In experimental operation of a small two-roll open mill, the speed ratio of the front roll to the rear roll, the temperature setting, and the roll gap are three process factors related to the mixing quality. These parameters affect the shear strength, dispersion uniformity, roll wrapping behavior, and thermal history of the material, ultimately impacting the performance of the rubber compound or composite material.

Three key parameters of a two-roll open mill: speed ratio, temperature, and roll gap explained in detail.

I. The Role of Speed ​​Ratio

The front and rear rolls of a two-roll mill typically rotate at different speeds, creating a speed ratio. Common ratios include 1:1.1 and 1:1.2. This speed ratio creates a speed gradient in the roll gap region, generating a shear flow field.

*If the speed ratio is too small, close to 1:1: the shear force is weak, making it difficult to break up material agglomerates and resulting in poor filler dispersion.

*If the speed ratio is too large, for example, exceeding 1:1.4: excessive shearing may lead to localized overheating and degradation of heat-sensitive materials.

*Recommended for general use: When compounding natural rubber, a speed ratio of 1:1.1–1:1.2 is commonly used; for highly filled systems, it can be appropriately increased to 1:1.25 to enhance filler dispersion.

The speed ratio also affects the tendency to coat the rolls. Generally, raw rubber is more likely to coat the slower rolls; therefore, the rear roll is often set as the slower roll to facilitate tapping and refining by the operator.

II. Impact of Temperature Setting

The front and rear rollers can be independently temperature-controlled, and the temperature distribution strategy needs to be adjusted according to material characteristics:

• Same temperature setting (front and rear roller temperatures are the same): Suitable for materials with good thermal stability and a wide processing window, such as SBR;
• Front roller temperature higher than rear roller temperature: Helps the melt maintain fluidity during sheeting, reducing surface roughness, commonly used in the processing of thermoplastic elastomers (TPE/TPU);
• Rear roller temperature higher than front roller temperature: Enhances the coating of raw rubber on the rear roller, facilitating powder absorption, often used in the initial mixing stage of natural rubber or EPDM.

Generally, the rubber mixing temperature is controlled at 40–70℃, while thermoplastic materials (such as TPU) may require 80–120℃. Excessive temperature can easily lead to scorching or degradation of the material, while excessively low temperature will reduce the material’s fluidity and make powder absorption difficult.

III. Significance of Roller Gap Adjustment

Roller gap refers to the minimum distance between the surfaces of two rollers, directly determining shear strength and material residence time:

*Small roller gap (e.g., 0.5–1 mm): High shear force, beneficial for filler dispersion, but results in thinner, more easily broken sheets, and higher motor load;

*Large roller gap (e.g., 2–3 mm): Weak shear, suitable for plasticizing or preliminary mixing, produces thicker sheets, facilitating subsequent operations;

*Typical process: First, use a large roller gap for powder mixing, then gradually reduce the roller gap for refining, and finally adjust to the target thickness for sheet production.

Roller gap also affects the amount of accumulated adhesive. Appropriate accumulated adhesive (located above the roller gap) helps form a stable mixing zone, but too much can lead to poor heat dissipation or localized over-mixing.

IV. Parameter Coordination and Material Adaptation

The above three parameters need to be adjusted synergistically. For example:

• When compounding high-filled calcium carbonate/PP systems: Use a higher speed ratio (1:1.25), medium temperature (80℃), and gradually reduce the roll gap;
• When plasticizing natural smoked sheet rubber: Start with a low speed ratio (1:1.1), low temperature (50℃), and a large roll gap;
• When processing TPU: The front roll should be slightly higher than the rear roll (e.g., 90℃/80℃), with a speed ratio of 1:1.15 to avoid roller sticking.

Operators should dynamically optimize parameter combinations by observing visual phenomena such as the roll wrapping condition, the morphology of accumulated rubber, and the surface finish of the sheet, combined with torque or tactile feedback.