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Torque Limiter Compound Food Processing Twin Screw Extruder

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Factors influencing mixing


a) Chemical nature of the additive. 


The surface chemistry (polarity) of powdered additives is very
important in determining their tendency to form
aggregates and agglomerates and conversely
defines the mechanical forces necessary to break
them down. In addition, the presence of moisture
can increase agglomerate strength. A number of
surface treatments are used to assist the dispersion
of powders, including silanes and stearates. These
can have the added benefit of lubricating the
molten mass and reduce melt temperatures or allow
higher levels of filler to be used.
Other factors, such as the chemical reactivity of the
resin system used can impose temperature and
residence time limits on the compounding process.


b) Premixing. 


All formulation ingredients are
usually premixed for operational simplicity and
lower installation costs. It is important that the
premix remains free-flowing and uniform in
consistency to ensure stability of the mixing
process and in particular, the mechanical energy
input provided by the extruder. Both low speed
ribbon blenders and high speed turbine mixers are
used but the high speed variant is more effective in
contributing to the overall mixing process due to
the increased attrition and impact forces on the dry
blend giving a more uniform particle size.


c) Resin viscosity. 


Higher shear stresses for
agglomerate breakdown are imposed with
increasing resin viscosity according to the formula:
t = g.h
where:
t = shear stress
h = viscosity
g = shear rate


Conversely, filler particles will be wetted more
easily by low viscosity resin melts. Viscosity is
related to temperature and shear rate, and can be
influenced by the interaction between the resin melt
and additive.


d) Shear history in the extruder. 


The shear historyof the material is defined as the combined effect of
the average shear rate imposed by the extruder (a
function of the screw speed) and the residence time
within this shear rate regime (related to screw
speed and output). These two factors determine the
probability of high shear stresses being imposed on
a given particle and hence the mixing efficiency.
The breakdown of agglomerates will only occur
after a certain minimum stress, or yield stress, has
been exceeded. Therefore, lower shear mixers, e.g.
single screw extruders, will be expected to need
higher residence times to achieve a comparable
shear history to the high average shear rate twin
screw extruder. The minimum yield stress for
efficient dispersion may never be achieved in low
shear mixers despite very long residence times if
the average shear rate is too low.

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