Banbury Internal Mixer Product

Overview

A Banbury internal mixer is a high-shear device that disperses chemical additives (accelerators, antioxidants, fillers, colorants) into rubber polymer through extreme mechanical working. The name derives from the original invention patented by Fernley Banbury in 1916. It remains the industry standard for rubber compounding before tire building or extrusion.

The Mixing Chamber is a sealed vessel heated to 120–160 °C. Inside, two counter-rotating figure-eight shaped rotors—the Fast Rotor and Slow Rotor—rotate at different speeds (typically 1.5:1 or 2:1 ratio), creating intense shear and folding action. A Ram Cylinder plunges from above, pressing raw rubber and additives against the rotor tips, preventing material from stalling and forcing repeated working. As the rotors turn, they drag material around the chamber cavity, stretching and folding it thousands of times per minute. The high shear generates heat—controlled by Temperature Control steam and water circuits. When mixing is complete, the Drop Door at the chamber bottom is opened, and the finished compound is discharged onto a mill or into a cooling tank.

Modern rubber compounding requires precise batch temperatures (±5 °C) and chemical ratios. The Control System PLC manages both by:

1. Storing recipes (rubber type, additives, target cure state)
2. Regulating chamber temperature via Steam Control Valve and Coolant Control Valve
3. Monitoring Discharge Sensor to detect end-of-cure (exotherm peak)
4. Adjusting Drive Motor speed via VFD as material viscosity changes

How it works

The operator measures raw rubber (e.g., 250 kg natural rubber) and additives (e.g., 25 kg carbon black, 5 kg curatives) according to a recipe entered into the HMI. Raw rubber is fed into the Mixing Chamber through a top hopper or manual placement. The Door Latch is secured, and the Ram Cylinder—a large hydraulic piston—is lowered to apply 200–500 kN force, sealing the chamber and compressing the material heap.

The Drive Motor, typically sized 40–75 kW, is started via the Variable Frequency Drive. The motor drives the Gear Train (an epicyclic gearbox) which meshes two rotor shafts together: as one shaft rotates at 60 rpm, the other rotates at 40 rpm (or similar ratio). The Fast Rotor and Slow Rotor begin their opposing figure-eight paths within the Rotor Cavity.

The speed differential is key: where the fast rotor moves ahead of slow rotor, material is dragged between them with extreme shear stress. The Ram Cylinder ram force (200–500 kN depending on material) ensures rubber stays packed and does not stall. As the rotors turn, they fold and stretch the rubber continually, breaking down its polymer chains (mechanically) and distributing pigments and chemicals uniformly through the mass.

Heat generation is inevitable in high-shear mixing. The Steam Control Valve maintains the Heating Jacket at setpoint—initially by heating (steam flow), then by cooling (chilled water via Coolant Control Valve) once internal friction raises temperature. The Chamber Temperature Sensor (an embedded RTD) reports wall temperature every 5 seconds. The Discharge Sensor measures the actual compound temperature as it exits. Vulcanization curatives (sulfur, accelerators) undergo a chemical reaction characterized by a temperature rise (exotherm). The PLC senses this exotherm peak and triggers discharge: once temperature reaches the target (typically 160–170 °C), vulcanization has proceeded far enough.

After 3–8 minutes of mixing (depending on rubber type, batch size, and target state), the Drive Motor is powered down and the Drive Brake engages, locking the rotors. The Ram Cylinder is retracted hydraulically. The Door Latch solenoid de-energizes, and an operator manually opens the Drop Door by pulling a lever or pressing a release button. The finished compound, now at 160–170 °C, falls by gravity into a two-roll Two-Roll Mill for cooling and final banding, or into a water-cooled batch dump cart.

The chamber is briefly cooled before the next batch. The Drain Valve may be cracked to release condensate. Raw rubber for the next batch is placed, the cycle repeats.

Mixing Mechanics

The figure-eight rotor geometry is critical. As the two lobes sweep around the chamber cavity:

• High-shear zone: The tip regions where rotor lobes approach each other at highest speed.
• Folding zone: Where the slower rotor leads material against the advancing fast rotor, causing stretching and folding.
• Kneading zone: Regions where the rotor lobe presses material against the chamber wall, exerting sustained pressure.

The rotor speed ratio (1.5:1 or 2:1) ensures that these zones continuously shift around the chamber, so no region of material stalls or receives insufficient working.

Shear rate in a Banbury is on the order of 100–500 s⁻¹ (strain rate), compared to 10–50 s⁻¹ on a two-roll mill. This high shear:

• Breaks down long polymer chains (plastication), making rubber flow-ready for extrusion or molding.
• Disperses carbon black, clay, and other pigments to colloidal fineness (~1 μm).
• Accelerates sulfur vulcanization by heating and mechanically degrading polymer bonds.

The outcome is a homogeneous, uniform-color compound ready for tire building or extrusion within 5 minutes.

Energy and Throughput

Power consumption:

• Drive motor: 40–75 kW continuous during mixing phase
• Ram hydraulic pump: 5–15 kW
• Heating/cooling circuits: 10–20 kW (steam boiler or chiller)
• Controls and fans: 2–5 kW

Total per mixer: 60–110 kW. A 500-ton/day tire plant requires 5–10 Banbury-equivalent mixers running in parallel shifts, consuming ~500–1000 kW continuously.

Batch throughput: A 300 kg batch cycle of 6 minutes yields 3000 kg/hour per mixer. A modern plant compounds 2–5 tonnes/hour to supply downstream tire builders continuously.