Analyze the formula materials and three major processes of EVA foaming

First, EVA foam 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 (polyethylene octene coelastomer), etc., and even add a little TPR (a combination of rubber and thermoplastic properties). Materials) to enhance certain physical properties. The main indicator of EVA is the VA content, and its high and low decision is almost directly related to the performance of EVA foam products. of course. Some can only be foamed with PE. 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. The maximum amount is 40 Phr (the percentage of the main ingredient) in the formulation I have seen.

Foaming agents are generally used to use AC series foaming agents, such as AC-3000H. AC series foaming agents are high-temperature foaming agents with a decomposition temperature of over 220 degrees. There are also low-temperature foaming agents, such as AD-300, with a decomposition temperature of 140 degrees, and a medium temperature foaming agent. Because the price difference is not large, and the high-temperature foaming agent will be relatively stable, so many old EVAs are now using high-temperature, and also used in combination. The amount of AC can be based on the specific magnification.

The bridging agent, which is currently used most, is DCP (di-octyl phthalate), and TAIC, PL400, etc. have been used before. The half-life of DCP is 1 minute at 180 degrees and 10 minutes at 130 degrees. Therefore, when training materials, 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. In the past, we used these two combinations. Now we only use zinc oxide. 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. We have a consensus that the amount of zinc oxide is generally not more than 0.2Phr, and the shrinkage of excessive products will be 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. 0.5Phr is recommended.

Second, EVA foaming process

EVA foaming generally has three processes, a conventional flat large foaming, a small foaming in the mold, and an injection.

(1) "Traditional flat large foaming" is now used in small factories, and 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. 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.

(2) The "in-mold small foaming" process is mainly used in the aspect of shoe materials, and the sports shoes are used for the first foaming of the secondary midsole. The granules are granulated according to the formula, and are weighed and placed in a mold. The foaming is the general appearance of the shoes.

The difficulty of this process 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 of this process are relatively flexible, depending on the shape and structure of the product, of course, mainly the change of time, and the temperature does not change much.

(3) The “injection process” is the mainstream of the future. One process is the product, but the precision of the mold is higher. The principle is similar to the injection molding in the plastics industry, except that the injection molding is immediate, and the mold temperature is different. In fact, the EVA injection is just to adjust the mold temperature and mold opening time of the plastic injection molding.