to the initial random winding state,the molecular chains are still in an orientation stretched state,so that a large internal stress is formed inside the product,and eventually leads to occur the warpage.