The microstructure and mechanical properties of large section zinc alloy worm gear were investigated by metallographic microscope and universal testing machine

and the pin-hole degree in the section face of the double-squeezing castings was evaluated.The optimized double extrusion casting process parameters of ZA27 worm wheel selected were presented as follows:holding temperature of 600～650 ℃

mold preheating temperature of 200～250℃

mold closing speed of 5～10 mm/s

main and auxiliary punch at the specific pressure of 70～100 MPa for 50～100 sand then auxiliary punch at the specific pressure of 50～70 MPa began to pressurize the worm wheel flange for 50～120 s.The results show that ZA27 worm gear with uniform microstructure and properties can be obtained by double extrusion casting.Compared with those of ones by gravity casting

the tensile strength and elongation of double extrusion worm gear are increased by 27%and by 100%

respectively

and the pinhole degree is increased by more than 1 grade.