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EVA is the English abbreviation of ethylene-vinyl acetate copolymer, which is a random copolymer composed of non-polar, crystalline ethylene monomer and strong polar, non-crystalline vinyl acetate monomer (also known as VA)
.
Compared with PE, EVA reduces crystallinity, improves flexibility, impact resistance, and improves filler mixing and heat sealing due to the introduction of vinyl acetate monomer into the molecular chain.
Generally, the density is between 0.
91 and 0.
93.
, good transparency and gloss
.
EVA is a thermoplastic polymer, so it can be used in injection molding, extrusion, blow molding, calendering, rotomolding, vacuum thermoforming, foaming, coating, heat sealing, welding and other processing and forming processes
.
EVA material has been widely used in shoe materials for a long time.
Its blended foam shoe material has the properties of softness, good elasticity, shock absorption and chemical resistance, and is widely used in middle and high-end travel shoes, hiking shoes, slippers and sandals.
the soles
, etc.
In terms of shoe materials, there are generally three types of EVA foaming processes: traditional flat plate large foaming, in-mold small foaming and injection cross-linking foaming, among which molding foaming and injection cross-linking foaming are used more
.
1.
Traditional flat large foam
Traditional flat large foam Traditional flat large foam
Small factories generally use this because the cost of machinery and equipment is relatively low
.
Figure EVA foam board, from the Internet
2.
Small foam in the mold
Small foam in the mold Small foam in the mold
Small foaming in the mold is also called molding foaming.
It is to granulate the material prepared according to the formula, weigh it and put it into the opened mold, and the foaming is the general appearance of the shoe
.
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 customized according to the needs of each shoe design.
Shape shapes into a variety of different >
.
The difficulty of the small in-mold foaming process is the symmetry of the mold and the formula.
Otherwise, it is difficult to control the magnification and hardness at the same time, and there will often be a contradiction between the qualified size and insufficient hardness, while the hardness is enough and the size is too small
.
Process conditions such as temperature, time, and pressure also have a great impact on the properties of the material
.
Figure EVA & midsole mold, from the Internet
3.
Injection cross-linked foaming
Injection cross-linking foaming Injection cross-linking foaming
Injection cross-linked foaming has now become the mainstream process for the preparation of EVA shoe midsole by large shoe material processing factories.
Only half the size of the finished midsole
.
The high temperature mold needs to stand for a period of time, 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 an injection-molded midsole, after expanding to a larger size, may shrink during subsequent cooling
.
Therefore, the process of processing expansion and contraction of the injection molding method is more complicated than that of the compression molding method
.
Compared with small in-mold foaming, this process has significant advantages such as high production efficiency and basically no leftovers, which can greatly reduce manufacturing costs and make product designs more diverse and functional
.
4.
Supercritical foaming
Supercritical foaming Supercritical foaming
The molecular chain of EVA is linear, so it is usually necessary to add a cross-linking agent during the foaming process to lock the gas through the cross-linked structure
.
Therefore, the supercritical foaming of EVA needs to solve the problem of how to lock the gas
.
Judging from patent documents and some enterprise data, the supercritical foaming process of EVA is basically such a route: cross-linking and granulation after banburying and blending, molding to obtain pre-vulcanized soles, and placing the soles in an autoclave for foaming In the device, a physical foaming agent is introduced into the kettle, and a microcellular foaming sole material is obtained by a pressure reduction method
.
Morphological changes of EVA materials: granulation-injection/injection-foaming-sole
.
Figure Polyolefin Microcellular Foaming Process, Shensai New Materials
Such a foaming process can be simply understood as replacing the chemical foaming agent with a physical foaming agent in the process of molding foaming, and adding a rapid pressure relief operation to obtain a supercritical foamed EVA sole
.
If you want the whole process to be environmentally friendly and clean, the previous cross-linking process can be replaced by chemical cross-linking methods such as radiation cross-linking
.
Figure cross-section of supercritical foamed EVA sole, photo courtesy of Strength Machinery
The supercritical foaming process has become more and more mature in the past two years, and the equipment and process have been gradually improved.
Among them, regulating the formula and process to make it perform excellent and control shrinkage and cost is the key
.
