Page 41 - Plastics News February 2020

FeAtures

primarily by the screw speed and head pressure, and noted—will save a lot of time and scrap. Even if designed
generally to a lesser extent by the barrel temperature. for the same polymer, the screws are engineered for a
However, the practice of adiabatic extrusion almost always certain melt temperature at a specific speed, which is
helps stabilize the whole process. Adiabatic extrusion usually at or near their capacity. Therefore, if you run
is seeking temperature settings where there is minimal one screw at one-third speed, another at two-thirds
thermal cycling of the barrel temperatures—i.e. no heat, speed and the third at full speed, they will produce
no cooling. substantially different melt temperatures. Since polymers
are poor conductors of heat and a good coextrusion block
The shear-thinning effect is designed to prevent intermixing of the layers, the layers
The shear-thinning effect is different for every polymer will maintain their temperature through the coextrusion
and grade. The individual curves are also offset up block and will thermally homogenize only in the die itself.
or down as the temperature decreases or increases, It can take considerable time for the layers to stabilize
respectively. How the shear rate and temperatures at a single temperature. For example, in the case of the
affect polymer viscosity in the tooling can result in thin HDPE cast film coex line cited above, it was necessary
considerable variations in pressure drop through the die. to run all three extruders to prevent degradation in the
Polymer viscosity mismatch leads primarily to distortion coextrusion block of resin from any extruders not being
of the layers. However, excessive shear in any of the operated.
layers can lead to melt fracture, usually corrected by
raising the melt temperature—but that results in having Advantages Of Co-extrusion:
to raise the temperature of each layer. Non-uniformity ● High quality mono-layer extrusion coatings in larger
of melt temperature causes zig-zag patterns or wave varieties of line speeds and widths
● Use of lower cost materials for filling purpose, assists
in saving on the amount of qualitative resins
● Capability of making multi-layer as well as multi-
functional structures that too in a single pass
● Reduction in the number of steps required in general
extrusion process
● Provides targeted performance with the use of
definite polymers in particular layers
● Reduction in setup and trim scrap
● Potential for use of a recycle layer
Disadvantages Of Co-extrusion:
● Minor differences in physical properties are
responsible for making a combination desirable, but
effects in the sheet, so the most stable system possible these differences are also responsible for making the
is needed. Die swell varies between polymers and is a combination incompatible
function of temperature and shear rate for a particular
polymer and may require changes in the layer thickness ● For this process, polymers must have similar melt
to meet profitable operating levels. These are meant as viscosities to sustain a laminar flow. All the viscosity
words of caution to those not experienced in coextrusion differences may be more or less tolerable, according
processing and do not apply in monolayer extrusions to the material location inside the composite
unless supplemental adapters and dies are also part of the structure along with the layer's thinness
equipment. With additional adapters and dies you can shut ● Requires more sophisticated extruder and its
off and disconnect any extruders not required, as well as operator. This implies extra maintenance cost of the
change or eliminate the feedblock or multi-manifold die. equipment.
Additionally, proper training of the operating personnel ● Demands considerable planning as well as forethought
on how to deal with the issues noted—as well as some not in the system design

41 Februar y 2020 Plastics News