Problems associated with internal mixers

The fill factor most suitable for the mixing of rubber is about 70-75%, which was determined through experience. In order for the mixer s contents to move uniformly, a 

In addition to the non-uniformity within a batch, there is a batch-to-batch reproducibility problem. In general the extent of non-reproducibility depends upon the configuration of the mixer, the size of the machine, operating conditions such as the temperature of the machine, and the viscoelastic nature of the rubber. Therefore, the two most important problems of the mixing operation are the process control and scale-up. 

As a solution to the reproducibility problem, energy rather than time-based control was proposed [2-6]. For examining the reproducibility, the dynamic stress relaxometer was used [4]. This idea is called the unit work concept or energy-based control. A power integrator is used for the control. The energy-based control is also reported to be useful 

Up until the mid 1970s, investigation on the operation of internal mixers relied on instruments for monitoring temperature, torque and energy input. Samples were removed at certain intervals to examine the state of mixing [10, 11]. With these methods, the movement of the contents of the mixer was not observed. 

Direct observation of mixing, so-called flow visualisation was reported by Freakley and Wan Idris [12] he adopted a transparent plastic chamber for a miniature mixer, the Brabender Plastograph, and observed the dynamics of mixing in detail using silicone rubber. Realising that silicone rubber is soft and has a rather unique viscoelasticity, he also examined an oil-extended natural rubber (NR) and an oil-extended, carbon blackfilled styrene-butadiene rubber (SBR). However, these were mixed in a metal chamber and by monitoring pressure at one location therefore, they were not visualised. 

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