EVA Injection Molding
Eva injection molding has the characteristics of high efficiency and energy saving, which can effectively shorten the plastic manufacturing cycle. Due to the good resilience and toughness of EVA materials, their products have excellent cushioning and shock resistance.
EVA Injection Molding Process
Compared to ordinary injection molding, EVA injection molding is more efficient and precise, and the manufacturer widely applies it to the manufacturing of insoles, films, wires, and cables. Its main principle is to quickly inject plastic powder or particles in a molten state into the mold under compressed air, and then through the joint action of the cooling system and exhaust system, solidify and demold to obtain the formed part. EVA material has strong fluidity, which can better control the details of the plastic injection molding process, and is not prone to uneven wall thickness, ensuring the precision of the formed part size.
Performance of EVA Injection Molding
EVA plastic is a common material in injection molding, with outstanding transparency and toughness. It has excellent performance and prominent advantages but also has certain limitations. Therefore, when using EVA plastic for injection molding, comprehensive consideration is needed.
Advantage
- Toughness: EVA material can have good elasticity and toughness in different temperature environments, providing cushioning for products and suitable for making various types of insoles.
- Lightweight: EVA has a lower density and produces products that are easy to carry.
- Low water absorption: EVA materials have low or even no water absorption, and can resist wet environments well.
- Good Chemical Properties: EVA has good corrosion resistance and does not react with certain chemicals. Suitable for various types of protective equipment.
- Electrical insulation: EVA injection molding products have good resistance and insulation performance.
- Convenient Production: EVA materials are easy to process and shape, and complex products can also be manufactured quickly, meeting the application needs of different fields.
- Slow Thermal Conductivity: EVA materials have a large number of porous structures inside, resulting in slow heat transfer and excellent insulation effect.
- Nontoxic and Harmless: EVA materials have strict hygiene standards during the production process and are harmless to human health.
- Anti-aging: products made of EVA material have a long service life, and are not easy to be age and brittle, and have a long service life.
- Widely Applicable: EVA materials are widely used in various fields such as insoles, car interiors, packaging, etc., meeting a wide range of product requirements.
Shortcoming
- Poor Heat Resistance: EVA materials stored in high-temperature environments are prone to losing their optimal performance.
- Easy to Stretch: EVA materials are prone to deformation and have weak tensile strength when working in high-strength environments.
- Fast Oxidation: Especially for EVA materials that are white or have high transparency, they oxidize quickly and are prone to yellowing, which affects their performance.
- Weak Heat Sealing Performance: EVA has poor heat sealing performance and may not be suitable for certain specific heat sealing needs in the packaging industry.
- Poor Oil Resistance: EVA foam materials are prone to chemical changes when in contact with oily substances, resulting in weakened material properties. Therefore, in oily environments, the use of EVA materials should be avoided as much as possible.
- Not Easy to Recycle: Due to the unique structure and properties of EVA materials, they are not easy to recycle and require specialized recycling techniques and equipment.
EVA's Purpose
EVA injection molding products have a wide range of uses. Generally, when the vinyl acetate content of EVA is less than 5%, it can mainly produce films, LDPE modifiers, adhesives, wires, cables, etc; When the content of vinyl acetate in EVA is between 5% and 10%, the product is mainly an elastic film; When the ethylene acetate content in EVA is between 20-28%, its products mainly consist of hot melt adhesives and coating products; When the content of ethylene acetate is between 35% and 45%, the main products produced are films (including agricultural films) and sheets, injection molded and molded products, foam products, hot melt adhesives, etc.
- Foam Shoe Materials
Shoe materials are the main application field of EVA resin in China. In the EVA material used in shoe manufacturing, the content of vinyl acetate is generally between 10% and 22%. Due to the flexibility, good elasticity, toughness, and chemical corrosion resistance of EVA material blended foam products, they are widely used in the soles, insoles, and interior materials of mid to high-end travel shoes, hiking shoes, slippers, sandals, and more. In addition, this material is also used in the fields of soundproof panels, gymnastics mats, and sealing materials. Dongguan, Shunde, and Zhongshan in Guangdong, Wenzhou in Zhejiang, and Jinjiang and Quanzhou in Fujian are the main production bases of China’s shoe industry, consuming a large amount of EVA materials to make products every year.
- Toys
EVA resin is also widely used in toys, such as children’s car wheels, seat cushions, etc. In recent years, China’s toy processing industry has developed rapidly, with more investment in production in coastal areas such as Shantou, Dongguan, and Shenzhen, mainly for export and external processing. According to analysis, these manufacturers consume approximately 5000 tons of EVA resin annually, with a usage level similar to that of shoe materials.
- Thin Film
The main purpose of EVA film is to produce functional greenhouse films. Functional greenhouse film needs to have good weather adaptability, long service life, and insulation performance (airtightness). Because polyethylene does not have this ability, even if a certain amount of anti-fog agent is added, its anti-fog performance can only be maintained for about 2 months, which is short in time and needs to be re-added after failure, making the operation cumbersome; The greenhouse film made by adding a certain amount of EVA resin not only has high light transmittance but also maintains anti-fog performance for a longer time, usually 4 months or even longer. In addition, EVA can also be used to produce packaging films, medical films, laminated films, casting films, etc. The vinyl acetate content of EVA film is generally around 5% -10%.
- Hot Melt Adhesive
Hot melt adhesive is mainly composed of EVA resin. Due to its solvent-free nature, good environmental performance, and high safety, it is very suitable for automated assembly line production. Therefore, it is widely used in the sealing and binding of books, edge sealing of furniture, assembly of cars and household appliances, shoe making, carpet coating, and anti-corrosion coating of metals. The main content of EVA vinyl acetate used in hot melt adhesive is around 25% to 40%. Although there are products with this content in China, production has not been arranged for a long time, so most of them use imported materials.
- Wire and Cable
Due to its excellent filler inclusiveness, electrical insulation, and crosslinkability, EVA resin is widely used in flame-retardant cables, semiconductor shielding cables, and two-step silane crosslinked cables. In addition, EVA resin is also used to make sheaths for some special cables. The EVA resin used in wires and cables generally has a vinyl acetate content of around 12% to 24%.
EVA Injection Molding Process
EVA injection molding process is relatively not complicated, but it also requires precision machining. EVA’s main processing techniques include injection molding, blow molding, extrusion, rolling, foaming, heat sealing, etc.
-Main Raw Materials:
LDPE can be used to produce high-magnification foam products, which have high hardness and weak resilience. It is mainly used for insulation materials, foam slippers, luggage production, etc; Natural rubber can provide good anti-slip performance and increase safety; EPDM can increase product elasticity.
-Color Selection:
The dispersion of the color masterbatch is good, and the color is not easily changed during high-temperature processing; The price of color powder is low, its dispersibility is poor, and the processing environment is prone to pollution; Color sand: has good dispersibility and strong environmental performance.
-Processing Foaming Additives:
There are three types of foaming additives available: high, medium, and low temperature. High temperature has good magnification, but it has a significant impact on the color of the product and is prone to color difference; Medium temperature is suitable for use in injection and small mold processes.
-Workmanship
- Ejection
This process is relatively advanced and can form products in just one step, but the molds used require high precision and will develop faster in the future. Its principle is similar to injection molding in the plastic product industry. The difference is that injection molding starts immediately after cooling, and the mold temperature needs to be controlled. However, EVA injection molding has changed this mold opening method and temperature control. Modern manufacturers mostly use this method when manufacturing sports shoes.
- Small Foam Inside the Mold
This process is mainly widely used in shoe materials. The first foaming of sports shoes is to granulate the refined material (7470M), weigh it, and then put it into the opened mold. The foam produced looks like a draft of a shoe. The difficulty is that the mold and formula need to be consistent, which requires simultaneous control of magnification and hardness. The foaming conditions of this process are relatively flexible and vary with the shape and structure of the product. Temperature changes do not require significant changes, mainly by controlling the time. The second molding process involves grinding off the skin of the previously foamed rough draft and then pressing it into the mold cavity. After heating and cooling, the product is formed. The pressure is 50 kilograms per square centimeter, and the heating temperature needs to be controlled at 125-135 degrees Celsius. After heating for a certain period, water cooling is carried out to take out the secondary molding of the midsole. This compressed product has a relatively stable size and good physical properties.
- Traditional Flat Panel Foam
Many small manufacturers in China use this technology, and the equipment cost is relatively low. This process produces sheet metal, which also requires processes such as punching and edge grinding to become a product. The foaming conditions are relatively stable, but the foaming time depends on the thickness of the mold.
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