The manufacturing process for EVA midsoles in footwear varies depending on the technology employed, but here's an overview of several common methods:
Traditional Two-Stage Molding (CP):
- Mixing & Calendering: EVA raw materials, along with potential additives like antioxidants and plasticizers, are mixed in a mixer and further refined through calendering to adjust thickness.
- Sheet Formation: The mixed material is formed into sheets.
- Flat Sheet Foaming: Sheets undergo high-temperature, high-pressure treatment for initial foaming, increasing volume.
- Cutting: Sheets are cut according to the midsole's required dimensions.
- Secondary Foaming: Cut midsoles undergo another foaming process, enhancing lightness and elasticity. Midsoles produced this way are sometimes referred to as PHYLON midsoles.
Microcellular Foam Injection Molding (CMP):
- Compounding & Pelletizing: EVA and additives are mixed according to a formula and then pelletized.
- Molding: Precisely weighed pellets are placed into molds, which are heated to foam and shape the midsoles in one or two stages.
Direct Injection Molding (DP):
- Injection Molding: EVA resin pellets are directly fed into an injection molding machine's mold, where they are rapidly foamed and shaped under high temperature and pressure in a single step.
One-Piece Injection of Midsole/Outsole (IP):
- Injection Molding: EVA pellets are introduced into an injection molding machine, melted, and injected into a mold under vacuum or specific gas conditions to quickly foam and cool, forming both the midsole and outsole simultaneously.
Throughout these processes, critical parameters such as temperature, pressure, foaming duration, and precise formulation control significantly impact the final product's qualities, including density, hardness, elasticity, and dimensional stability. Post-molding steps may include trimming, cleaning, and assembly with other shoe components.
