What are the different types of mixing machines?

The mixing machines are perhaps the heaviest, dirtiest and most energy gobbling



entities in the rubber industry.


There are three types of mixing machines.


1. Rolling mills or external mixers.


2. Continuous mixers.


3. Banbury – type or internal mixers.


Except for the second type, the other two types can either be continuous or discontinuous


type.


1. Rolling mill mixers.

The rolling mill is furnished with two rolls. The rolls have internal water-cooling. Both


rolls have slightly different peripheral speeds giving a more efficient mixing.



The ratio of the peripheral speeds is usually 1:1.2. Typical roll dimensions are l = 1.5 m,


Φ = 0.5 m. The power rating is around 200 kW with a roll speed of 20 rpm. Typical


mixing cycle takes 30 min. and batch sizes are 60 – 70 kg.



At mixing, the mix is following one of the rolls and the additives are added in between


the two rolls. During mixing the roll gap is slightly opened. After mixing, the rubber strip


is cut in slices and fed in between the rolls, a number of times. This is to ensure thorough


mixing.




An important difference between rolling mills and batch mixers is that the former gives a


“good” (quality) mix while the latter gives a “fast” (productivity) mix. Furthermore,


technological advancement opportunities in the rolling mill mixing procedure are nil


compared to the internal batch mixers. Automatization is also a hurdle in rolling mill


mixers.


2. Continuous mixers.


The mixing action in a continuous mixer is similar to that in a Banbury – type mixer. The


raw materials are fed into the mixer from hoppers. The rotors or “mixing screws” in


continuous mixers can be divided into three parts – screw conveyor, mixer and pumper.


The conveyer part of the rotor propels the ingredients to the mixing part of the rotor. The


overall forward movement of the material is actually obtained by the particular geometry


of the rotors and the constant change in material viscosity as its temperature increases


along the rotor (maximum where the mixing takes place).