Living Hinge Design for Celcon M90 ( Acetal Copolymer)

[BMDesign]

I am designing a 90° bend living hinge, But it is cracking, can anyone help or send me design guidlines for living hinge with Celcon material.

thanks
Maninder

 

[btrueblood]

I don't think acetal is "ductile" enough for a 90 degree hinge; usually these hinges are made from polyethylene or sometimes polypropylene if the cycle life is to be anything more than a few to 10's. That said, if a few dozen cycles is all that is required... are you notching both sides of the hinge (see second ref. below)?

see

http://plastics.dupont.com/plastics/pdflit/europe/design/L12565_6.pdf

this reference gives more detail on the why's and how's:

http://engr.bd.psu.edu/pkoch/plasticdesign/living_hinge.htm

 

[Pud]

You might get a few cycles (e.g. 5 -10) if you're lucky and have a good design. Acetal usually requires such a thin section to keep the stress down that it becomes impractical to mould.

As btrueblood mentions - PP or PE is a more suited to hinges of this type.


Cheers


Harry

 

[patprimmer]

As others have said, PP is the preferred material. PE also works fine.

There is lots of data out there on design, but it is pretty straight forward.

No sharp edges.

Gate on one side only so material MUST flow through the hinge and MUST NOT stop and form a weld in the hinge.

The hinge should not be to thick.

The land of the hinge should not be to short.

It is best if it is bent through maximum angle while the part is stll hot (ie automatically as the mould opens) so that the molecules are aligned parallel and to each other and perpendicular to the axis of the hinge.

It works best on fairly straight linear molecules. Anything that draws down well into a fibre will work. The higher the optimum draw down for fibre production, the better it will tend to work as a hinge. PP likes very high draw down ratios when making fibres. From vague memory, about 10:1 vs 6:1 for nylon.



Regards

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[BMDesign]

Thank you all for the suggestions. I know it has to PP or PE, but for this application it has to be Acetal only.

Design looks fine, no sharp edges, thin cross section, length of hinge is ine too. I took the guidlines from 'Designing plastic parts for Assembly' by Paul A Tres.

After the initial moldflow analysis, I out that the gate location of my part is not right, which cause to form a weldline across the hinge.

Also I am getting the help of Ticona to get into the root cause of the failure.

 

[dwightdixon]

Dupont at one time recommended that people do a secondary warm forging operation on parts made from their acetel. It makes the hinge thinner that you could normally mold. It also guarentees orientation of the polymer chains and now weld lines. Maybe you could get some help from them if they thought you could convert over to Delrin?

 

[patprimmer]

Homopolymer does have better mechanical properties, so Delrin or some other homopolymer might be somewhat better in your application, that is so long as thermal aging or chemical resistance is not also an issue.

Regards

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