MIXING TECHNOLOGY - BASIC METHODS

1.. Early oil addition method ::

involves addition of elastomer first and the dry ingredients at the
earliest. After 0.5 –1.0 minutes all liquids are added together. In this
method dispersion is improved if the addition of liquids are delayed
slightly, however this will extend the mixing cycles. This method is used
for compounds containing large volumes of liquid plasticizers.

2. Upside-down mix method::

Is the fastest and simplest way of mixing. This is useful for
compounds containing large volumes of liquid plasticizers and and
large particle size fillers.This method involves adding all ingredients
into the mixer before lowering the ram and commencement of mixing.
All dry ingredients are added to the mixer first, then the liquids, and finally the elastomer on the top.
MIXING TECHNOLOGY - DISCHARGE CRITERIA

Time and temperaturehave generally been the criteria for
determining the termination of mixing process, the primary aim being to
guarantee the quality of the end product, avoid over mixing and to reduce
variation between batches.
A more precise and reproducible control of mixing cycle can be
obtained by following the energy input at various stages in the cycle.
The latest development in the control of mixing parameters has been
the computerized process control where the control on the reproducibility of
the mixes are achieved by controlling the weighing,, temperature, charging of
raw materials and the mixing cycle itself
( time, temperature, power, ram, rotors, hopper and drop door) .

MIXING TECHNOLOGY -OPERATING VARIABLES

Rotor Speed ::

Totally affects the total shear strain or deformation
rate and thus the speed of mixing, which, in practice, is limited by the
maximum allowable temperature.

Ram Pressure ::
The main function of the ram is to keep ingredients in
the mixing chamber. As ram pressure is increased, voids within the mix are
reduced, and slippage between rubber and rotor surface is reduced. This is
because as pressure increases the contact force between the rubber and the
rotor surface increases, and flow begins at a lower temperature

Chamber loading ::

Also must be optimised. At the lower limits
sufficient material must be present for the ram pressure effect. If the
mixer chamber is overloaded, much longer mixing times are needed
to achieve good dispersion. The optimum batch weight for a
particular mix depends on the type and level of rubber, filler and
plasticiser.


FILL FACTOR

FILL FACTOR=BATCH VOLUME/BANBURY VOLUME
TYPICAL FOR NR COMPOUNDS - 0.75
General Mixing Practices:

Mixing Sequence for master/ Repass/ Final :

Mixing Rpm: 40 Min
60 Max

Fixed RPM followed for majority of compounds.

Sequence of Mixing:

• Load Rubber + Chemical + Carbon Black = 0.0 sec.

• Add oil

• Ram Up / Down.
• Discharge.