Apple put a heat pipe in the iPhone Duo's hinge. Here's why.

A folding phone has a heat problem that a regular phone doesn't. Apple says its fix is a bendable graphite pipe that runs through the hinge. But the hinge is only the last piece of a bigger redesign that starts at the chip.

The details come from PCMag's interview with two Apple engineers: Dan Murray, vice president of silicon engineering, and Banafsheh Barabadi, senior manager of thermal product design.

The problem

The iPhone Duo is thin when open. Apple lists it at 5.2 mm open and 11.3 mm folded. Close it and the two halves stack into one thick block with a hinge down the middle.

Inside is the A20 Pro, Apple's first 2nm chip for phones. It's fast, and fast chips make heat. As PCMag puts it, the less room you have, the harder it is to move heat away without losing speed. A phone that folds in half has very little room.

Step one: unstack the chip

The cooling story starts with how the chip is built.

Murray explained that every iPhone chip before the A20 Pro used package-on-package. The memory sat directly on top of the processor. That saves space, but it also covers the chip. Heat from the processor has a harder time getting out.

For the A20 Pro, Apple moved the memory to sit beside the processor. The technique is called a wafer-level multichip module, and Apple describes the design as inspired by its Mac chips. The tradeoff is a slightly larger package, so Apple worked with the system team to make room for it.

Barabadi said this let Apple connect the chip straight to the cooling system for the first time. The team then designed the whole thermal setup around that.

The four pillars

She described four parts to the system:

1. Direct access to the chip. With the memory out of the way, heat has a clear path to the vapor chamber. She said this removes the bottleneck that came with the old stacked design.
2. A material called metal over foam. It sits between the chip and the vapor chamber. It moves heat well and also handles the mechanical stress between the two parts. Apple says it took months to get the properties right.
3. A much bigger vapor chamber. It has three times the surface area of the iPhone 17 Pro's and holds more de-ionized water. On the iPhone 18 Pro, it also acts as the phone's structural backbone. The battery and logic board mount directly to it, and it's made with 80% recycled stainless steel for strength.
4. A new plate behind the display. Barabadi called it nanotwin copper. It's strong enough to support the screen and also conducts heat well. A graphite layer goes with it. Together they spread heat sideways across the display and cut down on hot spots. (PCMag's own summary calls this plate "nanotube" copper, so the exact term is unclear. I'm using the one from her direct quote.)

Her summary was that the four together improve thermal performance across every metric. She said that covers long jobs like gaming and video recording, and short bursts like on-device AI. She also tied the cooling to faster charging on the 18 Pro models.

What's different on the Duo

The Duo's chip isn't a copy of the 18 Pro's. Murray said Apple added a second display engine so the inner and outer screens can run at the same time. It also added support for a FaceTime camera behind the inner display. So it's the same core design with some extras.

The cooling changes too. The Duo gets a different vapor chamber. It's thinner than the 18 Pro's, and instead of being the structural center of the phone, it floats in the middle.

The graphite pipe

Here's where the hinge comes in. Barabadi said Apple routed a flexible graphite pipe through the hinge to carry heat between the two halves of the phone. That also gives the phone more surface area to shed heat from.

She didn't downplay how hard it was. She called it one of the most technically challenging aspects of the design.

Think about what the pipe has to do. A hinge moves every time you open or close the phone. The pipe has to keep carrying heat and stay in one piece through many hinge cycles over the life of the phone. That's why the thermal team and the mechanical team built it together.

How much better is it?

Barabadi said Apple got 35% higher sustained GPU performance than the iPhone 17 Pro in a form factor people didn't think was possible. Note that this is a GPU number, and it's Apple's own. For the 18 Pro, Apple's launch material claims up to 40% better sustained performance. The Duo's gain is smaller, which makes sense with less room inside.

What we don't know yet

Nobody has tested the Duo. PCMag says it hasn't run the Duo yet and expects to in the coming weeks. Its conclusion is only that, per Apple's engineers, the Duo should run fine without overheating. The Duo ships October 23.

There's some early evidence on the 18 Pro. PCMag says its own testing found record-setting benchmark scores for the A20 Pro and a battery longevity award for the 18 Pro Max. Separately, other testers ran Resident Evil 4 Remake at high settings. The 17 Pro Max reached a "Fair" thermal state in about 2 minutes 39 seconds. The 18 Pro Max took over 8 minutes. That's encouraging for the shared chip and cooling approach. But it's a different phone, with no hinge and a different vapor chamber.

The pipe has no public fold rating. Apple said "many hinge cycles" and no more. Some outlets printed specific numbers, from "thousands" to "hundreds of thousands." The interview doesn't give one, so I wouldn't trust any of them.

It's Apple's story so far. Every performance claim here comes from Apple or from interviews with Apple. Independent reviews and teardowns of the Duo will show whether the design works as described.

Where this fits

Other phone makers have chased the same problem. PCMag notes RedMagic uses a vapor chamber with a fan in its gaming phone. It also says Qualcomm made a similar memory-placement change in its newest chips, though PCMag hasn't tested those yet. So moving the memory aside isn't only an Apple idea.

What stands out about the Duo is how much of the phone's cooling was designed around the fold. The hinge isn't just a joint. It's part of the heat path.

Sourcing note: this version is based on the full PCMag interview, which Eric Zeman published on October 13, 2026. Phone specs and the gaming test come from other outlets' reporting, and the Duo's dimensions and price come from launch coverage.