The difference between mixing in an internal mixer and an open mixing mill

I. Comparison Between Open Mill Mixing and Internal Mixing: Core Differences, Advantages and Disadvantages

Operation Mode:

- Open mill: Adopts an open and continuous processing method, where materials are crushed and mixed in the gap between two relatively rotating rolls.

- Internal mixer: Adopts a closed and batch processing method, completing material mixing through the strong shearing and kneading action of rotors inside the chamber.

Mixing Efficiency and Quality:

- The internal mixer achieves better and more uniform mixing results due to its strong shearing force, longer effective action time, and closed environment. Its airtightness effectively prevents dust emission, reduces raw material loss, and significantly improves the working environment.

- The open mill has relatively low mixing intensity with slightly inferior results; its open operation leads to poor environmental control.

Production Capacity and Applicability:

- The open mill is more adept at handling large batches of materials and can directly calender them into thin sheets, facilitating subsequent processes.

- The internal mixer is more suitable for small-batch, high-efficiency mixing tasks, especially for formulations requiring high dispersion of fillers.

Temperature Control:

- The internal mixer generates a large amount of frictional heat during operation, causing the material temperature to rise sharply. Therefore, an effective cooling system must be equipped to prevent thermal decomposition of materials.

- The open mill has relatively mild heat generation, but the roll temperature is crucial—it directly affects whether the material can uniformly wrap the rolls and the dispersion effect of additives.

II. Key Operation Points for Internal Mixers

- Loading capacity: The feeding amount must be strictly controlled within an appropriate range. Either excessive or insufficient loading will affect the mixing pressure, failing to achieve the ideal mixing effect.

- Rotor speed: The speed setting must be reasonable. Excessively high speed will cause overheating and potential material degradation; excessively low speed results in insufficient shearing force, affecting mixing quality.

- Temperature management: The temperature of the mixing chamber must always be controlled below the material vulcanization temperature to prevent premature vulcanization (scorching). Simultaneously, cooling the rotors is critical.

- Safety regulations: During operation, strictly guard against scalds from high-temperature components and crushing risks from moving parts. Always comply with safety procedures.

III. Key Operation Points for Open Mills

- Roll gap adjustment: Ensure the gap (roll gap) between the two rolls is uniform and parallel to avoid direct contact and friction between rolls. Proper roll gap adjustment is the foundation for ensuring mixing effect.

- Safe operation: Maintain a safe distance when the equipment is running. Never allow any part of the body (e.g., limbs) or loose clothing (e.g., hair) to approach the rotating rolls to prevent entanglement and serious accidents.

IV. Reasons for Yellowish-Brown Discoloration of Rubber After Internal Mixing

Carbon black is often added as a reinforcing filler during rubber internal mixing. Residual carbon black that is difficult to completely remove from the mixing chamber and rotor interior will contaminate the new materials during subsequent mixing, causing the originally light-colored rubber compound to appear yellowish-brown.

V. Key Process Parameters Affecting Internal Mixing Effect

- Rotor design: The rotor structure type (e.g., shear-type or intermeshing-type) directly determines production efficiency and mixing quality—shear-type rotors offer high efficiency, while intermeshing-type rotors provide better dispersion.

- Rotational speed: Increasing the speed can shorten the mixing cycle, but a balance must be struck between the resulting temperature rise and shearing effect.

- Temperature: Mixing temperature directly affects material fluidity and viscosity; precise control is required to prevent material overheating, aging, or agglomeration.

- Time: Mixing time is related to filler dispersion and production efficiency. Insufficient time leads to poor dispersion, while excessive time may cause over-mixing.

- Addition order of additives: The feeding sequence of various additives (e.g., oil, small ingredients, fillers) is crucial for mixing effect. A reasonable sequence can significantly shorten time and improve mixing quality.

VI. Key Process Parameters Affecting Open Mill Mixing Effect

- Roll temperature: Roll temperature is a core parameter that determines material viscosity and roll wrapping behavior. It should be adjusted according to specific material properties.

- Feeding method: Uniformly spreading additives (e.g., powder) on the surface of the material on the rolls helps shorten the integration time, thereby improving overall efficiency.

- Addition order of additives: Similar to internal mixing, the feeding sequence affects the final quality of the mixed rubber and operational safety (e.g., preventing scorching), and must follow a scientific order.

- Roll wrapping property: Whether the material can stably wrap the rolls depends on its own properties, roll temperature, and shear rate. These factors need to be adjusted to achieve the optimal roll wrapping state.