What is the right 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.
First, what is foam? Foam is a soft plastic that is filled with air bubbles. The properties of different foams are created by using different types of plastic, controlling the size, and controlling the character of the air bubbles.
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
Density or Durometer is simply how firm or stiff the foam is. For EVA foam, a density of 25″C” is okay for upper padding but way too soft for the midsole.
Compression set resistance is simply the foam’s ability to bounce back after it’s compressed. A foam with a poor compression set will be crushed flat after just a few days.
Open Cell Foam:
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 shoe uppers. The PU foam allows the stitches to sink in and gives mesh some extra support while reducing wrinkles.
Reticulated foam for shoes
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 its own useful properties and will be explained below. 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 Pick Shoe Materials
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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.
Shoemaking Material Kit
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 soft and flexible or rock hard and stiff and 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 its 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, but
PE foam is closed cell and waterproof.
Other common foams 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. 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 sheets are often used to make die cut footbeds.
If you want to learn how to design shoes you must have an understanding of fabrics. Fabrics, or textiles, are a miracle material for footwear designers! With an infinite variety of weaves, knits, colors, patterns, and special features, textiles have a special place in footwear design. You will find fabric inside and outside on footwear, even on shoe bottoms. The polymer fibers such as nylon and polyester are lightweight and durable. Lycra is stretchable and cotton canvas is a must for vulcanized construction and has a look all its own.
The basic building block for fabric is…of course, thread! Denier is how thread weight is measured. 1 denier = 1 gram per 9000 meters of thread. Typical deniers are 110D for very lightweight fabric, 420D to 600D are common in shoes, 1000D for boots and bags.
There are many ways to “weave” the fibers
Textiles often incur a high import duty rate. Shoes made with 51% textile surface area are assessed for 20% duty. A textile shoe which costs less than US$12.50 will be charged 20% +.90 duty! (depending on the country of origin). However, there is a trick, by molding textile on the sole of the shoe you can avoid the high duty rates! Be sure to check the import rules for your shoes’ planned country of origin and destination.
Textiles really are a miracle material for shoes! With a rainbow of colors, patterns, and special features; textiles have a special place in footwear design. They are lightweight, strong, and flexible. There are so many options for your design.
We have included annotated cross sections of over 30 different shoe types. You can see exactly how each shoe materials are used inside real production shoes. Look inside basketball shoes, running shoes, track spikes, hiking boots, work boots, high heels, cowboy boots and many more!
Chapter 1: Leather for footwear
Shoe Dissection: See Inside Nike Kobe XI Elite Low
Introduced in 2012, Nike™ Flyknit™ technology, also called 4D knitting is changing the way athletic shoes are made.
Real Leather for Shoes
Synthetic leather for shoes
Textiles for Shoes
Lining Fabric for Shoes
Lasting Board
Metal Hardware
Shoe Laces/Webbing / Trims Etc./Reinforcements
Footbed foam
Lasts
Outsoles