Improvement of Glass Formability in Ultrasonic Vibration Assisted Molding Process

Micro optical elements with complex shapes are needed increasingly in optics based industries. Glass molding process is an effective way of fabricating these microstructures on glass surface. The quality of the surface produced strongly depends on the filling capacity of glass at high temperatures. When convectional mangrove molding is used to fabricate the microstructures, low form accuracy, high internal and surface defects easily occur due to glass incomplete filling and ease of adhesion between glass material and the mold surface during pressing.  In this forming process, the friction force is high thus leading to increased adhesion between the glass and the mold at high temperatures. It is worthy to note using ultrasonic vibration, the friction force can significantly be lowered, therefore, increasing glass formability in the molding process.

Researchers led by Tianfeng Zhou from the Beijing Institute of Technology, China, proposed a method on the improvement of glass formability by using ultrasonic vibration assisted molding process. Their research focused on improving the formability of glass in the molding process. Their work is now published in the peer-reviewed journal, International Journal of Precision and Manufacturing.

The researchers begun by carrying out two main experiments, first, the basic experiment, where the influences of ultrasonic vibration on reducing friction force were experimentally measured. Secondly, the Finite Element Method simulation was carried out to test the filling depth of the glass under the pressing condition with and without ultrasonic vibration, respectively. Eventually, the material formability of glass at high temperature was also experimentally tested with and without ultrasonic vibration correspondingly.

At first, the researchers had to devise a friction-reducing mechanism of ultrasonic vibration. Using self-developed ultrasonic vibration assisted molding machines, they were able to clarify the mechanism of ultrasonic vibration during the molding process by carrying out the friction force testing experiments.

The researchers also had to determine the effects of ultrasonic vibration on friction at high temperatures. It was noted that at high temperatures, glass behaves as a viscoelastic body rather than the brittle material it is at room temperatures. They observed that between the transition temperatures, glass showed significant viscoelasticity meaning it possess both elastomeric and fluid viscosity properties, therefore, making the temperature range suitable for glass molding.

They then simulated the material formability using the ultrasonic vibration assisted molding. They realized that the viscoelasticity of the glass showed significant relaxation at high temperatures, which is a time dependent response of stress or strain. Both the thermal relaxation and structural relaxation leads to increase of internal energy where the latter in turn translates to increase in the material formability.

This research paper demonstrates the benefits of using ultrasonic vibration to increase glass formability in the glass molding process. It has confirmed and introduced a promising method to fabricate microstructures with excellent accuracy. It is now clear that ultrasonic vibration has a significant effect in reducing the interfacial friction force between the glass pre-form and mold at both room temperature and softening temperature. It is also clear that Ultrasonic vibration can homogenize the stress distribution and reduce stress concentration inside the glass during the forming process. Lastly, ultrasonic vibration also increases the material formability during the hot forming process of glass at high temperature. The outcomes here present an important step towards scalable improvements in glass forming using ultrasonic vibration assisted molding process.