A computer-controlled process developed at Carnegie Mellon University enables flat pieces of plastic produced in an inexpensive 3D printer to assume predetermined 3D shapes when heated. Source: Carnegie Mellon University

Carnegie Mellon University researchers have used inexpensive 3D printers to create flat plastics that are self-folding. When these plastics are heated, they fold themselves into pre-determined shapes.

Lining Yao, assistant professor in the Human-Computer Interaction Institute and director of the Morphing Matter Lab, was the lead researcher behind the foldable plastics. Yao said the self-folding plastics are a big step toward new products like flat pack furniture that ships flat, and then shapes into furniture with help from a heat gun. Foldable products would also be helpful for emergency shelters.

Self-folding materials have a lot of potential because they are cheaper and quicker to produce than 3D projects. This could make it possible for self-folding materials to replace noncritical parts or product prototypes using structures that approximate the solid objects. Boat hulls and other fiberglass products could be produced inexpensively and quickly with the new materials.

Self-folding materials have been explored in the past, but these attempts have used exotic materials or depended on sophisticated processing techniques that are not available everywhere. Yao and her team created their self-folding material with the cheapest 3D printer available by taking advantage of warpage, a problem that usually plagued these printers.

"We wanted to see how self-assembly could be made more democratic — accessible to many users," Yao said.

The FDM printers first lay down a continuous filament of melted thermoplastic. The combination of these materials causes residual stress and when the material is cooled, the thermoplastic contracts, resulting in warped edges and surfaces, called warpage. The researchers used the warpage to their advantage.

Yao’s team controlled the warpage by varying the speed that the thermoplastic material is deposited by combining warp-prone material with rubber-like materials that resist contracture. When the objects come out of the 3D printer they are flat and hard plastic. When the plastic is hard enough, it turns soft and rubbery but not melted, therefore triggering the self-folding process.

The 3D printer that was used was a pretty standard 3D printer, but the team replaced the open source software with their own code. This code automatically calculated the print speed and pattern needed for the material’s folding angles.

"The software is based on new curve-folding theory representing banding motions of curved area. The software based on this theory can compile any arbitrary 3-D mesh shape to an associated thermoplastic sheet in a few seconds without human intervention," said Byoungkwon An, a research affiliate in HCII.

The early examples of these printers are desktop size, but larger scale manufacturing of self-folding objects is possible.

"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," said Jianzhe Gu, HCII research intern.

This new material will be presented at CHI 2018 in Montreal.