Study On The Cell Quality And Surface Quality Of Foaming Nylon 6 Composite Under Short-shot Injection Foaming Process

With light weight,good shock resistance and seismic resistance,microfoam polymer has been widely used in aviation equipment,medical equipment,household lamp holder,car seat and other fields.Microcellular foam molding methods mainly include compression molding,extrusion,injection,etc.Among those,the injection molding method has the advantages of short molding cycle and the ability to produce large scale parts with complex shape and precise size,so it has become the most promising lightweight product molding method.There are different foaming environments in the process of injection molding,mainly including the release pressure foaming environment and the short-shot foaming environment.The cell quality and surface quality in release pressure foaming environment are good,but it is difficult and expensive for the complicated shape,so it is not suitable for industrial production.Short-shot injection molding is developed on the basis of release pressure molding,which can produce large products without modifying the equipment.Although the short-shot foaming environment is suitable for industrial production,its poor quality of the cell and defects of air marks on the surface of products also limit the promotion and application of lightweight products.Therefore,it is of great significant to widen the application of lightweight products to adjust the short-shot foaming environment by certain process or technical means to make the products with good shape and high surface gloss.In this paper,based on the previous systematic study of foaming quality and surface quality of foaming PP composites in different foaming environments,combined with some research results and accumulated experience of foaming PP composites,the influence of foaming quality and surface quality of foaming PA6composites is studied with the emphasis on PA6 as the matrix material,from the perspective of material characteristics and mold cavity pressure,and the key influencing index factors in short-shot foaming environment are proposed.Furthermore,the influence of foaming quality and surface quality of foaming PA6composites in short-shot foaming environment is further studied by means of back pressure and gas counter pressure techniques,and the causes of deterioration of foaming quality and surface quality of foaming PA6 composites in short-shot foaming environment are revealed.The main results are as follows:(1)whether it was foaming PP composite or foaming PA6 composite,the foaming quality in the release pressure foaming environment was better than that in the short-shot foaming environment.Under the release pressure foaming environment,the cell shape of foaming PA6 composite material was regular.With the increase of weight loss,the average diameter and the density of the cell increased gradually.In the short-shot foaming environment,the average diameter and deformation of foam PA6 composites increased with the increase of weight loss.Under the condition of 20%weight reduction,the cell density reached the maximum value of 9.91×10~5cell/cm~3,and the weight loss further increased and the cell density decreased.(2)The surface quality of foaming PA6 composites in the release pressure foaming environment was better than that in the short-shot foaming environment.In the environment of releasing pressure foaming,the surface quality changed little with the increase of weight loss.However,under the condition of owe injection foaming,the melt pressure of mold cavity during the foaming process had obvious influence on the surface quality of foaming PA6 composites,but not on the surface quality of foaming PP composites.(3)Under the condition of the same weight loss,the cavity pressure changed little when the back pressure of foaming PA6 composite was increased.The back pressure had almost no effect on the size and shape of the cell,and could only increased the cell density to a certain extent.The cell density under the back pressure of 0bar increased by about 60%compared with that under the back pressure of 30bar.No matter at low or high weight loss,increasing back pressure could not improve the surface quality of the samples in short-shot foaming environment.(4)The injection of gas counter pressure into the short-shot foaming could effectively prevent the deterioration of the cell of the foaming PA6 composite.The larger the gas counter pressure was,the higher the nuclear barrier would be.As a result,the diameter of cell would increase,up to 96.22?m.The deformation degree of cell was reduced,and the minimum was only 4%.The cell density was reduced to 1.81×10~5cell/cm~3.Gas counter pressure can improve the surface quality of foaming PA6 composites.When the weight was reduced to 20%,the gas counter pressure increased from 0MPa to 0.3MPa,and the surface gloss increased from22.7 to 49.8.But the gas counter pressure continued to increase,the surface gloss was no longer changing.(5)Under the same weight loss,the greater the gas counter pressure was,the higher the melt pressure was;Under the condition of 5%weight loss,the melt pressure in the gas counter pressure of 0MPa was 7MPa.When the gas counter pressure increased to 0.6MPa,the melt pressure increased to 16.8MPa.Under the same gas counter pressure,the weight loss increased,and the melt pressure decreased slightly,but the overall change was not great,and the melt pressure was still high.Under the gas counter pressure of 0.2MPa,the melt pressure was13.8MPa in the condition of 5%weight loss.When the weight loss increased to25%,the melt pressure was about 10MPa.