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Preliminary Plastics Selection - p1

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)

Polysulphone, Acetal & ABS Parts

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.

 

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