Page 330 - Plastics Engineering

Processing of Plastics 313

Fig. 4.56 Thermofomed flower pot

Solution

=
(a) al= tan-' (E) 79.4"
12.5
Using the terminology from Fig. 4.39(b)
H = 35tana = 187.6 mm

From equation (4.33)
( 1 + cos 79.C) ( 187.6 - 40)(E79.4)-' = 0.203
who =
2 187.6
h = 0.203 x 2.5 = 0.51 mm
(b) The draw-ratio for a thennoformed moulding is the ratio of the area of
the product to the initial area of the sheet. In this case therefore
RJ[(R - r)2 + h2](R + r) + nr2
Draw ratio =
RR~
- nJ[(35 - 22.5)2 + 672](35 + 22.5) + ~(22.5)~
-
lr(35)2
= 3.6

4.5 Calendering
Calendering is a method of producing plastic film and sheet by squeezing the
plastic through the gap (or 'nip') between two counter-rotating cylinders. The
art of forming a sheet in this way can be traced to the paper, textile and metal
industries. The first development of the technique for polymeric materials was
in the middle 19th century when it was used for mixing additives into rubber.
The subsequent application to plastics was not a complete success because the