How does a folding phone close flat without creasing its screen?

Fold a sheet of paper flat and it creases for good. A folding phone’s screen can’t do that: pressed flat on itself, the thin glass inside it would crack. Yet Apple’s folding iPhone closes with its two halves flush. The trick is in the hinge.

▶ Try it yourself in Teardrop Hinge: close the phone with three kinds of hinge and compare the bend, the strain and the gap.

Why can’t the screen fold flat?

When a sheet bends, its outer surface stretches. That stretch, the strain, is the sheet’s thickness divided by twice the bend radius. Pressed flat, a screen’s tightest bend is only about as big as the screen is thick, around a tenth of a millimetre, so 30-micron glass would stretch about 15%. Glass cracks at a percent or two, and a hard fold leaves a permanent crease.

Where does the screen go when the phone closes?

Into the hinge. As the phone closes, the middle of the screen drops into a space inside the hinge and curls into a teardrop shape. HTX Studio CT-scanned an iPhone Duo and measured the teardrop at about 3.85 mm across at its widest. That keeps the tightest bend near 1.9 mm, where 30-micron glass stretches only about 0.8%. The neck of the teardrop narrows above the loop, so the two halves can meet flat.

What did older foldables do?

The first Samsung Galaxy Fold, in 2019, bent its screen into a U at the hinge. A U needs room as wide as twice its radius, so the halves stayed apart and left a visible gap when closed. Samsung says the Flex Hinge it introduced in 2023 virtually eliminated that gap.

What’s inside the hinge?

Apple says the hinge has more than 100 components. HTX Studio’s teardown shows gears that make both halves turn together, and springs and cams that hold the phone open at angles from about 15° to 165°. Near closed, magnets pull it shut. REWA Technology’s teardown counted four sets of springs.

Why the glass can bend at all, and why the screen’s layers glide, is in the companion tool, Folding Screen Lab.

The tool’s slab thickness, U radius and teardrop neck shape are illustrative, and its break line is approximate. Photos are AI-generated illustrations of a generic foldable.

Sources: HTX Studio, iPhone Duo review and CT scan; Apple, Introducing iPhone Duo; REWA Technology, iPhone Duo teardown; Marques Brownlee, The Broken Galaxy Folds: Explained; Samsung, Flex Hinge (2023); Apple, iPhone Duo.