Shoe Materials: EVA Midsoles

EVA midsoles have become the go-to cushioning material in contemporary sports footwear. Whether in running shoes, basketball sneakers, or various other athletic footwear, EVA stands out for its light weight, resilience against compression, ease of formation, and a wide availability in numerous colors. As a result, EVA has emerged as the predominant midsole material utilized in a wide array of shoes. From budget-friendly sneakers featuring die-cut EVA to high-performance basketball shoes worth $200.00 that incorporate molded EVA, this versatile material is a staple across the spectrum.

The Production of EVA Foam

Known scientifically as Ethylene-vinyl acetate, or PEVA (poly-ethylene-vinyl acetate), EVA is a copolymer derived from the fusion of ethylene and vinyl acetate. This unique combination results in a copolymer which exhibits enhanced characteristics depending on the ratio of its constituents. Typically, the weight percentage of vinyl acetate varies, ranging between 10% to 40%, while the remaining proportion is comprised of ethylene plastic. The specific percentage of vinyl acetate significantly influences the overall properties of the EVA material: higher-quality EVA features a greater percentage of vinyl acetate, yielding a more flexible and softer material. In contrast, EVA with a lower vinyl acetate content may exhibit hardness and brittleness, akin to cardboard. The foaming process of EVA is critical, converting this soft plastic into an ideal midsole material for shoes.

Methods for Expanding EVA

The traditional method for foaming EVA employs an expansion press. In this technique, the EVA compound is fashioned into a putty-like consistency and subsequently rolled into flat sheets. These sheets are then placed into square molds, typically around two meters in size and 10 cm in depth. When subjected to heat and pressure inside the press, the EVA expands to fill the mold. Adjusting the raw material quantity impacts the EVA's hardness, with added EVA yielding firmer results, while reduced EVA combined with an increased blowing agent leads to a softer product. Once cooled, the molds are opened, revealing substantial sheets of EVA.

Modern advancements have introduced the use of injection machines for EVA expansion. This method enables the creation of individual sheets of EVA rather than large blocks. The special injection machine heats the EVA material and injects it under pressure into a mold that is roughly 50% of the desired final size. Once cooled, the mold opens, allowing the EVA parts to rapidly expand, resembling popcorn bursting. This efficient method minimizes waste, resulting in products of varying sizes after the parts pop out of the mold.

Various EVA Expansion Techniques

The pellet expansion technique represents another approach for expanding EVA. During this process, raw EVA in pellet form is combined with azodicarbonamide and placed into a mold, similar in size to those used in injection molding. By applying pressure, the pellets expand, filling the cavity of the mold, which is retained in this state for a few minutes to enable cooling. Following this period, the mold opens rapidly, allowing the EVA part to achieve its final size. However, additional reprocessing is required to shape the expanded foam into precise forms needed for shoe production.

Cold Forming and Hot Forming EVA Processing

Cold forming techniques include die-cutting, where EVA sheets are shaped and placed within shoes. Additionally, traditional jogger midsoles often utilize cut and buff processes to refine the edges. The term "cold pressing" can be misleading, as this process involves heating the die-cut EVA sheets in an oven until they become pliable for shaping.

For hot forming, compression molding—also known as CMEVA or Pylon—serves as the leading method for creating mid-soles in athletic footwear. This process commences with an already-expanded EVA piece, either from an expansion press or pelleting method, placed in a heated mold to ensure complete filling and texture replication. Once cooled, the EVA parts are removed, requiring only a slight cleaning before use.

In terms of EVA hardness, it typically ranges from 5 to 70, with a recommended density of 40 to 65 for optimal performance. EVA densities are measured using a durometer tester, and most standard EVA densities for running and casual footwear hover around 55.

The integration of EVA midsoles occurs separately from the main shoe assembly line. Commonly referred to as stock fitting, this process includes washing EVA parts, affixing additional components like rubber parts or logos, and ensuring robust adhesion through specialized machinery and techniques.

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