Page 91 - Plastics Engineering

74 Mechanical Behaviour of Plastics

L=10 mm 15 t"Un
40
35
20 mm
30
25
5 ' 25 mm
5
E. 20
30 mm
15
35 mm
10
40 mm
5
0
0 1 2 3 4 5
Beam thickness, d (mm)
Fig. 2.24 Variation of stress with beam dimensions

2.9 Design of Ribbed Sections
It will be shown later (Chapters 4 and 5) that it is normally good practice to
design plastic products with cross-sections which are as thin as possible. This
is because thick sections cool very slowly and hence the moulding cycle times
are long resulting in uneconomic production. Also, thick sections tend to shrink
more and can lead to warpage and distortion. Of course thin sections tend to
have low stiffness and so engineers usually adopt geometrical configurations
which will enhance stiffness whilst retaining the required thinness in the wall
section.
In any particular material, the flexural stiffness will be defined by the second
moment of area, I, for the cross-section. As with a property such as area, the
second moment of area is independent of the material - it is purely a function
of geometry. If we consider a variety of cross-sections as follows, we can
easily see the benefits of choosing carefully the cross-sectional geometry of a
moulded plastic component.
All the sections have the same cross-sectional area (and hence the same
weight).

(a) - Solid Rectangle (B = 1OD)
(b) - Solid Rectangle (B = 40)
(c) - Solid Rectangle (B = 20)
(d) - Square (B = D)
(e) - Hollow rectangular box (B = 20, B = 1Oh)