Internal Mixers – Overview and Key Characteristics

Versatile Mixing Capability

Internal mixers are highly suitable for mixing a wide range of rubber and plastic compounds, including:

• Natural and synthetic rubbers (e.g., SBR, EPDM, NBR)
• Thermoplastic elastomers (TPE)
• Thermoset materials
• Additives like carbon black, silica, oils, resins, and curatives

Rotor Design & Friction Ratio

• • Internal mixers utilize rotors that rotate in opposite directions, with a defined friction ratio (typically between 1.1:1 and 1.3:1), meaning one rotor rotates slightly faster than the other.
    • This friction ratio helps in shearing and dispersing fillers and additives more efficiently.
    • Higher friction ratios generally lead to greater mixing intensity and better dispersion of ingredients.
• Rotor Types – 2 or 4 Wings
  • Rotors can be designed with 2-wing or 4-wing geometries:
    • 2-wing rotors: Simpler design, suitable for standard applications with moderate mixing requirements.
    • 4-wing rotors: Provide higher shear and better mixing uniformity, especially for more complex or highly filled compounds.
  • Rotor configuration affects:
    • Shear rate
    • Residence time of materials
    • Energy consumption
    • Temperature distribution during mixing
• Tangential vs. Intermeshing Rotors – Fill Factor
  • Tangential rotor mixers (e.g., Banbury-type mixers):
    • Rotors operate with a small gap and do not intermesh.
    • Higher fill factor – can be filled to 70–80% of chamber volume, making them more productive for large batch processing.
    • Suitable for high-viscosity compounds or materials requiring intensive shear and heat.
  • Intermeshing rotor mixers (e.g., Kneader-type mixers):
    • Rotors physically interlock, offering more efficient self-cleaning and gentler mixing.
    • Lower fill factor (~60–70%), more suited for heat-sensitive or low-viscosity materials.