1. Traditional rubber vulcanization process
1. The main factors affecting the vulcanization process:
Sulfur dosage. The larger the dosage, the faster the vulcanization rate and the higher the degree of vulcanization that can be achieved. The solubility of sulfur in rubber is limited, and excess sulfur will be precipitated from the surface of the rubber compound, commonly known as "sulfur spray". 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 use requirements of rubber products, the amount of sulfur in soft rubber is generally not more than 3%, the amount in semi-rigid rubber is generally about 20%, and the amount in hard rubber can be as high as 40% or more.
Vulcanization temperature. If the temperature is 10 °C higher, the curing time will be shortened by about half. Since rubber is a poor thermal conductor, the vulcanization process of the product varies due to the temperature difference of its various parts. In order to ensure a relatively uniform degree of vulcanization, thick rubber products are generally vulcanized at a gradual temperature and at a low temperature for a long time.
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 under-sulfur). If the time is too long, the degree of vulcanization is too high (commonly known as over-sulfur). Only a suitable 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 film products. The products are dipped in carbon disulfide solution containing 2% to 5% sulfur chloride, and then washed and dried.
2. When vulcanizing at room temperature, the vulcanization process is carried out at room temperature and normal pressure, such as using room temperature vulcanized mortar (mixed rubber solution) for bicycle inner tube joints and repairs.
3. Thermal vulcanization is the main method of vulcanization of rubber products. According to the different vulcanization medium and vulcanization method, thermal vulcanization can be divided into three methods: direct vulcanization, indirect vulcanization and air-mixed vulcanization.
①Direct vulcanization, the product is directly vulcanized in hot water or steam medium.
②Indirect vulcanization, the product is vulcanized in hot air, this method is generally used for some products with strict appearance requirements, such as rubber shoes.
③ For air-mixed vulcanization, air vulcanization is used first, and then direct steam vulcanization is used. This method can not only overcome the shortcomings of steam vulcanization affecting the appearance of products, but also overcome the shortcomings of long vulcanization time and easy aging due to slow heat transfer of hot air.
3. Rubber vulcanization process
Before the rubber is unvulcanized, there is no cross-linking between the molecules, so it lacks good physical and mechanical properties and has little practical value. When the rubber is added with a vulcanizing agent, the rubber molecules can be cross-linked by heat treatment or other methods to form a three-dimensional network structure, thereby greatly improving its performance, especially the tensile stress, elasticity, hardness, and tensile strength of the rubber. A series of physical and mechanical properties will be greatly improved. The rubber macromolecules chemically react with the cross-linking agent sulfur under heating, and the process of cross-linking into a three-dimensional network structure. The vulcanized rubber is called vulcanizate. Vulcanization is the last process in rubber processing, and it can obtain shaped rubber products with practical value.
Fourth, injection molding vulcanization process:
The most obvious difference between ordinary molding and injection molding is that the former compound is filled into the mold cavity in a cold state, while the latter is heated and mixed, and injected into the mold cavity at a temperature close to the vulcanization temperature. Therefore, during the injection molding process, the heat provided by the heating template is only used to maintain the vulcanization, and it can quickly heat the rubber compound to 190℃-220℃. In the molding process, the heat provided by the heating template is first used to preheat the rubber compound. Due to the poor thermal conductivity of rubber, if the product is very thick, it will take a long time for the heat to be transmitted 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 scorch on the edge of the product near the hot plate. Vulcanization by injection molding can shorten the molding cycle and realize automatic operation, which is most beneficial to mass production. Injection pressing also has the following advantages: it can save the preparation of semi-finished products, moulding and product trimming, etc.; it can produce high-quality products with stable dimensions and excellent physical and mechanical properties; reduce vulcanization time, improve production efficiency, and reduce the amount of rubber. Reduce costs, reduce waste, and improve enterprise economic benefits.
Five, injection molding vulcanization process matters needing attention:
Use reasonable screw speed and back pressure to control the appropriate temperature of the injection machine. In general, it is advisable to keep the difference between the temperature of the glue at the discharge port and the temperature of the control cycle not more than 30 degrees. The purpose of the injection machine screw is to prepare a sufficient amount of compound for each cycle at a selected and uniform temperature; it significantly affects the output of the injection machine. The back pressure is generated by slowing down the flow of the oil outlet in the injection cylinder, and restricts the pushing action of the injection cylinder for the rubber injected by the injection machine. In practice, back pressure will only slightly increase shear to the compound without causing a reduction in the physical properties of the cured product.
Nozzle Design:
The nozzle connects the injection head and the mold, and at the same time has a certain effect on the heat balance. The pressure loss through the nozzle is converted into heat through the injection. The compound must never be vulcanized in this area. Therefore, it is very important to choose the appropriate nozzle diameter, which affects the frictional heat generation at the nozzle, the pressure and filling time required for the injection of the rubber compound.
Appropriate mold temperature, optimal vulcanization conditions. After selecting the best combination of the rubber compound, it is important to cooperate with the injection molding conditions and the vulcanization conditions. Compared with compression molding, injection molding has different temperature distributions on the surface and inside of the mold. To achieve good vulcanization, it is necessary to control the temperature with high precision, so that the surface and the inside of the mold can achieve the best 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. Furthermore, in terms of forming pressure, high pressure forming is extremely advantageous because pressure is inversely related to shrinkage.
Safe and reasonable compound formulation design. For injection molding compounds, the following properties are required:
The Mooney scorch time of the compound should be as long as possible for maximum safety. Generally, the Mooney scorch time should be 2 times longer than the residence time of the compound in the barrel.
The vulcanization speed is fast. Through the reasonable selection of different rubber vulcanization systems and the addition of appropriate accelerators, the rubber has a satisfactory efficiency during injection vulcanization. Good fluidity, good flow properties reduce the residence time of the compound, reduce the injection time, and improve the anti-scorch ability.
6. Nitrogen vulcanization process
The main advantages of using nitrogen-filled vulcanization are energy saving and prolonging the life of the capsule, which can save 80% of the steam, and the service life of the capsule can be extended by 1 times. Tires consume a lot of heat and electricity during the vulcanization process, so it is of great significance to develop and promote energy-saving vulcanization processes. Due to the small molecular weight and small heat capacity of nitrogen, when nitrogen is filled into the inner cavity of the tire bladder, it will not absorb heat and cause the temperature to decrease, and it is not easy to cause the oxidative cracking damage of the bladder.
Seven, nitrogen vulcanization process characteristics
First pass high-temperature and high-pressure steam, and then switch to nitrogen after a few minutes, and use the "pressure-keeping and temperature-changing" process of nitrogen-filled vulcanization to vulcanize to the end. Because the heat of the first few minutes of steam is sufficient to keep a tire cured, theoretically, the temperature does not drop below 150°C before the curing is completed. However, when nitrogen vulcanization is used, high temperature and high pressure steam is first introduced, which will cause the temperature difference between the upper and lower sidewalls. To eliminate the vulcanization temperature difference between the upper and lower sidewalls, it is necessary to reasonably arrange the position of the vulcanization medium injection, and improve the sealing and thermal piping system. . The purity of nitrogen used for sulfidation is required to reach 99.99%, preferably 99.999%, and it is recommended that enterprises prepare their own nitrogen system to reduce the cost of use. Insufficient nitrogen purity will affect the service life of the capsule. The "pressure-changing temperature" vulcanization principle of nitrogen vulcanization is applied to the transformation of the traditional circulating superheated water vulcanization process. When vulcanizing, first pass high temperature and high pressure steam, and then switch to circulating superheated water after a few minutes, and then close the return valve to stop the circulation after a few minutes, until the latent heat vulcanization is completed. Using this new heating vulcanization method, according to theoretical calculation, its energy consumption is only 1/2 of the traditional vulcanization method.
8. Vulcanization process Temperature measurement.
Key Factors of High Temperature Vulcanization Process
Carry out vulcanization temperature measurement, find the slowest vulcanization point in the product, and use this point as the basis to determine the vulcanization time, the effect is better than the first two. Using this method, the vulcanization efficiency can be improved to different degrees and the uniformity of the vulcanization degree can be improved. However, since only the external temperature is examined in actual production, the actual temperature of each part of the tire is not known for sure, and the temperature is not fixed every time, so there is a large error between the results calculated according to the temperature measurement and the actual vulcanization results.
The simulation and prediction of the temperature field in the vulcanization process of thick rubber products show that the uneven temperature is the main factor causing the uneven degree of vulcanization of the tire casing. The rubber industry generally believes that constant external temperature is an important condition to ensure quality, and every effort must be made to achieve constant temperature from equipment. This is true 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 all parts of the tire through the model. Rubber is a poor conductor of heat, and the temperature rises slowly. In the early stage of heating, there is a significant temperature gradient in each part of the tire, and it takes a long time to reach equilibrium.