Three EVA foaming processes for shoes
EVA is an abbreviation of ethylene-vinyl acetate copolymer, which is a random copolymer composed of a non-polar, crystalline ethylene monomer and a highly polar, amorphous vinyl acetate monomer (also known as VA).
Compared with PE, EVA introduces vinyl acetate monomer into the molecular chain, which reduces crystallinity, improves flexibility, impact resistance, improves filler mixing and heat sealing, and generally has a density between 0.91 and 0.93. , transparency and gloss are good.
EVA is a thermoplastic polymer, so it can be used in injection molding, extrusion, blow molding, calendering, rotational molding, vacuum thermoforming, foaming, coating, heat sealing, welding and other processing processes.
EVA materials have been widely used in shoe materials. The blended foam shoes have softness, good elasticity, shock absorption and chemical resistance. They are widely used in medium and high-grade sneakers, hiking shoes, slippers and sandals. Sole and other aspects.
In terms of shoe materials, the EVA foaming process generally has three processes: traditional flat plate large foaming, in-mold small foaming and injection cross-linked foaming, in which molded foaming and injection cross-linking foaming are used more.
1. Traditional flat large foam
Small factories generally use this because the cost of machinery and equipment is relatively low. This process is made of sheet metal, and then made into products by punching, edging and other processes.
2. In-mold small foaming
In-mold small foam is also called molded foaming. It is granulated according to the formula and is put into a good mold after weighing. The foaming is the general appearance of the shoes. The process exhibits higher quality in detail and higher dimensional accuracy, but has lower production efficiency and more trim.
The process is further divided into one-time molding foaming and secondary molding foaming. For example, the PHYLON midsole is EVA secondary molding foaming, which has good dimensional stability and excellent variability, and can be tailored to the needs of each sneaker design. Shape shapes into a variety of different styles.
The difficulty of the small foaming process in the mold is the symmetry of the mold and the formula. Otherwise, it is difficult to control the magnification and hardness at the same time, and the dimensionally qualified hardness is not enough, and the hardness is enough to contradict the small size. Process conditions such as temperature, time, and pressure also have a large effect on the properties of the material.
3. Injection cross-linking foaming
Injection cross-linking foaming has become the mainstream process for the preparation of EVA shoe midsole in shoe processing enterprises. The process is to high-speed kneading EVA, modifiers, additives, etc., and then extruding and granulating, followed by injection molding. Only the finished midsole is half the size.
The high temperature mold needs to be allowed to stand for a while, otherwise, the EVA midsole will jump out when the mold is opened. Because the mold is much smaller than the actual midsole, the midsole expands rapidly as it is released from the mold, and the injection molded midsole may shrink during subsequent cooling after expanding to a larger size. Therefore, the injection molding process is more complicated than the compression molding process.
Compared with the in-mold small foaming, the process has significant advantages such as high production efficiency and substantially no trim, which can greatly reduce the manufacturing cost and make the product design more diversified and functional.
