6 Foaming Technologies for Footwear Elastomers: A Complete Guide

In the footwear industry, the performance of a midsole-its cushioning, energy return, and weight-is largely determined by the foaming technology used to create the elastomer material. This guide breaks down six distinct foaming processes, from established methods to cutting-edge innovations.

 

1. Flat Press Foaming (Large Foaming)

This is a very mature and traditional process. The material (like EVA) is mixed in a banbury mixer, formed into sheets, and then placed in a mold for foaming. The resulting foamed sheets are then cut and shaped into sole components.

 

Advantages: Low equipment cost and low technical requirements.
Disadvantages: High material waste (scrap), difficulty recycling cross-linked EVA, and low production efficiency.
Common Application: Primarily used by smaller processing factories; often for sheets, mats, or less complex components.

 

 

2. Compression Molding Foaming (Small Foaming)

A classic process for EVA midsoles. It includes one-shot and two-shot methods. The two-shot process produces PHYLON midsoles, known for superior cushioning and resilience compared to one-shot EVA. The process involves mixing pellets, placing them in a mold, and foaming. For two-shot, the initial "rough" midsole is skinned and then pressed into a final mold with heat and cooling cycles.

 

Advantages: Flexible foaming conditions, mature technology, less waste than flat press foaming.
Disadvantages: Lower efficiency and higher energy consumption compared to injection methods; process conditions heavily affect final properties.
Common Application: EVA midsoles for various footwear.

 

 

3. Injection Cross-Linking Foaming

This is currently a mainstream, advanced process for EVA midsoles, favored by large-scale manufacturers. EVA raw materials and additives are mixed, pelletized, and then directly injection-molded into the final midsole shape in one step.

 

Advantages: Significantly higher production efficiency, minimal to no scrap waste (one-shot molding).
Disadvantages: Dimensional control and shrinkage management can be more complex than compression molding, potentially leading to slight accuracy deviations.
Common Application: High-volume production of EVA midsoles.

 

 

4. Batch Autoclave Foaming

This is the primary method for producing E-TPU (Expanded Thermoplastic Polyurethane), famously known as "Boost" or "popcorn" TPU. TPU pellets are placed in a sealed autoclave with a foaming agent (often supercritical CO2) under high pressure and temperature. The pressure is then rapidly released or temperature raised, causing the pellets to expand into foam beads.

 

Advantages: Produces high-performance beads with excellent properties and expansion ratio; relatively simple process principles.
Disadvantages: High equipment cost requiring special explosion-proof measures, and it is a batch process (lower continuous efficiency).
Common Application: Producing E-TPU beads for subsequent steam molding into midsoles.

 

 

5. Continuous Extrusion Foaming

This process aims to overcome the batch inefficiency of autoclave foaming. TPU and additives are melted in an extruder, supercritical fluid is injected, and the mixture is cooled and extruded through a die, where it expands and is cut into foam beads continuously.

 

Advantages: Potentially higher, continuous production efficiency.
Disadvantages: High investment costs; domestic equipment and technology are still maturing; risk of producing beads with too many open cells, which affects subsequent molding.
Common Application: Emerging technology for continuous E-TPU bead production.

 

 

6. Supercritical Foaming for EVA

Building on autoclave and extrusion technologies using supercritical fluids (like CO2 or N2), this method is now being applied directly to EVA. Two main approaches are supercritical compression molding (where a preform is foamed in a mold by rapid decompression) and supercritical sheet foaming (where an EVA sheet is foamed continuously and then cut). Major brands like Skechers, Anta, and Peak have adopted this for high-performance midsoles.

 

Advantages: Environmentally friendly (no chemical residue), high performance, and good elasticity.
Disadvantages: Process control is critical and requires specialized equipment.
Common Application: High-performance, lightweight, and resilient EVA midsoles.