Let’s start with introducing the physical properties of EVA:
The physical performance indicators ofEVAgenerally include these: hardness, specific gravity, elongation, tensile strength, tear strength, compression set, and elasticity.
Hardness, everyone understands this. The degree of softness and hardness of the product is measured by Shore hardness. EVA foam can be used from more than 10 degrees to more than 80 degrees.
Specific gravity is density. Many people can find a way to measure this with simple tools. Generally, microcomputer specific gravity balances are used if conditions permit. The minimum specific gravity of the products I have made seems to be around 0.05 (g/cm3).
Elongation, that is, elongation at break. Make the EVA foam product into a certain specification, for example, 60*10*5mm, and then use a tensile testing machine to pull it at a constant speed, and divide the length when it breaks by the original length.
Tensile strength, simply put, is the load at the time of fracture. Measured simultaneously with elongation. Tear strength, this is really hard for me to explain. It is to make a small opening for the EVA sheet, and fix the two sides of the opening to the two pliers of the tensile testing machine, and let it be pulled later. The applied load is the tear strength.
Compression permanent deformation, some people call it compression distortion, generally referred to as compression. In fact, compression crookedness is the name of the instrument for testing compression permanent deformation. The EVA is made into a specified disc and placed in a compression chamber, adjusted to the required parameters, and then placed in an aging machine for several hours. Finally, take it out and test the thickness of the disc. The value calculated by several coefficients is compressed.
Elasticity, this is just an abstract name. Calling this resilience seems incomplete. If you want to talk about the image, you can call it Q (QQ candy). Some brands have a more professional name, which divides elasticity into two parts: cushioning and reflection. These two words should be easy to understand. I won’t go into details.
After the small foam granulation is not used for a long time, the magnification will increase. There is no very good way to deal with this. It depends on reasonable arrangements in production. Generally, the raw materials are relatively good, and should not be parked for more than a week, and the poor ones must be used up within three days, so as not to increase the magnification. If the time limit is exceeded, it can be returned to the furnace to re-granulate, and it will be fine to add some DCP appropriately.
For small foaming, there are holes in some thicker places, which is not cooked. From the condition, the foaming time can be extended appropriately, or the mold temperature can be increased, or the machine pressure can be increased; from the formula, it can be increased appropriately. The amount of DCP, or add auxiliary bridging agent.
EVA foam formula generally consists of the following raw materials: main ingredients, fillers, foaming agents, bridging agents, foam accelerators, lubricants.
The main material, obviously, is EVA and/or PE. Of course, in order to improve the physical properties of the product, some other materials can be added appropriately, such as rubber, POE, etc., and even a little TPR can be added to strengthen some physical properties. As for EVA, the main indicator is the VA content, and its level is directly related to almost all properties of EVA foam products. Of course, some can also be foamed with only PE. What main material to use depends on the requirements of the product. Fillers, calcium carbonate or talcum powder are generally used at present. Its purpose is to reduce costs, increase product rigidity, etc., and it can also play a role in heat conduction. Generally, the particle size is used as the quality index (of course, the water content is also one aspect), such as 120 mesh, 400 mesh, etc. In principle, the thinner the better, of course the price will be higher. The maximum dosage is 40Phr (the percentage of the main ingredient) in the formula I have seen.
Foaming agents are generally used to use AC series of foaming agents, such as AC-3000H, AC series of foaming agents are high-temperature foaming agents, and the decomposition temperature is more than 220 degrees. There are also low-temperature foaming agents, such as AD-300, whose decomposition temperature is 140 degrees, and medium-temperature foaming agents. Because the price difference is not big, and the high-temperature blowing agent is relatively stable, so now many old EVA manufacturers have switched to high-temperature blowing agents, and some use them together. The amount of AC used depends on the specific magnification. As the bridging agent, DCP is the most used now, and TAIC, PL400, etc. were also used in the past. The half-life of DCP is 1 minute at 180 degrees, and 10 minutes at 130 degrees. Therefore, when kneading materials, the temperature should be kept below 120 degrees as much as possible. Some products have requirements for odor, another bridging agent BIPB can be used, which is generally used in conjunction with TAIC. The dosage of DCP is generally 0.5-0.6 Phr in flat plate foaming and small in-mold foaming, and 0.8-1.0 Phr in injection foaming. Of course, there are also appropriate additions and subtractions, and there are also different usages.
There are two kinds of foam accelerators that are used more now, zinc oxide powder and zinc stearate powder. In the past, we also used these two combinations, but now we only use zinc oxide. A single type can also achieve the effect, 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 generally does not exceed 0.2Phr, and the shrinkage of too much product will be relatively large. Of course, if it is too little, the foaming speed will be too slow, so it is best not to be lower than 1.0Phr.
Lubricants, generally stearic acid. In fact, it does not have much effect, it is to make the material not stick to the machine when it is practiced. It is not good to use too much, because it can reduce the friction between the parts and reduce most of the physical properties. 0. 5Phr is recommended.
Adding accessories in EVA foam formula
The excipients that can be added to the EVA formula are very miscellaneous. Let’s just talk about a few main ones.
Leftovers are the excess tails and edge strips after making products. They are all foamed. This kind of thing is a cost killer. Using it can greatly reduce production costs. Of course, it will also have an impact on physical performance. This type is used by almost everyone who makes EVA, so I won’t go into details.
2. Antistatic agent
Some products are required to have antistatic function, and EVA foam products themselves have strong static electricity, so some antistatic agents need to be added to the formula. Generally, the antistatic agent used in EVA is a mixture, and the specific composition MS is very complicated. I can’t tell clearly at the moment, and the dosage is 3%-5%.
3. Flame retardant
EVA foam products used as some packaging materials may need to be flame retardant. Flame retardants can be properly added to the formula to achieve the effect of flame retardancy. Mg (OH) 2 and Al (OH) 3, commonly used flame retardants for plastics, also have certain effects on EVA foam products, but the dosage is relatively large, we all use 12%-20%, but for products with high flame retardant requirements , they also don’t work very well. There is a product in Japan with very good flame retardant effect, but it does not meet the ROHS directive.
4. Quick curing agent
The EVA foaming process takes time. If you want to increase the daily output and reduce the energy consumption of the product, you can add a quick-cooking agent to shorten the time required for foaming. The most common quick-curing agent on the market now is TAIC, which should be a bridging aid to be precise. Adding an appropriate amount of TAIC can indeed shorten the time, but with its addition, the size of the product will be reduced accordingly, and the time-shortening effect is not very ideal. I don’t know about Fujian, but there are several places in Guangdong that use another fast-cooking agent with a small amount of addition, and the time-shortening effect is quite considerable.
Foaming products are generally colored, so coloring agents need to be added to the formula to adjust the color. EVA foam products can be colored with toner, color glue, and of course it is best to use color masterbatch. MS products cannot be matched by computer, probably because the color changes too much before and after foaming, and the computer can’t match it.
EVA foam generally has three processes
Large foaming in traditional flat panel, small foaming and injection in mold.
1. Traditional flat foaming
Now small factories generally use this. The cost of machinery and equipment is relatively low. This process produces plates, which are then made into products through processes such as punching and edging. The foaming conditions are relatively fixed, the temperature is 160-170, the time is determined by the thickness of the mold, generally 90-110s/mm, and the pressure is 150KG/cm2.
2. Small foaming in the mold
This process is mainly used in shoe materials. Sports shoes do the first foaming of the secondary midsole. The material trained according to the formula is granulated, weighed and put into the opened mold, and the general shape of the shoe is formed after foaming. The difficulty of this process is the symmetry of the mold and formula, otherwise it is difficult to control the magnification and hardness at the same time. Often the size is qualified, but the hardness is not enough. The hardness is enough and the size is too small. The foaming conditions of this process are relatively flexible, depending on the shape and structure of the product, of course, mainly due to the change of time. The temperature does not vary much. Not to mention stressful conditions. Speaking of the secondary midsole, let’s introduce the second molding here. Grind off the skin of the previously foamed rough embryo, press it into the finished product mold, and form the product through two steps of heating and cooling. The heating temperature is more suitable at 125-135, the pressure is 50KG/cm2, and the water is cooled after heating for a certain period of time. Taking it out is the second midsole. The bottom size of this kind of compression is relatively stable, and the physical properties are relatively better.
Injection technology should be the mainstream in the future. A process is done to become a product. It’s just that the precision of the mold is higher. The principle is similar to injection molding in the plastics industry. It’s just that injection molding is to open the mold immediately, and the mold temperature is different. That is to say, the injection of EVA is nothing more than adjusting the mold temperature and opening time of plastic injection molding. It seems that a friend from Shanghai called me last time, and later let his injection molding machine be used to make foam baby car wheels. Most of the models of well-known sports shoes are made in this way. Compared with other methods, the efficiency is really poor.