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        Abstract: The effects of steady‑speed and variable‑speed mixing processes, the phase angle of synchronous rotors, and temperature control in a kneader on mixing efficiency were analyzed. Compared with steady‑speed mixing, a variable‑speed mixing regime—featuring rapid feed, medium‑speed dispersion, and low‑speed distribution—can enhance mixing efficiency. For synchronous‑rotor kneaders, a 90° rotor phase angle improves mixing performance while avoiding the stall phenomenon that occurs when the rotor phase angle is 180° and the long ribs of the rotors come into tangential contact. Moreover, maintaining an appropriate mixing temperature such that the rotor temperature remains lower than the mixing chamber temperature can boost the rotor’s shear action and overall mixing efficiency.
During the production of compounded rubber, enhancing the efficiency of internal mixers has long been a key concern for manufacturers. This study investigates how mixing technologies—such as variable-speed mixing, selection of rotor phase angles in synchronous rotors, and temperature control in internal mixers—affect both mixer productivity and the quality of the compounded rubber.

1. Uniform and variable-speed mixing:
During the steady‑state production of compounded rubber in a kneader, slippage of the rotor frequently occurs after the compound enters the machine. The power curve for steady‑state mixing of tread compound is shown in Figure 1 (with sulfurization, at a rotor speed of 30 r·min⁻¹ and a discharge temperature of 110°C).

From Figure 1 It can be observed that, upon entry into the internal mixer, the power curve exhibits a relatively low peak during the first 20 seconds, indicating a reduced shear force acting on the compound. On-site observations and analysis suggest that, when the mixer capacity is appropriately sized, the lower shear arises because, as the compound enters the mixer, large lumps are fragmented into smaller pieces while the material remains at a relatively low temperature. Additionally, the addition of carbon black, vulcanizing agents, and accelerators creates an isolating effect, leading to slippage between the rotors and the compound. Consequently, increasing the shear intensity during the early stages of mixing is an effective strategy for mitigating rotor slippage. In practice, press‑ram pressure and mixer capacity are typically fixed, whereas rotor speed is adjustable; however, excessively high rotor speeds can cause rapid temperature rise in the later stages of mixing, which is detrimental to the process. Variable‑speed mixing technology offers a viable solution to this issue.

Variable-speed mixing is typically divided into three stages: rapid feeding, medium-speed dispersion mixing, and low-speed distribution mixing.
Rapid feeding: At higher rotor speeds, the rotor rapidly shears large rubber lumps into smaller pieces, causing the rubber to break down, disperse, and blend under intense shear. Due to this rapid shearing, the rubber temperature rises quickly; therefore, it is essential to keep the temperature from becoming excessively high.
Medium-speed dispersion and mixing: A moderate rotor speed is selected to control the compound temperature, preventing rapid temperature rise and enabling optimal dispersion and uniform distribution of compounding agents under appropriate stress and shear conditions.
Low-speed dispersion mixing: To further slow the rate of temperature rise in the later stages of mixing, at a lower temperature, the processed material is made uniformly distributed; the rotor speed is further reduced, and the resulting low shear induces mild fragmentation, dispersion, and blending of the material.

        Figure 2 The figure shows the power profile for variable-speed mixing of tread compound (with rotor speeds of 40, 35, and 30 r·min⁻¹ and a discharge temperature of 110 °C). As illustrated in Figure 2, increasing the rotor speed results in a higher peak on the early-stage power curve, effectively eliminating rotor slippage. Compared with constant-speed mixing, the production time per batch is reduced by approximately 20 seconds, and the production capacity increases by about 20%.
Testing has shown that, compared with rubber compounds produced via the constant-speed mixing process, those manufactured using the variable-speed mixing process exhibit no significant differences in Mooney viscosity, hardness, density, rheometer data, or filler dispersion.
Increasing the rotor speed can enhance production capacity; however, an improperly selected rotor speed may compromise compound quality. Figures 3 and 4 show the power profiles for the second-stage mixing of tread compound using variable-speed mixing at 40/35 and 35/30 r·min⁻¹, respectively. The filler dispersion indices of the mixed compound are 4.78 and 5.01, while the distribution indices for oversized particles are 8.45 and 8.77.

        From Figures 3 and 4 As can be seen, the temperature in Figure 3 rises more rapidly than in Figure 4, resulting in a reduction of approximately 20 seconds in the mixing time; however, the filler dispersion is somewhat poorer. The primary reason for the inferior filler dispersion is that, when the compound temperature increases too quickly, the shear forces acting on the compound decrease, thereby compromising filler dispersion. Therefore, when selecting the rotor speed, it is essential to monitor the rate of temperature rise to determine the optimal rotor speed match.

2 Synchronous Rotor Phase Angle:
The synchronous‑rotor internal mixer is currently recognized as one of the most advanced shear‑type internal mixers. Its advantages include strong material‑feeding capability, robust shearing, tearing, and tensile actions, excellent dispersion and distribution mixing performance, high filler dispersion, and superior uniformity between batches and within each batch, along with minimal temperature differentials. Since the speed ratio of the synchronous rotors is 1:1, the phase angle between the two rotors directly influences the mixing outcome; common phase angles are 90° and 180°.
(1) Rotor phase angle 90°

When the rotor phase angle is 90°, the rubber compound experiences shear in the mixing chamber, as shown in Figure 5. Upon entering the mixing chamber, the compound is subjected not only to shear between the rotors and the chamber walls but also to compressive forces between the rotors themselves, thereby accelerating the blending and uniform distribution of the material. Figure 6 presents the mixing power curve for tread compound at a rotor phase angle of 90° (under steady‑state mixing conditions, with a rotor speed of 50 r·min⁻¹). As illustrated in Figure 6, once the compound enters the internal mixer, it undergoes significant shear. At a mixing time of 146 seconds and a discharge temperature of 160°C, the filler dispersion index is 5.3, and the ultra‑large particle distribution index is 8.9.
(2) Rotor phase angle 180°

When the rotor phase angle is 180°, the shear action experienced by the rubber compound in the mixing chamber is illustrated in Figure 7. When the long ribs of both rotors are horizontal—as shown in Figure 7(a)—the rotors exert a shearing effect on the compound. After the rotors have rotated through 90°—as depicted in Figure 7(b)—the long ribs of the two rotors become vertical, at which point the shear intensity between them decreases. Introducing the compound into the internal mixer under these conditions can increase the feeding rate. With each revolution of the rotors, the shear force acting on the compound varies as the rotor positions change. Figure 8 presents the mixing power curve for tread compound at a phase angle of 180° (under steady‑state mixing conditions, with a rotor speed of 50 r·min⁻¹). As shown in Figure 8, once the compound enters the mixing chamber, the shear action is relatively weak and the temperature rise is slow, thereby affecting the mixing process. At a mixing time of 178 s and an exit temperature of 160°C, the filler dispersion index is 4.1, and the distribution index for oversized particles is 7.8.
(3) Selection of the rotor phase angle:
Comparing the filler dispersion at rotor phase angles of 90° and 180°, with a discharge temperature of 160°C, it can be observed that when the synchronous‑rotor internal mixer employs a 90° phase angle, the mixing time is shortened and production efficiency is improved. Moreover, this configuration promotes more uniform distribution and blending of the rubber compound, enhancing the homogeneity of the compounded material’s constituent components.
Additionally, adopting a rotor phase angle of 90° helps prevent excessive motor load—and subsequent motor stall—when the two rotors’ long edges come into contact at a phase angle of 180°.

3. Internal Mixer Temperature Control:
Currently, both shear‑type and intermeshing‑type internal mixers are equipped with a three‑zone heating–cooling temperature control system. To achieve high‑efficiency mixing, the mixer’s temperature control should be maintained at a critical operating point. In the case of radial tire compounds dominated by natural rubber (NR), most of the compound tends to adhere to the warm surfaces rather than the cold ones during mixing. Therefore, to optimize the mixing process, the rotor temperature should be kept lower than that of the stationary surfaces (the mixing chamber) so that the compound adheres to the stationary surfaces with minimal slippage, enabling the rotor blades to shear the compound more effectively. When mixing isoprene rubber, raising the cooling water temperature from 10–15°C to 40–50°C reduces the mixing time by 7% and increases production efficiency by 5%, while also improving the quality of the compounded rubber. Tests conducted on a GK520E internal mixer showed that increasing the rotor temperature from 35°C to 40°C and the mixing chamber temperature from 40°C to 46°C—while keeping the discharge temperature constant—shortened the mixing time per batch by an average of 11 seconds and boosted production efficiency by 7.5% (for tread base compound). However, if the rotor temperature exceeds the mixing chamber temperature, the mixing time will lengthen. For instance, when the rotor temperature was set at 50°C and the chamber temperature at 40°C, the mixing time per batch increased by an average of 15 seconds compared with the pre‑adjustment condition, accompanied by uneven filler dispersion and the presence of raw rubber lumps in the compound. This demonstrates that when the rotor temperature is higher than the chamber temperature, the compound adheres to the rotor and, as the rotor turns, fails to bond with the chamber walls, thereby reducing the shearing action on the compound and adversely affecting the dispersion of the raw rubber.

4. Conclusion:
(1) Mixing is carried out using a variable-speed mixing process. Compared with constant-speed mixing, this approach enhances production efficiency, ensures stable compound properties, and delivers significant economic benefits.
(2) When using a synchronous rotor, the rotor phase angle should be set to 90°, which can enhance mixing efficiency and prevent excessive motor load—and subsequent stall—caused by the long lobes of the rotors coming into contact when the phase angle is 180°.
(3) By appropriately adjusting the temperatures of the rotor and the mixing chamber cooling water, it is possible to enhance both the production efficiency of the internal mixer and the quality of the compounded rubber, while also achieving energy savings.