On Mon, 01 Aug 2011 10:08:01 +0100, David Billington wrote:
anorton wrote:
"Christopher Tidy" cdt22... wrote ...
Gunner Asch wrote:
If its not dynamically loaded..should work fine. If its loaded
...it wont last as long as other better Stuff will.

Thanks for the thoughts. It's a piston in a miniature air pump. Here's
a picture. The plastic layer is over the knurling you can see:
http://www.mythic-beasts.com/~cdt22/pump_piston.jpg

I had wondered about oil-filled nylon. It's definitely not epoxy
as I've been told it was thermally moulded.

It is too clear to be filled with anything. It could be nylon,
polycarbonate or even low density polyethylene. If you can do
destructive testing, strong acids will eat nylon, and ketones (lacquer
thinner) will eat polycarbonate. Polyethylene will be more resistant to
both, but it is not compatible with mineral oil.
There are some easier tests for basic plastic types and more detail can
probably be found on the net. 3 of the basic ones I remember are nylon
when burned smells like burning hair, polypropylene and polyethylene
smell like a burning candle but one floats in water though I can't
remember which one, it'll be the less dense one anyway. There are a
number of other simple tests like these to help narrow down the basic
stuff.

I've seen the following chart several places, but most clearly at:
http://www.consultekusa.com/pdf/Tech%20Resources/New%20ID%20chart%20.pdf
The tests are destructive but can be done with slivers of plastic, via
techniques as at http://www.chymist.com/Polymer%20Identification.pdf.
(On my system, that page didn't display in browser until after I used
wget http://www.chymist.com/Polymer%20Identification.pdf
to fetch it.) It may be that traces of oil on the plastic will
interfere with flame-based and density-based tests given in those
references.

If OP (C.D.T.) hadn't been told that the piston cover "was thermally
moulded", I'd have thought it could have been made by sliding a
short length of vinyl tubing over a knurled brass part.

--
jiw