Foam for Shoes
There are many types of foam used to make shoes. We are going to review the foam found in the uppers and outsoles of shoes. Generally, foam is divided into two types; “Open Cell” and “Closed Cell” foam. Each foam’s density, compression set, resistance, and breathability are important attributes to understand when selecting foam for your shoes.
Density and Compression Set
Compression set resistance is simply the foam’s ability to bounce back after it’s compressed. A foam with poor compression set will be crushed flat after just a few days.
Open Cell Foam:
Open cell foam is exactly what it sounds like! The plastic compound that makes up the foam cells is open so air and water are free to enter and exit the foam just like a sponge.
Open cell foam is generally soft and can be made from Polyurethane plastic. This type of foam is commonly known as “KF” or “KFF” foam. This foam is available in different densities and in almost any thickness and color. This foam is used in the tongues and collars of shoes. Thin sheets of PU foam are used to back fabric in most shoes uppers. The PU foam allows the stitches to sink in and gives mesh some extra support while reducing wrinkles.
Reticulated foam is the most open style of foam. This foam for shoes is often used for ventilation features. Reticulated foam is almost skeletal looking, air and water flow freely through.
Memory foam is a variation of polyurethane with additional chemicals which increase its viscosity and density. Memory foam has the feature of a very slow return rate. Watch out, memory foam may freeze into a solid block in cold weather boots.
CLOSED CELL FOAM:
Closed cell foam is exactly that; the individual cells are closed or sealed, not allowing the foam’s internal gas to escape.
Closed cell foam is generally denser than open cell foam. The stretchability and hardness of the plastic material, combined with the size of the cells, determines the density of the foam. Midsoles of shoes are all made from closed cell foam. (midsole foam is covered further in our article Outsole Design.) The most common closed cell foams include EVA (ethyl vinyl acetate), PE (Polyethylene), SBR (Styrene butadiene rubber), PU (Polyurethane), Latex, and Neoprene. Each has their own useful properties. EVA foam is used for backing mesh materials. A 2mm sheet of EVA will make the fabric waterproof. Neoprene and SBR are used when elastic properties are required. Latex is common for collar linings. PE foam is very light, but not so durable, so its use is limited.
How to Measure Foam Density:
Measuring the density of very soft foams can be difficult. For soft foam, the density is measured in Lbs Per square foot. Very soft open cell foam is 1.2 Lbs per Sq. Foot. For denser foam, a Durometer tester is needed.
An Asker “C” scale Durometer tester is used to test the hardness of any foam components in your shoe. The Durometer tester will give you a reading of the density of the material. Try to test in flat spots and take several readings for each shoe part you are checking. For foam, try to cut the parts so you can test the center of the foam. EVA foam skin may give you a harder reading. A standard EVA midsole may be 55˚, a soft footbed 35˚. You can see the testing problem on the bottom is larger for softer foam.
When testing the hardness of rubber and plastic shoe parts you need a Shore scale “A” Durometer tester. The Durometer tester’s bottom measuring pin is simply pressed against the material. The Durometer tester will give you a reading of the density of the material. Try to test in flat spots and take several readings for each shoe part you are checking. You can see the testing problem is very small.
Common Foam in Shoes:
EVA (ethyl vinyl acetate): The most common midsole material for sports shoes. EVA is lightweight, durable, easy to form, and resists compression set. EVA can be hot pressed, cold pressed, die cut, injected, and machined to make midsoles or inserts. Available in a wide range of densities and formulations, EVA can be pillow soft and flexible or rock hard and stiff. EVA can be made in almost any color. It can be found in all different styles of shoes. EVA is also used to make footbeds, padded Stroble socks, and is often laminated as a fabric backing.
Closed cell PU (Polyurethane): Also a common foam for shoes. PU foam is “blown” into molds. The liquid compound expands and foam air cells fill the molds. PU foams are used to make durable midsoles for hiking boots and can be made into entire sole units, tread, and midsole all in one. Many women’s fashion shoe high heel outsoles are made from high-density PU. Very soft PU is used for footbeds due to is resistance to compression set.
PE (Polyethylene): Expanded into sheets, PE foam is easily die cut and laminated. Parts are then pressed into shape for internal pads and tongues. Due to its weakness in compression set, PE foam is not used underfoot.
PE foam is closed cell and waterproof.
More common foam for shoes…
SBR (Styrene butadiene rubber): A very soft foam, often laminated between two layers of fabric. SBR is closed cell and is used to make parts waterproof. SBR foam is often used as a lightweight replacement for Neoprene rubber but is not as stretchable.
Open Cell PU (Polyurethane): This may be the most common foam used in sports shoe footwear construction. This PU foam is open cell so you must be careful that it does not absorb the glue. Thin layers of PU are laminated to fabric to provide backing substance. PU foam is also used to make tongue foam and collar foam. Often called KFF foam. Due to its softness, open-cell PU foam cannot be used underfoot.
Latex Rubber foam: Latex foam is easily formed into complex shapes in open top molds. Latex is used for upper padding but not as a midsole material. High-density latex foam sheet is often used to make die cut footbeds.
For more information on foam and more detailed insight on all materials used for making footwear, check out our book the Shoe Material Design Guide. You can buy it or download a copy today.
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