Banbury – type or internal mixers.

Banbury - type mixer is the most common internal batch mixer.
The initial distributive stage of the mixer gives the “masterbatch”. There is high
temperature and pressure inside the giant blender; these conditions convert the raw
materials into the hot, black, sticky “masterbatch”. Named so as the mixing operation is
typically a batch operation in which around 200 kilograms of formulated compound can
be mixed in 3-5 minutes.


In the second dispersive stage of mixing the “curative” package (containing Sulphur) is
added in this stage the temperature should not rise above 100 – 110 degrees centigrade (~
150 degrees is the vulcanizing temperature). Another mixing operation sequence may
introduce additives into the raw rubber in the mixing chamber from the feed hoppers via
pipes and valves at predetermined times. The sequence in which the different ingredients
are added is very critical as is the mixing temperature, which can rise as high as 160 –
170 degrees centigrade. If the temperature is too high the compound may get damaged
due to “scorching”.

In both the stages of mixing, the “mass” is intensively ”kneaded” between the rotors. To
ensure thorough mixing, the rotors are assisted by a ram pressing on the material from the
top. The ram pressure can be varied as well as it can be repeatedly lifted and reapplied during the mixing process.

The ram is lifted after the mixing time has elapsed and a hydraulically actuated trap –
door is opened at the bottom of the mixing chamber. The mixed material goes for further
mixing in a rolling mill and the mixer is refilled as soon as the temperature returns to the
initial value. Thus the mixing is continuous, only the charging and discharging are
discontinuous.

Typical batch sizes vary between 10 – 650 liters. The mean rotor speed is around 50
rev/min.

The rotors, side doors and mixing chamber sides are provided with
heating/coolingchannels. These channels are provided with heating as well as cooling
channels. The heating channels carry hot water to heat the apparatus to ensure uniform
initial conditions for each batch. Cooling is provided during the mixing process to
dissipate the excess heat which if not removed may result in scorching of the “mass”.


From the invention of the Banbury by Fernley Banbury 75 years ago, the internal
batch mixer has undergone notable changes in terms of design and usage of technology.