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An Analysis of the Working Principles and Structure of Internal Mixers
Release time:
2026-04-13
Source:
Twin-shaft shredders originated in Europe in the 1980s, initially designed to process household and industrial waste.
In the rubber industry’s production chain, internal mixers play a crucial role in uniformly blending raw rubber with various additives. Operating in a sealed environment, this equipment completes the entire process from dispersion to plasticization, laying the foundation for subsequent processing.
The Evolution from Open to Closed-Milling
Compounding methods primarily include open-mill compounding and closed-mill compounding, both of which are batch-type processes. Unlike open mills, which rely on low-temperature mechanical mixing, closed-mills operate under high temperature and pressure, progressing through three stages: wetting, dispersion, and kneading. The process can be conducted using either a single-stage or two-stage compounding method to accommodate different rubber compound formulations. The closed environment of the internal mixer gives it distinct advantages in terms of mixing capacity, efficiency, and automation, while also reducing material loss and dust pollution.
Core Structure and Working Principle
An internal mixer primarily consists of a mixing chamber, rotors, rotor sealing devices, feeding and pressing devices, discharge devices, a drive system, and a base. Its core operating principle relies on the rotational motion of the rotor to generate shearing, compression, and friction within the mixing chamber, thereby achieving uniform mixing and plasticization. The closed-type internal mixer employs a sealed cylindrical vessel with a paddle-type impeller structure, which allows for temperature control of the material and minimizes dust dispersion. Some internal mixers are equipped with alloy steel mixing shafts and jacket-type temperature control systems to enhance wear resistance and process stability.
The Process of Feeding, Mixing, and Discharging
During operation, material is fed through the inlet after the upper plug is raised. The upper plug, driven by pneumatic pressure, pushes the material into the mixing chamber and maintains constant pressure. Inside the chamber, the material is subjected to shear forces generated by the gap between the rotor’s protruding ribs and the chamber wall. Under these shear forces, the compounding agents are worked into the rubber compound, achieving axial mixing. Once mixing is complete, the lower platen opens, and the material is discharged through the discharge port. The internal mixer continuously mixes, plasticizes, and vulcanizes the material through the rotating mixing chamber and rotor. The main types are divided into two categories: batch-type and continuous-type.
Precise Control of Process Parameters
During the mixing process, parameters such as loading capacity, platen pressure, mixing temperature, and discharge temperature directly affect the uniformity of the compound. The typical feeding sequence is: raw rubber → stearic acid → small additives → carbon black → oil-based plasticizers; sulfur must be added during the low-temperature stage of the calendering process. In recent years, green technologies such as low-temperature single-pass continuous mixing, wet mixing, and twin-screw continuous mixing have also been continuously advancing. Through the continuous interaction between the rotor and the material within a sealed chamber, the internal mixer enhances production efficiency while ensuring product quality stability.
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