Twin Screw Extruder for EVA Granulation and Mixing

What is EVA？

EVA stands for Ethylene-Vinyl Acetate. It is a copolymer, which means it is composed of two different monomers, ethylene and vinyl acetate. EVA can be produced with varying proportions of ethylene and vinyl acetate, resulting in a range of materials with different characteristics. The vinyl acetate content in EVA determines properties like flexibility, softness, and transparency. Higher vinyl acetate content generally leads to a softer and more flexible material.

Common applications of EVA include foam materials, packaging materials, sports equipment, medical equipment, automotive interiors, wire and cable insulation, etc. But using EVA without modification is not ideal for making various products.

EVA, in its raw form, lacks uniformity in terms of composition and properties. It typically comes in the form of powders or pellets and may not have the desired characteristics required for specific applications. Without proper mixing and granulation, the distribution of additives, fillers, and colorants would be uneven, leading to unpredictable product performance.

Powdered EVA is also difficult to process, they might not flow smoothly through manufacturing equipment like extruders or injection molding machines, leading to operational issues, clogs, and inconsistent results. Granulating EVA helps improve its processability, making it easier to work with during manufacturing.

So, how does EVA improve its performance? The machine commonly used for mixing and granulating the raw material EVA is called an “Extruder.” An extruder is a versatile piece of equipment used in various industries, including plastics and polymer processing. It’s particularly useful for compounding, mixing, and granulating materials like EVA. Mixing and granulating EVA before producing different products is an essential step in the manufacturing process.

What is the best extruder for EVA mixing and granulating?

The best extruder for EVA mixing and granulation is the twin screw extruder. Twin-screw extruders are widely used for compounding, mixing, and granulating polymers like EVA. They offer excellent mixing capabilities due to the intermeshing screws and are available in various configurations to accommodate different throughput rates and material formulations.

The different torques, screw diameters, and L/D of the twin screw extruder can all affect the compounding efficiency. How to choose?

Generally speaking, the longer L/D of the screw, the better the compounding effect. The smaller the diameter of the screw, the better the mixing effect of the screw. The greater the torque of the screw, the greater the production capacity of the extruder during the production process. At this time, the filling rate of the material inside the screw is higher, and the compounding effect of the material is better.

How to choose？

Choose based on the customer’s material system, material usage, and production capacity.

Firstly, the L/D is selected based on the compounding requirements of the material, and the screw diameter (extruder model) is selected based on the production capacity. Finally, the granulation method is selected based on the characteristics of the material; For example, water-cooling strand pelletizing unit, air-cooling die face hot pelletizing unit, water ring die face hot pelletizing unit, and underwater granulation, etc.

Comprehensive Extrusion Solutions for Various Situations

For EVA foam shoe materials, films, wires and cables, toys, hot melt adhesives, and suspension (adhesive) applications, Nanjing Kerui Extrusion has a complete set of material modification granulation, blending, and molding extrusion solutions.

Foam shoe material

In the EVA resin used in shoe materials, the content of vinyl acetate is generally between 15% and 22%. Due to the flexibility, good elasticity, and chemical corrosion resistance of EVA resin blended foam products, they are widely used in the soles and interior materials of mid to high-end tourism shoes, hiking shoes, slippers, and sandals. In addition, this material is also used in the fields of soundproof panels, gymnastics pads, and sealing materials.

In the granulation of EVA foam shoe materials, we adopt a granulation process of internal mixer+elevator+cone double feeder+single screw extruder+air-cooling die face hot pelletizing unit. Currently, it has been widely used in many factories.

Thin film

The main purpose of EVA film is to produce functional greenhouse films. Functional greenhouse films have high weather resistance, anti fogging and insulation properties. Due to the lack of polarity of polyethylene, even if a certain amount of anti fogging agent is added, its anti fogging performance can only be maintained for about 2 months; The greenhouse film made by adding a certain amount of EVA resin not only has a high transmittance, but also has a significant improvement in anti fog performance, which can generally exceed 4 months. In addition, EVA can also be used to produce packaging films, medical films, laminated films, casting films, etc.

We fully utilize the advantages of twin screw extruder plasticizing and mixing, and establish stable pressure through the melt metering pump. After passing through a T-shaped mold, the molding process of salivation or three roller calender is carried out.

Wire and cable

Due to its excellent filler inclusiveness and cross-linkability, EVA resin is widely used in halogen-free flame-retardant cables, semiconductor shielded cables, and two-step silane cross-linked cables. In addition, EVA resin is also used to make sheaths for some special cables. The content of EVA resin used in wires and cables is generally between 12% and 24%.

For the special industry of wires and cables, we generally adopt a granulation process of internal mixer+elevator+cone double feeder+single screw extruder+water ring die face hot pelletizing unit, ensuring stable and reliable material performance.

Toys

EVA materials have excellent elasticity, flexibility, transparency, insulation, low-temperature flexibility, weather resistance, and chemical corrosion resistance, and are also widely used in daily life. Our twin screw extruder blends and modifies the raw materials to achieve the required mechanical properties.

Hot melt adhesive

Hot melt adhesive with EVA resin as the main component is very suitable for automated assembly line production due to its solvent-free, environmentally friendly, and high safety. Therefore, it is widely used in wireless book binding, furniture edge sealing, automobile and household appliance assembly, shoe making, carpet coating, and metal anti-corrosion coating.

Produce refrigerator ducts, gas pipes, civil panels, containers, and daily necessities, as well as packaging films, gaskets, medical equipment, and can also be used as hot melt adhesives, cable insulation layers, etc.

There are many processes and processes for producing hot melt adhesives, but they all rely on underwater granulation systems. We have rich experience in the field of hot melt adhesives and provide various solutions for different production capacities and uses of hot melt adhesives.

At present, the main processes for producing hot melt adhesives include kneading machines, reaction kettles, and twin screw extruders. The kneading machines and reaction kettles are all produced intermittently, while the twin screw extruders are produced continuously. Our underwater granulation system has been widely used in the field of EVA hot melt adhesives.

EVA suspension

The content of vinyl acetate in the range of 70%~95% is usually in the form of lotion, which is called EVA lotion. EVA lotion is milky white or yellowish in appearance.

EVA lotion is the abbreviation of vinyl acetate ethylene copolymerization lotion. It is a high molecular lotion made of vinyl acetate and ethylene monomer as basic raw materials, and other auxiliary materials through lotion polymerization.

EVA lotion is mainly used in adhesives, coatings, cement modifiers and paper processing, with many excellent properties.

EVA lotion has permanent flexibility. EVA lotion can be regarded as the internal plasticizing product of polyvinyl acetate lotion. Because it introduces the vinyl molecular chain into the polyvinyl acetate molecule, the acetyl group is discontinuous, the rotational freedom of the polymer chain is increased, the space barrier is small, the polymer main chain becomes soft, and the plasticizer will not migrate, ensuring the permanent softness of the product.

EVA lotion has good acid and alkali resistance. EVA lotion can maintain stable performance in the presence of weak acid and weak base, so no matter it is mixed with weak acid or weak base, demulsification will not occur, and the product has a wide range of applications.

Real case examples of the effectiveness of Cowin twin-screw extruders for EVA

EVA production site for domestic customers in China

Process of EVA Granulation using Twin Screw Extruders

Technology introduction

EVA is the English abbreviation for ethylene vinyl acetate copolymer, which is a random copolymer composed of non-polar and crystalline ethylene monomers and strongly polar and amorphous vinyl acetate monomers (also known as VA). The content range of vinyl acetate (VA) in ethylene vinyl acetate copolymers can be wide, ranging from 5% to 95%. Different contents have different performance, so strictly speaking, there are different subdivisions. The commonly used EVA product refers to EVA resin, with a VA content of 5% to 40%. Most EVA manufacturers also produce this type of product. With regard to the synthesis process of EVA, after more than 50 years of development, there are currently four mature EVA production technologies at home and abroad: high-pressure continuous bulk polymerization, medium pressure suspension polymerization, solution polymerization and lotion polymerization.

Among them, solution polymerization and lotion polymerization are seldom used, and most enterprises adopt high-pressure continuous bulk polymerization process.

The high-pressure continuous bulk polymerization process usually uses a high-pressure kettle reactor or a tubular reactor, and the process principle is similar to the production process of low-density polyethylene (LDPE).

The typical processes of the kettle method include DuPont, USI, and Lyondell Basell (formerly Equistar) processes, which can produce EVA resin with a VA content of less than 40% and a one-way conversion rate of 10-20%;

The typical processes of the tubular method include Basf, Imhausem/Ruhrchemie, Basell’s Lupotech process, etc. Generally, EVA resin with a VA content of less than 30% is produced, with a one-way conversion rate of 25% to 35%.

Due to the fact that the polymerization mechanism of EVA under high pressure and high temperature is basically the same as that of LDPE, the difference between EVA products produced using high-pressure reactor and tubular reactor is similar to the difference between LDPE produced using these two processes.

Manufacturing processes

There are usually 7 options available, and customers can choose according to their own needs.

1. Reactor (kneader) – Melt pump – (screen changer) – Underwater granulator
2. Reactor (kneader) – (melt pump) – Single screw extruder – (screen changer) – Underwater granulator
3. Reactor (kneader) – Melt pump – Twin screw extruder – Melt pump – (screen changer) – Underwater granulator
4. Twin Screw Extruder – (Melt Pump) – (Screen Changer) – Underwater granulator (or water cooling strand pelletizing unit)
5. Reactor (kneader) – Melt pump – (screen changer) – Water cooling strand pelletizing unit
6. Reactor (kneader) – (melt pump) – Single screw extruder – (screen changer) – Water cooling strand pelletizing unit
7. Reactor (kneader) – (coarse filtration) – Melt pump – Air cooling steel conveyor pelletizing (only suitable for low molecular weight, very good flowability, and weak melt strength situations.)
8. Internal mixer+elevator+cone double feeder+single screw extruder+air-cooling die face hot pelletizing unit
9. Internal mixer+elevator+cone double feeder+single screw extruder+water ring die face hot pelletizing unit

What are the factors that affect the quality of EVA pellets produced?

Compared with PE, EVA reduces crystallinity, improves flexibility, impact resistance, and improves filler miscibility and thermal sealing due to the introduction of vinyl acetate monomer on the molecular chain. The general density is between 0.91 and 0.93, and the transparency and gloss are good. The performance of EVA resin mainly depends on the vinyl acetate content (VA content) and melt flow rate (MFI) in the copolymer. EVA products have good softness, low temperature resistance, impact strength, environmental stress cracking resistance, good optics, good breathability, average mechanical properties, and poor insulation performance over a wide temperature range.

Other frequently asked questions about EVA on twin-screw extruders

How to solve the problem of high melt temperature of EVA in twin screw extruders?

Reason: The screw has insufficient pressure building capacity, the head pressure is too high, and the screw speed is too high, resulting in high melt friction heat.

Solution: Adjust the screw combination, use special screw elements, and reduce the screw speed at the same time.

How to solve the problem of material overflow from the exhaust port of EVA on a twin screw extruder?

Reason: Problem with screw combination arrangement, clogged filter screen of extruder screen changer, and low head temperature.

Solution: Rearrange the screw assembly, replace the filter of the screen changer, and check the head temperature to reduce the head pressure of the extruder.

If necessary, adjust the arrangement of the barrels.

How to solve the secondary adhesion problem during EVA underwater granulation process?

Solution: 1. Add various isolation agents. 2. Reduce the water temperature of the cooling water, increase the length of the cooling pipeline, and make the EVA particles cool as fully as possible.

Advantages Of Twin Screw Extruder For EVA mixing and granulating

• Superior Mixing

Twin-screw extruders provide excellent mixing capabilities, this results in thorough dispersion of additives, fillers, and other components within the EVA matrix, leading to consistent material properties and enhanced product quality.

• Flexibility in Formulations

EVA can be mixed with various additives, fillers, and colorants to achieve specific characteristics. Twin-screw extruders accommodate a wide range of formulations, enabling manufacturers to produce materials with varying hardness, flexibility, UV resistance, and other properties.

• Gentle Processing

The relatively low shear rates and controlled processing conditions of twin-screw extruders make them suitable for sensitive materials like EVA. This helps prevent material degradation and ensures consistent material properties.

• Consistent Product Quality And Performance

EVA is often used in a mixture with other additives, fillers, or colorants to achieve specific properties in the final product. Proper mixing ensures that these components are uniformly distributed throughout the EVA, resulting in consistent product quality and performance.

• Reduced Waste

Homogeneous EVA mixtures result in fewer rejected products and a more efficient manufacturing process.

• Improved Performance

Different applications require EVA with specific characteristics, such as hardness, flexibility, or density. Mixing EVA with other materials and additives allows manufacturers to precisely control these properties by adjusting the composition of the mixture.

Mixing and granulating EVA also can enhance its overall performance by creating a uniform structure that maximizes its desirable properties, like flexibility, strength, and durability.

• Enhanced Processability

EVA pellets or granules are easier to handle and process than raw powders or liquid components. 

Granulated EVA is easier to transport, store, and measure accurately compared to powders or liquids. This ease of handling contributes to a smoother manufacturing process.