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The demolding process in micro-injection molding constitutes a critical phase, exerting a decisive influence on the quality of polymer microstructures. In this study, the demolding forces of PMMA microfluidic chips were measured under different demolding temperatures, packing pressures, and demolding speeds by using pure nickel (Ni) mold insert. The dimensional deviations of the microchannels with different aspect ratios were analyzed. In addition, the effect of mold insert modification on the demolding force was further analyzed. The results showed that the effect of demolding temperature on the demolding force was the most significant, and the peak demolding force decreased with the decrease in temperature. The width of the microchannels increased and the depth decreased after demolding, while the opposite results were observed for the micro-mixing structures. However, the dimensional deviation of the micromixing structures was larger than that of the microchannels. Using the optimal molding parameters, the peak demolding force could be reduced from 114.0 N at 110[degrees]C to 47.0 N, a decrease of 58.8%. It is noteworthy that, when the microchannel was narrower than 100 [micro]m, achieving precise replication of microchannel dimensions became progressively more challenging as the microchannel width decreased. Using the Ni-W[S.sub.2] mold insert with low surface energy and low friction coefficient, the demolding force could be further reduced by 38.3%. The effect of the molding quality of microfluidic chip microstructures on the mixing performance of heavy metals was demonstrated by micro-mixing experiments. Highlights * Demolding temperature crucially affects microstructure dimensions. * Optimal parameters and modified mold insert reduce demolding force by 38.3%. * Dimensional deviation increases with the increase of microchannel aspect ratio. * Optimized microfluidic chip enhances [Co.sup.2+] chemiluminescence by 6.42%. KEYWORDS demolding force, dimensional deviation, microfluidic chip, micro-injection molding, microstructure, 1 | INTRODUCTION Microfluidics is extensively employed in various fields such as biotechnology, chemical analysis, and medical diagnosis. (1,2) Microfluidic chips, which integrate an array of microstructures including microchannels, microcolumns, [...]

The landing gear should be able to achieve safe and rapid separation from the aircraft body under overload conditions. To clarify the effect of breakaway groove on the breakaway load of emergency fuse-pin for civil main landing gear, a series of FEM simulations are conducted to study the influence of structural changes of breakaway groove on the breakaway load for outer-groove, long outer-groove, inner-groove and short inner-groove emergency fuse-pin. The breakaway load and failure mode are comparatively analyzed between inner-groove and outer-groove emergency fuse-pin by experimental tests. The results show that the breakaway failure of fuse-pin is extended from inside to outside. The breakaway loads increase with the increasing of the sectional area and exhibit a linear relationship. The breakaway load variation with the sectional area（shear strength coefficient）of through-hole pin is the maximum, followed by outer-groove pin, and the inner-groove pin is the minimum. Compared with other pins, the breakaway load of inner-groove pin is easy to control and less affected by sectional area and overall size. The fracture morphology is changed from ductile fracture of outer-groove pin to brittle fracture of inner-groove pin. Accordingly, the design of inner-groove pin meets the aviation regulations requirement for civil main landing gear.

When designing the retractable landing gear of an aircraft oil-gas mixed type，the medium flow characteristics between the inner cavities should be fully considered. The damping aperture and fuel filling amount of a certain UAV landing gear buffer is taken as the research object，and the single factor experimental method is used to analyze the changes of gas-liquid flow inside the buffer caused by each factor. The gas-liquid characteristics of the buffer under different damping apertures and oil filling amounts in the lowering stage are simulated and calculated by Fluent software. The results show that the damping hole oil flow during the landing gear lowering process is only related to the size of the pore size，not affected by the oil filling amount of the buffer. Under the 637 mL oil filling required by this type of UAV，the buffer damping aperture should be greater than 6 mm. For other types of landing gear buffers，when the oil filling amount is determined，the gas-liquid filling of the buffer that meets the lowering stage should be used as one of the buffer damping hole design criteria.

Tire stiffness is one of the important parameters of tire dynamics model. The influence law of tire stiffness on the shimmy of large civil aircraft nose landing gear can provide reference for the anti-shimmy design of large civil aircraft nose landing gear. The nonlinear mathematical model of the nose landing gear shimmy is established, and the shimmy region diagram of different tire torsional stiffness and tire side bending stiffness is calculated by using Matlab/Matcont software. The influence law of tire torsional stiffness and tire side bending stiffness on the shimmy of the nose landing gear is studied, and the sensitivity of the two on the shimmy of the nose landing gear is compared. The results show that the sensitivity of tire torsional stiffness to the impact of nose landing gear shimmy is greater than that of tire lateral bending stiffness. When the positive torque coefficient is decreased by 1%, the maximum critical damping of torsional shimmy is decreased by 0.88%, and the minimum critical damping of lateral shimmy is increased by 33.87%. Reducing the torsional stiffness of tire and increasing the lateral stiffness of tire are beneficial to restrain the yaw of large civil aircraft nose landing gear.

System stiffness (longitudinal, lateral, and torsional stiffness) is a meaningful index for strut nose landing gear design.It arises from profound significance to explore the influence law of system stiffness on shimmy stability for the anti-shimmy design of landing gear.By changing the initial gas pressure of the buffer, the influence of buffer stroke on the system stiffness is analyzed, and the influence of the system stiffness on the initial torsion angle, convergence time, convergence ratio, and stability area of the strut nose landing gear shimmy is studied.Results show that increasing the stiffness of the strut-type nose gear system can improve the anti-shimmy performance, but will increase the initial torsional angle and shimmy convergence time, and reduce the ability of the landing gear to resist external disturbances.Therefore, the stiffness of the strut-type nose landing gear system cannot be excessively increased.

As one of the main factors affecting the shimmy stability of aircraft in high speed rolling,the structural clearance of landing gear determines the safety and comfort of aircraft rolling.In this paper,the existing form of landing gear structural clearance is summarized,and the formation reasons of landing gear structural clearance on shimmy are analyzed.The existing landing gear shimmy clearance analysis models are described in detail,and the limitations of the models and future modeling direction are pointed out.Furthermore,the solving methods of the shimmy model with clearance are summarized through literature analysis,and the applicability of different methods is illustrated.The future research direction is prospected.

Based on the practical problem of fatigue life prediction of landing gear, the fatigue life prediction model of landing gear components with surface treatment was studied. The calculation process of the traditional local stress-strain method was introduced, and the factors that affect the local stress-strain method for fatigue life prediction were systematically explained; The actual engineering research shows that the main factors for predicting the fatigue life of landing gear by using the local stress-strain method are surface treatment and surface roughness. By modifying the strain-life curve, a model for predicting the fatigue life of landing gear components is proposed. The two calculation examples show that the predicted results are in good agreement with the experimental results, and the proposed method for revising the strain-life curves under different surface conditions is reasonable.