EVA Foam Formulation of Raw Materials and Four Process Analysis
EVA, ethylene vinyl acetate copolymer, polymer materials. EVA midsole is lightweight and good physical properties, is a senior lightweight shoes, sports shoes, casual shoes use the most amount of material. Today, we will learn to learn EVA foam formulation of raw materials and the four processes.
A. EVA foam formulation of those things
EVA foam formulation generally consists of the following raw materials: main material, filler, foaming agent, bridging agent, foam accelerator, lubricant.
The main material, which is EVA or PE. Of course, in order to improve the physical properties of the product, you can also add some other materials, such as rubber, POE (polyethylene octene co-elastomer), etc., but also can even add a little TPR (is a combination of rubber and thermoplastic properties of the material) to strengthen certain physical properties.
EVA, the main indicator is the VA content, its high and low decision almost all the performance of EVA foam products are directly related. Of course some only use PE can also be foamed. Specific with what the main material or depends on the requirements of the product.
Filler, currently calcium carbonate or talcum powder is generally used. It is used to reduce the cost, increase the rigidity of the product, etc., but also can play a role in the thermal conductivity. Generally the size of the particle size as an indicator of quality (of course, the water content is also an aspect), for example, 120 mesh, 400 mesh, etc. In principle, the finer the better, of course, the price will also be higher. The maximum amount in the recipe I have seen 40Phr (the amount is the percentage of the main material).
Foaming agent, generally used to use AC series of foaming agent, such as AC-3000H, AC series of foaming agent belongs to the high temperature foaming agent, decomposition temperature in more than 220 degrees.
There are also low temperature foaming agent, such as AD-300, decomposition temperature at 140 degrees, and medium-temperature foaming agent. Because the price difference is not much, and high-temperature blowing agent will be relatively stable, so now a lot of old EVA are changing to high-temperature, but also with the use of the AC dosage, depending on the specific multiplier.
The most used bridging agent now is DCP (di-sec-octyl phthalate), previously also used TAIC, PL400, etc. The half-life of DCP is 1 minute at 180 degrees Celsius and 10 minutes at 130 degrees Celsius, so generally, when practicing the material, the temperature should be controlled below 120 degrees Celsius.
Some products that require odor can use another bridging agent, BIPB, which is generally used in conjunction with TAIC. The dosage of DCP is 0.5-0.6Phr for flat foam and in-mold small foam, and 0.8-1.0Phr for injection foam, but there are also differences in usage.
There are two kinds of foam accelerators, zinc oxide powder and zinc stearate powder, which are used a lot nowadays. We used to use these two together here, but now we only use zinc oxide. A single kind can also achieve the effect, and the stability of the product may also be better. Zinc oxide lowers the decomposition temperature of AC to about 160 degrees, which facilitates production. We have a consensus here, the general amount of zinc oxide does not exceed 0.2Phr, and too much product shrinkage will be relatively large. Of course, if too little, the foaming speed is too slow, so it is best not to be less than 1.0hr/.
The lubricant is usually stearic acid. In fact, it does not have much effect, is to let the practice material, not sticky machine. 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 and recommend 0.5Phr.
B. About the Process
EVA foam generally speaking there is four processes, traditional flat large foam, small foam in the mold, injection and supercritical foam.
1. Injection Process
This process is the mainstream of the future, a process out of the product, but the mold precision is higher. The principle is similar to the plastic industry injection molding, but injection molding is immediately open mold, and mold temperature is different, that is, EVA injection molding mold temperature and mold opening time is adjusted a little. Do now well known sports shoes most of the type of body have changed to do with this method. Other methods than it, efficiency is really a million miles away.
2. Traditional Flat Large Foam
Nowadays, small factories generally use this kind, and the cost of machines and equipment is relatively low. This process is made out of sheets, and then made into products through the process of punching, grinding and so on. Foaming conditions are relatively fixed, the temperature is 160-170, time is determined by the thickness of the mold, generally 90-110 seconds / MM, pressure 150KG / square CM.
3. Small in mold Foam
This process is mainly used in shoe materials, sports shoes to make the first foam of the secondary midsole. The material is granulated according to the formula, weighed and put into the open mold, and the foam comes out as the general appearance of the shoe.
The difficulty of this process is the symmetry of the mold and recipe; otherwise it is difficult to control the multiplicity and hardness at the same time. Often the size is qualified but the hardness is not enough, and the hardness is enough but the size is small.
The foaming conditions of this process are more flexible, depending on the product shape structure, of course, mainly the time is the change, and the temperature change is not much.
Previously, we talked about the second midsole, and here we introduce the second molding. The rough embryo of the previous foam is ground off the skin and pressed into the finished mold, and the product is formed by heating and cooling in two steps.
Heating temperature at 125-135 is more appropriate, pressure 50KG / square CM, heating for a certain period of time and then water cooled, take out is the second midsole. This compression into the size of the bottom is more stable, the physical properties are relatively better.
4. Supercritical Foaming
EVA molecular chain is linear, so in the foaming process usually need to add cross linking agent, through the cross linking structure to lock the gas. Therefore, EVA supercritical foaming will need to solve the problem of how to lock the gas.
From the patent literature and some corporate information, EVA supercritical foaming process is basically such a route: dense blending after cross-linking, granulation, molding to get pre-vulcanized soles, the soles into the autoclave foaming device, the physical foaming agent into the kettle, using the pressure reduction method to get microporous foam sole materials.
EVA material morphological changes: granulation – injection / injection molding – foam – soles.
Such a foaming process can be simply understood as the process of molding foam, the chemical blowing agent into a physical blowing agent, plus a quick pressure relief operation, you can get supercritical foam EVA soles. If you want the whole process to be environmentally friendly and clean, the previous cross-linking process can be replaced with a chemical cross-linking method such as radiation cross linking.
Supercritical foaming process has been developed more and more mature in the past two years, equipment, technology has been increasingly perfect, which regulates the formulation, process, so that it’s excellent performance and control shrinkage and cost is the key.