Shoe Materials: EVA MidsolesEVA midsoles are the most common cushioning materials in modern sports shoes. Used to make running shoes, basketball shoes, and almost any other sports shoe, EVA is lightweight, resists compression set, forms easily, and is available in nearly every color. EVA is now the most widely used midsole material for shoes. Die-cut EVA can be found inside the cheapest discount sneaker, while molded EVA can be found in $200.00 high-end basketball shoes.
How is EVA foam madeEVA (Ethylene-vinyl acetate) also known as PEVA (poly-ethylene-vinyl acetate), is the copolymer of ethylene and vinyl acetate. When two plastic types join 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.
Expansion Press or Injection MachineThe classic way to foam EVA is to use an expansion press. The EVA compound is mixed into a putty-like consistency then rolled into flat sheets. These sheets are then loaded into square molds about two meters square and 10 cm deep. These molds are stacked in a huge press then heated under pressure. As the EVA heats, it gradually expands to fill the mold. If more raw material is placed into the mold the EVA will be harder, less EVA and more blowing agent will make softer EVA. Once the EVA is cooled the molds are opened, and huge sheets of EVA are released.
The Ultimate Shoe Material Textbook
Start with an in-depth study of material types available for modern shoemaking. Learn the technical details of material specifications. 195 pages and 300 color photos.
on sale $39.99
The more modern way to expand EVA is by using an injection machine. Instead of making a large block, this process will make individual sheets. The EVA injection machine is very special. The EVA material is heated inside then pushed under pressure into a mold. This mold is about 50% of the finished part size. The mold is cooled just enough for the EVA skin to solidify. The mold opens by high-speed hydraulic ram in less than 1/4 of a second. With the pressure released, the EVA parts expand like popcorn. POP! This process is very efficient with little waste. When parts pop out of the mold, their sizes will vary. This is how EVA sandals like Crocs™ are made.