1. Project report—Running shoes’ Sole
Prepared by: Huijian Tian
December 1, 2015
USC ID: 2386525387
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AME 588 PROJECT REPORT
2. PROBLEMS DEFINITION
Objective
Material selection and process selection for running shoe sole, in particularly two parts, insole
and outsole.
Goals
Find the materials which offer both cushioning and stability proving to be the best match for
running. Shoe sole can be easily made to match foot’s shape.
• Insole: must be safe to direct contact skin, flexible but strong enough, good sweat and
body fluid resistant, and it is better to be antivirus. Minimize cost
• Outsole: maximize strength, minimize stiffness, minimize weight, minimize cost. Good
fracture toughness, must work under different temperature. Good durability against water,
acid, alkali, oil, complicated environment.
MATERIAL SELECTION
Insole
Function: direct contact with foot skin, cushioning
Constraints: flexible and strong enough under designed strength, biocompatible
Objective: maximize strength, minimize cost
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AME 588 PROJECT REPORT
3. 1, First step, we compare young’s modulus and yield strength. Figure 1 shows that only
elastomers are good candidates for running show insole, with yield strength higher than 1 MPa
and young’s modulus lower than 0.1 GPa. There are nine main elastomers: butyl rubber(IIR),
styrene butadiene rubber(SBR), Ethylene vinyl acetate (EVA), natural rubber(NR),
polychloroprene(Neoprene, CR), polyisoprene rubber(IIR), polyurethane(PU), silicone(medical
grade), silicone elastomer.
2, Second step, we consider the biocompatibility of nine materials.
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AME 588 PROJECT REPORT
Figure 1, Young’s modulus VS Yield strength
4. As it shows in figure 2, there are only four materials left, which are EVA, NR, polyurethane and
silicone.
Final step, we consider environmental properties and cost: carbon footprint VS cost. Also, we
limit the durability in water(fresh) and water(salt) atmosphere to acceptable.
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Figure 2, Biocompatibility of elastomers
5. As it shows in figure 3, all of them are satisfied in durability under water(fresh) and water(salt).
However, silicone has too high price and carbon footprint.
Conclusion: The best material for running shoe insole is Ethylene vinyl acetate(EVA). Also,
natural rubber and polyurethane can be used as an alternative choice.
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Figure 3, carbon footprint VS price
6. Outsole
Function: direct contact with ground when running, grip and stability, cushioning
Constraints: flexible and strong enough under designed impact. Work on variety temperature
from -30 celsius degree to 50 celsius degree. Good durability under different environment
Objective: maximize strength, minimize weight
1, First step, the outsole’s most important role is supporting runner’s weight and as much
flexible as possible. As it shows in previous part, only elastomers are satisfied in both stiffness
and strength. We consider thermal property at first:
In figure 4, durability are considered by limited material’s resistance to water(fresh), water(salt),
soil(acidic) and soil(alkaline) in acceptable. As it shows in figure 4, EVA and polyurethane are not
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Figure 4, minimum service temperature VS maximum service temperature
7. acceptable in durability and service temperature. There are five candidates left: styrene
butadiene rubber(SBR), Nature rubber(NR), Polychloroprene(Neoprene, CR), Polyisoprene
rubber(IIR), silicone elastomer.
2, Second step, running show outsole must be considerably resistant to fracture. We consider
fracture toughness against strength this time:
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Figure 5: fracture toughness VS yield strength
8. As it shows in figure 5, styrene butadiene rubber(SBR), polychloroprene(Neoprene, CR),
silicone elastomer are far more suitable than nature rubber and polyisoprene rubber(IIR) in
terms of damage tolerant.
3, Third step, we consider cost and environmental effects:
As it shows in figure 6, silicone elastomer is the most expensive one. Carbon black reinforced
styrene butadiene rubber is cheap but a relatively high carbon footprint.
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Figure 6, carbon footprint VS price
9. 4, Fourth step, running shoe would be as light as possible to give everyone runner the best
experience. Our goal is to minimize the weight while maintain the strength.
Figure 7 shows a strength limited design at minimum mass. We notice that silicone elastomer
has a high density while a low strength at the same time, which is not suitable as a material
using in running shoe.
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Figure 7, yield strength VS density
10. Conclusion: Carbon black reinforced styrene butadiene rubber(SBR) is the best material for
running shoe outsole, which is stiff enough, very strong, good fracture toughness, relatively low
density and a very cheap price. An alternative choice is polychloroprene.
PROCESS SELECTION
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Figure 8, castability of elastomers
11. Figure 8 is the cast ability of elastomers and figure 9 shows the moldability of elastomers,
where 5 means excellent and 1 means poor. In previous part, we choose Ethylene vinyl
acetate(EVA) and polyurethane as our insole materials, carbon black reinforced styrene
butadiene rubber(SBR) and polychloroprene(Neoprene, CR) as outsole materials. To Ethylene
vinyl acetate(EVA), molding is a better process than casting. To carbon black reinforced styrene
butadiene rubber, molding is the only good process selection.
So we choose molding as our review candidate:
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Figure 9, moldability of elastomers
