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Finite element analysis can help in the design phase. It will tell you about flow. Simulation can prevent mistakes and allow you to tweak the design before tooling. This saves a lot of time and risk, particularly because you can't trust prototypes. 3D printed and machined parts cannot give the right material properties in the flex area.

[Daniel Harris]

Hey. I’m working on an insecticide sprayer, using a living hinge battery compartment. I’d like thoughts on design and durability issues. I know material selection is a big deal – it needs to be tough, flexible and durable. What about Polypropylene? It’s supposed to have a good fatigue resistance and it’s not expensive.

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[Christoph]

Agree. Live hinges work well in polyethylene, but not for cosmetic products. PP isn’t really an engineering material and it’s tricky to get great finish – pick an experienced tool designer/molder. 

The thickness is also crucial. Start about 0.2 to 0.4 mm. I like to start the tooling with the hinge at the thinnest, as the thicker the metal off the tool, the thicker the metal.

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[Daniel Harris]

How do you ensure the hinge’s durability? This one won’t see huge cycles.

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[Christoph]

A parallel hinge will crack sooner than a tapered one. Smoothly thinning towards the center will reduce stress concentration.

Consider the bent radius of the hinge. A longer flange reduces the stress. Avoid all sharp transitions in the flex area. A bend radius of 0.25 to 0.5 times the hinge thickness is good.

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[CommunityMember]

Also avoid positioning it near areas of high stress. Keeping the hinge away from turbulent flow will allow better chain alignment, which improves crystallinity. Turbulent flow will make the flex part amorphous/fragile.

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[Daniel Harris]

Good point. He’s the model I created following these design tips. Now I want to make sure the molding process of the live hinges will be flawless. 

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[CommunityMember]

Finite element analysis can help in the design phase. It will tell you about flow. Simulation can prevent mistakes and allow you to tweak the design before tooling. This saves a lot of time and risk, particularly because you can’t trust prototypes. 3D printed and machined parts cannot give the right material properties in the flex area.

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[Daniel Harris]

This is really helpful, thank you. I’ll also run flexing tests for the hinge to confirm it can withstand repeated use.

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