Nylon Check for cracks

[Allegiance]

I was wondering what would be a good way to check an opaque plastic part (BAyer T40 Nylon-basically a Nylon 6) for high stress areas that may crack.

We believe we've tracked down the reason for inconsistent failure modes to insufficiently dried material (i.e. the part fails through yielding in most places but fails through brittle fracture in others) though we're open to suggestions there as well.

Nitric Acid works (not user friendly) as does Zinc Chloride (though the Zinc takes forever to work)-alcohol also seems to work but not consistently.

Thoughts?

Thanks!

MP

 

[patprimmer]

Allegiance

I am having some trouble following your reasoning.

I am trying to remember if T40 is a non nucleated type 6 that can be moulded transparent in thin sections in a cold mould, or if it is an aromatic nylon that inherently has a much more amorphous nature.

Either way,inadequate drying BEFORE processing, will reduce impact and elongation at break, all other thing being equal.

To test whether the nylon was moulded with 2 much moisture, do a melt flow index test on material from the brittle mouldings vs good mouldings or virgin material. An increase in MFI will indicate some degradation.

Zinc chloride is a safe way to test for moulded in stress. You might be able to accelerate it a bit by heating to say 50 deg C. Do not go to 60 deg C as you might start to anneal the part and thereby remove some of the stress you are testing for.

Another way to test for moulded in stress is to anneal the parts and measure changes in dimension, especially warpage.

Inadequate moisture conditioning AFTER moulding also has a dramatic effect on both impact strength and elongation at break. Are the samples you are testing properly moisture conditioned before testing.

Some areas being brittle and some ductile also indicates some problem with the part design, with notches or sudden change of section.

If it is moulded in stress, try a hotter mould or hotter melt temperature. Make sure your ring check valve is in good condition, and pack the part well.







Regards
pat pprimmer@acay.com.au
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