How to Solve Warpage in Tempered Glass – A Practical Look at 7 Common Issues

During tempering, glass warp may cause for a number of reasons. It usually comes down to how the glass moves through the heating and cooling stages. Below are the most common types of deformation we see on the shop floor, along with what actually causes them and how to fix them.

1. Concave Warpage (Usually Called "Upward Bending")

After tempering, the glass curves upward along its long and short edges relative to the roller table. This typically happens when the top surface gets hotter than the bottom - assuming cooling rates are the same on both sides. It can also occur when the bottom cooling is more aggressive than the top, even if temperatures are equal. The fix? Adjust your heating profile or balance the cooling air pressure.

Solution: Tweak heating temperatures and cooling air pressure.

2. Convex Warpage (Usually Called "Downward Bending")

Here, the glass bends downward after tempering. The main culprit is a hotter bottom surface compared to the top (again, with equal cooling rates). Or, the top cooling might be stronger than the bottom despite equal temperatures. Same fix as above.

Solution: Adjust heating and cooling balance.

3. Unstable Bending (Often Called "Wobbly Bending")

The glass ends up looking like a pan bottom and can be bent back and forth by hand. This tends to happen when the glass is nearly square (aspect ratio close to 1:1) and larger than about 1.5 square meters. The root cause: the edges are hotter than the center. Adjusting the heating curve usually does the trick.

Solution: Fine-tune the heating curve.

4. Saddle-Shaped Warpage

The glass twists so its four edges curve in different directions, looking like a horse saddle. This is basically the opposite of the previous issue - the edges end up cooler than the center. Again, adjusting the heating curve helps a lot.

Solution: Adjust the heating curve.

5. Wave-Like Deformation

When you look at the glass surface, reflections appear wavy, like ripples on water. This happens when the glass is heated too hot or for too long, causing it to soften and warp while still inside the furnace. Lower the temperature and shorten the heating time to fix it.

Solution: Reduce furnace temperature and heating duration.

6. Edge Frame Effect (Often Called "Dish Shape" or "Folded Edge")

This one is common when tempering Low-E glass. After tempering, all four edges curl up noticeably, and the glass looks like a dish. Because of how Low-E coatings behave, the glass first develops a severe convex bend during cooling before partially flattening out. The key here is to adjust temperature to control flatness – don't just rely on cooling balance.

Solution: Focus on heating adjustments rather than overcompensating with cooling.

7. Other Special Cases

Some deformation types depend on unique equipment or product conditions. These need to be handled on a case-by-case basis.