A Quick
Guide to Plastics Selection
A few
years ago I needed to some reading-up on plastics and
their selection in engineering design. For an engineer
who practised mainly with steel and its alloys this was
an interesting experience. I made notes at the time and
thought I'd post a summary of them here in the hope they
will be of interest.
I make reference to charts and diagrams
from a handful of excellent technical references - these
are listed at the page bottom and you'll need to buy or
borrow them to see the diagrams for yourself.
Approach
There are probably three points to make
here about the overall approach I took -
1) The range of polymers is vast. It
helps to bring a bit of initial structure to the problem
by learning that a differentiation is often made between
three broad categories of plastics. These are general
purpose or "Commodity"
plastics, "Engineering"
plastics and "High
Performance" plastics. The
categories tend to overlap a bit but generally they
appear
to relate to volume/cost and performance trends -
Commodity plastics being
cheap and plentiful but with modest properties through
to High Performance
plastics which are expensive but with the best levels of
performance. I've colour coded specific types with their
category to make them easier to place.
2) Try to identify the KEY properties you
need from the material - these arise from your design
requirements, ie from the design problem. These KEY
properties may serve to identify particular plastics as
being suitable or unsuitable and to reduce the range of
choice. The properties I thought where most likely to
come up are (in no particular order) -
There will be others, eg appearance,
flammability, toxicity etc but those above seemed to me
to be most important.
3) Use these property requirements to
produce a list of candidate materials, note which ones
fall into the different categories mentioned above, ie
which have the lowest cost and are most readily
available and which candidates are more expenvie or
difficult to get. You can then read-up on the most promising
candidates to prove their suitability or eliminate them.
The aim is to try to reduce the vast range of
possibilities to a few candidate materials that you can
actually get your head around and to pick a plastic that
does the job and is as low a cost as possible .
Cost & Availability
Both B&B and
Strong categorise
thermoplastic polymers into two broad groups - general
purpose or "Commodity"
plastics and "Engineering"
plastics.
Strong uses a further subdivision to
differentiate High Performance
plastics which is useful.
Commodity / General Purpose Plastics
PE Polyethylene
(of various weights)
PP Polypropylene
PVC Polyvinyl Chloride
(Rigidised form mainly)
PS Polystyrene
ABS Acrylonitrile
Butadiene Styrene
B&B also includes
Acrylic (PMMA) and Perspex
in this category. PE is
listed by B&B but it doesn't feature much in technical
use discussions.
Engineering Plastics
PA Polyamides
(Nylons)
(Related are Aromatic Polyamides eg
Kevlar & Nomex)
POM
Polyoxymethylenes (Poly Acetals eg Delrin)
PET Polyethylene
Terephthalate (Polyesters)
|
PSO, POM &
ABS Parts |
PBT PolyButylene
"
PC Polycarbonate
PTFE Poly Tetra Fluro
Ethylene
And High-Performance
PPO Polyphenylene
Oxide (Polyphenylenes)
PPS Polyphenylene
Sulphide
PEEK Polyetherether
Ketone
PSO Polysuphones
PES Polyether Sulphone
PI Polyimide
PAI Polamide-imide
There are others, and many grades within.
But these are the principal polymers mentioned in the
references. B&B figure 19-5 shows DEC 2000 unit volume
costs for several plastics. There is some overlap
between categories but generally the
Commodity plastics are cheaper than the
Engineering plastics which
are cheaper than the High
Performance plastics.
In general mechanical properties,
dimensional stability, high temperature use etc improve
as you move up through the categories. This isn't
entirely consistent however and better properties can be
accompanied by manufacturing difficulties in addition to
higher cost.
