This post by Dave Sherman originally appeared on the PORON Cushioning Blog.

Many customers are often surprised when we explain to them that all PORON® Performance Cushioning Materials are open cell polyurethane foams. It seems that the standard cushioning foams that they see on the market are typically closed cell EVA foams or closed cell polyurethane foams.

In fact, the open cell nature of PORON Cushioning foams is one of the material properties which help to make PORON Materials have the best resistance to compression set (C-Set) or for the non-foam geeks out there: resistance to break down after multiple uses.

Closed Cell Foams vs. Open Cell Foams

What do you need, a tennis ball or a spring?

EVA foams, commonly used in insoles and sports padding, are closed cell foams, meaning that all the bubbles of trapped air in the foam are complete bubbles, with cell walls all around, like a million balloons all stuck together. This kind of foam gets most of its properties from the air trapped inside the bubbles. When the foam is compressed, say when you’re walking on an insole or when you land on your elbow pad, the air inside the bubbles is compressed, and the return force is caused by the decompression of the air. This behavior is just like the behavior of a tennis ball, which gets its bounce from the air inside the ball. Like a tennis ball, the air eventually leaks out of the cell, and the foams go flat, or “take a set” in foamese. That means the foam insole is less comfortable on the next step, or less protective on the next hit.

Closed Cell Foam:

PORON Cushioning materials are an open cell foam, which has little connections or portals between the cells that allow air flow between them. So PORON materials are not dependent on the air for their properties, but instead on the properties of the materials in their cell walls. They operate more like spring, and return to their original position after being compressed, time after time because the air moves freely in and out of each cell.

Open Cell Foam:

Being closed cell offers some advantage and disadvantages in applications. The foams can be very light, as their cell walls can be very thin, but are usually stiff because of the incompressibility of the air inside them They can also be better at resisting liquid penetration.

Likewise, in addition to being resistant to taking a set, open cell foams have some other advantages, like being breathable and soft (better Compression Force Deflection – CFD). The table below summarizes some of the advantages (marked with an A) each foam has.

Foam Properties

Measures

Closed

Open

CFD Softness/Conformability A
Compression Set Resistance Life of Properties A
Anti Microbial Integral Coating
Breathability MVTR-Yes/No A
Water Absorption % Uptake After Some Time A
Washability Cycles at Setting A
Closed/Open Cell

So as you design something that will use a foam, ask yourself what are the most important properties you want. If you want light weight and great washability, then use a closed cell foam. If you want reliability over time, softness and breathability, choose PORON? Performance Cushioning Materials.

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One Response to Foam Properties: What Makes One Foam Different From Another?

  1. I am an artist, designing an inexpensive medical device out of foam.
    Have a lot of foam questions to ask, can a structural resilient foam be cast with a skin soft layer in the same operation? And still be inexpensive per item?
    Also, could you please furnish me with samples that might fit this need?
    This is so I can look at foam “swatches” to better understand the names and properties?
    Larry Oswald

    THANK YOU.

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