The improvement of mechanical properties of high-density polyethylene(HDPE) and polypropylene (PP) blends was studied by controlling the phase compatibility versus phase separation．In the original injection molding processes

super mechanical properties of blend specimens have not been goteasily．Researches have been carried out in injection molding with high shear injection conditions．It has beenpointed out that the compatibility of blends will increase under the strong shear in injection processes due to the raise of LCST culwe．After the melt is injected into the cold mold．its temperature and compatibilitydecrease simultaneously

phase separation takes place and becomes frozen in the specimen．High mechanicalproperties will be got when the phase separation develope to some extent

but it is difficult to control thephase compatibility and separation The special method of oscillating packing injection molding under lowpressure provides a condition to control the phase separation degree．Because the phase morphology and crystal morphology of HDPE／PP blends compete with each other

it is difficult to control the micro-phaseseparation．In this paper

the fine struture combined with the micro-phase separation and orientation were controlled by using that special injection method and high strength and high toughness blends were obtained．DSC and WAXD results indicate that the improvement of tensile strength is dependent on the number of shish-kebab crystals and the degree of macromolecule orientation caused by the oscillating packing injection molding．SEM results indicate that the impact strength is dependent on the degree of macro-and micro-phase separation．The high tensile strength 97．1MPa and high toughness 45．5kJ／m2 of HDPE／PP blends were achieyed When the PP content in the blends was 8％and 20％．respectively．Which are 4.3 times and 9.5 times as high as the corresponding values of their static specimens respectively．The future work Will be carried out to investigate the phase morphology and crystal morphology as well as thepossibility of cocrystallization of PP and HDPE under oscillating packing injection conditions．