Appnote: Simple, acccurate control for enhanced preform surface polishing

An easy to use, low cost system for enhancement of existing glass lathes

Nextrom glass working lathe with high accuracy mass flow controller flame shown; image courtesy of Rosendahl-Nextrom

Ensuring the highest quality of preforms used in the fiber draw process is a critical step for all types of fiber optic cable. The preform is a specially constructed glass rod, from which a desired fiber optic cable is drawn. The preform is normally mounted at the top of a fiber draw tower and the end melted using a specialized furnace. Molten glass is then drawn to the desired thickness and is typically coated with a buffer and exterior jacket to complete the cable.

A common problem in fiber draw is contamination of the preform surface. Any such contamination, whether resulting from impurities in the preform itself or from the surrounding environment, can compromise the quality of the resulting fiber optic cable. Problems directly resulting from surface contaminants can include poor uniformity of appearance (especially important for medical grade fibers), brittle or easily broken fiber, as well as poorly performing cladding layers.

A common way to ensure that the surface is free of contamination is fire polishing the preform on an appropriate glassmaker’s lathe. A typical system would include burners flowing oxygen and hydrogen to generate a flame between 2500-3300 degrees Fahrenheit.  These temperatures will melt the surface of the glass, relaxing any strain, vaporizing most impurities, and smoothing the surface to prepare the preform to be drawn.

Precise control of the input hydrogen and oxygen is crucial to proper fire polishing of any preform. In many systems, simple rotameters are used to achieve this level of burner fuel flow control. While this level of control is inexpensive, it also suffers from imprecision, a lack of automation, and potentially dangerous leakage of combustible gases. More sophisticated systems utilize automated process controls, including mass flow controllers, to ensure that the burner fuel input is under precise and repeatable control.

Alicat’s Mass Flow Controllers & Meters can provide a simple way to precisely and repeatably control the mixture of input gases. Most MFC’s require costly computer or process control equipment. While Alicat mass flow controllers can be utilized with virtually any PLC or computer based system, they also include a convenient front panel-based control scheme. This provides a simple upgrade path for a system where more precise control is desirable. Desired flow rates for the input gases can be easily set and controlled directly from the unit, without any need for additional hardware. Once set, the Alicat mass flow controller will ensure smooth, precise delivery of the gas. The resulting mixture can then easily be optimized to ensure ideal burner flame for perfect polishing conditions.

Diagram of typical configuration for the described application.  Note: Gas colors displayed conform to CPK coloring standards, see

Zero point drift baseline

Alicat’s mass flow meters can provide an even lower cost solution when the current system requires only validation or calibration rather than improved control. Fuel flow rates can be measured and validated directly from the unit’s front panel display. All Alicat meters and controllers also provide the capability to log multiple measured parameters simultaneously, including flow rate, temperature, and pressure of the fuel lines to facilitate data logging and process monitoring. The very low zero drift of these units, coupled with a lifetime warranty, mean that the calibration is always reliable. Even on modern systems, calibration of mass flow control remains a critical step. Many mass flow controllers require frequent and time consuming calibration steps. With zero drift baseline, Alicat Mass Flow Meters can go years without any loss of accuracy or change in zero point. And these systems all include Alicat’s limited lifetime warranty.

Alicat mass flow controllers help improve and enable the most precise and demanding technologies.

Doesn’t your fiber draw process deserve proper mass flow control?

Find out how we can improve your glass & fiber processes. Speak to an applications engineer at 1.888.290.6060 or request a recommendation.