When I held the new Motorola Razr foldable phone in my hand, all I wanted to do was open and close it a thousand times. Not because I wanted to test the screen or durability (like we did with the Galaxy Fold and will do again with the Razr on Thursday), but because that's what I did all the time when I had the original Razr flip phone in 2005.

I toured Motorola's headquarters in Chicago last fall, specifically a lab filled with industrial machines used to test and build the foldable screen phone. On one side of the lab there were handwritten measurements scribbled on a wall like markings you make to keep track of a kid's growth. Next to that was a menacing machine used to drop the Motorola Razr repeatedly to test durability. My guess is that the measurements were the different heights Motorola dropped the phone. Also, I wondered if I should be wearing safety glasses.

It was early November and I was at Motorola to see how the hinge on its new Razr was designed, iterated and implemented. As I toured the lab, I kept flicking the Razr open and closed with my thumb. The "thud" sound it made wasn't exactly the same as the original, but it was still immensely satisfying. If the Razr's folding screen is the star of the show, then the hinge is the stage manager making it all work.

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Hiding the folding-screen bulge

Motorola wanted to create a pocketable and slim foldable phone that didn't have a permanent crease down the middle of the screen. One of the biggest hurdles to overcome was how the display would fold. Flexible displays are made of thin plastic and limited in how far they can bend before breaking or creasing.

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After years of experimentation, Motorola determined exactly how far it could bend a screen without leaving a crease. It's similar to folding a piece a paper in half. If you crease it all the way, it'll cause a wrinkle. But if you leave a curve in the fold instead, most of the paper can lie flat and there won't be a crease. Plastic flexible screens have been developed to the point where they can have a pretty tight curve. The trade-off is incorporating the display's curve into the chassis of the phone.

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Huawei solved the display curve problem by putting its screen on the outside of the Mate X. Essentially, the tablet's body fills the gap in the fold curve like a taco shell.

Like Motorola, Samsung put the Galaxy Fold's display inside, more like a book. But unlike the Razr, which closes perfectly flat, the Fold has an open space between the two halves. If you look at a side view of the Galaxy Fold, you can see its body follows the fold curve of the display.

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Motorola took a different approach than Samsung and built its phone to house the display's fold curve. The result is a hinge that's built on the sides of the screen to allow space for the fold curve to live when in the closed position. It's similar to a hinge design Motorola's parent company Lenovo used for its Yoga laptops.

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Getting rid of the gap

The Galaxy Fold and Motorola Razr have two very different designs. The Fold opens from a regular-size smartphone into a small tablet. The Razr folds down from a smartphone into a pocket-friendly shape. Motorola wanted the Razr to sit perfectly flush in the closed position without a gap between the folded ends of the screen. In contrast, when the Fold is in the closed position there is a wedge-shaped air gap.

To prevent a gap, Motorola used steel plates under the screen that go across the width of the phone. When the Razr is closed the plates slide out of the way to make room for the display's teardrop fold. When the phone is open, the plates push up against the screen to help it lie perfectly flat.

Motorola

The display has a system of spring cams positioned on the edges to guide and pull it completely flat. As you open and close the Razr, the display actually moves up and down ever so slightly behind the phone's chin.

The cams are made from hardened steel and have a diamondlike coating for durability. Think of these cams as being like the gears on a watch. When you look at the phone close up, you can see the cams moving as you open and close it.

As I finished my day up at Motorola, I handed back the phone. The Razr made a solid first impression on me. I have an appreciation for the amount of problem-solving and engineering challenges Motorola went through. But until the phone gets into the hands of reviewers and customers, the durability of the screen and hinge remains unknown. For example, the openings on the sides of the screen where you can see the hinge cams seems vulnerable to dust, crumbs and other small particles. The same concern applies to a space behind the display that can be seen when opening or closing the phone.

Juan Garzon/CNET

Photos and videos of the Razr in the middle of being opened or closed can make the space behind the display seem quite exposed. In reality, opening and closing the phone takes a fraction of a second, which minimizes the chances of collecting small debris behind the screen. I look forward to testing the Razr in depth when it comes out this week.

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Originally published Nov. 21, 2019.