How Foam Mattresses Work, According to a Chemical Engineer

By David Holm  |  Jun 18, 2017
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If you type “foam mattress unboxing” into your favorite internet search tool, you’ll see thousands of homemade videos of people taking their newly-arrived, compressed foam mattress out of the box, cutting away the plastic, and watching as it unfurls. This was one of the key differences in Casper’s business when we opened in 2013. It allowed us to save on shipping costs, reduce the cost of the mattress to customers, and give people a cool at-home science experiment to witness.

The reason why you can compress a foam mattress and fit it in a box is the same reason why they can be incredibly comfortable: their unique molecular structure. Polyurethane foam (the kind of foam the Casper mattress is constructed from) is made up of small little “boxes” — on a microscopic level — that vary in shapes and sizes. These “boxes” are really a polymeric structure of cells that are full of air. Each cell is made up of “struts,” which offer support, and “windows,” which are the spaces between the struts. Depending on the chemical composition of the foam, the microscopic windows can be open, closed, or somewhere in between. When you compress a polyurethane foam mattress, you push all the air out of the cells. This is what allows us to compress a 10-inch-thick mattress down to around one inch, roll it up, and deliver it to your door.

A majority of the unboxing magic happens right after you take the plastic off. The polymeric structure of the mattress wants to regain its shape, and as it does so, it creates a vacuum effect that draws in air from the environment. So as the mattress gets its first “breath of air” out of the box, air rushes into all the cells within the foam, and the mattress begins to take shape almost instantaneously. Within minutes, the cells have all sorted themselves out and you can test it out.

The unique molecular composition of a foam mattress is the same reason they can be incredibly supportive and comfortable. One of the largest advantages of a foam mattress over a spring mattress is weight distribution, which comes back to its molecular composition. In a spring mattress, the surface area spreading out the weight of your body comes down to the diameter and number of coils. As you have more of your weight pushing down on these coils, you will feel them pushing back on you more, leading to pressure points. The cellular structure of foam creates nearly infinite contact points for your body. Viscoelastic polyurethane foam (a.k.a. memory foam), for instance, is great for pressure distribution and energy absorption and contours to your body. It does have some disadvantages though as it has slow recovery, leading to that “stuck” feeling some people experience. However, there are a lot of different ways to “layer,” or otherwise structure a foam mattress, to get it to have the characteristics an engineer like me looks for.

I can best explain how overall composition works in terms of how the Casper mattress functions. The Casper mattress is constructed from foam layers of increasing firmness, with varying properties. On a molecular level, as you lay on the Casper, the weight of your body collapses and pushes air out of the cells as the more supportive foams begin to engage. So as you compress our springy layer on top (which we developed to offer good airflow, support, and bounce), the viscoelastic layer beneath it will start to compress, and as the viscoelastic layer compresses the transition layer begins to compress, and ultimately the base foam of our mattress engages as the final support. But the key is that the body experiences all of these layers virtually simultaneously, just to varying degrees. It’s why we’re really cognizant of the overall construction of the Casper mattress.

We have also come up with a variety of tests to quantify (and optimize) the way it feels. The most important one we utilize is a force displacement profile. This allows us to visualize the force of the mattress pushing back on a sleeper as they compress our four layers. One of the largest improvements we’ve made to our mattress over the last 12 months is adding a transition layer to help ease the transition from the viscoelastic foam to the base layer, making it a more comfortable and “absorbing” sleep experience. We use many other tests to grade and improve our mattress as well, such as grade level and full mattress durability tests, to ensure that each Casper mattress still feels like it does on day one after a decade of sleep.

The fact that it can maintain peak fitness for that long — without any exercise — is almost enough to make you jealous.

David Holm is an engineer at Casper. He studied chemical engineering at the University of Minnesota and loves to talk molecules.

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