This phenomenon is especially related to thin and large Low-E glass sheets. The reason is when you’re heating these large Low-E glass sheets, the glass tends to warm more on the sides than from the center. When this happens you need to overheat the glass or the glass becomes bi-stable after tempering. The way to avoid this is to have good crosswise and lengthwise profiling to heat the glass more in the center than the edges.
Some sophisticated tempering furnaces do this automatically, but you still need to pay special attention to the profiling.
This phenomenon is actually the same when you’re heating the soup in a microwave oven. Everyone knows that the soup gets really hot around the edges and the center stays cold which is extremely annoying. And the reason is that the microwave does not have any profiling so the edges will always heat up more.
The problem with heat strengthening thick glasses is that you have to use extremely low quenching pressure. It does not matter if your furnace is small or large but you have to have a configuration that allows you to have sufficiently low quenching pressure. So, your furnace needs to have a possibility to adjust the nozzles independently. Without independent nozzle adjustment it is really hard to process these thick heat strengthened glasses. The other thing is that your inverters have to support low pressures.
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