Page 84 - Plastics Engineering

Mechanical Behaviour of Plastics 67

Fig. 2.18 Equivalent section for sandwich moulding

exist between density (p) and modulus (Moore et ul.).

2
(2.19)

From the equivalent section the second moment of area can then be calcu-
lated as
I=--- 12(1213 12(813 + 12 (-)2 m(
12 12 909
= 1439 UTI^

The solid polypropylene beam which would have the same stiffness when
loaded in the same way would need to have the same 2nd moment of area. So
if its depth is d then

12(d)3
-- - 1439, d = 11.3 ~UTI
12
The weight per unit length of the solid beam would be
W, = 12 x 11.3 x x 909 x lo3 = 123 g
The weight per unit length of the foamed beam is

Wf = (909 x 2 x 12 x 2 x + (600 x 12 x 8 x
= 101.2 g

Hence the weight saving is 17.7%
Once the foamed plastic moulding has been converted to an equivalent
section of solid plastic then the long term design procedures illustrated in the