Bob wrote:
On Tue, 16 Sep 2008 16:37:05 -0700 (PDT), Scott
wrote:


Forgive me for asking a plastic question in a metal newsgroup, but it
seemed to be the most applicable place.

I have to machine some round bushing-like parts. The inner diameter of
the bushing is going to hold a rubber packing seal (similar to a
chevron or V seal). Fluid inside the mechanism will be low pressure
water. One option I have is to machine these seals out of a metal like
aluminum or brass. Another option is to machine them out of plastic.
I'm thinking that for prototyping plastic is the way to go (presumably
faster to work with than aluminum or brass). However, I've never
worked with plastic before.

So I'm looking to buy some plastic rod stock, in 3" diameter.

Checking a few online merchants like OnlineMetals.Com, I see that
Nylon is pretty pricey, about 25% more expensive than the same
dimension of aluminum (this surprises me). UHMW is much cheaper, about
70% less than the same dimension of aluminum.

Can someone give me some info on machining these plastics? Does UHMW
machine fairly easily? Is there any reason why I should buy the
significantly more expensive Nylon instead of UHMW? What about
durability? I'd like the prototype to hold up to some hours of use.

Thanks,
Scott


What Dave said, but you might want to try acetal (Delrin). Holds
dimension, and is more rigid than UHMW and nylon.

Bob

Also IIRC acetal has very low water absorption whereas for nylon it can
be significant and causing dimensional swelling. I have been told of one
case where this caused seizure of a bearing when the nylon bearing
equipped machine was left in a damp atmosphere. The OP mentioned water
immersion so it needs to be considered.