Fluid-layer-assisted mold temperature control for injection-molded bridge-type compliant mechanisms: Experimental thermal validation and simulation-based benchmarking against conventional cooling channels

The injection molding of bridge-type compliant mechanisms is sensitive to non-uniform mold temperature because the thin flexure hinges are prone to premature solidification and incomplete filling. This study evaluates a fluid-layer-assisted mold temperature-control architecture using experimental thermography, transient thermo-fluid simulation, a representative straight-channel thermal baseline, melt-filling analysis, and displacement testing of molded HDPE mechanisms. The fluid-layer thermo-fluid model was validated against infrared measurements, yielding a mean absolute error (MAE) of 5.01 °C and a root mean square error (RMSE) of 6.44 °C across six validation points. Under the numerical benchmark condition of an 80 °C inlet temperature, a mass flow rate of 59.5 g/s, and 40 s heating, the fluid-layer architecture was predicted to increase the average cavity-surface temperature by approximately 4.0 °C and reduce the maximum longitudinal temperature difference by 28.6% relative to the straight-channel model. Independent thermography of a representative steel straight-channel insert under the same inlet-temperature and flow-rate conditions showed a lower and more non-uniform surface-temperature field than the steel fluid-layer configuration, confirming the direction of the predicted thermal advantage. Melt-filling simulations subsequently predicted incomplete filling at the thin flexure region for the straight-channel configuration and complete filling for the fluid-layer configuration; this filling comparison remains simulation-based. For HDPE blends containing 0-25 wt% recycled material, the molded mechanisms retained an approximately linear input-output displacement response over the investigated range. At an input displacement of approximately 175 µm, the mean output displacement decreased from 857 µm for virgin HDPE to 745 µm at 25 wt% recycled HDPE. The results support the fluid-layer architecture as a conventionally machinable approach for improving mold-surface thermal control, while polymer morphology, fatigue durability, and experimental straight-channel filling remain subjects for future work.