Shoe Materials: EVA Midsoles

EVA midsoles are the most common cushioning materials in modern sport shoes. Used to make running shoes, basketball shoes, and almost any other sports shoe, EVA is lightweight, resists compression set, is available in almost any color, and is easily formed. EVA is now the most widely used midsole material for shoes.

How is EVA foam made
EVA (Ethylene-vinyl acetate) also known as PEVA (poly-ethylene-vinyl acetate), is the copolymer of ethylene and vinyl acetate. When two plastic types are joined in the same polymer chain, the polymer is called a copolymer.

The weight percent of vinyl acetate plastic usually varies from 10% to 40%, with the remainder being ethylene plastic. The amount of vinyl acetate greatly affects the character of the material. Higher quality EVA will have a higher percentage of vinyl acetate while EVA with very little vinyl acetate can be hard, brittle, and feel like cardboard. Ethylene-vinyl acetate is simply a soft plastic, it’s the foaming process or “blowing” of EVA that makes it perfect for a shoe midsole.

EVA foaming and “blowing” is the introduction of air bubbles into plastic. In the mixing process, a blowing agent is added to the EVA mixture. Azodicarbonamide is the most common blowing agent in EVA production. When heated, the solid azodicarbonamide breaks down or decomposes into a gas. When trapped inside a semi-liquid plastic EVA compound the gas expands to create tiny bubbles. The plastic is now foam.

EVA in shoe production
The EVA midsole is combined with the shoe outsole away from the main shoe assembly line. In a process called stock fitting, the outsole sub-assemblies are completed. Before stocking fitting, EVA parts may be washed to remove mold release and bonding surfaces maybe buffed to cement adhesion.

EVA midsoles may be painted, plastic shanks fitted, rubber parts bonded, and logos added during stock fitting. The stock fitting line will have heat tunnels for drying primer and cement. This line will be very short, maybe 1/3rd the size of the main assembly line, but will have presses to ensure strong bonding of the sole parts.