3.4 Effect of ultrasonic vibration on improving glass formability

As mentioned above, hot glass embossing is a novel process to produce microstructures on glass substrate. Although the fabrication of microstructure should be performed by conventional process, the accuracy of the final shape of products is hard to achieve due to the surface defect or the adhesion between the glass and the mold as glass is filling into the micro cavities. These disadvantages especially appear

with three-dimensional microstructures such as micro-grooves, micro-pyramids, micro-prisms, and micro-lenses which are increasingly needed in optical, optoelectronic, and biomedical industries. These difficulties could be resolved with the effect of ultrasonic vibration. Figure 14 shows the experimental data results which illustrate that applying ultrasonic vibration to hot embossing process can increase the filling ability of glass material significantly (up to 17%). Under the effect of ultrasonic vibration, the glass formability would even improve better than the case of using compression mold, which is usually a solution to help the glass fill more into the microwave (Figure 15). Similarly, ultrasonic vibration could also increase

the embossing speed. Experiments show that the amount of glass filled into the micro cavities at high speed during the ultrasonic process was even more than that at lower speed during conventional process [12, 13]. Similar to the above discussion, these phenomena could be explained by the heat generation when glass absorbed

Glass pressing experiments with the assistance of ultrasonic vibration at room temperature and at elevated temperature have been performed to show effect of ultrasonic vibration on friction force [14]. Some hot embossing experiments were first performed at room temperature. Since ultrasonic vibration is applied to the glass as a sinusoidal displacement, the time that the glass contacts to the mold would be much shorter than that without ultrasonic vibration. Another finding was that the contacting time could be decreased more as increasing the amplitude of ultrasonic vibration. Those findings should be considered as the reasons for the friction reduction during the hot embossing process assisted by ultrasonic vibration [15]. Figure 16 shows the results from the above experiments. Two kinds of glass with different surface qualities were used as specimens for both conventional and ultrasonic experiments. Although the resistance force grew linearly with the increase of pressing load, the resistance force in case of ultrasonic experiment was much lower than that in case of conventional case. Further, the reduction of resistance force was also more with better quality of glass surface (60% with smooth surface compared to 50% with rough surface in average). As the resistance force is smaller, the life of

4. Finite element analysis of ultrasonic vibration-assisted hot glass

Glass molding, also hot embossing, is a replicative process that allows the production of high-precision optical components from glass without grinding and polishing. Many researchers have been studying the glass molding process using finite element analysis. However, very few studies have focused on the hot glass embossing process assisted by ultrasonic vibration. Since the only difference between the conventional process and the ultrasonic vibration-assisted process is in the embossing stage, the glass model for the embossing stage should be created. This model could not only describe the glass behavior under embossing force but also express the effect of ultrasonic vibration. Standard linear solid (SLS) model, one kind of viscoelastic models, which combines a Maxwell model and a spring in series, was proposed for the glass deformation behavior during the embossing stage [8] (as shown in Figure 17). Substituting complex strain and complex stress

3.5 Effect of ultrasonic vibration on decreasing friction force

the energy from ultrasonic vibration.

DOI: http://dx.doi.org/10.5772/intechopen.86546

Ultrasonic Vibration-Assisted Hot Glass Embossing Process

the mold should be prolonged [16].

from Eq. (5) into constitutive equation [8]:

ð Þ E<sup>0</sup> þ E<sup>1</sup> E1 η1 � 1 � �

3

<sup>5</sup>exp � <sup>E</sup><sup>1</sup>

η1 ε � �

þ v L

where E0, E1, and η<sup>1</sup> are constants, determined by fitting experimental data; v is the embossing speed; and L is the initial height of glass sample. Storage and loss

ð Þ E<sup>0</sup> þ E<sup>1</sup> E1 η1 � 1

� � exp ð Þþ �<sup>ε</sup> <sup>E</sup>0<sup>ε</sup> � <sup>E</sup>0η<sup>1</sup>

E1

(10)

� v L

moduli can be calculated as

embossing process

<sup>σ</sup> <sup>¼</sup> <sup>E</sup>0η<sup>1</sup> E1

27

2 4

with three-dimensional microstructures such as micro-grooves, micro-pyramids, micro-prisms, and micro-lenses which are increasingly needed in optical, optoelectronic, and biomedical industries. These difficulties could be resolved with the effect of ultrasonic vibration. Figure 14 shows the experimental data results which illustrate that applying ultrasonic vibration to hot embossing process can increase the filling ability of glass material significantly (up to 17%). Under the effect of ultrasonic vibration, the glass formability would even improve better than the case of using compression mold, which is usually a solution to help the glass fill more into the microwave (Figure 15). Similarly, ultrasonic vibration could also increase

Noise and Vibration Control - From Theory to Practice

Resistance force decrease by ultrasonic vibration with smooth end surface (a) and rough end surface

Figure 16.

26

(b) glass [14].

the embossing speed. Experiments show that the amount of glass filled into the micro cavities at high speed during the ultrasonic process was even more than that at lower speed during conventional process [12, 13]. Similar to the above discussion, these phenomena could be explained by the heat generation when glass absorbed the energy from ultrasonic vibration.

#### 3.5 Effect of ultrasonic vibration on decreasing friction force

Glass pressing experiments with the assistance of ultrasonic vibration at room temperature and at elevated temperature have been performed to show effect of ultrasonic vibration on friction force [14]. Some hot embossing experiments were first performed at room temperature. Since ultrasonic vibration is applied to the glass as a sinusoidal displacement, the time that the glass contacts to the mold would be much shorter than that without ultrasonic vibration. Another finding was that the contacting time could be decreased more as increasing the amplitude of ultrasonic vibration. Those findings should be considered as the reasons for the friction reduction during the hot embossing process assisted by ultrasonic vibration [15]. Figure 16 shows the results from the above experiments. Two kinds of glass with different surface qualities were used as specimens for both conventional and ultrasonic experiments. Although the resistance force grew linearly with the increase of pressing load, the resistance force in case of ultrasonic experiment was much lower than that in case of conventional case. Further, the reduction of resistance force was also more with better quality of glass surface (60% with smooth surface compared to 50% with rough surface in average). As the resistance force is smaller, the life of the mold should be prolonged [16].
