Rubber Processing: Banbury Mixing, Calendering Kinetics & Die Swell Extrusion

Processing uncured rubber is a complex challenge because rubber is a highly viscoelastic material. It displays strong non-Newtonian flow, elastic recovery, and temperature-sensitive crosslinking (scorch risk). In manufacturing operations, from tyres to hoses and conveyor belts, process parameters must be tightly controlled during mixing, calendering, and extrusion. Major Indian manufacturers such as MRF, JK Tyre, and Pix Transmissions optimize these operations to minimize energy consumption and eliminate processing defects like scorching or sheet blistering.

• Analyze the shear flow and velocity profile of rubber melt in the nip of a two-roll calender.
• Explain the difference between dispersive and distributive mixing in internal Banbury mixers.
• Calculate sheet thickness variations during calendering using roll separation force factors.
• Solve die swell ratios in rubber extrusion using viscoelastic recovery equations.
• Reference rubber processing standards such as ASTM D1646 (Mooney viscosity) and ISO 2393.

3.1 Banbury Mixing Mechanics

Rubber compounding occurs in internal mixers (Banbury) where shear is applied to break down rubber bundles and disperse additives:

• Dispersive Mixing: High-shear zone near the rotor tips breaks solid agglomerates (e.g., carbon black aggregates) into primary particles.
• Distributive Mixing: Low-shear zones distribute particles uniformly throughout the rubber matrix. The process must remain below the scorch temperature (typically < 120°C for final batches) to prevent premature vulcanisation.

3.2 Calendering Kinetics & Flow Profile

Calendering presses rubber compound into sheets or embeds it into textile/steel cords. The flow between two counter-rotating rolls is governed by lubrication theory:

Where:

• $h_0$: Minimum gap (nip clearance)
• $R_g$: Roll radius
• $\theta_{max}$: Entrance angle where the rubber enters the roll nip

Bank Control: Excess rubber forms a rotating "bank" at the roll entrance. If too large, it heats up and scorches; if too small, it causes air entrapment and voids in the sheet.

3.3 Extrusion & Die Swell (Viscoelastic Recovery)

Uncured rubber exhibits significant elastic memory. When extruded through a die, normal stress differences ($N_1$) relax, causing the extrudate to expand in thickness and contract in length:

Where:

• $B$: Die swell ratio
• $D_e$: Extrudate diameter
• $D_d$: Die diameter
• $S_R$: Recoverable shear strain at the die wall

Viscosity is measured using the Mooney Viscometer per ASTM D1646 (written as e.g., ML(1+4) at 100°C).

Problem: An EPDM rubber compound is extruded through a circular die of diameter $D_d = 8.0$ mm. The recoverable shear strain of the compound at the processing shear rate is $S_R = 0.64$. Calculate:

1. The die swell ratio $B$.
2. The expected diameter of the uncured extrudate ($D_e$).

Solution:

1. Compute the die swell ratio using the empirical viscoelastic equation:

1. Compute the extrudate diameter $D_e$:

Interpretation: The rubber sheet/profile will expand by 32% in diameter upon exiting the die. Tool makers must configure the die profile smaller (approx. 7.58 mm) to achieve a target profile of 10.0 mm after elastic relaxation.

Pix Transmissions (Nagpur) produces industrial V-belts and hoses. They use multi-roll calendering lines to coat polyester cords with polychloroprene (Neoprene) compounds. Controlling roll temperature and nip pressure is critical to prevent air bubbles and fabric misalignment.

Indian manufacturers test compound processability using Mooney viscometers conforming to IS 3660 guidelines to ensure consistency before launching extrusion runs.

• Mooney Viscosity: Torque measurement of raw rubber flow resistance; standard unit is Mooney units.
• Die Swell: Viscoelastic diameter recovery when leaving a die; driven by normal stress difference.
• Calender Bank: Rotating pool of excess rubber compound at the calender roll nip entrance.
• Dispersive mixing: High shear breaking agglomerates; crucial for carbon black incorporation.
• Scorch: Premature vulcanisation occurring during compounding or processing steps.

1. ASTM D1646 — Standard Test Methods for Rubber — Viscosity, Stress Relaxation, and Pre-Vulcanization Characteristics (Mooney Viscometer)
2. ISO 2393 — Rubber test mixes — Preparation, mixing and vulcanization — Equipment and procedures
3. IS 3660 — Methods of test for natural and synthetic rubbers (BIS)
4. ASTM D3182 — Standard Practice for Rubber — Materials, Equipment, and Procedures for Mixing Standard Compounds

1. Draw a schematic of the velocity and pressure profiles of a rubber compound passing through a two-roll calender nip.
2. Explain the molecular mechanism behind die swell. How does increasing shear rate or molecular weight distribution affect it?
3. Describe the operation of a Mooney viscometer. What do the terms "ML", "1", "4", and "100°C" represent in a reading of 50 ML(1+4) @ 100°C?

Q1. Mooney viscosity of rubber is measured at what standard rotor speed?

• B) 2 RPM

Q2. Which type of mixing is responsible for breaking carbon black agglomerates in a Banbury mixer?

• A) Dispersive mixing

Q3. The swell of rubber extrudates upon exiting the die is primarily due to the relaxation of:

• C) Normal stress differences (elastic recovery)

Q4. To prevent scorching during calendering operations, the processing temperature should be kept:

• C) Below the onset of vulcanisation curatives activation

Q5. In the Mooney reading "ML(1+4) @ 100°C", the number 4 stands for:

• C) 4 minutes of rotor run time before reading the torque value