Page 23 - Plastics Engineering

6 General Properties of Plastics

mechanical properties are not heat sensitive. Examples of thermosets are phenol
formaldehyde, melamine formaldehyde, urea formaldehyde, epoxies and some
polyesters.

1.3 Plastics Available to the Designer

Plastics, more than any other design material, offer such a wide spectrum of
properties that they must be given serious consideration in most component
designs. However, this does not mean that there is sure to be a plastic with the
correct combination of properties for every application. It simply means that
the designer must have an awareness of the properties of the range of plastics
available and keep an open mind. One of the most common faults in design
is to be guided by pre-conceived notions. For example, an initial commitment
to plastics based on an irrational approach is itself a serious design fault. A
good design always involves a judicious selection of a material from the whole
range available, including non-plastics. Generally, in fact, it is only against a
background of what other materials have to offer that the full advantages of
plastics can be realised.
In the following sections most of the common plastics will be described
briefly to give an idea of their range of properties and applications. However,
before going on to this it is worthwhile considering briefly several of the special
categories into which plastics are divided.

13.1 Engineering Plastics
Many thermoplastics are now accepted as engineering materials and some are
distinguished by the loose description engineering plastics. The term probably
originated as a classification distinguishing those that could be substituted satis-
factorily for metals such as aluminium in small devices and structures from
those with inadequate mechanical properties. This demarcation is clearly arti-
ficial because the properties on which it is based are very sensitive to the ambient
temperature, so that a thermoplastic might be a satisfactory substitute for a metal
at a particular temperature and an unsatisfactory substitute at a different one.
A useful definition of an engineering material is that it is able to support
loads more or less indefinitely. By such a criterion thermoplastics are at a
disadvantage compared with metals because they have low time-dependent
moduli and inferior strengths except in rather special circumstances. However,
these rather important disadvantages are off-set by advantages such as low
density, resistance to many of the liquids that corrode metals and above all, easy
processability . Thus, where plastics compete successfully with other materials
in engineering applications it is usually because of a favourable balance of
properties rather than because of an outstanding superiority in some particular
respect, although the relative ease with which they can be formed into complex
shapes tends to be a particularly dominant factor. In addition to conferring the