All About Footwear Foam Physical Properties

As foam technology advances, the selection available for use in footwear keeps growing.  Choosing which foam best suits your footwear design is challenging enough already. Luckily, standardized testing provides a benchmark for quality assurance, allowing you to draw meaningful comparisons across the different foam types.

When you see American Society for Testing and Materials (ASTM) sourced for testing physical properties, you know the material has been subjected to rigorous testing with very specific rules around equipment used, sample sizes, and measurement procedures. Here, we will introduce you to the top four physical foam physical properties customers prioritize in underfoot comfort and the behind-the-scenes testing that can help you understand those features.

Foam Physical Properties and the Customer Experience

Hardness

• Definition: Hardness describes how difficult the foam is to compress. Harder foams are more difficult to compress than softer foams.
• Test Name and Process: ASTM D2240-15e1 test using an Asker C durometer. A durometer gauge with a sensor penetrates the foam and measures the amount of indentation.
• Comfort Outcomes: Combined with specific underfoot shapes, harder foams can provide more arch support and stability compared to softer foams. As a result, high performance or work-related footwear often benefits from foams like poured PU over memory foam. Meanwhile, customers may find softer foams more comfortable in shoes designed for relaxation, such as slippers and other leisure footwear.

Density

• Definition: The mass of the foam within a specific volume, or, how tightly packed in the foam is. Less dense foams like open cell PUs and EVAs will typically feel airier than denser foams like poured PU.
• Test Name and Process: ASTM D3574, Test A. Mass and volume are measured on a defect-free foam sample. Density is found by dividing the mass by the volume of the sample.
• Comfort Outcomes: Density can affect the compression set and overall weight of footwear. Foams with higher density tend to offer better shape retention over time and are less likely to pack out. They can also feel heavier, affecting comfort perception. Meanwhile, low-density foams may have a softer feeling out of the box, but can suffer from a shorter lifespan.

Compression Set

• Definition: Compression set measures how much foam loses its shape after being compressed and released. Compression-resistant foams have lower percentages than foams that compress more easily.
• Test Name and Process: ASTM D3574-17, Test D. Testers measure the original thickness of a foam sample before placing the sample within a metal compression device in a heated lab. The foam is compressed to 50% thickness for 8 hours, and is then removed and released to decompress for another 12 hours. Then, testers measure the foam’s new thickness to calculate the percentage of the original measurement.
• Comfort Outcomes: Compression affects how a shoe feels over time. Wearers will notice that their cushioning may be less effective the longer they wear it, and underfoot support may feel uneven as movement patterns affect shape. Lower compression set percentages indicate the foam recovers well and may offer a longer lifespan.

Resilience

• • Definition: Resilience reflects the amount of energy returned after the foam is compressed.
• Test Name and Process: ASTM D3574-17, Test H. In this test a steel ball is dropped through a tube onto a foam sample and the height of the rebound is measured using a light curtain.
• Comfort Outcomes: Foams with better rebound often feel more responsive to movement. They are designed to give back a higher percentage of the energy put into them with each footfall, making activities like running and jogging feel more effortless. While PU foams have a wide range of resilience properties, foams like EVA, ETPU, and supercriticals are known for exceptional rebound, which is why they are often found in athletic footwear.

Foam Knowledge is Power

Now that you have been armed with the info around foam physical properties, you can test your new knowledge by checking out some tech sheets. Find what you’re looking for in our foam selection.