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Materials

Materials. Plastics. Introduction. What are Plastics. Polymer “Poly” – many “mer” – unit Many Units Carbon based, high molecular weight, versatile synthetic materials that are built up from monomeric units. How plastics are made. Addition or Condensation Reaction Addition

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Materials

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  1. Materials Plastics

  2. Introduction

  3. What are Plastics • Polymer • “Poly” – many • “mer” – unit • Many Units • Carbon based, high molecular weight, versatile synthetic materials that are built up from monomeric units

  4. How plastics are made • Addition or Condensation Reaction • Addition • A simple combining of molecules without generating byproducts • Vinyls • PE • PP • PS

  5. Addition Reaction - Polyethylene

  6. How plastics are made • Condensation • Involves removing certain atoms from each molecule, allowing the molecules to combine • Byproducts are generated that must be removed • Nylons • PC

  7. Condensation Reaction - Polycarbonate

  8. Types of Plastics • Thermoplastic • Soften with heated, then solidify when cooled • Only physical changes • Thermoset • Polymers that chemically react when heated to form a cross-linked polymer chain network • Not reformable with heating

  9. Thermoplastics • Amorphous • Random Structure • Tg • Polystyrene, Polycarbonate • Semi-Crystalline • Organized Molecular Arrangement • Tg, Tm • Polyethylene, Polypropylene

  10. Crystallinity

  11. Thermoplastics • The ability of plastics to form crystals is largely dependent on the structure of the plastic molecule • Linear plastics with small side groups can form crystalline regions • HDPE, LDPE, Acetals, Nylon and PET

  12. Structure Property Relationship • The Property of a Plastic Material formulation can be tailored to meet most end use applications • The properties are dependent on • The chemical composition of the polymer • Additives

  13. Structure Property Relationship • Chemical Composition varies by • Structure of the repeat unit • Average molecular weight • Molecular weight distribution • Linear, branched or cross-linked

  14. Structure Property Relationship • PMMA and PS are very different in behavior and properties because their repeat units are different

  15. Molecular Distribution

  16. Structure Property Relationship • Number-Average Molecular Weight (Mn) • Mn = NiMi/ Ni • where Ni is the number of molecules of the ith species of molecular weight Mi. • Measured from colligative properties such as: • freezing point depression for low molecular weight • osmotic pressure for higher molecular weight • gel permeation or size exclusion chromatography

  17. Structure Property Relationship • Weight-Average Molecular Weight (MW) • MW= NiMi2/ NiMi • where Ni is the number of molecules of the ith species of molecular weight Mi. • Measured using techniques such as: • light scattering • gel permeation or size exclusion chromatography.

  18. Structure Property Relationship • Polydispersity(MWD) = MW / Mn • A measure of the distribution of molecular weights of polymer chains.

  19. Low shear – lots of entanglements, Mw has direct effect on viscosity Medium shear – reduced entanglements Mw has less effect on viscosity High shear – few entanglements, Mw has no effect on viscosity Effect of Mw on Viscosity High Shear Medium Shear Low Shear Log  Log  Log shear rate

  20. Effects of MWD on Viscosity Narrow MWD Viscosity Broad MWD Shear Rate

  21. Structure Property Relationship • Additives • Used to enhance specific properties • Combustion modifiers • Release agents • Blowing Agents • UV stabilizers • Fillers • Reinforcements • Colors • Additives are like medications, they have side effects

  22. Plastics Behavior and Properties

  23. Plastics Behavior and Properties • Mechanical Behavior • Flow Behavior • Short Term Mechanical Properties • Long Term Mechanical Properties • Thermal Properties • Electrical Properties • Environmental Properties • Other Properties

  24. Mechanical Behavior • Viscoelasticity • Creep • Stress Relaxation • Recovery • Loading Rate

  25. Viscoelasticity • Elastic • The material returns to original shape after the load has been removed • Linear stress strain response • Viscous • The material will deform or flow under load • Nonlinear stress-strain response • Plastics show both responses • Short term load • elastic • Long term load • viscous

  26. Creep • One of the most important results of plastics’ viscoelastic behavior • Deformation over time when a material is subjected to a constant stress • The polymer chains slip past one another • Some of the slippage is permanent

  27. Creep

  28. Stress Relaxation • Gradual decrease in stress at constant strain • Same polymer chain slippage as in creep

  29. Recovery • The degree to which a plastic returns to its original shape after a load is removed

  30. Temperature and Loading Rate Effects • Loading Rate • The rate at which the part is stressed or strained • Thermoplastics become stiffer and fail at smaller strain levels as the strain rate increases • At higher temperatures plastics lose their stiffness and become more ductile

  31. Temperature and Loading Rate Effects

  32. Flow

  33. Types of Flow • Drag Flow • Caused by the relative motion of one boundary with respect to the other boundary that contains the fluid • Two major boundaries in injection unit are the barrel and screw surfaces • Since the screw is rotating in a stationary barrel, one boundary is moving relative to the other boundary • This causes drag flow to occur

  34. Types of Flow • Pressure Flow • Caused by the presence of pressure gradients • Pressure flow is what occurs downstream of the injection unit • Sprue, runner, gate and cavity • Flow occurs because the pressure is higher at the injection unit discharge than in the mold

  35. Types of Flow • For the overall system • The injection unit uses drag flow to move the material and build pressure • This pressure buildup at the discharge of the injection unit results in pressure flow through the mold

  36. Shear Flow Induced by Drag Flow • Different layers of plastics move at different velocities with the maximum velocity being at the moving boundary and zero velocity at the wall

  37. Shear Flow Induced by Pressure Flow • Different layers of plastics move at different velocities with the maximum velocity being at the centerline of flow and zero velocity at the walls

  38. Shear Rate • Difference in velocity per normal distance • The change in shear strain with time • Units of seconds-1 • Drag Flow • Pressure Flow

  39. Shear Stress • The stress required to achieve a shearing type flow • Force divided by the area over which it acts • Units of Pascal or psi • Drag Flow • Pressure Flow

  40. Shear Viscosity • Internal resistance to shear flow • Ratio of shear stress to shear rate • Units of poise or Pa-sec

  41. Shear Heat • Viscous heat generation • Heat generated due to shear flow • Conversion of mechanical energy to heat through friction • Amount is equal to the product of the viscosity and the shear rate squared

  42. Effect of Temperature on Viscosity

  43. Types of Fluids • Newtonian • A fluid whose viscosity is independent of shear rate • Shear thinning(pseudo-plastic) • A fluid whose viscosity decreases with increasing shear rate • Shear thickening(dilatants) • A fluid whose viscosity increases with increasing shear rate

  44. Flow Behavior

  45. Power law Fluids • Polymer melts are shear thinning fluids • The fact that the viscosity reduces with shear rate is of great importance in the injection molding process • Important to know the extent of the change of viscosity with shear rate • m is the consistency index • n is the power law index

  46. Mechanical Properties

  47. Mechanical Properties • Important in all applications • Stiffness • Hardness • Toughness • Impact Strength • The ability to support loads

  48. Mechanical Properties • Mechanical property data is used to • Select materials • Estimate part performance • Predict deformation and stresses from applied loads

  49. Mechanical Properties • Most data have been derived from laboratory tests and may not directly apply to your application • Data should be used for comparison purposes only because • Difference between testing and end use conditions • Material and processing variability • Unforeseen environmental or loading conditions

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