Foam for Shoes
There are many types of foam used to make shoes, here we are going to review the types of foam found inside the uppers of shoes and outsoles. Generally foam is divided into two types, “Open Cell” and “Closed Cell” foam. These foams have different attributes to understand. Density, compression set resistance and breathability are important to understand when selecting foam for your shoes.
Compression set resistance is simply the foams 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 is exactly what is sounds like, the plastic compound that makes up the foam cells is open, air and water are free to enter and exit the foam just like a dish washing sponge.
Closed cell foam is exactly that, the individual cells are closed or sealed not allowing the foams internal gas to escape. The strechability and hardness of the plastic material combined with the size of the cells determins the density of foam. Open cell foam is generally softer these foam are made of 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 type is often used for ventilation features. Reticulated foam is almost skeletal looking. Air and water flow freely through reticulated foam.
Memory foam is a variation of polyurethane with additional chemicals increasing its viscosity and density. Memory foam has the feature of 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 generally denser. Midsoles of shoes are all made from closed cell foam. Midsole foam is covered the 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 are the most common types. Each has their own properties. EVA foam is used for backing mesh materials, 2mm sheet EVA will make the fabric water proof. Neoprene and SBR are used when elastic properties required. Latex is common for collar linings. PE foam is very light but not so durable so it’s use is limited.
When working with foam it important to know what foam is appropriate for the shoe upper and what is appropriate for cushioning the shoe outsole.
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 shoe components. The Durometer tester will give you a reading of the density of the material. Try to test in flat spots, 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.
A Durometer tester is used to test the hardness of any rubber or plastic shoe component. For testing shoe rubber and plastic you need a Shore scale “A” tester. The Durometer testers 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, 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 light weight, 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 ranges of densities and formulations. EVA can be pillow soft and flexiable or rock hard and stiff. EVA can be made in almost any color. Can be found in all different styles of shoes. EVA is also used to make footbeds, padded stoble socks and is often laminated as a fabric backing.
Closed cell PU (Polyurethane): Also a common foam for shoes. PU foam is “blow” into molds. The liquid compound expands and foams air cells to fill molds. Used to make durable midsoles for hiking boots and cam be made into entire sole units, tread and misdole 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 easly die cut and laminated. Parts are then pressed into shape for internal pads and tongues. Due to it’s weakness in compression set PE foam is not used under foot.
PE foam is closed cell and water proof
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 water proof. SBR foam is often used as a lighter replacement for Neo-Prene rubber but is not as stretchable.
Open Cell PU (Polyurethane): Maybe the most common foam used in sports shoe footwear construction. PU foam is one cell so care must be taken so that it does not absorb glue. Thin layers of PU are laminated to fabric to provide backing substance. PU foam is also use to make tongue foam and collar foam. Often called KFF foam. Due to it’s softness open Cell PU foam can not be used under foot.
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.
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