Page 292 - Plastics Engineering

Processing of Plastics 275

From (4.2) the drag flow, Qd, is given by

From (4.6) the pressure flow, Qp, is given by

1 dP
Q ----TH
- 124 dz
So combining these two equations, the total output, Q, is given by

(4.19)

This must be equal to the volume of coating on the wire so
Q = nVd((R + h)* - R2)
Q = nVdh(2R + h) (4.20)

Combining equations (4.19) and (4.20)
TH3 P
.
-
nVdh(2R + h) = iTHVd + -
12q L
from which
p=- 6qLvd (2H - H) (4.21)
H3
This is an expression for the pressure necessary at the extruder exit and
therefore enables the appropriate extrusion conditions to be set.
(f) Recent Developments in Extrusion Technology
(i) Co-Extrusion As a result of the wide range of requirements which occur
in practice it is not surprising that in many cases there is no individual plastic
which has the correct combination of properties to satisfy a particular need.
Therefore it is becoming very common in the manufacture of articles such
as packaging film, yoghurt containers, refrigerator liners, gaskets and window
frames that a multi-layer plastic composite will be used. This is particularly true
for extruded film and thermoforming sheets (see Section 4.4). In co-extrusion
two or more polymers are combined in a single process to produce a multi-
layer film. These co-extruded films can either be produced by a blown film or
a cast film process as illustrated in Figs 4.28(a) and (b). The cast process using
a slot die and chill roll to cool the film, produces a film with good clarity and
high gloss. The film blowing process, however, produces a stronger film due
to the transverse orientation which can be introduced and this process offers
more flexibility in terms of film thickness.