1. Introduction

#### 1.1 Conventional hot glass embossing process

Hot embossing process is one of the common replication technologies for the replication of microstructures in micro optic systems, such as micro-lens array, micro-prism array, photoresist columns on a glass substrate, etc. The microreplication process is known as a process replicated from a microstructured master. With the development of microsystem technology, low cost is one of the important requirements for manufacturing optical components in a micro range. Besides injection molding process, hot embossing has been favored because of its operation as well as the convenience in producing mold tools. Especially, it is the best choice to create micro components with complex geometry and high aspect ratio. In addition, hot embossing is also suitable for mass volume fabrication with high quality.

Material is the main component in molding process, also hot embossing process. Most applicable materials for hot embossing are thermoplastic polymers. Nevertheless, alternative molding materials like glass, metals, or ceramics will also be used [1]. Glass is a material commonly used in optical microsystem. Compared to optical polymers, optical glass has higher transparency, higher scratch, and humid resistance. Another advantage of the optical glass is that its thermal expansion coefficient is smaller than that of the optical polymers. This reduces significantly the

deviation between the design and the final optical components. Moreover, the refractive index of the optical glass is much higher than that of optical polymers (a range from 1.5 to over 2.0 compared to a range from 1.3 to 1.7). With the higher refractive index, optical glass could bend the light rays to focus in a smaller range, which is suitable to optical lenses. With the above advantages, optical glass has been favored for high-precision applications.

The hot glass embossing process is divided into four major steps (Figure 1). The process starts with heating the sample to the molding temperature, which is usually above transition temperature Tg or the annealing temperature At of the material, followed by an isothermal molding by embossing with speed- and/or forcecontrolled, the cooling of the molded part to the de-molding temperature, and finally de-molding of the component. Heating and embossing stages are usually performed in vacuum environment to protect molds from oxidation, while the cooling stage is usually supported by high-pressure nitrogen. Compared to injection molding, hot embossing process has more advantages, such as shorter flow distances and lower velocity, which decreases shear stress of material significantly. As a result, the decrease of shear stress of material as filling into micro cavities should reduce the residual stress of the embossed parts. In addition, hot embossing process could be the best choice for manufacturing microstructures, which are hardly performed by injection molding. Since the molding temperature is set constantly, the conventional hot glass embossing process is isothermal. The temperature setting of this process is shown in Figure 2.

1.2 Ultrasonic vibration-assisted hot glass embossing process

Temperature setting for the ultrasonic vibration-assisted glass hot embossing process.

Steps for the ultrasonic vibration-assisted hot glass embossing process [2].

Ultrasonic Vibration-Assisted Hot Glass Embossing Process

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

2. Components for ultrasonic vibration-assisted hot glass

In general, an ultrasonic vibration-assisted hot glass embossing process has three main components: heating furnace, compression tester, and ultrasonic vibration

embossing process

17

Figure 3.

Figure 4.

Ultrasonic vibration technology has been widely applied in various industrial processes, such as machining, welding, and forming. Recently, this technology has been also utilized for processes working at high temperature like hot upsetting and hot embossing. The steps of an ultrasonic hot glass embossing process are like those of the traditional one, except the embossing stage. During the embossing stage, an ultrasonic source is located on the top of the mold to generate high-frequency vibrations (Figure 3). The high energy of ultrasonic vibration rapidly increases the temperature at the contact area between the glass and the mold. Because of this principle, the temperature distribution between the traditional process and the ultrasonic process is different. In the conventional process, the temperature distribution inside the glass during the embossing stage is identical, whereas in the ultrasonic process the localized heat-affected zones are concentrated on the contact area of the mold and the glass. Therefore, this ultrasonic embossing method is not an isothermal process. Temperature setting of this process is shown in Figure 4.

Figure 1.

Steps for the conventional hot glass embossing process [2].

Figure 2. Temperature setting for the conventional glass hot embossing process.

Ultrasonic Vibration-Assisted Hot Glass Embossing Process DOI: http://dx.doi.org/10.5772/intechopen.86546

Figure 3.

deviation between the design and the final optical components. Moreover, the refractive index of the optical glass is much higher than that of optical polymers (a range from 1.5 to over 2.0 compared to a range from 1.3 to 1.7). With the higher refractive index, optical glass could bend the light rays to focus in a smaller range, which is suitable to optical lenses. With the above advantages, optical glass has been

The hot glass embossing process is divided into four major steps (Figure 1). The process starts with heating the sample to the molding temperature, which is usually above transition temperature Tg or the annealing temperature At of the material, followed by an isothermal molding by embossing with speed- and/or forcecontrolled, the cooling of the molded part to the de-molding temperature, and finally de-molding of the component. Heating and embossing stages are usually performed in vacuum environment to protect molds from oxidation, while the cooling stage is usually supported by high-pressure nitrogen. Compared to injection molding, hot embossing process has more advantages, such as shorter flow distances and lower velocity, which decreases shear stress of material significantly. As a result, the decrease of shear stress of material as filling into micro cavities should reduce the residual stress of the embossed parts. In addition, hot embossing process could be the best choice for manufacturing microstructures, which are hardly performed by injection molding. Since the molding temperature is set constantly, the conventional hot glass embossing process is isothermal. The temperature setting

favored for high-precision applications.

Noise and Vibration Control - From Theory to Practice

of this process is shown in Figure 2.

Steps for the conventional hot glass embossing process [2].

Temperature setting for the conventional glass hot embossing process.

Figure 1.

Figure 2.

16

Steps for the ultrasonic vibration-assisted hot glass embossing process [2].

Figure 4.

Temperature setting for the ultrasonic vibration-assisted glass hot embossing process.

#### 1.2 Ultrasonic vibration-assisted hot glass embossing process

Ultrasonic vibration technology has been widely applied in various industrial processes, such as machining, welding, and forming. Recently, this technology has been also utilized for processes working at high temperature like hot upsetting and hot embossing. The steps of an ultrasonic hot glass embossing process are like those of the traditional one, except the embossing stage. During the embossing stage, an ultrasonic source is located on the top of the mold to generate high-frequency vibrations (Figure 3). The high energy of ultrasonic vibration rapidly increases the temperature at the contact area between the glass and the mold. Because of this principle, the temperature distribution between the traditional process and the ultrasonic process is different. In the conventional process, the temperature distribution inside the glass during the embossing stage is identical, whereas in the ultrasonic process the localized heat-affected zones are concentrated on the contact area of the mold and the glass. Therefore, this ultrasonic embossing method is not an isothermal process. Temperature setting of this process is shown in Figure 4.
