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Materials selection and design

Which is an appropriate materials selection and design policy for improving the recyclability of a product?

  1. ABond different kinds of plastics and metals into one piece with adhesives instead of screws to reduce the number of parts, making them impossible to disassemble
  2. BUse thermosetting resins, which cannot be melted and reshaped by heat, in as many parts as possible, from the housing to the internal components
  3. CReduce the number of material types, use easily disassembled joints such as screws, and mark plastic parts with their material identification
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Answer: C. Reduce the number of material types, use easily disassembled joints such as screws, and mark plastic parts with their material identification

In recycling, whether collected products can be separated by material strongly affects quality and cost. Reducing the number of material types, using easily removed joints such as screws and snap fits, and marking plastic parts with their material code make dismantling and sorting easier and yield high-purity recycled material. Bonding dissimilar materials into one piece may help reduce weight and part count, but it makes separation difficult and lowers recyclability. Thermosetting resins have a network structure and do not melt when heated, so unlike thermoplastics they cannot be melted and remolded and are difficult to recycle as materials. Performance and recyclability often compete, so both must be compared as objectives.

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More Materials selection and design questions

  1. You want to minimize two objectives at once, the mass and the cost of a part, but lighter materials are more expensive and no material is best on both. Which is an appropriate way to choose a material in this case?
  2. In Ashby's method of materials selection, the design requirements are organized into function, constraints, objective and free variables. In selecting the material for a bicycle frame, which of these is the objective?
  3. A tie (tension member) of fixed length L must carry a tensile load F without yielding, and its cross-sectional area may be chosen freely. To make it as light as possible, which performance index should be maximized, with σf the yield strength and ρ the density of the material?
  4. A beam of square cross section with fixed length and bending stiffness is to be made as light as possible by freely varying the size of the cross section. Which performance index should be maximized, with E the Young's modulus and ρ the density?
  5. On a log–log materials selection chart (Ashby chart) with Young's modulus E on the vertical axis and density ρ on the horizontal axis, what is the slope of the guideline along which the performance index for a light, stiff beam, √E/ρ, is constant?