1、 Traditional rubber vulcanization process

1. The main factors affecting the vulcanization process are:

Sulfur dosage. The larger the dosage, the faster the vulcanization speed, and the higher the degree of vulcanization that can be achieved. The solubility of sulfur in rubber is limited, and excessive sulfur will precipitate from the surface of the rubber material, commonly known as "sulfur spraying". In order to reduce the phenomenon of sulfur spraying, it is required to add sulfur at the lowest possible temperature or at least below the melting point of sulfur. According to the usage requirements of rubber products, the amount of sulfur in soft rubber generally does not exceed 3%, in semi hard rubber it is generally about 20%, and in hard rubber it can be as high as 40% or more.

Sulfurization temperature. If the temperature is 10 ℃ higher, the vulcanization time will be shortened by about half. Due to the poor thermal conductivity of rubber, the vulcanization process of products varies due to the temperature differences in different parts. In order to ensure a relatively uniform degree of vulcanization, thick rubber products generally adopt gradual heating, low temperature, and long-term vulcanization.

2. Vulcanization time: This is an important part of the vulcanization process. If the time is too short, the degree of vulcanization is insufficient (also known as undersulfurization). If the time is too long, the degree of vulcanization is too high (commonly known as oversulfurization). Only an appropriate degree of vulcanization (commonly known as positive vulcanization) can ensure the best comprehensive performance.

 

2、 According to the vulcanization conditions, it can be divided into three categories: cold vulcanization, room temperature vulcanization, and hot vulcanization.

1. Cold vulcanization can be used for the vulcanization of thin film products. The products are immersed in a carbon disulfide solution containing 2% to 5% sulfur chloride, and then washed and dried.

2. During room temperature vulcanization, the vulcanization process is carried out at room temperature and normal pressure, such as using room temperature vulcanized rubber slurry (mixed rubber solution) for bicycle inner tube joints, repairs, etc.

3. Thermal vulcanization is the main method of vulcanization for rubber products. According to the different vulcanization media and methods, thermal vulcanization can be divided into three methods: direct vulcanization, indirect vulcanization, and mixed gas vulcanization.

① Direct vulcanization involves placing the product directly into hot water or steam medium for vulcanization.

② Indirect vulcanization, where products are vulcanized in hot air, is generally used for certain products with strict appearance requirements, such as rubber shoes.

③ Mixed air vulcanization, first using air vulcanization, and then switching to direct steam vulcanization. This method can not only overcome the drawbacks of steam vulcanization affecting the appearance of products, but also overcome the drawbacks of long vulcanization time and easy aging due to slow heat transfer of hot air.

 

3、 Rubber vulcanization process

Before vulcanization, rubber molecules do not undergo cross-linking, thus lacking good physical and mechanical properties and having little practical value. After adding vulcanizing agents to rubber, heat treatment or other methods can cause cross-linking between rubber molecules, forming a three-dimensional network structure, greatly improving its performance, especially a series of physical and mechanical properties such as constant elongation stress, elasticity, hardness, and tensile strength of rubber. The process of rubber macromolecules reacting chemically with the crosslinking agent sulfur under heating to form a three-dimensional network structure. The rubber after vulcanization is called vulcanized rubber. Vulcanization is the last process in rubber processing, which can produce shaped and practical rubber products.

 

4、 Injection molding vulcanization process:

The most obvious difference between ordinary molding and injection molding is that the former fills the mold cavity with rubber in a cold state, while the latter heats and mixes the rubber and injects it into the mold cavity at a temperature close to the vulcanization temperature. Therefore, during the injection process, the heat provided by the heating template is only used to maintain vulcanization, and it can quickly heat the rubber material to 190 ℃ -220 ℃. During the molding process, the heat provided by the heating template is first used to preheat the rubber material. Due to the poor thermal conductivity of the rubber, if the product is very thick, it takes a long time for the heat to transfer to the center of the product. The use of high-temperature vulcanization can also shorten the operation time to a certain extent, but often leads to scorching at the edges of products near the hot plate. The use of injection pressure vulcanization can shorten the molding cycle and achieve automated operation, which is most advantageous for mass production. Injection molding also has the following advantages: it can eliminate processes such as semi-finished product preparation, demolding, and product trimming; Can produce high-quality products with stable dimensions and excellent physical and mechanical properties; Reduce vulcanization time, improve production efficiency, reduce rubber usage, reduce costs, reduce waste, and improve enterprise economic benefits.

 

5、 Precautions for injection molding vulcanization process:

Use reasonable screw speed and back pressure to control the appropriate injection machine temperature. Generally, it is advisable to maintain a temperature difference of no more than 30 degrees Celsius between the discharge port and the controlled circulation temperature. The purpose of the injection machine screw is to prepare a sufficient amount of adhesive for each cycle at a selected and uniform temperature; It significantly affects the output of the injection machine. Back pressure is generated by slowing down the flow rate of the oil outlet in the injection cylinder, and limiting the pushing and squeezing effect of the injection cylinder on the rubber material ejected by the injection machine. In practice, back pressure only slightly increases the shear of the rubber material, without causing a decrease in the physical properties of vulcanized products.

Design of nozzle:

The nozzle connects the injection machine head and the mold, and has a certain effect on thermal balance. The pressure loss through the nozzle will be converted into heat through injection. Rubber is never allowed to vulcanize in this area. Therefore, selecting the appropriate nozzle diameter is very important as it affects the frictional heat generation at the nozzle area, the pressure required for rubber injection, and the mold filling time.

Suitable mold temperature and optimal vulcanization conditions. After selecting the best combination of rubber materials, it is important to coordinate the injection molding conditions with the vulcanization conditions. Compared with molding, injection molding has different temperature distribution on the surface and inside of the mold. To achieve good vulcanization, it is necessary to carry out high-precision control on the temperature, so that the mold surface and inside can reach the optimal vulcanization conditions at the same time. High temperature will increase the shrinkage rate of rubber, but the relationship between the two is linear and should be fully estimated before production. In addition, in terms of forming pressure, high-pressure forming is extremely advantageous because the pressure is inversely proportional to shrinkage.

Safe and reasonable rubber formula design. For rubber materials undergoing injection pressure vulcanization molding, the following characteristics are required:

The Mooney scorch time of the adhesive should be as long as possible to achieve maximum safety. Usually, the Mooney scorch time should be twice longer than the residence time of the rubber material in the barrel.

The vulcanization speed is fast, and by selecting different vulcanization systems and adding appropriate accelerators, the rubber material has satisfactory efficiency during injection vulcanization. Good flowability, good flowability reduces the residence time of the rubber material, reduces injection time, and improves anti coking ability.

 

6、 Nitrogen vulcanization process

The main advantages of using nitrogen filled vulcanization are energy saving and extending the lifespan of the capsule, which can save 80% steam and extend the lifespan of the capsule by one time. Developing and promoting energy-saving vulcanization processes is of great significance as tires consume a large amount of thermal and electrical energy during the vulcanization process. Due to the small molecular weight and heat capacity of nitrogen, when it is filled into the inner cavity of the tire capsule, it will not absorb heat and cause a decrease in temperature, nor is it easy to cause oxidative cracking and damage to the capsule.

 

7、 Process characteristics of nitrogen vulcanization

First, pass through high-temperature and high-pressure steam, and then switch to nitrogen after a few minutes. Use the "pressure maintaining and temperature changing" process of nitrogen filled vulcanization to vulcanize until the end. Because the initial heat generated by a few minutes of steam injection is sufficient to maintain vulcanization of a tire, theoretically, as long as the temperature does not drop below 150 ℃ before vulcanization is completed. However, when using nitrogen for vulcanization, the first thing to be introduced is high-temperature and high-pressure steam, which can cause a temperature difference between the upper and lower tire sides. To eliminate the vulcanization temperature difference between the upper and lower tire sides, it is necessary to reasonably arrange the injection position of the vulcanization medium, improve the sealing and thermal pipeline system. The purity requirement for nitrogen used for vulcanization is 99.99%, preferably 99.999%. It is recommended that enterprises prepare their own nitrogen system to reduce usage costs. Insufficient nitrogen purity can affect the lifespan of the capsule. The "pressure maintaining and temperature changing" vulcanization principle of nitrogen vulcanization has been applied to the transformation of traditional cyclic superheated water vulcanization process. People have also developed a vulcanization process that uses high-temperature and high-pressure steam with superheated water to replace the conventional cyclic superheated water vulcanization process. During vulcanization, high-temperature and high-pressure steam is first introduced, followed by a few minutes of switching to circulating superheated water. After a few more minutes, the return valve is closed to stop circulation until the latent heat vulcanization is completed. According to theoretical calculations, the energy consumption of using this new heating vulcanization method is only 1/2 of that of traditional vulcanization process methods.

 

8、 Temperature measurement of vulcanization process

Key factors in the high-temperature vulcanization process

Perform vulcanization temperature measurement to find the slowest vulcanization point in the product, and use this point as a basis to determine the vulcanization time. The effect is better than the first two. By using this method, the vulcanization efficiency can be improved to varying degrees and the uniformity of vulcanization degree can be improved. However, due to the fact that only external temperature is considered in actual production, the actual temperature of various parts of the tire is not known with certainty, and the temperature is not fixed every time, there is a significant error between the calculated results based on temperature measurement and the actual vulcanization results.

The simulation and prediction of the temperature field during the vulcanization process of rubber thick products indicate that uneven temperature is the main factor causing uneven vulcanization degree of tire outer tires. The rubber industry generally believes that constant external temperature is an important condition for ensuring quality, and every effort should be made to achieve constant temperature in equipment. This is correct for non thick rubber products, but not for thick rubber products such as tire casings. The tire is heated and vulcanized in the model, and the heat is transmitted to various parts of the outer tire through the model. Rubber is a poor conductor of heat, with a slow temperature rise. In the early stages of heating, there are obvious temperature gradients in various parts of the outer tire, which take a long time to reach equilibrium.