Selection of Plastic Materials

• Read up to section 10.5• What factors are important for proper

plastic selection?

Start with Curbell Plastic Catalog!

What factors are important for proper plastic selection?

• Operating temperature– Stay away from Tm (s/c)– Tg all over the map

• Mechanical Stress– Ultimate strength and

stress rupture• Creep• Stress Relaxation• Stiffness (modulus)

• Fracture toughness• Impact• Environmental

exposure• Dimensional stability• Fatigue

– Repeated loading• Flammability

What factors are important for proper plastic selection?

• Wear• Chemical exposure• Cosmetics (color)• Aging• Product Design!!

• Secondary:– Thermal conductivity– Electrical conductivity– Transparency– Surface finish– Manufacturing process

Industry Examples:

• Bearing surface – seismic isolator• Rack deflector• Gear shifter• Sterilizer

Why is designing with plastic more complicated than metal?

• Highly non-linear materials!– Stress-strain curves– Sensitivity to temperature, frequency, strain, aging,

etc.– Anisotropic!

• So many players (trade names)• Lack of published data• Too many materials to choose from!• Too many properties to worry about!• Impact of design and manufacturing method!

Anisotropy!

Plastic vs. Metals:1. Higher thermal expansion – nearly 10X that of steel.2. Unfilled polymers = 30 times less stiff3. Filled polymers (i.e. 40% gf Nylon) = 4 to 5 times less

stiff4. More flammable5. Deteriorate (degrade) more readily with aging (can

improve with antioxidants)6. Electrical/thermal insulators7. Much softer8. Can not be shaped by cold forming processes

Plastic vs. Metals:

10.Required tolerances greater than metals11.Warping issues (heat, aging and

moisture)12.Lower mechanical properties (strength,

impact strength, etc.)13.Absorb moisture unlike steels14.Creep (or stress relax) more than steel15.Aging issues to deal with!

Potential Benefits over Metal:

• Lower cost!– Material cheap (most), can inject complicated

parts! Case study: 1950 Ford• Don’t need to be painted• Better corrosion resistance• Easily made cosmetically pleasant!

Aluminum toothbrush???• Lighter

Amorphous vs Crystalline

Impact resistance – both ways but as a general rule S/C are more brittle than amorphous.Weather Resistance – amorphous polymers slightly better.

Material TypesAmorphous

Polyvinyl Chloride (PVC)

General Purpose Polystyrene (GPPS)

Polycarbonate (PC)

Polymethylmethacrylate (PMMA or Acrylic)

Acrylonitrile Butadiene Styrene (ABS – a terpolymer)

Material Types

Semi-crystallinePolyethylene (PE, HDPE, LDPE, etc.)

Polypropylene (PP)

Polyamides (PA – Nylon)

Polyesters

Polyethylene Terephthalate (PET)

Polybutylene Terephthalate (PBT)

Polyoxymethylene (POM - Acetal)

Polytetrafluoroethylene (PTFE – Teflon)

polymers – stay below Tm!!

Allowable operating Temp? Depends on Tm and Tg:

Polymer Creep and Temperature Effects

• Creep (viscoelastic flow) = change in strain as a function of time usually under constant load and temperature:

Polymer Creep and Temperature Effects

• Stress Relaxation = change in stress as a function of time usually under constant deformation (strain) and temperature:

What other components might see stress relaxation?

Factors that effect creep/stress relaxation (see T 10-5):

• Polymer structure: amorphous or crystalline (amorphous usually better)

• Fillers or reinforcements (better with glass filler up to a point)• Temperature – stay below Tg by at least 50C for amorphous

polymers• Stress level• Environment (moisture, humidity, chemicals) – avoid swell due to

moisture!

How to measure Creep:

ASTM D2990:

Creep Modulus = Ec = i/i

Stress Rupture and Environmental Cracking

• Can happen even at low stress levels (<<Su).

• Due to sustained tensile stress. Can be static stress!!

• Cracks form under constant stress, propagate until failure! (i.e. this is the classical metal failure except dynamic stress!!)

Stress Rupture and Environmental Cracking

• Very unique to polymers – all polymers susceptable to failure via stress rupture.

• Accounts for 30 – 40% of all plastic part failures.

• Can be greatly influenced by temperature, environment (chemical exposure) and of course stress!

Design for Stress Rupture and Environmental Cracking

• Reduce stress to value below Rupture Stress (Table 7.7) or design to 1/10 to 1/6 of Su.

• Anneal parts to relieve residual stresses (Table 7.9)

• Use fiber reinforcement or select alternate polymer

• Use metallic component instead!• Knit lines should be parallel to tensile stress

field.• Avoid Kt• Run tests on material or your part!

Impact Strength

• Issue for parts that see impact loading. Examples??

• Impact Toughness measured with Izod test (energy per thickness) or Gardner test (burst strength).

How to design for Impact?? – see Table 10-8

• Design– Minimize Kt– Watch part thickness– Design parts that flex

• Usage– Rate of Loading– Environment

• Processing– Residual stress, molding lines– Consider annealing

• Material– Use PVC, PC, UHMWPE, ABS, etc. – Impact modifiers– Fillers

Fatigue Failure

• Can be an issue with repeated dynamic loading – plastics may or may not have an endurance limit).

• Frequency can be an issue due to excessive heat build up.

• Design to below the fatigue strength (or endurance limit) if possible.

Figure 10-10: Fatigue Curves for Various Plastics

Dimensional Stability

• Check your design!– Consider high temperature and calculate

dimensional changes using coef of thermal exp.

– Look at moisture absorption rate (Table 7-6) and calculate dimensional change.

– Redesign if above present problems!

Flammability

• Related to composition of polymer higher hydrogen to carbon ratio higher the combustion!

• Consider self extinguishing polymers.• Look at limiting oxygen index – want

polymer greater than 21% (air). Example PTFE = 90% (Table 10-10).

• Run tests• Consider anti-flammability additivies

Approximate stiffness of most materials

10.3 Wear and Friction in Plastics

• Remember wear/friction is a “system effect”!!

• Consider adding lubricant: PTFE, silicone oil, graphite

• Consider reinforcement: carbon or glass fiber (Figure 10.16)

• Consider material (Figure 10.15)

Figure 10-15 – abrasion wear of various plastics

Figure 10-16 – Effect of filler on wear

Figure 10-17 – Wear Test

4 Main Types of Wear:

1. Adhesive Wear – Due to adhesion between surfaces: sesimic sliding system

W = k x (sliding distance) x (load)

Specific wear rate

Archard Equation

4 Main Types of Wear:

2. Abrasive Wear – hard surface imposed on softer surface (i.e. think file) – hand tool sliding across a concrete floor

W = k x (sliding distance) x (load) 3 (tan a)

Specific wear rate

Inclined angle of imposed tip of abrasive particle

Note: Many system can be combination of Adhesive and Abrasive Wear!!

4 Main Types of Wear:

3. Erosion – wear produced by interaction of fluid.An issue for PVC pipes, etc.

4 Main Types of Wear:

4. Surface Fatigue – Wear due to repeated compressive stress (i.e. gear teeth).

W (constant)/ (max stress)9

Figure 10-18

10.4 Corrosion (Environment) Control

• Plastics in seawater? Better than steels, But….– Permeation – liquids can move through– Dissolution – chemicals dissolve polymer

chain – be careful!– Absorption – can absorb water or chemicals

which weaken or soften polymer AND cause dimensional changes due to swell.

10.4 Corrosion (Environment) Control

– Environmental Stress Cracking. Chemical attack + mechanical stress = premature cracking

– Physical aging – certain polymers susectable to certain degradation modes – See Table 10-12!

– Chemical attack – See table 10-14! Difficulty – predict long term behavior with short term testing in chemicals – reference InSinkErator coupler.