Chapter 10 Selection of Plastic Materials Read up to section 10.5What factors are important for proper plastic selection?Start with Curbell Plastic Catalog!

What factors are important for proper plastic selection?Operating temperatureStay away from Tm (s/c)Tg all over the mapMechanical StressUltimate strength and stress ruptureCreepStress RelaxationStiffness (modulus)Fracture toughnessImpactEnvironmental exposureDimensional stabilityFatigueRepeated loadingFlammability

What factors are important for proper plastic selection?WearChemical exposureCosmetics (color)AgingProduct Design!!

Secondary:Thermal conductivityElectrical conductivityTransparencySurface finishManufacturing process

Industry Examples:Bearing surface seismic isolatorRack deflectorGear shifterSterilizer

Why is designing with plastic more complicated than metal?Highly non-linear materials!Stress-strain curvesSensitivity to temperature, frequency, strain, aging, etc.Anisotropic!So many players (trade names)Lack of published dataToo many materials to choose from!Too many properties to worry about!Impact of design and manufacturing method!

Anisotropy!

Plastic vs. Metals:Higher thermal expansion nearly 10X that of steel.Unfilled polymers = 30 times less stiffFilled polymers (i.e. 40% gf Nylon) = 4 to 5 times less stiffMore flammableDeteriorate (degrade) more readily with aging (can improve with antioxidants)Electrical/thermal insulatorsMuch softerCan not be shaped by cold forming processes

Plastic vs. Metals:Required tolerances greater than metalsWarping issues (heat, aging and moisture)Lower mechanical properties (strength, impact strength, etc.)Absorb moisture unlike steelsCreep (or stress relax) more than steelAging issues to deal with!

Potential Benefits over Metal:Lower cost!Material cheap (most), can inject complicated parts! Case study: 1950 FordDont need to be paintedBetter corrosion resistanceEasily made cosmetically pleasant! Aluminum toothbrush???Lighter

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

Material TypesAmorphousPolyvinyl Chloride (PVC)General Purpose Polystyrene (GPPS)Polycarbonate (PC)Polymethylmethacrylate (PMMA or Acrylic)Acrylonitrile Butadiene Styrene (ABS a terpolymer)

Material TypesSemi-crystallinePolyethylene (PE, HDPE, LDPE, etc.)Polypropylene (PP)Polyamides (PA Nylon)PolyestersPolyethylene 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 EffectsCreep (viscoelastic flow) = change in strain as a function of time usually under constant load and temperature:

Polymer Creep and Temperature EffectsStress 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 polymersStress levelEnvironment (moisture, humidity, chemicals) avoid swell due to moisture!

How to measure Creep:ASTM D2990:Creep Modulus = Ec = si/ei

Stress Rupture and Environmental CrackingCan happen even at low stress levels (

Stress Rupture and Environmental CrackingVery 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 CrackingReduce 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 polymerUse metallic component instead!Knit lines should be parallel to tensile stress field.Avoid KtRun tests on material or your part!

Impact StrengthIssue 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 10-8DesignMinimize KtWatch part thicknessDesign parts that flexUsageRate of LoadingEnvironmentProcessingResidual stress, molding linesConsider annealingMaterialUse PVC, PC, UHMWPE, ABS, etc. Impact modifiersFillers

Fatigue FailureCan 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 StabilityCheck 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!

FlammabilityRelated 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 testsConsider anti-flammability additivies

Approximate stiffness of most materials

10.3 Wear and Friction in PlasticsRemember wear/friction is a system effect!!Consider adding lubricant: PTFE, silicone oil, graphiteConsider 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:Adhesive Wear Due to adhesion between surfaces: sesimic sliding systemW = k x (sliding distance) x (load)Specific wear rateArchard 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 floorW = k x (sliding distance) x (load) 3 (tan a)Specific wear rateInclined angle of imposed tip of abrasive particleNote: 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) ControlPlastics in seawater? Better than steels, But.Permeation liquids can move throughDissolution 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) ControlEnvironmental Stress Cracking. Chemical attack + mechanical stress = premature crackingPhysical 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.

*http://www.fsdb.k12.fl.us/rmc/content/images/bucket_worms2.jpg

*http://www.wonderquest.com/fig2-geode.jpg*Amorphous at Tg and above they turn to mush