An effective method to control the non-linear shrinkage of micro-injection molded small-module plastic gears by combining multi-objective optimization with Moldflow simulation is proposed. The accuracy of the simulation model was verified in a micro-injection molding experiment using reference process parameters. The maximum shrinkage ($Y_1$), volume shrinkage ($Y_2$), addendum diameter shrinkage ($Y_3$), and root circle diameter shrinkage ($Y_4$) were utilized as optimization objectives to characterize the non-linear shrinkage of the studied gear. An analysis of the relationship between key process parameters and the optimization objectives was undertaken using a second-order response surface model (RSM-Quadratic). Finally, multi-objective optimization was carried out using the non-dominated sorting genetic algorithm-II (NSGA-II). The error rates for the key shrinkage dimensions were all below 2%. The simulation results showed that the gear shrinkage variables, $Y_1$, $Y_2$, $Y_3$, and $Y_4$, were reduced by 5.60%, 8.23%, 11.71%, and 11.39%, respectively. Moreover, the tooth profile inclination deviation ($f_{H\alpha T}$), the profile deviation ($f_{f\alpha T}$), and the total tooth profile deviation ($F_{\alpha T}$) were reduced by 47.57%, 23.43%, and 49.96%, respectively. Consequently, the proposed method has considerable potential for application in the high-precision and high-efficiency manufacture of small-module plastic gears.

Ultrasonic plasticizing micro-injection molding (UPMIM) technology has been considered as an effective means of UHMWPE molding. However, the cumbersome forming process, the degradation of mechanical properties and the poor consistency of molding and property seriously restrict further application. In this study, a new ultrasonic molding method of UHMWPE micro-parts is proposed. Firstly, the UHMWPE ultrasonic plasticizing material was prepared simply and quickly by ultrasonic technology. Secondly, the UHMWPE tensile samples were molded by an innovative UPMIM structure with a large diameter ratio of the ultrasonic sonotrode to plasticizing cavity. Then, the optimum molding process parameters were obtained by grey relational analysis (GRA). After that, the influence of system stability and process parameters on mechanical properties and consistency was studied by contribution analysis. Finally, compared with the typical UHMWPE molding method (compression molding) and the existing research results, the influence and feasibility of the process are analyzed in detail. The results show that the ultrasonic technique can effectively prepare UHMWPE tablets with almost unchanged properties (molecular weight decreased by 0.31 %). A large diameter ratio of the ultrasonic sonotrode to plasticizing cavity can expand the process window for complete filling of UHMWPE tensile samples, and the filling stability of the ultrasonic system is increased by about 1.8 times. Meanwhile, this ultrasonic system structure can also inhibit the oxidative degradation of UHMWPE, reduce the break of molecular chain. The elongation at break (EB) of tensile samples increased from 5.56 % to 12.2 %, while the tensile strength (TS) decreases from 136.54 % to 68.11 %. Moreover, the contribution of process parameters to the mechanical properties and consistency for UHMWPE tensile samples is 55.97 %-88.37 %, while the contribution of ultrasonic system stability is 11.63 %-44.03 %.