Chapter 10 – 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 • Fracture toughness
– Stay away from Tm
• Impact
(s/c)
• Environmental
– Tg all over the map
exposure
• Mechanical Stress
• Dimensional stability
– Ultimate strength and
• Fatigue
stress rupture
• Creep
• Stress Relaxation
• Stiffness (modulus)
– 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.
2.
3.
4.
5.
6.
7.
8.
Higher thermal expansion – nearly 10X that of steel.
Unfilled polymers = 30 times less stiff
Filled polymers (i.e. 40% gf Nylon) = 4 to 5 times less
stiff
More flammable
Deteriorate (degrade) more readily with aging (can
improve with antioxidants)
Electrical/thermal insulators
Much softer
Can not be shaped by cold forming processes
Plastic vs. Metals:
10. Required tolerances greater than metals
11. Warping issues (heat, aging and
moisture)
12. Lower mechanical properties (strength,
impact strength, etc.)
13. Absorb moisture unlike steels
14. Creep (or stress relax) more than steel
15. 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 Types
Amorphous
Polyvinyl Chloride (PVC)
General Purpose Polystyrene (GPPS)
Polycarbonate (PC)
Polymethylmethacrylate (PMMA or Acrylic)
Acrylonitrile Butadiene Styrene (ABS – a terpolymer)
Material Types
Semi-crystalline
Polyethylene (PE, HDPE, LDPE, etc.)
Polypropylene (PP)
Polyamides (PA – Nylon)
Polyesters
Polyethylene Terephthalate (PET)
Polybutylene Terephthalate (PBT)
Polyoxymethylene (POM - Acetal)
Polytetrafluoroethylene (PTFE – Teflon)
Allowable operating Temp? Depends on Tm and Tg:
polymers –
stay below
Tm!!
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 =
si/ei
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
Note: Many system can be
combination of Adhesive and
Abrasive Wear!!
Inclined angle of
imposed tip of
abrasive particle
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 1012!
– Chemical attack – See table 10-14! Difficulty –
predict long term behavior with short term
testing in chemicals – reference InSinkErator
coupler.