What Are Internal Mixers And Open Mill Mixers? What Are The Differences Between Them?

1. Open Mill Mixer

An open mill mixer, also known as an open-type rubber mixing machine, is widely used in rubber factories to prepare masticated rubber, compound rubber, or for thermal refining and sheet forming. The machine features exposed rollers.

The core working parts are two hollow or bored rollers rotating inward in opposite directions. The roller positioned in front of the operator is called the front roller and can be manually or electrically adjusted to move horizontally, allowing for the control of the gap between rollers based on operational needs. The rear roller is fixed in position. Both rollers are usually the same size but rotate at different speeds. As the rollers turn, the raw rubber is fed into the gap between them, where it undergoes strong shearing forces for mastication or mixing. Open mills are also used in plastic processing industries.

2. Internal Mixer

An internal mixer (or kneader) is a closed-type mixing machine primarily used for the mastication and compounding of rubber. It features a pair of specially designed rotors that rotate in opposite directions inside a closed chamber. The machine works under adjustable temperature and pressure, processing polymers intermittently.

An internal mixer is composed of the mixing chamber, rotors, rotor seals, feeding and pressing mechanisms, discharge systems, drive systems, and the base. Developed from the open mill, the internal mixer became widely known after the invention of the Banbury mixer in 1916. It brought numerous advantages to the rubber mixing process, including:

• Larger mixing capacity

• Shorter processing time

• Higher production efficiency

• Reduced dust and material loss

• Improved product quality and working environment

• Enhanced safety and reduced labor intensity

• Easier automation and mechanization

Therefore, the internal mixer is considered a major milestone in rubber machinery development and continues to evolve as a key equipment in mastication and compounding processes.

3. Differences Between Open Mill and Internal Mixer

• Structure:
  The open mill generates shearing force using two rollers rotating at different speeds. It has a small mixing volume and low mixing efficiency. In contrast, the internal mixer uses intermeshing rotors to create systematic spatial compression, producing stronger shearing force. It is more efficient, requires less manual operation, and has a larger mixing chamber.

• Cost:
  Open mills have simpler structures and lower costs. Internal mixers are more complex and expensive, ranging from tens of thousands of RMB for domestic models to over a million RMB for imported Japanese machines.

• Labor Intensity:
  Open mills have low automation levels and require dedicated operators, resulting in high labor intensity. Internal mixers are more mechanized and only need minimal human intervention, such as feeding and discharging. They can also integrate with automation systems.

• Environmental Impact:
  Open mills tend to generate dust and cause material waste, negatively impacting the environment and worker health. Internal mixers mitigate these issues and support a cleaner production environment.

In summary, internal mixers offer numerous advantages over open mills. They represent an advanced evolution of rubber mixing technology and continue to play a vital role in the industry. Some variants, like the vacuum internal mixer, are equipped with vacuum systems to mix materials that are reactive to air under high temperatures—further showcasing the specialization and development of internal mixer technology.

Although internal mixers remain relatively expensive, ongoing advancements in manufacturing and increasing demands for quality and efficiency will likely make them more affordable. They are expected to gradually replace older open mills and become the mainstream mixing equipment in the industry.

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