Tempering Very Thin Glass
What Radio Waves Mean for the Glass Industry
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
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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