We need the σf − E chart again (Figure 6.14). Candidates are
identified by using the guide line of slope 1; a line is shown at the position
M = σf/E = 2 × 10−2. The best choices for the hinge lie to the right of this
line: They are all polymeric materials. The shortlist (Table 6.14) includes
polyethylene, polypropylene, nylon, and, best of all, elastomers, though
these may be too flexible for the body of the box itself. Cheap products
with this sort of elastic hinge are generally molded from polyethylene,
polypropylene, or nylon. Spring steel and other metallic spring materials
(like phosphor bronze) are possibilities: They combine usable σf/E with high
E, giving flexibility with good positional stability (as in the suspensions of
relays). The table gives further details.