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Selection of Plastic Materials
• Read up to section 10.5• What factors are important for proper
plastic selection?
Start with Curbell Plastic Catalog!
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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
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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
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Industry Examples:
• Bearing surface – seismic isolator• Rack deflector• Gear shifter• Sterilizer
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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!
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Anisotropy!
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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
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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!
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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
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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.
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Material TypesAmorphous
Polyvinyl Chloride (PVC)
General Purpose Polystyrene (GPPS)
Polycarbonate (PC)
Polymethylmethacrylate (PMMA or Acrylic)
Acrylonitrile Butadiene Styrene (ABS – a terpolymer)
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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)
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polymers – stay below Tm!!
Allowable operating Temp? Depends on Tm and Tg:
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Polymer Creep and Temperature Effects
• Creep (viscoelastic flow) = change in strain as a function of time usually under constant load and temperature:
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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?
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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!
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How to measure Creep:
ASTM D2990:
Creep Modulus = Ec = i/i
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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!!)
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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!
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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!
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Impact Strength
• Issue for parts that see impact loading. Examples??
• Impact Toughness measured with Izod test (energy per thickness) or Gardner test (burst strength).
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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
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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.
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Figure 10-10: Fatigue Curves for Various Plastics
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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!
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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
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Approximate stiffness of most materials
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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)
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Figure 10-15 – abrasion wear of various plastics
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Figure 10-16 – Effect of filler on wear
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Figure 10-17 – Wear Test
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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
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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!!
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4 Main Types of Wear:
3. Erosion – wear produced by interaction of fluid.An issue for PVC pipes, etc.
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4 Main Types of Wear:
4. Surface Fatigue – Wear due to repeated compressive stress (i.e. gear teeth).
W (constant)/ (max stress)9
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Figure 10-18
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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.
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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.
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