Abstract: Aimed to the premature failure of the tri-thread near a slot for releasing stress of a tie-bar of a plastic injection machine, this paper analyzed the stress distribution of the tri-thread and its adjacent slot of a tie-bar with finite element method in ANSYS software, and built up a parameter optimum model ac- cording to the geometry characteristic of the adjacent slot of the tri-thread end of a tie-bar. A multi-ob- jective function was used to reduce the maximum stress of a tri-thread and to control the maximum stress and the minimum machining allowance ofits adjacent slot of a tie bar.Through the multi-objective opti- mization of the parameters of a tie-bar under the condition of one-way tension, the maximum stress on the root near the slot of the tri-thread of a tie-bar decreases 55 percent. Under the condition of deflective load,its maximum stress decreases 21.4 percent.Under the condition of one-way tension, the maximum stress of the slot of a tie-bar increases 26.5 percent in comparison with the average one-way tensile stress.It demonstrates that the optimum method can make the slot play a better role on releasing stress.

Key words: plastic injection machine, tie-bar, slot for releasingstress, multi-o均ective optimization, trithead