In addition to the feeding sequence, the quality of the rubber compounded by the internal mixer mainly depends on the mixing temperature, the charging capacity, the rotor speed, the mixing time, the top bolt pressure and the type of the rotor.
Loading capacity
That is, the mixing capacity. Insufficient capacity will reduce the shearing effect and kneading effect on the rubber compound, and even the phenomenon of rubber compound slippage and rotor idling will occur, resulting in poor mixing effect. On the contrary, if the capacity is too large, it is difficult to turn the rubber material, which makes the position of the top bolt inappropriate, so that a part of the rubber material is stagnant at the feeding mouth and neck, so that the rubber material is mixed unevenly, the mixing time is long, and the equipment is easily overloaded. High energy consumption. Therefore, the mixing capacity should be appropriate, usually 60% to 70% of the total effective volume of the closed chamber. The material capacity during mixing of the internal mixer can be calculated by the following empirical formula:
Q——loading capacity, Kg;
K——filling coefficient, usually 0.6～07;
V——the total effective volume of the airtight chamber, L;
ρ——The density of the rubber compound, g/cm3.
The selection and determination of the filling factor K should be determined according to the type of raw rubber and the characteristics of the formula, the characteristics of the equipment and the degree of wear, and the pressure of the top bolt. For NR and formulas with high glue content, K should be appropriately increased; for synthetic rubber and formulas with low glue content, K should be appropriately reduced; for old equipment with a large degree of wear, K should be increased; new equipment should be smaller; meshing The K of the type rotor internal mixer should be smaller than that of the shear type rotor internal mixer; the pressure of the top bolt increases, and the K should also increase accordingly. In addition, the K of the backmixing method must be as large as possible.
Feeding order
In the mixing of the internal mixer, the order of adding the raw rubber, carbon black and liquid softener and the mixing time are particularly important. Generally, the raw rubber is added first, followed by carbon black, and mixed until the carbon black is basically in the rubber compound. After dispersing, add liquid softener, which is beneficial to kneading, improves kneading effect and shortens kneading time. If the liquid softener is added too early or too late, it is unfavorable for mixing, and it is easy to cause uneven dispersion, prolong mixing time and increase energy consumption. The addition time of the liquid softener can be determined by the partition coefficient K. Sulphur and superspeed accelerators are usually added late in mixing, or discharged onto the tablet press to reduce the risk of scorch. Small medicines (solid softener, activator, accelerator, anti-aging agent, anti-scorch agent, etc.) are usually added after raw rubber and before carbon black.
top bolt pressure
When the mixer is mixing, the rubber must be subjected to a certain pressure from the top bolt. It is generally believed that the pressure of the top bolt is 0.6-0.8MPa. When the rotor speed is constant, further increasing the pressure has little effect. When the kneading capacity is insufficient, the pressure of the top bolt can not fully exert its effect. Increasing the top-bolt pressure can reduce the unfilled space in the closed chamber, making it about 10% more filled. With the increase of capacity and speed, the pressure of the top bolt must be increased.
The increase in the top bolt pressure will accelerate the heat generation of the compound during the mixing process and increase the power consumption during mixing.
Rotor Construction and Type
The geometry and size of the rotor's working surface largely determines the capacity and mixing quality of the mixer. There are two basic configurations of internal mixer rotors: shearing rotors and meshing rotors. Generally speaking, the shear rotor mixer has high production efficiency, which can be used for fast feeding, fast mixing and fast debinding. The meshing rotor mixer has the characteristics of high dispersion efficiency and low heat generation rate, and is suitable for the manufacture of hard rubber and one-stage mixing. The dispersing and homogenizing effect of the meshing rotor internal mixer is better than that of the shearing rotor internal mixer, and the mixing time can be shortened by 30~50%.
Rotating speed
Increasing the speed of the mixer rotor is one of the most effective measures to intensify the mixing process. The speed of rotation is doubled, and the mixing cycle is shortened by about 30% to 50%. Increasing the rotation speed will accelerate the heat generation, resulting in a decrease in the viscosity of the rubber compound and a decrease in the mechanical shearing effect, which is not conducive to dispersion.
Mixing temperature
High mixing temperature is conducive to the plastic flow and deformation of raw rubber and rubber compound, and is conducive to the wetting of the surface of the solid compounding particles by the rubber and mixing and eating of powder, but it also reduces the viscosity of the compound, which is not conducive to the crushing and mixing of compounding particles. Dispersion mixing. If the mixing temperature is too high, it will also accelerate the thermo-oxidative aging of the rubber, which will reduce the physical and mechanical properties of the vulcanizate, that is, the phenomenon of over-mixing; Cooling measures; but the temperature should not be too low, otherwise there will be a phenomenon of rubber compression.
mixing time
Under the same conditions, the mixing time required for mixing the rubber with an internal mixer is much shorter than that of an open mixer. When the mixing quality is required to be certain, the required mixing time is shortened with the increase of the internal mixer speed and the pressure of the upper ejector. Improper feeding sequence and unreasonable mixing operation will prolong the mixing time.
Extending the mixing time can improve the dispersion of the compounding agent in the compound, but it will also reduce the production efficiency. If the mixing time is too long, it is easy to cause the rubber compound to be over-mixed and the physical and mechanical properties of the vulcanizate will be damaged, and it will also cause the "thermal history" of the rubber compound to increase, which is prone to scorch. Refinement time.
