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Traditional rubber vulcanization process
1. Main factors influencing the curing process:
The higher the dosage is, the faster the curing rate is, and the higher the curing degree can be achieved.
The solubility of sulfur in rubber is limited, and excess sulfur can be precipitated from the surface of the glue, commonly known as "sulphur spraying".
To reduce sulphur spraying, sulphur is required at the lowest possible temperature, or at least below the melting point of sulphur.
According to the use requirement of rubber product, the dosage of sulphur in soft rubber is generally not more than 3%, the dosage in semi-hard rubber is generally about 20%, and the dosage in hard rubber can be as high as 40% or more.
If high temperature 10 ℃, shortens the curing time in half.
As rubber is a poor heat conductor, the process of vulcanization varies with the temperature of each part.
In order to ensure a more uniform degree of vulcanization, thick rubber products generally use gradually heating, low temperature for a long time vulcanization.
2. Curing time: this is an important part of curing process. The curing time is too short and the degree of curing is insufficient (also known as under sulphur).
Too long, too high degree of sulphur.
Only appropriate degree of vulcanization (commonly known as positive vulcanization) can guarantee the best comprehensive performance
Rubber vulcanization process
According to curing condition, it can be divided into three categories: cold curing, room temperature curing and hot curing.
1. Cold curing can be used for curing of thin film products, which are soaked in carbon disulfide solution containing 2% ~ 5% sulfur chloride and then cleaned and dried.
2. When curing at room temperature, the curing process is carried out under room temperature and room pressure, such as using room temperature vulcanizing glue (mixing glue solution) for repairing and repairing the inner tube joint of bicycle.
3. Hot vulcanization is the main method for vulcanizing rubber products.
According to the different curing medium and curing method, thermal curing can be divided into direct curing, indirect curing and mixed gas curing.
Direct vulcanization, products directly placed into hot water or steam medium vulcanization.
Place the product in hot air to vulcanize, this method is used commonly for some products with strict appearance requirements, such as rubber shoes.
Dry mixing gas is cured first by air and then by direct steam.
This method can not only overcome the defects of steam vulcanization affecting product appearance, but also overcome the disadvantages of slow heat transfer of hot air, long curing time and easy aging.
Iii. Rubber vulcanization process:
Before vulcanization, there is no cross-linking between the molecules, so the rubber lacks good physical and mechanical properties and has little practical value.
When rubber is added with vulcanizing agent, the crosslinking between rubber molecules can be formed through heat treatment or other means to form a three-dimensional network structure, which greatly improves its performance, especially a series of physical and mechanical properties such as tensile stress, elasticity, hardness and tensile strength of rubber can be greatly improved.
Rubber macromolecules undergo chemical reactions with the crosslinking agent sulfur under heating, and the crosslinking becomes a three-dimensional network structure.
The rubber after curing is called vulcanized rubber.
Vulcanization is the last step in rubber processing, and rubber products with practical value can be obtained.
Iv. Injection molding curing process:
The most obvious difference between ordinary moulding and injection moulding is that the former is fed into the mould cavity in a cold state, while the latter is heated and mixed with the glue and placed in the mould cavity near the curing temperature.
Therefore, in the process of injection molding, heating template provided by the quantity of heat only to maintain sulfide, it will soon be rubber heated to 190 ℃ to 220 ℃.
In the mould pressing process, the heat provided by the heating formwork should be used to preheat the adhesive first, because of the poor thermal conductivity of rubber, if the product is very thick, the heat should be transferred to the product center for a long time.
High temperature curing can also shorten the operating time to a certain extent, but often results in the pyrolysis of products near the hot plate.
It can shorten the molding cycle and realize automatic control, which is the most favorable for mass production.
Injection pressure also has the following advantages: the preparation of semi-finished products, mold and product repair procedures;
It can produce high quality products with stable size and excellent physical and mechanical properties.
Reduce curing time, improve production efficiency, reduce glue dosage, reduce capital cost, reduce waste, and improve enterprise economic benefits.
5. Notes for injection molding and vulcanization process:
Use moderate screw speed, back pressure, control the appropriate injection machine temperature.
Generally, it is advisable to keep the gap between discharge port glue and controlled circulation temperature no more than 30 degrees.
The purpose of the injector screw is to prepare sufficient amount of glue for each cycle at selected and uniform temperatures.
It obviously affects the output of the injector.
Back pressure is generated by slowing down the flow of the oil outlet in the injection cylinder, and the injection machine's injection molding is limited to the pushing action of the injection cylinder.
In practice, the back pressure only slightly increases the shearing of the adhesive without reducing the physical properties of the vulcanized product.
The nozzle is connected with the injector head and the mold, and has a certain effect on the thermal balance.
Pressure loss through the nozzle is converted into heat by injection.
The glue does not allow for vulcanization at this site.
Therefore, it is very important to select the proper nozzle diameter, which affects the friction heat generation in the nozzle area, the pressure required for glue injection and the filling time.
Suitable mold temperature, optimum curing condition.
After choosing the best combination of the glue, it is important that the injection molding condition and the curing condition cooperate with each other.
Compared with moulding, due to the different distribution of mold surface and internal temperature, high precision control of temperature must be carried out to achieve good vulcanization, so that the mold surface and internal at the same time reach the optimum vulcanization conditions.
High temperature will increase the shrinkage of rubber, but the relationship between them is linear and should be fully estimated before production.
In addition, as far as molding pressure is concerned, high-pressure molding is extremely beneficial because of the inverse relationship between pressure and contraction.
Safe and reasonable formulation.
The following characteristics are required for the rubber materials for injection and vulcanization molding:
The curing time of the glue should be as long as possible to obtain maximum safety.
In general, the curing time of Mooney should be twice as long as that of glue in the cylinder.
Fast curing speed, through the reasonable selection of different vulcanizing system, adding appropriate promoters, so that the rubber materials in the injection pressure curing efficiency is satisfied.
Good activity, good performance reduce the retention time of the glue, reduce injection time, and improve the ability to prevent coke burning.
Nitrogen sulfurization process
The main advantages of nitrogen sulfurization are energy saving and prolonged capsule life, which can save steam 80%, and the capsule life can be doubled.
The tire consumes a lot of energy and electricity in the process of vulcanization, so it is of great significance to develop and popularize the energy-saving vulcanization process.
Because of the small molecular weight and small heat capacity of nitrogen, when nitrogen is filled into the inner cavity of the tire capsule, it will not absorb heat and cause temperature reduction, and it is not easy to cause the destruction of the capsule oxidation cracking.
Process characteristics of nitrogen sulfurization
First through high temperature and high pressure steam, and then switch on nitrogen after several minutes.
Because the original into a few minutes steam heat enough to keep a tyre vulcanization, theoretically as long as before the complete curing temperature does not fall below 150 ℃.
However, when nitrogen is used for sulfurization, high-temperature and high-pressure steam is first introduced, resulting in the temperature difference between the upper and lower tire sides. To eliminate the temperature difference between the upper and lower tire sides, it is necessary to arrange the injection location of vulcanizing medium reasonably, and improve the sealing and thermal pipeline system.
The purity of nitrogen sulfide is required to reach 99.99%, preferably 99.999%, and it is recommended that enterprises make their own nitrogen system to reduce the cost of use.
Nitrogen is not pure enough, which will affect the service life of the capsule.
The principle of "pressure maintaining and temperature changing" sulfurization of nitrogen sulfurization is applied to the transformation of traditional recycling process of hot water vulcanization.
When curing, first pass into high temperature and high pressure steam, then switch into hot water after several minutes, and then close the backwater valve after several minutes to stop circulation, until the use of latent heat vulcanization to end.
According to the theoretical calculation, the energy consumption of the new method is only 1/2 of that of the traditional method.
Key factors in the process of curing
Shorten the curing time according to physical property test and production experience.
This reduces the degree of overvulcanization to some extent.
High temperature curing.
In recent years, the curing process of small tires has been developing towards high temperature curing. Considering the effect of post-curing, the curing time is short, which has a certain effect on reducing the uniformity of over sulphur and the improvement of curing degree.
The temperature of vulcanization was measured to find the slowest vulcanization point in the product.
The method can be used to improve the efficiency of vulcanization and the uniformity of vulcanization.
However, the actual temperature of each part of the tire is not known with the external temperature only observed in the actual production, and the temperature is not fixed at each time. Therefore, there is a big error between the calculated results based on the temperature measurement and the actual vulcanization results.
Simulation and prediction of temperature field during vulcanization of thick rubber product show that temperature unevenness is the main factor causing the unevenness of vulcanization of tire outer tire.
The rubber industry generally believes that constant external temperature is an important condition for quality assurance.
This is true for non-thick rubber products, but not for thick rubber products such as tire covers.
The tire is heated and vulcanized in the model.
Rubber is a poor conductor of heat, the temperature rises slowly, there is obvious temperature gradient in all parts of the tire at the early stage of heating, and it takes a long time to reach equilibrium.
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