Re: [CAD_CAM_EDM_DRO] plastics

Hi Ian:
Acrylic has a pronounced tendency to crack under stress.
This is an inherent shortcoming of the material and is only partially
alleviated by stress relieving in an oven.
For times and temperatures, you will need to refer back to the manufacturer
of the plastic, because they vary significantly, and there are many.
A better material choice is polycarbonate.
It too is available in many grades; 144R is a very tough but somewhat
flexible grade and would be my choice.
Polycarbonate is a bit more difficult (but not significantly so) to machine
than acrylic because it is gummier, but it
polishes nicely, and is far more resistant to fracture than acrylic.
Polycarbonate is what the clear spectator shields at the end zones of hockey
arenas are made of.
Cheers

Marcus

PS: the commercially made mouthpieces you refer to are probably injection
molded from polycarbonate.

----- Original Message -----
From: "Ian Wright" <Ian@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Saturday, January 12, 2002 1:34 PM
Subject: [CAD_CAM_EDM_DRO] plastics


> Forgive me for raising a question which is not specifically related to CAD
> or CAM but I have a problem which is perplexing me and preventing me from
> finishing my mill !! ;o)
>
> I have been making a new mouthpiece for my euphonium out of acrylic
> (perspex, lucite?) and I have turned it from a solid 2" diameter rod. When
I
> finished it and polished it it looked the bees knees and I was delighted
> but, since I have been playing on it for a few days, it has developed a
> network of fine cracks around the narrowest portion of the bore. I am
> assuming that these must be due to vibration from the sweet musical notes
> but, since such plastic mouthpieces are made and sold commercially in the
> US, I assume that they shouldn't be there and are probably due to stresses
> already in the plastic.
> Does anyone know if there is a way to stress relieve plastics after
> machining?
>
> Ian
> --
> Ian W. Wright
> Sheffield UK
> www.iw63.freeserve.co.uk
>
>
> Addresses:
> FAQ: http://www.ktmarketing.com/faq.html
> FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/
>
> Post messages: CAD_CAM_EDM_DRO@yahoogroups.com
> Subscribe: CAD_CAM_EDM_DRO-subscribe@yahoogroups.com
> Unsubscribe: CAD_CAM_EDM_DRO-unsubscribe@yahoogroups.com
> List owner: CAD_CAM_EDM_DRO-owner@yahoogroups.com, wanliker@...
> Moderator: jmelson@... timg@... [Moderator]
> URL to this page: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO
> bill,
> List Manager
>
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>