Research On The Mechanisms And Key Technologies In Ultrasonic-assisted Molding Of Microstructured Glass Elements

With the recent advances in the fields of micro-opto-electro-mechanics and biomicrofluidics,increasing attention has been paid to glass-based elements featuring diffractive,anti-reflective,hydrophobic microstructures or microchannels.Compared with polymers,inorganic amorphous glasses have inherent advantages in light permeability,thermochemical stability and biocompatibility.Due to the hardness,brittleness and high softening temperature of glass,however,great difficulties remain in removal machining or thermal forming of glass microstructures.Precision glass molding(PGM)should be one of the most promising methods for fabricating spherical,aspherical and smooth free-form glass lenses.Compared with conventional removal processes,PGM has pronounced advantages in material utilization rate,forming accuracy and forming efficiency.However,PGM may encounter obstacles in molding microarrays with sharp edges or high aspect ratios(e.g.V grooves,rectangular grooves,pyramids,or Fresnel lens).These obstacles are:(?)the flow and deformation of glass materials are trapped inside the mold microgrooves,leading to insufficient filling ratios of the molded glass microstructures;(?)the long filling time and excessive reaction force of glass during pressing result in decreased efficiency and stability of the molding system.In light of these defects,high-power ultrasonic technique is herein introduced to conventional PGM process.The thermo-mechanical effects,induced by the superimposed vibration,are expected to improve the deformability and filling capacity of the glass inside mold microstructures,which will contribute to enhanced replication accuracy and efficiency of glass microstructures.It should be noted that,however,ultrasonic-assisted glass molding(UGM)remains at an exploratory stage,due to the lack of adequate material properties,unrevealed forming mechanisms,unstable molding systems,and unmatched process parameters.To address these scientific /technical issues,in this dissertation,bottom-up studies concerning glass thermorheological mechanism,molding equipment and molding parameters are performed,which cover theoretical derivation,material characterization,numerical simulation and comparative experiment.The main contents herein are summarized as follows:(1)To obtain the thermal and mechanical properties of optical glass at molding temperatures,theoretical analyses and experimental characterization s of a typical low Tg glass(D-ZK2)were performed.First,impulse excitation technique(IET)was employed to determine the initial moduli of the glass at elevated temperatures.Subsequently,compression creep tests(CCTs)of D-ZK2 glass cylinders were conducted to quantify the glass-platen interface friction at high temperatures;using the quantified friction,the creep data of the glass were corrected,whereby a viscoelastic stress relaxation model with weakened frictional disturbance was developed.Finally,dilatometry(DIL)and differential scanning calorimetry(DSC)were employed to determine the thermal expansivity and specific heat of the glass.Furthermore,the structural relaxation parameters of the glass were obtained by fitting the DSC data with the Tool-Narayanaswamy-Moynihan(TNM)model.Hence,a systematic scheme for thermo-viscoelastic characterization of glassy materials was developed.(2)Based on the finite element software MSC.Marc,numerical simulations of the above CCT processes were performed,which validated the accuracy of the obtained static viscoelastic parameters.Inheriting the CCT simulation results,the subsequent annealing process of the compressed glass was numerically modeled,which demonstrated the effect of structural relaxation on the glass' s inner stress.Considering high-frequency energy dissipation and its coupled thermal effect,dynamic viscoelastic model of the glass was derived.Based on the static and dynamic viscoelastic models,numerical simulations of PGM and UGM processes were performed,which visualized the micro-filling process of the glass inside the V-shaped microgrooves,and numerically demonstrated the mechanism of the enhanced microformability of the glass under ultrasonic vibration.(3)For the implementation of the UGM process,a novel UGM apparatus,composed of an infrared heating system,an electro?servo driving system,an ultrasonic vibration system and a cooling system,was successfully developed.To address the eigenfrequency mismatch of the vibrating parts at elevated temperatures,theoretical and numerical analyses of the modal evolution of a partially heated horn were conducted,whereby a systematic eigenfrequency tuning method was proposed.In situ eigenfrequency measurements of the optimized horn were further performed,whereby the effectiveness of the proposed eigenfrequency tuning method was validated.Furthermore,the axial deformation of the loaded molding system and the temperature difference between the upper and lower molds were in-situ measured.The results indicated that the UGM system had satisfactory axial stiffness and heating uniformity,adaptable for molding the majority of low-Tg optical glasses at high temperatures and low/moderate loads.(4)Based on the self-developed molding apparatus,fundamental UGM/PGM experiments of microgrooved glass arrays(hundred-micron-sized)were conducted under a series of constant pressing speeds/loads.The comparative results showed that ultrasonic vibration could significantly reduce the molding force in constant-speed molding and the filling time in constant-load molding,as well as improve the filling depth of the formed glass microstructures.This overall enhancement was consistent with the simulated results.Meanwhile,direct quantifications of the ultrasonic-induced changes in mold temperature and glass-mold friction were performed,whereby the ultrasonic-induced non-uniform thermal softening and friction-reducing effects on the thermo-viscoelastic responses and microformability of the glass were revealed.Additionally,application-oriented UGM experiments of two typical microarrays were conducted using the molding parameters determined by fundamental experiments.The experimental results further confirmed the good applicability and great potential of UGM technique in fast fabrication of precision glass-based elements with micro/fine surface structures.