Study On Mechanism Of Polyethylene Induced Crystallization During Injection Molding

Injection molding is the most important processing method of polyethylene,we can directly affect the performance of the product by controlling the evolution of the microstructure of the polyethylene during the molding process.Usually,injection molded polyethylene products have poor mechanical properties,low crystallinity and other defects.The increase of the nucleation density can improve the performance of the product by improving the degree of crystallinity and changing the crystal form.Through two ways of mixing different molecular weight inducing crystallization and shear stress inducing crystallization,this paper discussed crystallization mechanism of polyethylene in injection molding,which based on the experimental phenomenon of induced crystallization.The main research contents include:(1)Using a direct blending method,the polyethylene of different molecular weight were mixed by twin-extruder to prepare blends.The effects of mixing different proportions and different molecular weight polyethylene into the blends on the products were investigated.The melt flow index,molecular weight distribution,mechanical properties and crystallinity of the new material were characterized.The changes of molecular weight and the role of long chain and short chain to induce crystallization were discussed by analyzing the changes of melt flowing and molecular weight.The results show that the addition of 4% low molecular weight polyethylene(2911)or 4% high molecular weight polyethylene(1158)in medium molecular weight polyethylene(5000S)can greatly improve the crystallinity of the product.Add 4% 2911 and 4% 1158 to 5000 S at same time,crystallinity can be found obviously improved in the product,and the tensile properties are greatly enhanced,elongation reached 846%.(2)We change the shear stress in experiment which use medium molecular weight polyethylene,the shear stress applied to the polyethylene melt that is adjusted by changing the injection pressure and the mold temperature of the injection molding machine.The relationship between shear stress and crystal form is analyzed by characterization of the samples with electronic universal testing machine,differential scanning calorimetry and scanning electron microscopy.The orientation and crystallization of shear layer and core layer were studied based on the analysis of the crystallization process of polyethylene.As is shown in the result,the shear layer of the product will continue to thicken and the mechanical properties will increase when the melt is subjected to continuous shear.The shear rate can directly affect the structure,shape and size of the crystal.When the shear rate is greater than a certain value,the molecular chain begins to appear obvious orientation.(3)The molecular behavior of the polymer chains in the shear stress field and tensile force field were studied by means of molecular simulation software.The molecular chain model and the coarse-grained model were used to simulate the degrees of orientation and tensile properties of the polymer.The deformation of the microstructure under external force is observed.In the mechanics test,the relationship is investigated between the tensile rate and the stress-strain curve using two different systems.The influence of external stress on the crystal shape is analyzed according to the change of non-bond potential and total energy.The results show that the NVT system can effectively avoid the formation of hole phenomenon.The molecular chain structure will undergo obvious orientation behavior under the action of shear stress or tensile stress.