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Selection of plastic Materials
The key factors for proper plastic selection include: - Operating temperature in relation to the material's melting point (Tm) and glass transition temperature (Tg) - Mechanical stresses such as strength, creep, stress relaxation, stiffness, impact resistance, and fatigue resistance - Environmental exposures such as chemical resistance, wear resistance, dimensional stability, and flammability - Manufacturing considerations such as the production process Proper material selection requires evaluating all these factors based on the design requirements and intended use of the plastic part. Failure to consider all relevant properties could lead to premature part failure.
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Selection of plastic Materials
- 1. Selection of PlasticMaterials • Read up to section 10.5 • What factors are important for proper plastic selection? Start with Curbell Plastic Catalog!
- 2. What factors areimportant 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
- 3. What factors areimportant for proper plastic selection? • Wear • Chemical exposure • Cosmetics (color) • Aging • Product Design!! • Secondary: – Thermal conductivity – Electrical conductivity – Transparency – Surface finish – Manufacturing process
- 4. Industry Examples: • Bearingsurface – seismic isolator • Rack deflector • Gear shifter • Sterilizer
- 5. Why is designingwith 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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- 7. Plastic vs. Metals: 1.Higher thermal expansion – nearly 10X that of steel. 2. Unfilled polymers = 30 times less stiff 3. Filled polymers (i.e. 40% gf Nylon) = 4 to 5 times less stiff 4. More flammable 5. Deteriorate (degrade) more readily with aging (can improve with antioxidants) 6. Electrical/thermal insulators 7. Much softer 8. Can not be shaped by cold forming processes
- 8. Plastic vs. Metals: 10.Requiredtolerances 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!
- 12. Potential Benefits overMetal: • 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
- 13. Amorphous vs Crystalline Impactresistance – both ways but as a general rule S/C are more brittle than amorphous. Weather Resistance – amorphous polymers slightly better.
- 14. Material Types Amorphous Polyvinyl Chloride(PVC) General Purpose Polystyrene (GPPS) Polycarbonate (PC) Polymethylmethacrylate (PMMA or Acrylic) Acrylonitrile Butadiene Styrene (ABS – a terpolymer)
- 15. 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)
- 16. polymers – stay below Tm!! Allowableoperating Temp? Depends on Tm and Tg:
- 18. Polymer Creep andTemperature Effects • Creep (viscoelastic flow) = change in strain as a function of time usually under constant load and temperature:
- 19. Polymer Creep andTemperature 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?
- 20. Factors that effectcreep/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!
- 22. How to measureCreep: ASTM D2990: Creep Modulus = Ec = σi/εi
- 23. Stress Rupture andEnvironmental 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!!)
- 24. Stress Rupture andEnvironmental 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!
- 25. Design for StressRupture 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!
- 27. Impact Strength • Issuefor parts that see impact loading. Examples?? • Impact Toughness measured with Izod test (energy per thickness) or Gardner test (burst strength).
- 28. How to designfor 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
- 29. Fatigue Failure • Canbe 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.
- 30. Figure 10-10: FatigueCurves for Various Plastics
- 31. Dimensional Stability • Checkyour 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!
- 32. Flammability • Related tocomposition 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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- 34. 10.3 Wear andFriction 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)
- 35. Figure 10-15 – abrasionwear of various plastics
- 36. Figure 10-16 –Effect of filler on wear
- 37.
- 38. 4 Main Typesof Wear: 1. Adhesive Wear – Due to adhesion between surfaces: sesimic sliding system W = k x (sliding distance) x (load) Specific wear rate Archard Equation
- 39. 4 Main Typesof 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!!
- 40. 4 Main Typesof Wear: 3. Erosion – wear produced by interaction of fluid. An issue for PVC pipes, etc.
- 41. 4 Main Typesof Wear: 4. Surface Fatigue – Wear due to repeated compressive stress (i.e. gear teeth). W ≅ (constant)/ (max stress)9
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- 45. 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.
- 46. 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.