t&d

• Strength


  The ability to resist a force without breaking
• Types of strength


  • Compression
  • Tension
  • Bending
  • Torsion
  • Shear
• Compression strength


  A mine roof-prop will need compressive strength
• Tensile strength


  A stretched wire will need tensile strength
• Bending strength


  Bending forces will try to bend a structure e.g. books on a shelf will apply a bending force
• Torsion


  A twisting force, the use of a screwdriver demands the application of forces called torque
• Shear strength


  A train coupling bolt will need to resist the equal and opposite forces pulling against each other across a component
• Hardness


  The ability of a material to withstand wear, scratching, abrasion and indentation
• Hard materials


  • Cutting tools
  • Drill bits
  • Files
• Of all the materials, diamonds are the hardest
• Elasticity


  The material's ability to return to its original shape after being deformed, this depends on the material and the load carried
• Plasticity


  The readiness of a material to deform to a stretched state when a load is applied, the plastic deformation is permanent even after the load is removed
• Plastic materials


  • Plasticine
  • Red hot metal
• Malleability


  The ability of a material to stretch in all directions without fracture, a malleable material can be hammered into shape
• Ductility


  The ability of a material to be drawn out longitudinally to a reduced cross section, a ductile material must also have a high level of plasticity
• The ductility of metals is an important property that enables sheet metal articles to be pressed into deep shapes such as car body panels, kitchen sinks, etc.
• Brittleness


  Those materials that show little to no deformation before fracture are said to be brittle
• Brittle material


  • Glass
• Stiffness


  The ability of a material to resist elastic deformation, steels are stiffer than aluminium alloys
• Corrosion resistance


  The ability of a product to withstand chemical attack, unprotected mild steel soon rusts in a damp atmosphere, whereas polished stainless steel (contains chromium) will not rust under the same conditions
• Thermal conductivity


  How heat travels through materials, metals especially copper possess high thermal conductivity, fire bricks and ceramics in general only melt at extremely high temperatures
• Electrical conductors


  Materials with a very low resistance to the flow of an electrical current are good electrical conductors e.g. silver, copper and gold, materials such as wood or plastic do not conduct electricity easily are therefore called electrical insulators

  
• Thermoplastics


  Can be easily reshaped with the application of heat and, for this reason they cannot be used in situations where a lot of heat is present
• Despite their low heat resistance, thermoplastics have been used to make many hundreds of products where resistance to heat is not a problem, such as packaging in chocolate boxes, tops on aerosol cans and unbreakable rulers
• Thermoplastic materials


  • ABS
  • Rigid Polystyrene
  • PVC
  • Nylon
  • Acrylic
• ABS


  Stiff, very impact resistant, available in a wide range of colours, low melting point, hard
• Rigid Polystyrene


  Stiff, wide range of colours, can be made impact resistant, hard
• PVC


  Wide range of colours, tough at room temperature, can be used outdoors, stiff/hard, good for fabricating
• Nylon


  Hard, tough, rigid & creep resistant, self-lubricating, resistant to chemicals, good fatigue resistance, high melting point, wear and friction resistant
• Acrylic


  Stiff, glass-clear, very durable outdoors, hard, easily machined, cemented and polished, good electrical insulator, safe with food, impact resistant, brittle
• Thermosetting plastics


  Once formed into a shape they cannot be re-shaped, generally used in situations where strength and resistance to heat is required, although they are generally quite hard, they are easily chipped or cracked

  
• Thermosetting plastic materials


  • Melamine
  • Polyester resin
  • Epoxy resin
  • Urea formaldehyde
• Melamine


  Strong, hard, tough, chemical resistant
• Polyester resin


  Stiff, brittle, chemical & heat resistant, hard
• Epoxy resin


  Soft & rubbery, high tensile strength, heat resistant, versatile adhesives
• Urea formaldehyde


  Stiff, strong, brittle, electrical fittings
• Vacuum forming


  A technique used to shape a variety of plastics, usually rigid polystyrene, used when an unusual shape like a 'dish' or a box-like shape is needed
• Vacuum formed product


  • Toy car
• The simple car mould has been placed in a vacuum former and a compressed polystyrene sheet has been placed above it, the polystyrene has been heated and then vacuum formed to the shape of the mould
• Frequently asked exam questions about vacuum forming include: What do you notice about this mould? Will it be suitable to use? What factors need to be considered when making a mould? If a mould becomes difficult to remove after forming what can be done?