EVA (Ethylene-Vinyl Acetate Copolymer) is a high-performance thermoplastic material produced through the random copolymerization of non-polar crystalline ethylene monomers and highly polar non-crystalline vinyl acetate (VA) monomers.
Compared with polyethylene (PE), EVA introduces vinyl acetate segments into its molecular structure, which reduces crystallinity and significantly improves flexibility, impact resistance, filler compatibility, elasticity, and heat-sealing performance. EVA typically has a density of approximately 0.91–0.93 g/cm³, featuring excellent transparency, gloss, and processing performance.
As a thermoplastic polymer, EVA can be processed through various manufacturing methods, including injection molding, extrusion molding, blow molding, calendering, rotational molding, vacuum forming, foaming, coating, heat sealing, and welding.
Among these processes, foaming technology is one of the most important applications of EVA in the footwear industry. EVA foam materials are widely used in sports shoes, running shoes, hiking shoes, slippers, sandals, and other footwear products due to their excellent cushioning performance, lightweight structure, flexibility, shock absorption, and chemical resistance.
Currently, EVA footwear foaming technologies can mainly be classified into four categories:
Traditional Flat-Sheet Large Foam Molding
In-Mold Foam Molding Technology
EVA Injection Foaming Technology
Supercritical Fluid Foaming Technology
Traditional Flat-Sheet Large Foam Molding Technology
Traditional flat-sheet foam molding is one of the earliest and most widely used EVA foaming methods. Due to its relatively low equipment investment and simple production process, it remains popular among many small and medium-sized footwear manufacturers.
In this process, EVA raw materials are first compounded and foamed into large EVA sheets. These sheets are then processed through cutting, grinding, shaping, and other secondary operations to produce finished footwear components.
The foaming process is relatively stable. Typical processing conditions include:
Foaming temperature: 160–170°C
Pressure: approximately 150 kg/cm²
Foaming time: generally around 90–110 seconds per millimeter of thickness
Advantages of this technology include mature processing experience, stable quality, and lower initial investment costs. However, the process involves additional cutting and shaping steps, resulting in higher material waste and lower production efficiency compared with newer technologies.
In-Mold Foam Molding Technology
In-mold foam molding is one of the most commonly used EVA foaming technologies in the footwear industry. It is widely applied in the production of sports shoe midsoles, especially secondary midsoles requiring excellent cushioning performance.
The process begins with EVA materials being compounded according to a specific formulation and processed into granules. After accurate weighing, the EVA granules are placed directly into a customized mold for foaming and molding.
During the foaming process, the material expands inside the mold and forms the basic shape of the footwear component.
However, the biggest challenge of this technology is achieving the correct balance between expansion ratio, hardness, and dimensional accuracy.
If the formulation and mold design are not properly matched:
The product may achieve the required size but lack sufficient hardness.
The product may achieve the required hardness but fail to reach the expected dimensions.
Therefore, precise control of EVA formulation, mold structure, temperature, and foaming conditions is essential.
For secondary midsole production, the initial EVA foam blank is usually trimmed and placed into a final mold for compression molding.
Typical molding conditions include:
Heating temperature: 125–135°C
Pressure: approximately 50 kg/cm²
Cooling method: water cooling after heating
The final compression-molded EVA sole provides excellent dimensional stability, elasticity, and mechanical performance.
EVA Injection Foaming Technology
EVA injection foaming technology is becoming an increasingly important manufacturing method in modern footwear production.
Similar to conventional plastic injection molding, this process injects EVA material directly into a precision-designed mold, where foaming and molding are completed in a single step.
Compared with traditional molding methods, injection foaming offers several advantages:
Higher production efficiency
Reduced material waste
Lower labor requirements
More flexible product design possibilities
Better consistency in mass production
However, this technology requires highly precise molds and advanced temperature control systems. The mold temperature, injection parameters, and opening timing must be accurately controlled to ensure uniform foam structure and product quality.
Many international sports footwear brands have adopted EVA injection foaming technology for high-performance footwear manufacturing.
Supercritical Fluid Foaming Technology
Supercritical fluid foaming represents one of the most advanced EVA foaming technologies currently available.
This technology mainly utilizes supercritical carbon dioxide (CO₂) as a physical blowing agent. Due to its excellent diffusion ability and high solubility in polymers, supercritical CO₂ can create extremely fine and uniform cellular structures.
During the foaming process:
The polymer matrix absorbs supercritical CO₂ under high temperature and pressure.
The dissolved gas acts as a nucleating agent.
Rapid pressure release creates a large number of microcells.
This process produces EVA foam materials with:
High cell density
Uniform cell distribution
Lightweight structure
Excellent cushioning performance
Supercritical foaming technology mainly includes two methods:
Supercritical Fluid Bead Foaming
Supercritical Fluid Sheet Foaming
Supercritical Fluid Bead Foaming Process
Bead foaming technology can be divided into autoclave foaming and extrusion foaming.
The autoclave foaming process is commonly used as an example:
EVA polymer particles are placed into a high-pressure vessel.
Supercritical CO₂ is introduced under high temperature and pressure.
CO₂ gradually dissolves and diffuses into the polymer matrix.
Rapid pressure release triggers cell nucleation and expansion.
Microcellular EVA foam beads are produced.
This technology allows precise control of foam density, cell structure, and cushioning performance, making it suitable for advanced footwear applications.
Supercritical Fluid Sheet Foaming Process
Supercritical sheet foaming can be achieved through autoclave foaming or compression molding.
The compression molding process generally includes:
The mold is heated to the required foaming temperature.
EVA material is placed into the mold and sealed.
Supercritical CO₂ is injected into the mold.
The gas dissolves and diffuses into the polymer matrix for approximately 30–180 minutes.
The mold is opened rapidly to release pressure and complete foaming.
The final product is a microcellular EVA foam material with controllable cell size, density, and mechanical properties.
With the continuous development of footwear manufacturing technology, EVA foaming processes are becoming more advanced and diversified. Traditional sheet foaming remains widely used due to its simple operation and low investment cost, while injection foaming and supercritical foaming technologies are gaining popularity because of their higher efficiency, better performance, and greater design flexibility.
For modern footwear manufacturers, selecting the right EVA foaming technology depends on factors such as product requirements, production capacity, equipment investment, and desired performance characteristics.
