Inexpensive 3D printers often have a problem called "warpage," in which objects printed by them tend to curl up as they harden. Now, however, scientists have harnessed the power of warpage to create flat items that self-fold into predetermined 3D shapes when heated.

Fused Deposition Modelling (FDM) 3D printers are the cheapest kind, and build objects up by laying down a continuous filament of melted thermoplastic. Unfortunately, as that plastic cools and residual stress within it is relieved, it can contract and thus warp.

Led by assistant professor Lining Yao, a team from Carnegie Mellon University has developed a technique known as Thermorph, that takes advantage of this phenomenon. It uses a standard FDM printer to create flat sheets, by depositing two types of thermoplastic – one that's warp-prone, and another that's warp-resistant.

Based on what shape the user wants the finished folded product to have, custom software automatically controls the speed at which the warp-prone plastic is laid down in specific areas of the sheet, plus it controls the order in which the two plastics are deposited in different places.

When the resulting flat, rigid sheet is placed in water that is hot enough to soften it (but not enough so to actually melt it), it will warp in the areas where the first plastic was deposited more rapidly, causing it to permanently fold at those locations – the faster it was deposited, the more it will fold. The direction of the fold is determined by the order in which the two plastics were laid down.

So, what's the point? Well, according to the university, self-folding flat materials are quicker and cheaper to produce than solid 3D objects, plus they're easier and less expensive to ship. And while the objects created so far have been relatively small, the team believes that the technology could be easily scaled up, to create flat-pack products that are "assembled" simply using a heat gun.

"We believe the general algorithm and existing material systems should enable us to eventually make large, strong self-folding objects, such as chairs, boats or even satellites," says research intern Jianzhe Gu.

You can see the folding action for yourself, in the video below.

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