Page 31 - Plastics Engineering

14 General Properties of Plastics

Table 1.3
Comparison between die casting alloys and nylons
Points for comparison Die casting alloys Nylon

Cost of raw materidtonne LOW High
Cost of mould High Can be lower - no higher
Speed of component Slower than injection Lower component
production moulding of nylon production costs
Accuracy of component Good Good
Post moulding operations Finishing - painting. Paint Finishing - not required -
chips off easily painting not required.
Compounded colour
retention permanent.
Surface hardness Low - scratches easily Much higher. Scratch
resistant.
Rigidity Good to brittleness Glass reinforced grades as
good or better
Elongation LOW GR grades comparable
unfilled grades excellent
Toughness (flexibility) LOW GR grades comparable
unfilled grades excellent
Impact Low All grades good
Notch sensitivity LOW LOW
Youngs modulus (E) Consistent Varies with load
General mechanical Similar to GR grades of 66 Higher compressive strength
properties nylon
Heat conductivity High Low
Electrical insulation Low High
Weight High Low
Component assembly Snap fits difficult very good

Typical applications for nylon include small gears, bearings, bushes,
sprockets, housings for power tools, terminal blocks and slide rollers. An
important design consideration is that nylon absorbs moisture which can
affect its properties and dimensional stability. Glass reinforcement reduces this
problem and produces an extremely strong, impact resistant material. Another
major application of nylon is in fibres which are notoriously strong. The density
of nylon is about 1100 kg/m3.
Acetals. The superior properties of acetal in terms of its strength, stiffness
and toughness have also earned it a place as an engineering plastic. It is more
dense than nylon but in many respects their properties are similar and they can
be used for the same types of light engineering application. A factor which may
favour acetal in some cases is its relatively low water absorption. The material
is available as both a homopolymer and a copolymer. The former is slightly
stronger and stiffer whereas the copolymer has improved high temperature
performance. This latter feature makes this material very attractive for hot
water plumbing applications and as the body for electric kettles.