Effects Of Gas Counter Pressure On Wheat Straw/High Density Polyethylene Foaming Injection Molded Parts

Wood plastic composites(WPC)is widely used which is in line with the concept of sustainable development.WPC can effectively use agricultural waste,reduce plastic consumption and save raw materials.Foaming injection molding has the advantages of saving materials,reducing energy consumption,shortening the molding cycle and so on.And it also improves the dimensional stability and molding accuracy of parts.It is a highly industrialized and rapidly developing molding processing method.Gas counter pressure(GCP)technology is a dynamic gas pressure control technology to improve the quality of foaming injection molded parts.High density polyethylene(HDPE)is very popular in industry,but its interface binding ability with wood fiber is poor.Thus,the wheat straw/HDPE composites were prepared firstly by using maleic anhydride grafted HDPE as the matrix material and the abundant wheat straw in Henan province as the filler.The DSC and rheological properties of wheat straw/HDPE composites were studied.Then the effects of material components(maleic anhydride grafting,silane coupling agent modification,different wheat straw contents and different wheat straw size)and process(GCP)on the cell morphology and properties of foaming injection molded parts were studied.The main research results are shown as follows:(1)The melting point of wheat straw/HDPE composites decrease and the storage modulus of composites increase after grafting maleic anhydride.The storage modulus of composites are enhanced with increasing of wheat straw content and decreasing of wheat straw size,but the melting point of the composites do not change significantly.The straw modified by silane coupling agent has little effect on the rheological properties and melting point of the composites.(2)Maleic anhydride grafting,silane coupling agent modifying,wheat straw content increasing and wheat straw size decreasing are helpful to improve the cell quality of foaming injection molded parts,decrease cell diameter,increase cell density,and enhance tensile strength.Among the above mentioned factors,maleic anhydride grafting has the greatest effect on the mechanical properties of composites foaming injection molded parts.The tensile strength is increased by 14.0% and the notched Izod impact strength is increased by 111.4% after maleic anhydride grafting.(3)By combining numerical simulation with classical nucleation theory,the effects of GCP on the cell morphology,surface quality and mechanical properties of foaming injection molded parts were studied.It is found that the deformation degree,density and size of the cell can be effectively controlled by adjusting the parameters of GCP during the process of GCP foaming injection molding.GCP technology effectively improves the surface quality of foaming injection molded parts,and the effect of GCP on the surface quality of parts is more obvious.When GCP is increased to 10 MPa,the surface roughness decreases by 61.8%;and when GCP holding time is prolonged to 6.5 s,the surface roughness decreases by 53.7%.By changing the GCP process parameters,the thickness and mechanical properties of foaming injection molded parts can be regulated.Increasing the GCP or prolonging the GCP holding time can increase the skin layer thickness and the tensile strength of foaming injection molded parts(up to 29.3%),but also can decrease the notched Izod impact strength(up to 22.6%).However,the effect of GCP is more obvious than the GCP holding time.The change of GCP process parameters has a greater impact on the mechanical properties of composites foaming injection molded parts compared with the change of material composition(wheat straw content and wheat straw size).