Page 35 - Plastics Engineering

18 General Properties of Plastics

toughness, less shrinkage during curing, better weatherability and lower mois-
ture absorption. A major area of application is in the aircraft industry because
of the combination of properties offered when they are reinforced with fibres.
They have an operating temperature range of -25 to 150°C.

1.4 Selection of Plastics
The previous section has given an indication of the range of plastics available
to the design engineer. The important question then arises How do we decide
which plastic, if any, is best for a particular application? Material selection
is not as difficult as it might appear but it does require an awareness of the
general behaviour of plastics as a group, as well as a familiarity with the special
characteristics of individual plastics.
The first and most important steps in the design process are to define clearly
the purpose and function of the proposed product and to identify the service
environment. Then one has to assess the suitability of a range of candidate
materials. The following are generally regarded as the most important charac-
teristics requiring consideration for most engineering components.

(1) mechanical properties - strength, stiffness, specific strength and stiffness,
fatigue and toughness, and the influence of high or low temperatures on these
properties;
(2) corrosion susceptibility and degradation
(3) wear resistance and frictional properties;
(4) special properties, for example, thermal, electrical, optical and magnetic
properties, damping capacity, etc;
(5) moulding and/or other methods of fabrication.
(6) total costs attributable to the selected material and manufacturing route.
In the following sections these factors will be considered briefly in relation
to plastics.
1.4.1 Mechanical Properties
Strength and Stiffness. Thermoplastic materials are viscoelastic which means
that their mechanical properties reflect the characteristics of both viscous
liquids and elastic solids. Thus when a thermoplastic is stressed it responds by
exhibiting viscous flow (which dissipates energy) and by elastic displacement
(which stores energy). The properties of viscoelastic materials are time,
temperature and strain rate dependent. Nevertheless the conventional stress-
strain test is frequently used to describe the (short-term) mechanical properties
of plastics. It must be remembered, however, that as described in detail in
Chapter 2 the information obtained from such tests may only be used for an
initial sorting of materials. It is not suitable, or intended, to provide design data
which must usually be obtained from long term tests.