At present, there are two mainstream foaming processes: one is injection crosslinking foaming, which is mainly used for EVA foaming; The second is intermittent kettle foaming, which is mainly used for TPU foaming. ETPU is called popcorn and has a lot to do with this process.
below, we will explain each foaming process listed separately.
first, flat foaming (large foaming)
flat foaming, also called large foaming, is a very mature foaming process. EVA products made by this foaming process are mainly used in cushions, coils and other aspects, and there are also shoes materials, but less.
(EVA foamed sheet)
In terms of shoe materials, the flat foaming process can be summarized as follows: mixing and mixing foamed materials-making sheets to be foamed in a mold-cutting and edging the foamed sheets-finished soles.
The advantages of flat foaming process are relatively low cost of machinery and equipment, and low technical requirements. The disadvantages are that there are a lot of leftover materials, and it is difficult to recycle EVA after cross-linking foaming, resulting in waste of raw materials and low production efficiency. Therefore, it is mainly used in small shoe material processing plants at present.
2. Molding foaming (small foaming)
Molding foaming, also called small foaming, can be said to be a classic process of EVA foaming for shoes. It is divided into primary foaming and secondary foaming, among which the secondary foaming EVA midsole is also called PHYLON, commonly known as Feilong, which is far better than the primary foaming EVA midsole in terms of shock absorption and elasticity.
(EVA midsole)
The molding and foaming process of shoe material elastomer can be summarized as follows: mixing and granulating the formulated materials-weighing and putting them into the mold-one-time foaming. If secondary foaming molding is needed, the skin of the primary foaming rough blank is ground off, pressed into the finished product mold, and the product is molded through two steps of heating and cooling.
The advantages of this process are flexible foaming conditions and very mature technology. Compared with flat foaming, it reduces the waste of scraps. However, compared with the injection crosslinking foaming process, its disadvantages are mainly low production efficiency, high energy consumption and great influence of process conditions on product physical properties.
III. Injection crosslinking foaming
Injection crosslinking foaming process is an advanced foaming process for the middle soles of EVA shoes at present, but there are few public research materials about this process, and the disclosure only involves the basic part. According to people in the shoe material industry, the mainstream EVA foaming process in shoe material manufacturers is injection crosslinking foaming at present.
injection cross-linking foaming process: EVA raw materials, modified materials, chemical additives, fillers, etc. are mixed in an internal mixer-the materials are poured twice on the wheel table and then put into a granulator to form particles-the EVA raw materials are inspected and injected into injection molding-E8VA midsole finished product.
The injection crosslinking foaming process can greatly improve the production efficiency, and it can be molded by injection at one time, and there is almost no corner waste. However, the process of dealing with expansion is more complicated than that of molding foaming, and the accuracy of finished products will be slightly different due to dimensional expansion.
fourth, intermittent autoclave foaming
ETPU is called popcorn, on the one hand, it is related to its structure like popcorn, on the other hand, it is related to the foaming process. At present, the TPU foaming process for shoes is still mainly kettle foaming, that is, intermittent kettle foaming.
(popcorn sole)
As for intermittent autoclave foaming, there are two methods: one is step-by-step heating method, and the other is rapid pressure release method. Among them, the rapid pressure relief method is similar to the process of making popcorn by an old popcorn machine.
the technological process of batch kettle foaming can be summarized as follows: putting TPU particles, foaming agents and various additives into a closed foaming kettle-saturating at a certain temperature and pressure-and then preparing TPU foaming beads by heating or depressurizing. The foaming agent is generally supercritical carbon dioxide fluid, so some people call it supercritical foaming.
there are two molding processes after TPU foamed beads are made: steam molding and PU gluing. However, due to environmental protection, weight loss and other factors, the subsequent shoe material molding process is basically steam molding.
The foaming ratio of ETPU made by this process is more than 1 times, with excellent properties, and the equipment and process are relatively simple, but there are some problems such as high cost and special explosion-proof treatment for equipment.
v. continuous extrusion foaming
intermittent autoclave foaming still has some problems such as low processing efficiency and unstable product performance, so continuous extrusion foaming, which can improve production efficiency relatively, has become a hot research topic of TPU foaming at present, and its main purpose is to further improve production efficiency, just like why EVA uses injection crosslinking foaming.
The foaming process flow can be summarized as follows: evenly mixing TPU particles and additives-injecting supercritical fluid into polymer melt-cooling, extruding, underwater ring cutting-making TPU foamed beads-steam molding --ETPU shoe midsole finished product.
At present, the bottleneck of the development of this process in China lies in the high investment cost, the immature domestic related equipment and technology, and the high open-cell content of the obtained foamed beads, which affects the subsequent molding.
generally speaking, both intermittent autoclave foaming and continuous autoclave foaming are related to supercritical fluid, and they are all supercritical foaming processes, which are environmentally friendly and efficient.