Question:
I have a nylon pivot which has suffered brittle failure in a bathroom
environment. The peice is a hollow cylinder diameter 14mm with a 3mm
wall. This peice cannot be made to fail with an impact loading either
under a streight blow or with a twist. Are there any specific
materials/cleaning products that might promote stress cracking in
Nylon?
Answer:
Some shampoos or cleaning products, both of which contains soaps and
surfactants, may stress-crack some nylons, but you would have to run tests
on the specific nylon (6 or 6,6 or 6,12, etc.) and the specific
stress-cracking agent.
If you have a magnifying glass you may want to look at the broken surface.
Does the surface have many hair-like cracks, which is typical of most
stress-cracking? Is there a void in the surface, indicating incomplete
filling or a piece of contaminant fall out?
The other possible issue with some nylons is water absorption. Absorbed
water can act like a plasticizers, making the nylon tougher. If the part
broke when it was very dry (in the winter?) and since that time it
re-absorbed some water, the part may be significantly tougher now.
Go to the DSM Polymer, Ensinger-Hyde (A.L.Hyde) or Nytef websites and see if
they have chemical resistant charts--look specifically for clorox, oils etc.
Also, Nylon swells in the presence of moisture--which could cause cracking or
failure if it's in a tight tolerance application.
Might consider Nylon 12 (low moisture absorption), Acetron GP (Delrin) or even
UHMW which has better chemical resistance and no moisture absorption to speak
of but not all the strength characteristics of the other two.
