The production and application of China's EVA foam materials started late, but due to the excellent processing performance and wide application of the products, the demand growth has been very rapid in recent years, attracting many manufacturers to enter the industry. Benefiting from the stable development of China's manufacturing industry, the upgrading of downstream industries, and the wider application areas will be developed, China's EVA foam materials market will maintain rapid growth.
What is EVA?
EVA is produced by copolymerization of ethylene (vinyl acetate) copolymer (also known as ethylene-vinyl acetate copolymer) from ethylene (E) and vinyl acetate (VA), abbreviated as EVA. The polymerization method uses high pressure bulk polymerization, solution polymerization, emulsion polymerization, suspension polymerization.
What is EVA used for?
EVA granules can be produced into various types of EVA sheets, and then the sheets are processed through various processes. For example, the embossing type can be used as a packaging lining, and the hot pressing can be made into a sole. EVA materials and so on can be used in the electronics, hardware and toy industries.
EVA foaming formula
EVA foaming formula generally consists of the following raw materials: main material, filler, foaming agent, bridging agent, foaming accelerator, lubricant.
The main ingredient is EVA or PE. Of course, in order to improve the physical properties of the product, it is also possible to add some other materials, such as rubber, POE, etc., and even add a little TPR to enhance certain physical properties. The main indicator of EVA is the VA content, and its level is directly related to the performance of EVA foam products. of course. The specific material used depends on the requirements of the product.
Fillers, currently commonly used calcium carbonate or talc. Its use is to reduce the cost, increase the rigidity of the product, etc., and can also play a role in heat conduction. Generally, the particle size is used as a quality index (of course, the water content is also one side), for example, 120 mesh, 400 mesh, and the like. In principle, the finer the better, of course, the price will be higher.
Foaming agents are generally used in high-temperature foaming agents, and the decomposition temperature is more than 220 degrees. There are also low-temperature foaming agents, the decomposition temperature is 140 degrees, and there is a medium temperature foaming agent. Because the price difference is not large, and the high-temperature foaming agent is relatively stable, so many of the EVAs are now used in high temperature, and also used in combination. The specific dosage can be based on the specific magnification.
The bridging agent, the most used now is DCP (di-octyl phthalate). Generally, when the material is used, the temperature should be controlled below 120 degrees. Some products have odor requirements, you can use another bridging agent BIPB, which is generally used in conjunction with TAIC. The amount of DCP used is generally 0.5-0.6 Phr in flat foaming and in-mold small foaming, and generally 0.8-1.0 Phr in injection foaming. Of course, there are also some additions and subtractions, and there are different usages.
There are two types of foaming accelerators, zinc oxide powder, and zinc stearate powder. A single one can also achieve results, and the stability of the product may be better. Zinc oxide can reduce the decomposition temperature of AC to about 160 degrees, which is convenient for production. In general, excessive zinc oxide will shrink the product relatively large. Of course, if too little, the foaming speed is too slow, so it is best not to be lower than 1.0hr/
Lubricant, generally stearic acid. In fact, it doesn't make much difference. It is to let the machine not stick to the material. It is not good to use more, because it can reduce the friction between the points, so that most of the physical properties are reduced.
Three major crafts
EVA foaming generally has three processes, a conventional flat large foaming process, an in-mold small foaming process, and an injection process. Let's take a look at everyone under the new parking lot!
1. Injection process
This kind of process will gradually become the mainstream. The process is made in one process, which is convenient and simple, but the precision of the mold is relatively high. The process principle is similar to the injection molding in the plastics industry, except that the injection molding is immediate, and the mold temperature is different. The EVA injection is to adjust the mold temperature and mold opening time of the plastic injection molding. Most of the well-known sports shoes are now using this method. high efficiency!
2. Traditional flat large foaming process
At present, the scale of the factory is generally small, and the cost of machinery and equipment is relatively low. This process is mainly used as a sheet, and then made into a product by processes such as punching and edging. The foaming conditions are relatively fixed, the temperature is 160-170, and the time is determined by the thickness of the mold, generally 90-110 seconds/MM, and the pressure is 150KG/square CM.
3. In-mold small foaming process
This process is mainly used in the aspect of shoe materials, and the sports shoes are the first foaming of the secondary midsole. The granules are granulated according to the formula and put into the mold, and the foaming is roughly the same. The difficulty is the symmetry of the mold and the formulation, otherwise it is difficult to control the magnification and hardness at the same time. Often the size is qualified, the hardness is not enough, and the hardness is enough to be small. The foaming conditions are more flexible, depending on the product structure, of course, mainly due to the influence of time.
Summary: EVA is biodegradable; similar to PVC; light weight; no odor; no heavy metals; no phthalates; high transparency, softness and toughness; super low temperature resistance (-70C) ; resistant to water, salt and other substances; high heat adhesion; low bonding temperature; silk screen and offset printing. The future is bright.