How Is Foldable Glass Made Thin Enough to Bend?

Glass seems like the last material you would choose for a folding phone. Drop a drinking glass and it shatters; try to bend a windowpane and it breaks. Yet some foldable screens contain real glass that curves each time you close the phone.

The trick is not to make glass behave like rubber. It is to make it extremely thin, protect it from tiny flaws, and control how tightly it bends. And despite the name “ultra-thin glass,” the surface you touch may still be plastic—for good reason.

Why thin glass can bend

When you bend a sheet of material, its outer face has to travel around a longer curve than its inner face. The outside is stretched, while the inside is squeezed.

Engineers call that stretching strain: the change in length compared with the original length. Glass can tolerate a little strain, but too much can cause a crack to grow.

Here is the crucial point: the thinner the sheet, the smaller the difference between the paths followed by its inner and outer surfaces. That means less stretching for the same curve.

Think of bending a single sheet of paper versus a thick stack. In the stack, the outer sheets need to travel noticeably farther than the inner ones. Making the whole structure thinner reduces that mismatch.

For a simple sheet, the stretching at the outer surface is approximately:

Here, $t$ is the sheet’s thickness, and $R$ is the bend radius—a measure of how broad or tight the curve is.

In words, the formula says two things:

• Thinner glass experiences less surface stretching at the same bend radius.
• A broader curve produces less stretching than a tight curve in the same glass.

A real display has several bonded layers, so this is a simplified picture rather than a complete description of the screen. But it captures the central idea.

Foldable glass does not become stretchy. Making it thin keeps the stretching required for a bend small enough to tolerate.

How thin is “ultra-thin”?

Samsung reported that its first foldable ultra-thin glass, often shortened to UTG, was about 30 micrometres thick. A micrometre is one-thousandth of a millimetre, so that is just $0.03$ millimetres.

For comparison, the rigid cover glass Samsung used in that explanation was roughly 500 micrometres, or $0.5$ millimetres, thick—about 17 times thicker. These are specific examples, not universal dimensions for every phone. (Samsung Display)

At the same bend radius, reducing thickness by that factor also reduces the surface strain by roughly that factor. Thinness therefore makes a dramatic difference without changing glass into a different kind of material.

Making it thin is only the beginning

Usable foldable glass is not simply an ordinary phone cover shaved down until it bends. Manufacturers must produce a very thin sheet while carefully controlling its surfaces, edges, and strength.

The broad process involves:

1. Forming or processing thin glass sheets to achieve the required thickness.
2. Cutting and finishing them carefully, including polishing where needed.
3. Minimizing surface and edge flaws that could become starting points for cracks.
4. Strengthening the glass so it can better withstand the stresses of bending.

The exact manufacturing route varies. What matters is that thickness alone is not enough: a thin sheet with damaging flaws may still break.

Why tiny flaws matter so much

Glass failure often begins at a scratch or another microscopic defect. When the outer face of the glass is stretched during bending, that defect can be pulled open.

A useful comparison is a small tear at the edge of a sheet of paper. Pulling on the sheet can make the tear spread. Glass behaves differently from paper, but the analogy highlights why a small defect can determine where a larger failure begins.

Careful finishing reduces potential crack-starting sites. Strengthening then adds another line of defence.

Ion exchange: giving the surface a protective squeeze

One way to strengthen glass is ion exchange. An ion is an atom or group of atoms with an electrical charge. In a common glass-strengthening process, the glass is placed in a hot bath of molten salt.

Larger potassium ions from the bath replace smaller sodium ions near the glass surface. Because the replacement ions take up more room within the glass structure, they create a surface layer under compression—a built-in squeezing force. (Corning)

Why does squeezing help? Because tension, the pulling force on the outside of a bend, tends to open cracks. Starting with a compressed surface means that the pulling force must first overcome that protective compression before it can readily pull a surface flaw open.

Ion exchange does not make glass soft. It gives the surface a head start against the forces that open cracks.

This allows the glass to tolerate more stress before a crack grows. But it does not make the sheet unbreakable. A deep scratch, an excessively tight bend, or too much force can still cause failure. (Corning)

How ultra-thin glass differs from early plastic covers

Early foldable phones used flexible plastic cover windows, including polyimide, a type of polymer. A polymer is a material made from long chains of linked molecules; many familiar plastics are polymers.

Plastic bends more readily than glass and is less likely to fracture under a sharp bend. Its drawback is that it is generally more vulnerable to scratching and surface distortion.

Ultra-thin glass provides a harder, smoother layer, but it should not be confused with the thick glass on a conventional rigid phone.

Samsung’s original Galaxy Fold used a polyimide cover before later models adopted ultra-thin glass. That change improved the cover material, but it did not eliminate the need for a layered protective structure. (Corning)

Why there is still plastic on top

A foldable screen is a stack of layers, not a single exposed sheet of glass. Glass and plastic contribute different qualities.

The ultra-thin glass provides a smooth, firm layer that bends with the display. Plastic layers above it help protect the delicate glass from everyday contact and impacts. In some designs, they also help keep fragments in place if the glass cracks.

This explains an apparent contradiction: a phone can legitimately have a glass-based folding screen while the outermost surface you touch is plastic.

It also explains why a fingernail or sharp object can damage the surface despite the presence of glass underneath. A harder layer below does not make the softer protective material above it scratch-proof.

Should you remove the factory film?

Do not peel the factory-applied film off the inner folding screen yourself. It is not merely shipping packaging; it is part of the screen’s protection.

Manufacturers warn that removing it can damage the display and may also advise against adding an unapproved protector over it. If the film lifts, bubbles, or becomes badly scratched, follow your phone maker’s replacement instructions or use an authorized service provider. Guidance for a separate, rigid outer screen may be different. (Samsung Support)

The hinge completes the solution

Even carefully made, strengthened glass cannot tolerate every possible bend. The hinge must guide it through a controlled curve rather than force it into a sharp corner.

That brings the whole design together:

• Thinness reduces the stretching caused by bending.
• Careful finishing limits flaws where cracks might start.
• Ion exchange helps resist crack-opening forces.
• Plastic protective layers shield the delicate glass.
• The hinge controls the bend radius.

The result is not ordinary phone glass made magically flexible. It is a carefully engineered protective system in which very thin glass can bend safely within defined limits.