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Conditions

Preliminary Plastics Selection - p4

A Quick Guide to Plastics Selection

Gas Filled ABS Agitator

Gas Filled ABS Agitator

Approach (read this first)

 

Criteria -

 

Cost & Availability

Operating Temperatures

Transparency Needs

Impact and Toughness

Abrasion & Wear Issues

Strength

Environmental Resistance

 

References

Strength

The vast majority of polymers are substantially weaker than mild steel. Tensile strengths up to 20 - 50 MPa are common in some commodity plastics, up to around 50 - 80 MPa for engineering plastics and in some high performance plastics up to 150+ MPa.

 

Due to effects such as stress relaxation, creep and stress induced crazing and cracking B&B recommend that structural plastic parts have working Factors of Safety on tensile strengths of between 6:1 and 10:1 (ratio of actual working stress to material tensile strength).

 

Fibre and particulate fillers can affect tensile strengths positively so check details for particular grades offered by suppliers.

 

In the references -

 

B&B, table 6-4 provide approximate tensile strengths for several polymers.

Strong, table 7.1 shows strength data for some Engineering & High Performance polymers.

CC&F, table 6.5 shows short term tensile strength for several plastics.

 

To show some effects of fibre reinforcement CC&F, table 6.7 shows tensile strengths for some reinforced Nylons, Polycarbonates and Polyesters. Note also the effect on elongation (ductility), whereas strength generally improves with reinforcement ductility reduces.

 

CC&F, table 3.4 shows some properties of a number of polymers and their blends.

 

 

 

Environmental Resistance

 

The ability of the polymer to maintain its properties after exposure to the environment may be significant. Particular aspects of this that are of interest are -

 

- Behaviour on exposure to UV, for exterior use.

- Moisture absorption, effect on properties and dimensional stability.

- Chemical exposure, special resistance needs.

 

 

Exposure to UV Light

 

Prolonged exposure to UV light can cause deterioration of properties including discolouration and embtrittlement. Polymers reported to be susceptible are -

 

PE Polyethylene - degrading mechanical properties

PP Polypropylene - ditto

PVC Polyvinylchloride - however UV resistant grades are available

PS Polystyrene - yellows with exposure

 

POM Polyacetals

 

PI Polyimides - stability

PPS Polyphenylene Sulphide

 

It is reported that most polymers do not deal well with prolonged UV exposure. There are some that do better -

 

PMMA Acrylic

CA/CAB Cellulosics

UPVC

 

Strong, table 7.1 indicates UV resistance od some engineering plastics.

 

 

Moisture Absorption

 

Water absorption can be marked in some polymers. It can result in dimensional changes and loss of mechanical properties. The dimensional instability can be detrimental to mechanism components such as gears, bearings etc.

 

Polymers particularly susceptible are -

 

PMMA Acrylics

PVC's

pa Nylons

CA Cellulosics

 

B&B, table 6-6 gives moisture absorption data

B&B, table 6-12 shows more data plus general environmental data.

 

 

Chemical Resistance

 

Plastic exposed to chemicals, for example in fuel tanks, medical equipment, processing equipment etc will be additionally constrained.

 

B&B, table 6-12 indicates the effects of chemicals.

 

Some polymers are resistant to many chemicals, they are -

 

Fluoroplastics (including PTFE)

Vinyl Polymers eg CPVC

Acrylics

Polyethylenes PE

Polypropylene PP

 

+ some other high performance polymers

 

 


 

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