How EVA Foam Material Is Used in Footwear Manufacturing

Introduction

As an EVA raw material supplier to the Footwear Industry, we detail how EVA is made into foam, and then into various components of finished footwear such as midsoles, insoles, heel counters etc. 

What Is EVA and Why Is It Preferred for Foam Manufacturing?

Ethylene vinyl acetate (EVA) is a copolymer made of ethylene and vinyl acetate. It is a demanding material for manufacturers as it can be made into a very light foam that is extremely flexible and is very cost effective. The chemical structure of EVA is highly compatible with crosslinking agents and with blowing agents used to create foams.

How Is EVA Foam Made? (Manufacturing Process)

EVA foam raw material is converted into foam through a structured chemical and mechanical process:

• Compound  mix – The basic EVA resin is blended with DCP (dicumyl peroxide) cross-linker, chemical foaming agent (azodicarbonamide) and thermally expandable microspheres
• Mixing – In order to increase tensile-elasticity and impact resistance a polyolefin  elastomer (POE) is added to the compound.
• Moulding – The compound is injected into a so-called compression mould or an injection mould and is allowed to cure at temperature (160–180°C) for a few minutes.
• Expansion-This development is caused by the thermal expansion of the thermally expandable particles as well as by the chemical foaming agent, leading to a volume increase of approximately 2 to 3 times the initial volume of the material.
• Cooling & Trimming – The part is allowed to cool after it has been ejected from the mold. After the part has been trimmed to size it is then inspected for density and hardness.

What Properties of EVA Foam Make It Ideal for Footwear?

EVA foam material offers specific physical properties that suit footwear applications:

• Low density — lightweight comfort for all-day wear
• Closed-cell structure — resists moisture and compression set
• High resilience — returns to original shape after impact
• Thermal stability — maintains performance across temperature ranges
• Chemical compatibility — bonds well with adhesives and rubber outsoles

How Is EVA Foam Used in Footwear? (Applications by Component)

EVA foam serves three primary footwear components:

• Component – Midsole: The shock and absorbency in footwear is achieved by the midsole. The midsole is primarily made from high density EVA foam. The VA content of midsole is typically in the range of 18% to 28%.
• Insole — a layer of a shoe, inside of the shoe, for the comfort of the foot, the softest cushioning, the lightest form of cushioning, lowest density.
• Outsole / Bottom: an outer surface of a performance or outdoor product, usually made of rubber blended with EVA.
• EVA raw material suppliers provide  Footwear Manufacturers with the mandatory grade of EVA resin for the relevant shoe component (e.g. midsole, insole, outsole part blended with rubber) complete with pre-set targets for Shore C hardness and density

How Do Manufacturers Choose the Right EVA Grade for Footwear Foam?

Selecting the correct EVA foam raw material grade depends on three technical parameters:

• Vinyl acetate (VA) content -VA content – Higher VA content (28-33%) gives softer more elastic characteristics whilst lower VA content (18-22%) gives firmer midsole characteristics.
• Melt flow index (MFI) — MFI data help to determine the molding characteristics of EVA and thus indicate whether a specific type of EVA is suited for compression molding. As a rule of thumb, low MFI EVA is preferred for such processing
• The higher the DCP loading, the higher the crosslink density and therefore the harder the foam.

The EVA supplier will also have provided the user with a datasheet for that specific EVA which will detail information such as the VA% content, the MFI value and other information relating to recommended processing conditions for various applications in footwear. 

Today EVA foam is the most used material for the footwear cushioning. EVA foam is very easy to process and is very versatile in use.

​Conclusion

EVA foam material remains the dominant choice for footwear cushioning due to its proven processability and performance versatility.

​Frequently Asked Questions

1. ​What is EVA foam and how is it different from regular EVA material?

EVA foam is the cellular or expanded form of EVA material (Ethylene Vinyl Acetate resin) commonly used in solid form such as films, cables, adhesives etc. However this material in EVA foam form is in the form of light weight, cushion like material with a lot of voids. The EVA resin is usually compounded with a crosslinking agent (Dicumyl Peroxide etc) and also blowing agents such as chemical foaming agents and thermally expandable microspheres.

2. ​What is the manufacturing process for EVA foam used in footwear?

EVA foam is made by combining EVA raw material with DCP crosslinking agent, chemical foaming agent and thermally expandable microspheres to form a compound. This compound is then compression or injection moulded at a temperature of around 160–180°C. The heat from the tool in the moulding process causes the EVA resin to blow, forming a lightweight cellular structure. The EVA foam is then allowed to cool before being removed from the tool. The individual component of foam is then trimmed to size as required. 

3. ​Which parts of a shoe are made from EVA foam and why?

EVA is used in the shoe’s midsole for primary impact cushioning, in the insole for underfoot comfort and occasionally in the outsole, in particular in blended EVA/rubber sections. Different parts of the shoe are made from different EVA foams with different EVA materials for the appropriate softness, hardness and bounce.

4. ​What EVA grade is recommended for midsoles and insoles?

For midsoles a sturdy EVA resins with a VA content of 18–28% and low MFI are recommended. For insoles EVA with a higher VA content (28–33%) with a softer compression characteristic is recommended. Many EVA raw material suppliers offer specific grades of EVA resins with full technical datasheets. The manufacturer of EVA then selects the appropriate grade for midsoles and insoles.

5. ​What are the limitations of EVA for high performance sports shoes?

As EVA undergoes a very much compression set after long time use, in high performance sports shoes which require to return very much energy in every step, hard type materials such as POE (Polyolefin Elastomer) based materials or other type of foams are needed. EVA also softens at high temperatures over 60°C and hence is not suitable for very high performance usage.