Study On The Cell Morphology And Surface Quality Of Own Injection Molding Polypropylene Foams

The study of microcellular polymer foam materials is in line with the current situation of resource shortage,material saving,cost reduction and energy consumption reduction,and its application value has been manifested in many fields.The molding methods of microcellular polymer materials include mould pressing,extrusion and injection molding method,which injection molding foam due to short molding cycle,size accuracy,high degree of automation,in industrial production has good prospects for development.At present,release pressure injection molding and own injection molding are the two main molding methods of microcell injection molding.Release pressure injection molding needs to instantly change the volume of the mold cavity to release the pressure,the microcellular material has good cell quality.However,release pressure injection molding required complexity mold structure,high equipment cost and difficult operation,and inability to mold complicated parts.Own injection molding overcome the shortcomings of the release pressure injection molding,which in the ordinary injection molding equipment can achieve the complex products foam.Has become a hot research now.Although own injection molding has unique advantages in terms of cost and molding,but the defects of cell foam deformation and surface silver marks on the surface of foam products seriously restrict its industrial production.So revealing the mechanism and process of cell deformation and surface defect formation,and through the appropriate control preparation good cell quality and no-surface defects foam material is the key issues urgently needed to be solved nowadays.So,this article uses polypropylene as a matrix material,discussion the foaming environment and foaming process of release pressure injection molding and own injection molding foaming,the main causes of the differences in the cell morphology and surface quality were explored.Foaming environment affect the mechanism of cells formation and the surface defects were revealed by a self-designed gas counter pressure device.The main achievements of this paper are as follows:(1)Comparing the cell morphology of release pressure foams and own injection foams,it is found that the cells in the release pressure foams samples have good uniformity in different regions,high cell density and small cell size.However,the cell morphology in different positions of cells in the own injection foams sample is quite different.The cell nucleation and resin migration are both carried out in the mold-filling stage,with resin migration the cells grows and it is affected by the flow field.The specific performance of near gate cells affected by small flow field and cell morphology better,far gate cells deformation serious phenomenon.By increasing the melt pressure of the resin,reducing the migration distance of the resin and increasing the binding force of the cell expansion,it can effectively improve the flow and deformation form of the bubble in the filling stage,and the cell tends to be rounded.(2)Through the design of the gas counter pressure device,the pressure in the melt filling process can be effectively controlled.The application of gas counter pressure increases the melt pressure during resin filling,and effectively improves the cell morphology,the thickness of the cell deformation layer decreases,which broadens the thickness of regular cell layer.(3)The gas content increases,the cell nucleation rate increases,the cell density increases,and the cell diameter decreases,but the gas counter pressure increases when the deformed cell becomes a regular circular structure;Considering the cell density,cell diameter,and gas counter pressure,the foaming agent content is 0.75%and the gas counter pressure is 0.6MPa under the existing experimental conditions.The foamed material obtained the best cell morphology.At this time,the cell diameter was 44.97?m and the cell density was 2.48×10~8 cells/cm~3.(4)In the process of own injection foam molding,the main reason for the deterioration of the surface quality is the cell breakage and gas evolution at the melt front during filling.The increase of gas counter pressure increases the melt pressure during filling,decreases the cell nucleation rate,reduces the chance of cell breakage and gas evolution,and improves the surface quality of the product.The content of blowing agent decreases,the surface quality becomes better,and the required critical gas back pressure decreases.