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A Closer Look at Foam: Part II – Closed-Cell Foam

Top to Bottom: Polystyrene (EPS), Polyethylene, Polyethylene Roll, Gym Rubber, Cross-Linked Polyethylene (XLPE), Neoprene

For the second half in our review of the cellular structure of our foam materials, we take a look at the more rugged and durable of our two main foam types – closed-cell foam!


While there is little variation among open-cell foams, closed-cell foam structure offers a range of cellular characteristics that can be differentiated from one another with the naked eye.

Expanded Polystyrene (EPS) has the most unique foam cell structure in the closed-cell category, as well as the most easily recognized. The ubiquitous white foam, often seen as coffee cups and packaging, is sometimes called bead board as a result of its build. During its formation, tiny beads of styrene are expanded to more than 40 times their original size and compressed together into blocks, sheets, and shapes. These beads are evident in the structure of the material, and can even be picked out of a form and held in your hand. Because of these BB-sized beads, smooth cuts of EPS require hotwire cutters, otherwise the material tears and flakes.

Polyethylene is a closed-cell foam that has a more plastic-like appearance than its counterparts. In its various colors, the material carries a sheen, as opposed to the matte finish of more rubbery-looking closed-cell materials. The cells of polyethylene are large, similar in size to Dryfast, but are completely sealed off to one another as bubbles. The size of the cells in polyethylene also show that the bubbles that make up foam aren’t perfectly spherical, but complex and irregular polygons that bond together to form the material.

Very similar to standard polyethylene is polyethylene roll. This material can be thought of as standard polyethylene with a much finer and consistent cellular structure. It retains some of the plastic-like feel and appearance of polyethylene, but has some sponge-like characteristics as well. Also unique is the skin on the top and bottom of sheets of polyethylene roll. It isn’t perfectly smooth as cells can still be seen, but is firmer than the interior cells of the material.

Also similar in cellular appearance to polyethylene is gym rubber, though that appearance can vary drastically. As the name implies, gym rubber is more rubbery than polyethylene, giving it less of a sheen aesthetically. The cells on the top and bottom of gym rubber are obscured by the smooth skin of the material, but the cross-section shows the bubbly cellular structure of the material. More than any other material Foam Factory stocks, gym rubber has the most variation in appearance, from miniscule to large cells, which are all dependent on the reactions taking place during its formation.

The two materials with the finest cellular structures are neoprene and cross-linked polyethylene (XLPE). These materials are both cellular, but the size of their cells are so small they’re almost impossible to see with the naked eye. They are still there obviously, but the resulting appearance produces a material that looks more like a solid piece of plastic or rubber.

Both neoprene sheets and XLPE are consistent all the way through their forms, without getting denser near the outer skins. Despite the visual similarities, the compounds used in these two materials produce different characteristics. The neoprene is a more rubbery material, softer in most formulations than XLPE, with more flexibility in most cases. Cross-linked polyethylene meanwhile, is more rigid and board-like while usually being lighter and less dense. In thinner sheets, these two foam materials can be cleanly cut with a sharp, flat blade, such as precision crafting knives.

While there are many other factors that go into dictating how a foam material can perform, having a little insight on a material’s structural makeup can give you a good starting point for deciding what might work for you.

For the previous post, click here: A Closer Look at Foam: Part I – Open-Cell Foam

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