Page 128 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

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Encyclopedia of Physical Science and Technology EN012c-593 July 26, 2001 15:56

634 Polymer Processing

of 300 sec and is related to self-diffusion of polymer lar, or rectangular with an aspect ratio, W/H, greater than
molecules. 10.0. At present three types of profile dies are used: orifice
One way of reducing the effect of the flow lines is dies, multistage dies, and tapered profile dies. Basically
through the design of the support system. Instead of ar- the orifice die consists of a die base and a die plate in which
ranging the spider legs radially, tangential arrangement the profile is formed. These dies are used for the extrusion
as shown will displace the defect circumferentially over of inexpensive profiles where dimensional accuracy is not
the extrudate. A better way for reducing the flow marks necessary. Because of the abrupt change in cross-sectional
is the use of offset spider legs. Here the flow marks do area, there is usually a buildup of stagnant material be-
not extend all the ways through the wall of the extrudate, hind the die plate and high extrusion rates are not possible.
which offers at least mechanical improvements. Finally, These dies are not commonly used for most thermoplastics
another way to reduce flow marks is through the use of a but are restricted primarily to PVC and rubber. Multistage
screen plate. In this design the melt passes into the annu- dies exhibit step changes in the cross-sectional area of the
lar region by first passing through a plate with many small flow channel. They consist of a series of die plates of simi-
holesboredinit.Ineffect,theannulardieisfedbymultiple lar geometry but of a decreasing cross-sectional area. Cer-
capillaries. tainlytheserepresentanimprovementovertheorificedies,
The side-fed mandrel die is used in both blown film but they still suffer from some of the same deficiencies
and pipe extrusion. The main consideration is to provide a as the orifice die. Whenever profiles of high-dimensional
uniform flow rate at the die land. This is done in much the accuracy are to be produced at high extrusion rates, pro-
same way as for flat film extrusion by the use of a mani- file dies with a gradual change of cross-sectional area are
fold. The spiral mandrel die seems to offer the best possi- required.
bility for providing a uniform flow rate circumferentially
at the annular die exit. The die is usually fed from the ex-
5. Multiple-Layer Extrusion (Coextrusion)
truder by means of either a star-shaped or ring-shaped dis-
tributing system. The melt then passes into spiral-shaped It is becoming more common to combine multiple layers
channels that are machined into the mandrel. The depth of different polymers to form products with properties that
of the channels decreases with spiral distance, which en- take advantage of the best properties of each component.
sures that there will be mixing of the melt from channel Forexample,packagingfilmmightbecomposedofseveral
to channel as the result of leakage. Because of the design different types of polyethylene along with a layer of adhe-
of this system there are no mandrel support elements, and sive and a barrier polymer. There are basically three types
hence flow lines are eliminated completely. of multiple-layer extrusion (or coextrusion) techniques:

1. Melt streams flow separately.
3. Wire-Coating Dies
2. Melt streams flow separately and then together.
Wire-coating dies also involve annular cross sections. The 3. Melt streams flow together.
meltusuallyentersfromthesideandsoresemblestheside-
fed annular dies. The goal in the design of wire coating In type 1, polymer streams are extruded through sepa-
dies is to provide a coating that is of the desired thick- rate flow channels and then joined outside the die. The
ness, free of imperfections, and with the wire centered advantage in this type of multilayer extrusion is that poly-
in the insulation. Again the even distribution of the melt mers with widely different processing temperatures and
from the die entry is the key design element. Two basic rheological properties can be used. The major problem
types of coating dies are used at present: pressure coat- is that of generating satisfactory adhesion between the
ing and tube coating dies (Fig. 29). In the pressure coat- components. Usually the technique is only used for two
ing die the wire is coated under pressure in the die. This polymers. In the second technique the streams are brought
technique is usually used for the application of the pri- together inside the die, and then they pass through a com-
mary coating where good adhesion is important. In the mon land region. Because the streams are brought to-
case of the tube coating die, the polymer coating is ap- gether under pressure, adhesion is improved. However,
plied outside the die and is used for applying a secondary it is not possible to have the streams at widely dif-
coating. ferent temperatures. Likewise the rheological properties
cannot be too widely different or flow instabilities will
arise. Furthermore, at the point where the streams con-
4. Profile Extrusion Dies
verge, interfacial instability problems may arise. In the
Profile extrusion refers to the extrusion of polymer melts third method, which is not too dissimilar from the sec-
through dies of cross sections that are neither round, annu- ond method, the polymer streams are brought together