How does a folding phone’s glass screen bend without breaking?

A folding phone’s screen folds in half, and on Apple’s folding iPhone there is glass inside it. Glass is brittle. Two ideas let it bend anyway: make the glass extremely thin, and let the layers of the screen slide.

▶ Try it yourself in Folding Screen Lab: change the fold radius and glass thickness, then let the layers slide.

Why does bending stretch glass?

When a sheet bends, its outer surface stretches and its inner surface squeezes. The stretch at the surface, the strain, is the sheet’s thickness divided by twice the bend radius. Ordinary phone cover glass is about half a millimetre thick. Folded around a 2 mm radius, its surface would stretch about 12%. Even strengthened glass cracks at a percent or two.

How thin is thin enough?

Foldable screens use ultra-thin glass. Samsung’s first, announced in 2020, was 30 microns thick, thinner than a typical hair, and was tested through 200,000 folds. Folded around the same 2 mm radius, its surface stretches only about 0.75%.

Why do the layers need to slide?

A folding screen is a stack. Apple says its inner display has more than 10 ultrathin layers, joined by optically clear adhesives that let the layers glide as the screen folds. Glued solid, the stack would bend like one slab, and glass near its top would stretch several percent. When the layers can slide, each one bends about its own middle, like a deck of cards, so the strain falls back to that of the thin glass alone and the screen is far easier to fold.

What about the crease?

The fold still leaves a shallow dent. HTX Studio measured about 48 µm on a new phone and 167 µm after 3,000 machine folds. Apple says the top of the inner display is a polymer, not glass, with a matte texture that scatters reflections, which makes the dent harder to see.

The tool’s 10-layer stack is illustrative, not Apple’s published specification, and its break line is approximate. Photos are AI-generated illustrations of a generic foldable.

Sources: Apple, iPhone Duo; Samsung Display, foldable display with ultra-thin glass (2020).