Produkt-Neuheit

Precise monitoring of viscosity during the melting process is crucial for quality assurance in quartz glass production. In this project, a torque transducer (SGR541) was integrated to enable real-time conclusions to be drawn about material homogeneity, purity and the progress of material bonding.

In modern glass technology, precise control of melting conditions is of fundamental importance: the viscosity of a melt in particular has a decisive influence on its further processing, quality and homogeneity. In the laboratory, viscosities are often determined using rotational viscometers: a spindle is rotated at a constant speed in the sample, the required torque is measured and the viscous resistance – i.e. the viscosity – is determined from this.

In industrial melting processes, especially in glass melting, this form of measurement is often not directly practicable. Instead, monitoring methods are used that provide real-time data and thus allow immediate process control.

In our customer's current project, this concerns the melting of quartz glass, where viscosity is not only a measure of fluidity, but also allows conclusions to be drawn about the internal state of the melt – for example, cavities (air bubbles) or incomplete material bonding.

Project and objectives
An industrial company focusing on precision glass components planned to improve the manufacturing process for quartz glass in terms of quality and process reliability. In this specific application, the challenge was that the glass is first melted at high temperatures and then has to reach a defined viscosity zone for further processing. Only in this range can it be guaranteed that

- the melt is homogeneous (no air bubbles),
- the connection or fusion of different material batches has taken place completely,
- the transition to the shaping or quenching/pre-forming process starts reliably.


The objective was therefore to reliably monitor the viscosity of the quartz glass melt in order to detect deviations in the melting process at an early stage and to establish process-reliable feedback for control and quality assurance.

Significance of viscosity in quartz glass
The viscosity of molten glass plays a key role in processing, as it determines the flowability and thus the moulding behaviour, bubble removal, degassing and homogenisation. In the case of quartz glass, the conditions are particularly demanding: high temperatures and viscosities, as well as a strong influence of temperature and composition on the viscosity curve.

Typical characteristics:

- As the temperature rises, viscosity decreases significantly, making the material more fluid.
- Small differences in composition or melting state (e.g. particles that have not completely dissolved, air pockets) can have a major impact on viscosity and thus indirectly on quality and processing.
- With quartz glass, precise knowledge of the viscosity-temperature curve is necessary, for example to define ‘working range’ temperatures at which the material can be processed safely.

In the project, viscosity was used as an indicator to monitor the homogeneity of the melt, the presence of undesirable inclusions (e.g. air bubbles) and the complete combination of several material batches.

Solution approach and sensor selection
Two torque sensors or measuring systems were considered for viscosity monitoring: the SGR520 and the SGR541. After thorough analysis, the SGR541 was selected and specially adapted for the quartz glass melting application.

Key reasons for choosing the SGR541:
- Greater robustness against extreme process conditions (temperatures, thermal shocks, possible vibrations) in the melting range of quartz glass.
- Option for customer-specific adaptation – e.g., reduced measuring shaft, separate sensor unit and electronics via a plug connection.
- Support from Althen regarding documentation, integration into process control, and interfaces for data acquisition and monitoring.

Adaptation to the application
The sensor was mechanically and thermally designed for the melting conditions of quartz glass melt. Calibration was performed using reference samples or model melts in order to map the torque ↔ viscosity relationship in detail in the specific process setup. The sensor was integrated into the process—typically between the motor/agitator (or melt mixing system) and the impeller (or melt stirrer)—and connected to the process control system in real time. Since torque transducers can be sensitive to lateral forces, the paddle must not be coupled directly to the sensor. Instead, double bearings should be used to avoid transverse forces.

Results and Benefits
The introduction of monitoring using the SGR541 led to the following positive effects:

- Increased process reliability: The melt could be continuously monitored for homogeneity and absence of air bubbles. Deviations were detected early and corrected.

- Quality assurance: The common error source “air bubbles in the quartz melt” was significantly reduced, as their formation manifested through increased viscosity or abnormal viscosity development.

- Improved efficiency: Less scrap, reduced rework, and fewer stops for manual inspections.

- Scientific feedback: The collected data (torque/viscosity curves) provided valuable insights into the behavior of the quartz glass melt – enabling future process optimizations.

Further Applications
This method is also successfully used in other applications – such as gypsum mixtures, coal slurries, and liquids with magnetic particles – where conventional viscometers fail and viscosity monitoring is required. In industries like cosmetics or pharmaceuticals – e.g., in the production of shampoos or medical solutions – it is also essential to monitor viscosity during mixing. This makes it possible to determine when the process is complete and the desired consistency has been reached.

Conclusion
This project clearly demonstrates how the combination of sensor technology (SGR541), viscosity expertise, and process integration can lead to significant improvements in quality processes within quartz glass manufacturing. For technical test engineers, process managers, and quality assurance teams, this solution offers a robust foundation for increasing efficiency, quality, and safety.

More information: https://www.althensensors.com/projects/viscosity-monitoring-in-quartz-glass-melting/