Total volume of the internal mixer - the volume of the cavity formed by the mixing chamber, side brackets (or retaining rings), discharge door, and pressure ram, minus the volume of the two rotors in the mixing chamber. It is the main parameter indicating production capacity and is generally adopted according to series standards.

Working volume (rated volume, charging volume) is the total volume of the internal mixer multiplied by the filling factor ψ. The filling factor is selected according to the rotor type, rubber type, process formula, process parameters, and pressure ram pressure, generally ranging from 0.55 to 0.75.

Charging capacity, i.e., mixing capacity. Insufficient capacity will reduce the shearing and kneading effects on the rubber, and may even cause rubber slippage and rotor idling, resulting in poor mixing effects. Conversely, excessive capacity makes it difficult for the rubber to turn over, causing the upper ram to be improperly positioned, causing some of the rubber to be retained at the neck of the charging port, resulting in uneven mixing of the rubber, long mixing time, and easy equipment overload and high energy consumption. Therefore, the mixing capacity should be appropriate, usually 60% to 70% of the total effective volume of the closed chamber.

The charging capacity during mixing in an internal mixer can be calculated using the following empirical formula: Q—charging capacity, Kg; K—filling factor, usually 0.6～0.7; V—total effective volume of the closed chamber, L; ρ—density of the rubber, g/cm3. The selection and determination of the filling factor K should be based on the type of raw rubber and the characteristics of the formula, the characteristics and wear degree of the equipment, and the upper ram pressure. For NR and formulas with high rubber content, K should be appropriately increased; for synthetic rubber and formulas with low rubber content, K should be appropriately decreased; for old equipment with a high degree of wear, K should be increased; for new equipment, it should be smaller; the K of an intermeshing rotor internal mixer should be smaller than that of a shearing rotor internal mixer; as the upper ram pressure increases, K should also be increased accordingly. In addition, the K for reverse mixing should be as large as possible.