Project report—Running shoes’ Sole Prepared by: Huijian Tian December 1, 2015 USC ID: 2386525387

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AME 588 PROJECT REPORT

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

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

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

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

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

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

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

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

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

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

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Figure 9, relative cost for small batch

Figure 10, relative cost for large quantity