From LEGOs to Ziploc: The Science of the Snap Fit

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Pen caps. IKEA furniture. Snaps on a baby’s onesie. Our world is filled with everyday examples of the snap fit. That’s the term used to describe bringing things together by clicking separate parts. (“Listen for the strong click,” my kid’s teacher tells them in kindergarten to encourage pen caps to find their proper home.)

Yet as ubiquitous as the snap fit is, the mechanics of how it works hadn’t been studied deeply. A new paper changes that, by diving into the physics of snap fits.

Hirofumi Wada, a physicist at Ritsumeikan University, in Kusatsu, Japan began to see snap fits everywhere he looked in the house he shared with his wife and three kids: Lego blocks, Ziploc bags in the kitchen, plastic covers when replacing batteries in kids toys. He started to wonder how it works. Not long after, Wada’s student, Keisuke Yoshida, was playing around with objects in the lab, and found a simple cylinder made from a curved sheet of plastic created the dynamics for a snap fit experiment.

The researchers used that system to show a rich variety of snapping behavior, which Wada said was totally unexpected. They used a cylinder and a thin plastic sheet that had been treated with hot water to bend to the shape of the cylinder, and measured the forces at work as the sheet bent over the cylinder and eventually slammed into place. They identified at least four different sequences as the snap happened —the strongest force happened when the sheet bent wildly, about to click into place. The paper was published last fall in the journal Physical Review Letters.¹

“We are physicists and like to study a simplified system in detail,” said Wada. He said that while the team doesn’t have a specific application in mind, they hoped to reveal how a snap fit may function at its simplest level, which could be useful for creating better industrial designs in the future.

In any snap system, the key thing is that you want it to be easy to push on and harder to pull off—known as the locking ratio—said Dominic Vella, an applied mathematician at Oxford University in the U.K., who was not associated with the new paper. He added that the new paper shows how getting a good snap fit depends on the interaction between the geometry of the object and its ability to deform and then return to its original shape.

The research combines beautiful work: a simple experimental setup and the collection of striking results, said Pierre-Thomas Brun, a chemical and biological engineer at Princeton University, who wasn’t involved in the research. “That’s the signature of this lab—using super-elegant tabletop experiments which unpack a lot of interesting and not necessarily simple results,” he said, adding that the project takes mundane objects and shows that there is more than meets the eye.

While it focuses on the fundamentals, the paper does mention a few new applications where snap fits could take the place of excess plastic or glues that are harmful to the environment. If pieces of construction materials could simply lock together in place, there would be no need for adhesives to stick them together.

Another potential application is in robotics. Instead of grasping an item—something that has proven to be tricky—a robot in a warehouse or on a job site could simply click-lock onto an object. It wouldn’t work for helping humans move from the bed to the shower, but could be used for moving groceries or packages. “It’s an easy way to grasp on to a crate and move it somewhere,” said Brun.

There’s not a really good answer to why physicists haven’t gotten inside the question of snap fits before, said Vella. One reason could just be that these systems have worked—and there wasn’t much impetus to investigate why. “You try it, it works and so people might not feel the need to go deeper than that,” said Brun. “But of course, when you go deeper, you can have a better understanding of the fundamentals of these things and in turn that can lead to more optimized applications.”

Wada is also interested in investigating what he calls the “type-II snap” which has the opposite qualities: it’s easier to pull apart and challenging to push together. He thinks it could be a building block for new metamaterials. He’s also interested in rigorously investigating how a plastic ball joint works. “In any structure, form and function are always intimately coupled and much needs to be studied from the viewpoint of physics.”

1. Yoshida, Keisuke and Hirofumi Wada. “Mechanics of a Snap Fit,” Phys. Rev. Lett. 125, 19 (6 November 2020). DOI:https://doi.org/10.1103/PhysRevLett.125.194301 .