EVA Foam Injection Molding Process Operation Guide
Ethylene Vinyl Acetate (EVA) foam injection molding is a specialized process that utilizes the foaming characteristics of EVA pellets to produce lightweight, elastic, and shock-absorbing plastic parts. It is widely used in shoe soles, sports equipment, packaging buffers, automotive interior parts, and other fields. The core of the process lies in precise control of temperature, pressure, and foaming agent decomposition to ensure uniform cell structure, stable density, and consistent physical properties of the finished product.
Core Process Requirements
1. Raw Material Preparation
EVA pellets (VA content 15%-40% is common) must be fully dried to avoid moisture-induced bubbles and uneven foaming. Dry at 60-80℃ for 2-3 hours using a hot air dryer; avoid high-temperature drying to prevent premature melting of pellets.The foaming agent (usually AC foaming agent) should be uniformly mixed with EVA pellets, with an addition ratio of 1%-5% according to the required foaming ratio. Ensure no agglomeration of the foaming agent to prevent local over-foaming or under-foaming.
2. Barrel Temperature Control
EVA has low melting point and poor thermal stability; excessive temperature will cause decomposition of the foaming agent and aging of EVA, while insufficient temperature will lead to incomplete melting and uneven foaming.Recommended segmented temperatures:
Feeding zone (rear): 80-100℃ (preheat and convey materials, avoid sticking to the screw)
Compression zone (middle): 100-120℃ (melt materials and uniformly disperse the foaming agent)
Homogenization zone (front): 110-130℃ (complete melting, control the initial decomposition of the foaming agent)
Nozzle temperature: 105-120℃ (slightly lower than the front zone to prevent foaming agent decomposition during material flow)
3. Mold Temperature Control
Mold temperature directly affects the foaming speed and cell structure of EVA:
Low-ratio foaming (density 0.2-0.5g/cm3): Mold temperature 40-60℃
High-ratio foaming (density 0.05-0.2g/cm3): Mold temperature 60-80℃
Too low mold temperature will lead to slow foaming, small cell size, and high product density; too high mold temperature will cause rapid foaming, easy cell rupture, and surface collapse of the product. It is recommended to use a mold temperature controller for stable temperature control.
4. Injection Pressure and Speed
EVA foam molding requires low to medium injection pressure (50-80MPa) to avoid crushing the formed cells; the injection speed adopts a slow-to-medium speed (50-80mm/s) to ensure uniform filling of the melt and prevent local shear overheating.Back pressure should be controlled at 1-3MPa (low back pressure) to reduce shear heat and avoid premature decomposition of the foaming agent in the barrel; holding pressure is kept low (20-30MPa) or even canceled to leave enough space for the melt to foam and expand in the mold cavity.
5. Foaming and Cooling Control
After injection filling, the mold needs to be kept closed for 10-20 seconds to allow the foaming agent to fully decompose and form stable cells; the cooling time is 30-60 seconds (adjust according to product thickness), ensuring the product is fully shaped before mold opening to avoid shrinkage and deformation of the foam structure.For open-mold foaming (high-ratio products), the mold is opened by 2-5mm after a short holding time to release pressure, allowing the EVA melt to expand fully and form a low-density foam structure.
Common Defects and Solutions
| Defect Type | Causes | Solutions |
|---|---|---|
| Uneven cell structure | Uneven mixing of foaming agent, unstable temperature | Optimize mixing process, calibrate barrel temperature, reduce injection speed |
| Low foaming ratio / high density | Insufficient temperature, short holding time | Increase front barrel temperature appropriately, extend mold closing holding time |
| Product surface collapse | High mold temperature, fast cooling | Reduce mold temperature, slow down cooling speed, increase cooling time |
| Foam rupture / air bubbles | Moisture in raw materials, excessive injection pressure | Strengthen drying, reduce injection pressure, optimize venting slots |
| Poor elasticity of product | Low VA content, incomplete foaming | Adjust EVA raw material grade, control foaming agent ratio, ensure full decomposition |
Equipment and Mold Requirements
The injection molding machine should be equipped with a low-shear screw (compression ratio 1.5:1-2.5:1) to avoid excessive shear causing foaming agent decomposition;
The mold cavity should be polished smoothly, with sufficient venting slots (depth 0.02-0.05mm) to discharge gas generated during foaming;
For high-ratio foaming products, the mold should have a mold opening adjustment mechanism to precisely control the mold opening distance and ensure uniform foaming expansion.
Safety and Production Notes
EVA foaming agent will release gas during decomposition; ensure good ventilation in the workshop to avoid gas accumulation;
Regularly clean the barrel and nozzle to remove carbonized EVA residues and prevent blockage;
The finished foam products should be placed in a ventilated environment for 24-48 hours to release residual gas and stabilize dimensions.
