The Four Major Processes of EVA Foaming

In the field of polymer materials, ethylene-vinyl acetate copolymer (EVA) foam has become a versatile material with widespread applications due to its excellent flexibility, shock absorption, and chemical resistance. From small items like athletic shoe insoles and children's toys to larger applications such as automotive interiors and packaging cushioning materials, EVA foam is ubiquitous. However, the quality and performance of EVA foam largely depend on its foaming process. Whether manufacturers are optimizing processes, industry professionals are enhancing expertise, or buyers are selecting suitable products, a deep understanding of the four core EVA foaming processes is essential. This article comprehensively dissects these four processes—covering principles, specific steps, advantages, limitations, and practical applications—while addressing common industry questions and analyzing future trends. It provides readers with a systematic and practical technical guide.
 

 What is EVA Foaming?

 
 EVA is a copolymer of ethylene and vinyl acetate. The foaming process essentially involves introducing bubbles into the EVA base material through specific methods, creating a porous structure. It is precisely this porous structure that endows EVA foam with unique properties such as lightweight, shock absorption, and thermal insulation.
 

 Four Core EVA Foaming Processes

 

 Mold Foaming Process

 

 Process Principle

 Mold foaming stands as one of the most traditional and widely applied techniques in EVA foaming. Its core logic involves placing the EVA raw material mixture into a sealed mold. Heating causes the chemical foaming agent to decompose and generate gas, while pressure is applied to ensure uniform gas distribution within the EVA substrate. Once the material solidifies, opening the mold yields a foam product matching the mold cavity's shape. Simply put, it is a process of "heating to generate gas + pressurizing to control foaming + cooling to set the shape."
 

 Specific Steps

 

1.  Raw Material Preparation: First, uniformly blend EVA resin, chemical blowing agent (commonly azobisisophurane, ADC), crosslinking agent (e.g., diisopropylbenzene peroxide, DCP), plasticizer, stabilizer, and any required colorants or fillers according to the formulation. To facilitate subsequent processing, the mixture is typically extruded into pellets or flakes.
2.  Mold Preheating: Preheat the mold with customized cavities to 160-180°C according to product requirements. Preheating ensures uniform heating of the raw material, preventing incomplete foaming in localized areas.
3.  Material Injection: Place the prepared EVA mixture into the preheated mold cavities. Material quantity must be precisely controlled to ensure cavities are fully filled after foaming while preventing excess material and waste.
4.  Pressurization and Heating: Close the mold and apply 10-30 MPa pressure while maintaining mold temperature. During this stage, chemical foaming agents decompose at high temperatures, releasing gases like nitrogen and carbon dioxide, while the EVA base material gradually melts. The pressure prevents premature gas escape, forcing bubbles to disperse uniformly within the base material. This "Pressurization and Heating" stage (industry term: "Pressure Holding and Temperature Maintenance") typically lasts several to over ten minutes, with duration depending on product thickness—thicker products require longer to ensure thorough internal foaming.
5.  Cooling and Demolding: After pressure and heat retention, the mold is cooled using cold water or air to lower the EVA foam temperature below its curing threshold. Once the product is fully cured, the mold is opened to remove the foam product.
6.  Post-processing: Demolded products may exhibit flash or overflow, requiring trimming for removal. Some products also undergo "aging treatment" (holding at specific temperatures for a period) to stabilize foam physical properties and prevent shrinkage or deformation during subsequent use.

 
Process Advantages

 

1.  Excellent Product Uniformity: The sealed mold and precise temperature/pressure control ensure consistent bubble size and distribution within the foam. This results in minimal variation in hardness, elasticity, and other properties across all product sections.
2.  High Design Flexibility: Complex shapes—such as athletic shoe midsoles with grooves and protrusions or irregularly shaped protective pads—can be produced as long as molds matching the required cavities are available.
3.  Superior Mechanical Properties: During heating, crosslinking agents induce the formation of crosslinked structures within EVA molecular chains, significantly enhancing the foam's tensile strength, abrasion resistance, and aging resistance, thereby extending product lifespan.

 
Process Limitations

 

1.  Long production cycle: The entire process—from preheating and molding to pressure holding and cooling—takes considerable time. Single-mold production efficiency is low, making it difficult to meet large-scale, fast-paced production demands.
2.  High mold costs: Custom molds with complex cavities require high-precision machining, resulting in high initial investment costs. If product updates occur frequently, mold replacement costs will increase further, making this process more suitable for batch production of fixed styles.

 
Extrusion Foaming Process

 

 Process Principle

 Extrusion foaming is a continuous production process. Its core involves using an extruder to melt, blend, and convey EVA raw material, where chemical foaming agents decompose within the barrel. When the molten EVA mixture is extruded through the die (nozzle) into atmospheric pressure, the internal pressure rapidly drops. Gases previously dissolved in the base material expand instantly, forming continuous foam sheets or profiles. This process achieves continuous "feedstock input to foam output," significantly surpassing the production efficiency of compression molding.
 

 Specific Steps

1.  Raw Material Feeding: EVA resin pellets are mixed with chemical blowing agents, crosslinking agents, and additives in specified proportions, then continuously fed into the extruder barrel via the hopper.
2.  Melting and Mixing: The screw inside the extruder barrel rotates at high speed. Through the combined action of shear force and the barrel heating system, the raw materials gradually melt. Simultaneously, the screw's conveying action thoroughly mixes the molten material with the foaming agent and additives, forming a uniform melt.
3.  Foaming Agent Decomposition: As the melt advances through the barrel, its temperature gradually rises to 170-200°C. At this temperature, the chemical foaming agent decomposes to produce gas. The barrel maintains high pressure (created by the screw and die), forcing the gas to dissolve into the melt without premature expansion.
4.  Extrusion Expansion: When the dissolved-gas-containing melt is pushed through the extruder die (which can be a flat die, tubular die, etc.) and extruded into the air, the external pressure suddenly drops. The dissolved gases rapidly escape and expand, causing the EVA melt to form a porous foam structure. The die shape determines the initial form of the foam product—flat dies produce foam sheets, while tubular dies produce foam tubes.
5.  Cooling and Setting: The extruded foam product must be immediately cooled through an air or water cooling system to rapidly solidify the foam structure, preventing bubble coalescence or product deformation. For foam sheets, a calendering roller is also used to control thickness and surface flatness.
6.  Cut-to-length: The cooled continuous foam sheets or tubes are conveyed by a traction machine to cutting equipment, where they are cut into finished products at preset lengths.

 
Process Advantages

 

1.  High Production Efficiency: The extrusion process enables continuous production without repeated mold loading/unloading. A single production line can achieve daily output of several tons, making it ideal for large-scale manufacturing of standardized products.
2.  Cost Control: Continuous production reduces labor and time costs per unit. Minimal equipment investment is required (compared to multiple molds in compression molding), making it particularly suitable for batch production of conventional shapes.
3.  Strong Product Consistency: The extruder's stable conveying and temperature control ensure uniform foam thickness and density with high surface flatness, making it an ideal base material for subsequent processing (e.g., foam boards for packaging).

 
Process Limitations

 

1.  Significant Design Limitations: Extrusion foaming can only produce products with fixed cross-sectional shapes (e.g., sheets, tubes, profiles) and cannot manufacture complex irregular components, resulting in relatively limited application scenarios.
2.  Difficult bubble structure control: Rapid pressure drop during extrusion can cause uneven bubble sizes or "large bubbles," affecting mechanical properties. This risk is higher when producing low-density foam.

 
Injection Molding Foaming Process

 

 Process Principle

 Injection foaming combines the advantages of injection molding and foaming technology. Its core process involves injecting molten EVA raw material mixtures at high speed into a sealed mold cavity via an injection molding machine. The mold's heating causes the chemical foaming agent to decompose and generate gas, which expands within the cavity to fill it completely. After cooling and solidification, the raw material forms a foam product. Compared to compression foaming, the "injection-foaming-curing" process of injection foaming is more efficient, making it particularly suitable for mass production of small, precision products.
 

 Specific Steps

1.  Material Preparation: Mix EVA resin, chemical foaming agent, cross-linking agent, and additives to form pellets. Ensure pellet uniformity to maintain process stability during injection.
2.  Mold Preheating and Cleaning: Preheat the custom cavity mold to 150-170°C while cleaning the cavity interior to prevent impurities from affecting product surface quality.
3.  Melt Injection: Feed EVA pellets from the hopper into the barrel, where heating melts the pellets. The screw then pressurizes and rapidly injects the molten material into the mold cavity. Injection volume must be precisely calculated—typically less than the cavity volume (allowing expansion space for foaming), termed "under-injection." This ensures the cavity is fully filled after foaming.
4.  Pressure Holding & Foaming: After material injection, the machine maintains a holding pressure (50-150 MPa) to prevent material backflow. Simultaneously, mold heat triggers chemical foaming agents to decompose and generate gas. This gas expands within the cavity, forcing the molten material to fill every corner and form a porous structure. Holding time is typically brief (seconds to tens of seconds), depending on product size.
5.  Cooling and Demolding: After pressure holding ends, the mold's cooling channels activate to rapidly lower mold temperature, solidifying the EVA foam. Once the product is fully set, the mold opens, and the ejector mechanism removes the part.
6.  Post-Processing: Some products may exhibit minor flash (caused by mold clearance gaps), requiring precise trimming for removal. For products demanding high surface precision (e.g., electronic components), additional surface grinding or coating processes may be applied.

 
Process Advantages

 

1.  Short Production Cycle: The high-speed injection and efficient cooling system of injection molding machines significantly reduce the production cycle per product (typically completing a cycle in tens of seconds), making it suitable for mass production of small items.
2.  High Product Precision: High-pressure injection ensures molten material fully fills intricate mold cavities, resulting in minimal dimensional tolerances (typically ±0.1mm), smooth surfaces, and reduced post-processing requirements.
3.  High material utilization: Precise injection control (underfill calculation) minimizes material waste, achieving over 95% material utilization and reducing production costs.

 Process Limitations

 

1.  High equipment investment: Injection foaming requires specialized injection molding machines (with high-pressure injection and precise temperature control capabilities), whose procurement costs are significantly higher than those of extruders or standard compression molding equipment. Additionally, mold design is complex (requiring structures like gates and cooling channels), leading to high customization costs.
2.  Product Size Limitations: Due to constraints in clamping force and injection volume, large foam products (e.g., oversized cushion pads) are difficult to produce. This process is primarily suited for small, precision components.

 
Typical Applications

 Injection molding foaming is primarily used for producing small, high-precision EVA foam products, including: electronic device housings (remote control casings, game controller grips), medical device accessories (hearing aid earpieces, surgical instrument handle cushions), automotive small parts (dashboard buttons, seat adjustment knobs), and precision components for children's toys (e.g., toy car wheels, puzzle snap-fits).
 

 Continuous Belt Foaming Process

 

 Process Principle

 Continuous belt foaming (also known as conveyor belt foaming) is an innovative continuous foaming process. Its core principle involves using a heat-resistant conveyor belt to continuously transport the EVA raw material mixture through heating, foaming, and cooling zones. The entire process—heating, foaming, cross-linking, and cooling—is completed during dynamic conveyance, ultimately producing large-area, continuous foam sheets. This process is particularly suitable for producing large-sized, thin or medium-thickness foam substrates.

 Specific Steps

1.  Raw Material Preparation: Mix EVA resin, chemical foaming agent, crosslinking agent, and additives into a paste or sheet form (achievable via mixer or calender), ensuring uniformity without lumps.
2. Raw Material Spreading: Prepared EVA raw material is evenly spread onto a heat-resistant conveyor belt (typically made of Teflon or stainless steel, resistant to high temperatures and non-stick) via a spreading device. The spreading thickness is set according to the final foam product's thickness (accounting for expansion rate during foaming; typically 1/3 to 1/5 of the finished product's thickness).
3.  Gradient Heating: The conveyor belt carries the material into a heating tunnel divided into multiple zones, where temperatures gradually increase from 120°C to 180°C. This gradient heating prevents localized overheating and degradation of the material while allowing the chemical foaming agent to decompose slowly, ensuring uniform bubble formation.
4.  Foaming and Crosslinking: In the middle to rear section of the heating tunnel, material temperature reaches the peak decomposition point of the foaming agent. Massive gas generation causes the material to expand into foam. Simultaneously, the crosslinking agent begins to act, crosslinking the EVA molecular chains. The foam structure gradually stabilizes, acquiring sufficient strength and elasticity. During this stage, the conveyor belt maintains a steady speed (typically 0.5-2 m/min) to ensure thorough foaming and crosslinking.
5.  Cooling and Setting: After foaming, the foam sheets enter the cooling zone via the conveyor belt. Rapid cooling is achieved through cold air or cold water spraying, solidifying the foam structure and preventing deformation during subsequent handling.
6.  Winding or Cutting: The cooled continuous foam sheet can be wound for subsequent slitting or directly cut into fixed-size panels (e.g., the common 1m×2m size for yoga mats) using longitudinal or cross-cutting equipment.

 
Process Advantages

 

1.  Large-Format Production Capacity: Customizable conveyor belt widths (typically 2-4m) enable continuous production of large-area foam sheets, overcoming the limitations of molding and injection molding processes in manufacturing oversized products.
2.  Uniform Foam Structure: Gradient heating and stable conveyor speed ensure even material heating, consistent bubble size, and minimal foam thickness deviation (typically ±0.2mm), making it ideal as a substrate for high-precision secondary processing.
3.  High Process Flexibility: Easily adjust foam density (ranging from 20 kg/m³ to 200 kg/m³) and hardness by modifying raw material formulations (e.g., blowing agent dosage), heating temperatures, or conveyor belt speeds to meet diverse application requirements.

 
Process Limitations

 

1.  Limited Shapes: Similar to extrusion processes, continuous belt foaming can only produce flat sheet products and cannot manufacture complex shapes, limiting its application to sheet-like products.
2.  High Energy Consumption: The heating tunnel requires maintaining stable temperatures across multiple heating zones, while the conveyor belt drive and cooling systems consume significant energy. Consequently, energy consumption per unit is higher than in compression molding or injection molding processes.