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|Title: ||Solid state extrusion of polymers through convergent-divergent dies|
|Authors: ||Zhao, Jianhong|
|Issue Date: ||1991|
|Publisher: ||© J. Zhao|
|Abstract: ||Compared with metals, polymer materials have Iow strength and stiffness. However,
molecular orientation can enhance many mechanical properties of polymer materials in
the direction of orientation. Studies on solid state polymer extrusion (Le. extrusion
carried out at temperatures below the melting point of the polymer) through convergent
dies show that it is possible to produce extrudates exhibiting a high degree of
monoaxial orientation in the extrusion direction. Although the strength of these
extrudates has been greatly enhanced in the orientation direction, the strength decreases
in the transverse direction to the axial orientation.
Biaxially oriented polymer materials, on the other hand, show increased mechanical
properties in more than one direction. But so far, extrusion processes that confer
orientation in more than one direction have not received much attention.
The present work is concerned with the development of biaxial orientation in thick
thermoplastics extrudates by extrusion through dies exhibiting simultaneously
converging and diverging walls perpendicular to each other and with a cross-section
area at the die entry being the same as at the exit. Four die designs are examined,
known respectively as the dual-taper die, the expansion fish-tail die, the constant
fish-tail die and the cross die, using polytetrafluoroethylene and ultra-high molecular
Measurements of birefringence and tensile strength on sections of the extrudates have
shown that a preferential orientation along the transverse direction is normally achieved
with the fish-tail dies and the dual-taper die, while the extrudates obtained with the
cross die were found to exhibit a cross-ply orientation pattern with a bias in the
extrusion direction for outerlayers and a preferential orientation in the transverse
direction for the middle layers.
The mechanics of the processes has been analysed by a plasticity approach for solid
state extrusion, and by using variable wall boundaries for the melt extrusion analysis.
The extrusion pressure predicted by the analysis compares very well with the values
|Description: ||A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.|
|Appears in Collections:||PhD Theses (Materials)|
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