Bend But Don't Break

[iwmob]

I'm designing a part that will be molded (injection) in the "flat" and then bent at 90deg angles for assembly (the part would be too expensive to mold in the final configuration). This is not a "living hinge". It will only be bent one time, probably right out of the mold while it's still warm.

Is there a source that describes the best way to design the bend area? I've found dozens of sites that show living hinge design parameters, but they are not acceptable.

I'm planning to use a nylon type material (possibly GF) for its dimensional stability but also need the flexability to bend out of the mold.

Thanks for the help.

 

[dhuffy]

iwmob,

It seems you would be asking for alot out of a GF nylon. I don't think you would be able to bend that material at all without it breaking, even if you do so right out of the mold. You would need to heat the bend area in order for it to even have a chance at working - and even then it may be difficult. I can't say for sure because I never tried it, but...
the configuration you describe does not sound to expensive to mold in the final shape. Or are there other things we were not told about the part, such as wall thicknesses, other part features, etc...?

Any other input you can share with us?

 

[iwmob]

Thanks for the response.

If molded flat, it would be a simple two-piece straight-pull mold. If it were molded in the bent config., it would require a lot of right angle pulls for thru holes and recessed areas. In the flat its overall height (thickness) is only .375". After folding its 1.75" tall. Huge difference in tooling cost.

What material would you recommend? I was looking at an impact modified Nylon 6 (Capron 8253 HS) for its increased flexibility. Do you know a material that has good mechanical integrity and decent flexibility?

 

[patprimmer]

If it must be nylon, then impact modified will be best regards the bending.

Glass filled will never work, as it's elongation at break is to low.

A critical property will be that the elongation at yeild will need to be less than the stress in the outer fibre of the bend area

If you use a toughened nylon, a high molecular weight version will work better as it will have a considerably higher elongation.

Also, DO NOT bend the nylon straight after moulding, as it will be dead dry. Some grades of nylon increase in elongation by around 3 times after conditioning in water. Their flex modulus can also decrease by over 70%, or back to about 1/3 the dry value. Thoroughly moisture condition, then bend. If you want to bend it hot, boil the water for a while just before removing the moulding and bending

Regards
pat

 

[patprimmer]

I can source High molecular weight impact modified Capron for you if you require it, as I am currently doing trials with it for an extruded nylon net which need maximum elongation at break.

My e-mail is patprimmer@swiftdsl.com.au

Regards
pat