Materials for engineering, 3rd Edition - (Malestrom)

Organic polymeric materials 163
cold mould closes around it (Fig. 5.3). The top of the parison is snipped off
by the closing of the mould and the bottom becomes the mouth of the bottle.
Air is injected and the inflated parison solidifies in the mould which then
opens to eject the bottle and the whole cycle is repeated.
5.3 Directionality of properties
The methods used to fabricate polymers have a strong influence on the
properties and performance of the product. The variations in properties are
largely associated with the differences in molecular orientation in the
manufactured object. Molecules in the melted polymer flow in a non-Newtonian
manner and tend to line up in the direction of flow. Molecules in thermoplastic
polymers are much longer than those in the unreacted liquid resin precursors
to thermosets.
The orientation effect in the injection moulding of thermoplastics is frozen
as the melt solidifies upon cooling. The resulting tensile strength in the direction
of molecular alignment is much greater than that in the cross-flow direction.
5.4 Mechanical properties
We will categorize the mechanical properties of polymers by stiffness, strength,
and toughness. Figure 5.4 shows schematic stress–strain curves obtained at
room temperature for the main classes of polymer. Curve 1 shows the longrange
elasticity typical of elastomers. These are wholly amorphous polymers,
but the chains are lightly cross-linked so that the large extensions are wholly
recoverable.
Extruded
tube
Parison
Moulding
Split mould
5.3 Blow moulding of plastic bottles.
Compressed
air