Research On Structure And Properties Of PP/HDPE Foamed Products By Co-injection Molding

Co-injection molding is a process in which two different polymers are sequentially injected into the mold cavity to manufacture the multi-layer polymer products. The melt firstly injected produces the skin layer of the part and the melt secondly injected induces the core layer. The foamed products made by co-injection molding has good economic, environmental and social benefits, which can overcome the disadvantages of poor surface quality in traditional injection process. Other advantages are as materials saving, production costs reducing, better properties of soundproofing and heat insulation. The above mentioned merits can valuably extend the application field of the foaming materials.In this study, HDPE(High density polyethylene) and OMMT(Nano-montmorillonite) were blended with PP(Polypropylene) firstly to improve the foaming properties of PP. Then, a lightweight product with a dense skin layer and foamed core layer was prepared by co-injection molding using a chemical foaming agent. In this paper, the influences of foaming agent content, HDPE content, OMMT content, temperature of core melt on the mechanical properties(apparent density, tensile strength and impact strength), cell morphologies(porosity, cell density, average cell size) and crystallization behavior had been systematically studied.1、HDPE is able to improve the impact strength of products. High content of foaming agent will lead to a sharp decline in tensile strength and impact strength of the products for combination and collapse of cell. The right amount of OMMT is helpful to enhance the tensile strength and impact strength of the products. When foaming agent content is 2wt.%, HDPE content is 30 wt.%, OMMT content is 3wt.%, temperature of core melt is 190oC, products have better comprehensive mechanical properties. In this case, the tensile strength is 25.80 MPa, the impact strength is 12.483 k J/m2.2、The cross-section microstructure of samples were observed by scanning electron microscopy(SEM), The experimental results show that: The right amount of HDPE and OMMT are conducive to increase the cell density and refine the cell size. With the increase of foaming agent content, the cell density increases. However, too high content of foaming agent will result in combination of cell. Higher temperature of core melt will exacerbate combination of cell with low melt strength of blends system. When foaming agent content is 2wt.%, HDPE content is 30 wt.%, OMMT content is 3wt.%, temperature of core melt is 190oC, cell size reaches the minimum value. In this case, cell size is 85.32 um, cell density is 2.486×105 unit/cm3.The melting point and crystallinity of two-phase have a decreasing trend after PP/HDPE were blened.While other factors have little effect on crystallization behavior.