Volume 45, Issue 3 - March 2010

feature

Tempering Very Thin Glass
What Radio Waves Mean for the Glass Industry
by Prem Boaz

In the current thermal tempering process, the body of a soda lime glass lite will have to reach a softening temperature of about 660°? C —this temperature will vary with the composition of the glass—before quenching operations start. The quench, which is usually done with forced air on the glass surface, extracts heat at a rapid rate to freeze the outer surface of the glass while the inner portion is still soft. As the cooling continues, the thermal contraction of the inner portion creates a strong compressive stress on the frozen outer surface, while a tensile stress is induced on the inner portion. The resulting panel is said to be heat tempered.

However, some physical difficulties arise during this sequence due to the required 660° C temperature required for the start of the quench operation. Typically, glass is heated in a conventional oven and then is transferred to a chamber where it is shaped, if required, and then transferred to a separate chamber for the quenching operation (see figure 1). At this location, the glass needs to be at a minimum of 660°? C. However, the glass loses its temperature very rapidly—as much as 25 degrees per second in thin glasses of less than 3-mm thick—before the quenching can start. The conventional solution to overcome this loss in temperature is to superheat the last zone of the preheat oven by as much as 50° to 100° C and pass the glass through this zone at a rapid rate. This strategy makes use of the fact that during a rapid rise in glass temperature the viscosity of the glass lags behind. However, in the case of glass thinner than 3-mm, this solution has been found to be inadequate.

At temperatures higher than the 660° C required for the start of the quench, the viscosity of the glass drops very rapidly and the physical handling of the glass tends to distort the panel. This problem becomes even more severe when the panels are less than 3-mm-thick. The viscosity of the glass actually drops by a factor of more than three between 600° and 660° C. It is this extreme drop in viscosity that presents a severe processing problem as this soft glass has to be transported from the heating zone into the quenching zone without inducing any optical distortion.



A New Process
To combat these difficulties a new process has been developed for shaping and/or tempering glass (see figure 4). In this new process, the glass is pre-heated in a conventional oven up to a temperature that allows the glass to be transferred or shaped. It is transferred at a convenient temperature below the softening temperature of 660° C. At this lower temperatransfer handling. This relatively cold glass of about 650° C (preferably between 620° and 650° C) is then transferred to the radio wave (RW) energy oven. Within the RW oven the glass is heated up to the required 660° C for the quenching action to take place. Since the inside of the RW oven is at room temperature, it also houses the quenching system and the quenching takes place within the chamber before the glass loses any temperature. This is the big difference in the new process from the conventional thermal tempering process.

The RW energy is preferably applied at about 20 megahertz. At this frequency, the apparatus is commercially available and has a proven record of plant safety. It is used extensively, for example, in moisture extraction application in food and paper processing industries.

Inside a RW chamber is typically a configuration of electrodes (see figure 6). The two beams of electrodes, the positive and the negative, are placed close to the glass and across the width of the glass to provide maximum exposure to the RW field that exists between the two terminals. Air blowers are provided above and below the glass to quench the glass when the required temperature of 660° C is reached. The timings (as to when the heating stops and cooling starts) are controlled within fractions of a second. The glass is supported on rollers that consist of ceramic rings specially designed for the RW application. The rings and the rollers allow free flow of air, essential for the quench process.

This process can also temper glass into curves (see figure 7). In some instances, turonce the glass is at about 630° C, it can be transferred from the pre-heat section to a set of rollers. The rollers can be arranged in an arc representing a radius to which the formed and tempered glass is meant to adhere, such as the curve required for automotive doorlites. After the glass arrives at this location the RW energy is activated and, as described above, heats the glass to about 660° C. At this temperature the glass begins to “sag” into the arc of the rollers. At the appropriate time the quench air is turned on to temper the panel.

The glass, when heated at a rapid rate with RW energy, does not exhibit the same sag/slump behavior usually expected when glass is heated at the same rate by conventional methods such as radiation, convection and conduction. For example, when the glass is heated by RW at the rate of 10° C/second, it tends to sag almost immediately when the softening point is reached. Additionally, RW heating seems to allow for a high level of tempering stress to be developed, even with conventional air quench after a rapid rise in temperature, up to the softening point. I suspect that the RW heating of glass is able to vary the viscosity of the body of the glass in unison with the rise in temperature even when the glass is being heated at a high rate.



Tempering Thick Glass
Another feature of this process is applicable to large commercial window glass in the range of 6 to 10 mm in thickness. These thick lites present a unique problem to temper since a large temperature gradient is introduced along the length during the transfer from the pre-heat to the quench chamber. The leading end loses much of the temperature before the trailing end is out of the pre-heat oven. This gradient can be compensated for by increasing the exit temperature from the pre-heat oven. However, this results in surface distortion. By moving the glass at relatively low temperature such as at 620° C e, approximately 650° C, the glass is pliable enough to be shaped but stiff enough to resist distortion from normal to the RW chamber and re-heating the glass to a uniform temperature of 660° C with RW heat and then turning on the quench system, a uniform temper can be introduced on the thick glass. For example, the electrodes can be placed to selectively heat the leading portion of the glass at a higher rate than the trailing portion (see figure 10). By varying the distance between the two electrodes, the intensity of the RW field that is applied to the glass is varied. This results in the leading end of the glass receiving more RW energy and its temperature gets closer to that of the trailing end.


However, one of the most significant advantages of the RW tempering process appears to be in its application to thin glass. While conventional process approaches its limits at 2.9 and 3 mm glass, the RW process appears to be capable of tempering glass down to 2-mm or less in thickness.



Thin Glass Applications
The proper means of exposing the pre-heated glass to a high flux density of controlled RW energy field allows very thin (less than 2-mm) glass to be tempered. The ability to temper thin glass measuring 2-mm or less might be expected to have an enormous impact in the flat glass industry in terms of the demand for raw material, energy consumption at the melting furnace, weight reduction of tempered window panels and more. The very thin tempered glass also could be expected to assist the photovoltaic cell industry in its quest for more durable and lighter panels with higher light transmission. Yet another area of application is in the fabrication of light laminated panels with very high impact-resistance. More exciting yet, this process can be added onto most existing tempering equipment.

Prem Boaz is president of Glass Products Consulting LLC in Port Orange, Fla. This paper was originally presented at Glass Performance Days 2009 on behalf of Vitro Group of Mexico. Mr. Boaz’s opinions are solely his own and do not necessarily reflect the views of this magazine.


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