Page 88 - Plastics Engineering

Mechanical Behaviour of Plastics 71
The stiffness ratios (Le. stiffness of the foam sandwich beam relative to
the original solid beam) are also given in Fig. 2.21. In both cases the values
given are independent of the original solid material or its dimensions, so this
provides a good design chart. The design of solid/foam sandwich structures is
also considered in Chapter 3 in the laminate analysis.

2.8 Design of Snap Fits
A major attraction of plastics to designers is the ease with which fast assembly
mechanisms can be incorporated into the end-product. A very good example
of this is the snap fit. A typical design is shown in Fig. 2.22 although there
are many variations. Snap fits exploit the fact that thin plastic sections can
undergo relatively large flexural deflections for a short period of time and
exhibit complete recovery. The design of snap fits is straightforward and does
not involve creep curves since the time-scale of the deflectionlstress is small.
The point that will be illustrated here is that in a real design situations it is
necessary to choose combinations of dimensions which provide the necessary
function but which do not overstress the plastic. In the following example a

Fig. 2.22 Snap fit for Example 2.10