Spinning Processes: Simulation Production Glas Wool / Fibers

Glass fibers are a central component of modern materials technology, for example in glass wool. They are widely used in sectors such as the construction industry in insulating materials, automotive manufacturing and in the production of consumer goods. The precise and efficient production of these fibers is crucial to ensure the performance of the end products while keeping production costs low. At the Fraunhofer ITWM, we develop innovative simulation and optimization methods that make it possible to precisely analyze the spinning process of glass fiber production, make it more efficient and sustainably improve product quality.

A key aspect in the production of the fibers is to ensure a uniform diameter and a specific length. This is essential for optimum product quality.

The production of glass fibers poses numerous challenges for companies, especially when it comes to:

precisely controlling material properties during the manufacturing process
optimize energy efficiency
reduce production waste
continuously improving product quality while simultaneously reducing production costs

At Fraunhofer ITWM, we develop innovative simulation technologies and optimization methods that help companies overcome these challenges, improve their processes and strengthen their competitiveness in a dynamic market.

Precise Simulation of the Glass Melt and Air Flow

Our innovative overall model is based on the commercial simulation software ANSYS Fluent to precisely capture the dynamics of the molten glass in a rotating disk and the surrounding air flow. In addition, we use the simulation software VISPI developed at Fraunhofer ITWM to simulate the dynamics of the glass fibers extruded from the nozzles of the disk. Both software solutions work together in an iterative process to achieve a convergent equilibrium solution that ensures high accuracy and reliability of the simulation results.

We particularly focus on accurately modeling the heat and momentum transfer between the disk and the airflow. To make the complexity of the system manageable and to increase accuracy, we usually calculate a periodic section of the disk. In this way, we record the filling level in the disk and precisely analyze the flow through the nozzles with their different mass flows and temperatures.

Interaction With Established Software VISPI

We use the boundary conditions determined in ANSYS Fluent to simulate the fiber dynamics outside the disk in VISPI. We take into account the interactions between the numerous fibers and the flow through an iterative coupling of the two simulation environments. This coupling is done with the help of User Defined Functions (UDFs) in ANSYS Fluent and an alternating calculation process that exchanges the necessary source information. After about 10 coupling iterations, we usually achieve a stationary solution. This coupled model not only gives us a deep insight into the production process, but also allows us to make precise adjustments to significantly increase efficiency and quality.

The simulation makes it possible to precisely determine the final diameters of the fibres in each nozzle and optimize them to achieve an even distribution. By adjusting the speed and temperature, we can also counteract abrasion and thus ensure process stability. In addition, the analyzed air turbulence and tension in the fibres provide valuable information that helps us to predict the possible fibre lengths – a decisive factor for the quality of the end product.

Simulation and Optimization of Optical Fibers

Simulation of the glass wool production process

In this video we show the simulation results of a glass disc that is used in the production of glass wool. At the beginning, the entire machine is shown, in which the air temperature and the glass fibers are brought into equilibrium. We then focus on a section of the pane in which the different paths of the glass fibers are clearly visible. Finally, we fade out the pane to show the filling level of the melt. This also reaches equilibrium and is marked by a vertical white line. The air temperature inside the pane has a similar profile to that inside the pane, although the temperature above the pane is slightly higher.

Industrial Application: Software for Glass Fiber Production at Wolz GmbH

Wolz GmbH uses specific process parameters and a precisely configured spinning device for the production of glass fibers. The speed of the disk is set to 2200 rpm. The glass melt is fed into the disk at a speed of 575 kg per hour and a temperature of 1065 °C.

To maintain the fiber temperature outside the wheel, burners are used that convey a mass flow of 1565 kg per hour at a temperature of 1440 °C. The spinning disk has 770 holes per row and a total of 35 rows. The hole diameters vary: Rows 1-6 and 33-35 have holes with a diameter of 0.74 mm. Rows 7-24 have a hole diameter of 0.69 mm. Rows 25 to 32 have holes with a diameter of 0.63 mm.

Simulation of the Production Process in Practice

We have simulated the glass fiber production process of the company Wolz GmbH with a simplified model assumption in which we have assumed rotational symmetry in order to perform the simulation efficiently in two dimensions.

Here we present the results of this simulation: Figure 1 shows the temperature distribution influenced by air flows and filaments. Hot air with a temperature of 1440 °C flows in from above and heating both the disk and the exiting fibers. As soon as the air touches the fibers, it cools slightly and is deflected radially. Further away from the disc, a fast, cold flow creates a strong expansion of the fibers and effectively cools them. The air flow pushes the fibers downwards and Coriolis forces cause the fibers to deflect laterally.

© Fraunhofer ITWM
Figure 1: Fiber curves outside the disc

Two-Phase Flow in the Disk

In Figure 2, the disk geometry is hidden to make the two-phase flow within the disk visible. A white vertical line indicates the filling level of the glass melt. In the equilibrium state, the temperatures of the air and the melt are equalized, whereby the air temperature above the pane is slightly higher.

Figure 3 shows the temperature curves of the fibers. Directly at the nozzles, the upper fibers are warmer, as the burner air flows in from above and heats the upper part of the disc more intensively. Shortly after leaving the nozzles, all fibers experience additional heating to ensure optimum stretching of the fibers. In the course of production, the fibers gradually cool down and solidify.

Figure 2: Equilibrium of the melt level of the two-phase flow disk — © Fraunhofer ITWM
Figure 2: Level equilibrium of the disk of the two-phase flow.

Abbildung 3: Temperaturverlauf verschiedener Fasern — © Fraunhofer ITWM
Figure 3: Temperature profile of various fibers

Abbildung 4: Fasergeschwindigkeiten für verschiedene Lochpositionen — © Fraunhofer ITWM
Figure 4: Fiber speeds for different hole positions

The different temperatures lead to different viscosities of the material, which in turn results in different mass flows at the nozzles. To counteract these different flow velocities of the fibers, different nozzle hole diameters are drilled. Figure 4 shows how the initial and final velocities of the fibers change as a result of this adjustment.

Optimization of the Holes for Uniform Fibres

Finally, Figure 5 shows the final diameters of the fibers. The influence of the varying mass flows on the final diameter is particularly evident in the first seven nozzles. While the initial diameters are the same, the final diameters differ. This precise assignment of the final diameters to the starting nozzles enables us to redesign the hole bores in order to produce fibers with a uniform diameter in the end.