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The
planning of the process part of MARIS co-rotating twin-screw extruders
concerns a group of functional modules composed of barrels and, at their
inside, of transport and kneading elements at variable geometry, which
are mounted on shafts for the motion/shear transmission.
The concept of the functional modules originated from the necessity
of achieving a perfect correspondence between the extruder hardware
component and the process phases that can be identified with a whatever
compounding or reactive extrusion.
The equipment capacity to meet with the process requirements can be
found, first of all, in the technicians ability to precisely establish
the physical or physico-chemical phenomena occuring inside the machine,
both in qualitative and in quantitative terms.
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The
functional modules composing the compounding extruder can be detailed
in the following way:
Main vertical feeding:
it is composed of a barrel with a hole on the top and
a hopper. In this area are generally mounted screw elements at high
transport capacity.
Polymer/s transport, heating and melting:
one more
or more barrels are foreseen. At their inside, there are transport
elements of proper geometry and length apt to generate a rapid thermic
increase through the shear stress and, in this way, to melt or soften
the polymer. The entity and the time of shear application depend on
the polymer thermic and rheological features.
Homogenizing: this phase generally
follows the melting and requires the presence of elements for the
distributive mixing of the viscous floods.
Additional feedings : they are
composed of a barrel having a presetting for the vertical or side
introduction of additives and resins and/or an injection point for
liquids.
The design of the internal transport, homogenizing and melting elements
depends on the nature of the materials to be fed.
These additional feedings can be more than one and different among
them.
Degassing: it consists
of one or more barrels having a proper venting port and a screw profile
suitable to the formation of pneumatic sealing chambers, which can
have a normal or a reduced pressure (up to 10 torr in the latter case).
In this area the volatile fractions, the humidity and their air, being
present in the compound, are removed.
Pumping:
this phase consists of a continuous barrels and of transport and compression
elements apt to eject the material from the die-head.
Finally, as far as the screw geometry is concerned, it is a most important
process factor. As a metter of fact, self-cleaning, positive or negative
transport capacity, material shear stress and relevant application
time of this same are strongly affected by this factor.
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