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Rubber Mixing Process: How to Choose Between Open Mill and Internal Mixer
2026/08/04
Rubber Mixing Process: How to Choose Between Open Mill and Internal Mixer
1. Basic Definition
- Internal Mixer (Banbury Mixer) It is a closed pressurized mixing equipment. Inside the sealed mixing chamber, the rotor generates shear force, and the ram applies pressure to intensively blend rubber stock with compounding ingredients.
- Two-Roll Open Mill It features an open twin-roll structure. Shear force is formed through the roll gap. Rubber is repeatedly milled for mixing under atmospheric conditions without a closed chamber.
2. Comparison of Core Advantages & Disadvantages
表格
| Comparison Item | Internal Mixer | Two-Roll Open Mill |
|---|---|---|
| Mixing Efficiency | High. Supports mass batch or semi-continuous production with large single-batch capacity. | Low. Intermittent manual operation limits production output. |
| Shearing Strength | Powerful mixing under pressure, delivers outstanding dispersion of powdery fillers. | Mild shear force. Poor dispersion performance for fine powders such as carbon black. |
| Temperature Control | Poor heat dissipation in closed chamber. Rubber heats up quickly, with high risk of scorch; mixing duration must be strictly controlled. | Open structure facilitates heat dissipation. Temperature is easier to stabilize, lowering scorch risk. |
| Dust Emission | Closed chamber minimizes powder flying, improves workshop environmental conditions. | Open feeding causes obvious dust pollution, bringing higher environmental compliance pressure. |
| Formula Adaptability | Ideal for formulas with high loading of carbon black and fillers that require sufficient dispersion; suitable for mass production. | Suitable for lab sampling, color matching, supplementary sheeting, re-milling; works well for heat-sensitive rubber prone to scorching. |
| Operation & Investment | High automation, parameterized stable control; higher equipment investment cost. | Simple operation with lower equipment cost; mixing quality heavily relies on operator experience. |
| Subsequent Processes | Discharged compound normally requires an open mill for sheeting, cooling and supplementary refining. | Can complete mixing independently, mainly used for lab sample preparation and auxiliary working procedures. |
3. Selection Guide for Practical Production
✅ Choose an Internal Mixer if:
- You operate mass production lines for tires, rubber hoses, seals and other bulk rubber products.
- The formula contains high content of carbon black, calcium carbonate and other fillers requiring uniform dispersion.
- Stable rubber quality is required to reduce deviation caused by manual operation.
- Dust emission control and environmental standards are strictly required in the factory.
Recommended configuration: Main internal mixer + sheeting open mill + cooling conveyor line
✅ Choose a Two-Roll Open Mill if:
- Conducting laboratory R&D, formula testing and small-batch trial production (kilogram-level samples).
- Processing heat-sensitive rubber compounds that are easy to scorch, such as certain CR and EVM formulations.
- Performing color mixing and color matching to visually check the uniformity of rubber color.
- Executing re-milling, hot milling, thin passing and pre-mixing auxiliary operations.
- Running multi-variety, small-batch production with frequent formula changeovers.
4. Widely Adopted Combined Process in the Industry
Rough mixing via internal mixer → Final mixing & sheeting on open mill
- The internal mixer completes intensive dispersion of raw rubber, carbon black and fillers in rough mixing stage.
- Discharged stock is transferred to the open mill, where vulcanizing agents and accelerators are added and blended at low temperature via thin passing.
Critical Note: Vulcanization systems cannot be added inside the internal mixer under high temperature, otherwise premature scorch will occur. This process route is adopted by most rubber manufacturers.
5. Common Misconceptions
- Misconception: An internal mixer can fully replace open mills → Incorrect. Operators cannot observe rubber status intuitively inside the mixer. High-temperature conditions forbid adding vulcanizing agents, so open mills are indispensable for final mixing.
- Misconception: Mixing effect of an open mill equals that of an internal mixer → Incorrect. Under the same mixing time, open mills achieve far poorer filler dispersion. Mass-produced products may suffer inconsistent physical properties.
- Misconception: All rubber compounds can be processed by internal mixers → Incorrect. Heat-sensitive and low-viscosity soft rubber may degrade or scorch after long-time closed high-temperature mixing, making open mills the better option.
