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 litres. The mean rotor

speed is around 50rev/min. The rotors, side doors and mixing chamber sides

are provided with heating/cooling channels. 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.