Date:         Mon, 13 May 1996 23:34:42 -0700
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         jwakefield@4dmg.net (john wakefield)
Subject:      Re: Need:instument pod frame

Ed Berroth wrote:
  "Maybe it was a run of bad plastic?????????????"

  I thought a reply I wrote earlier about platic pieces like this had 
been to this list, but apparently not.

  In another life, to pay for grad school, I worked for Sheller Globe.  
They made lots of plastic auto parts, skins, wheel covers, injection 
molded configurations, etc.  I wanted to know everything about everything 
in the plant, and had many unauthorized lenghty learning sessions while 
on the clock.  There I learned about "vehicle" and "resin."  For many 
plastic parts, these two groups form the plastisal from which the part is 
formed.  The vehicle evaporates to leave the resin.  Well, the trouble 
is, that this process goes on for years.  That's why you can smell some 
new plastic parts - vehicle is evolving.  When virtually all the vehicle 
is gone, elasticity disappears, and with it plasticity.  So I was told 
that their parts had a built in clock, so that with time, they'd become 
inelastic and brittle.  Heat speeds vehicle evolution, and UV rockets it 
forward.  I was told that UV exposure is like a jack hammer at the 
molecular level.

  Well, a plastic water part made of this kind of material will run its 
clock out and fail with the first need to flex, just as will the 
dashboard parts.  As to specific application of these general rules, I 
would defer to specifice manufacturer technical data.  But their design 
objectives are NOT to build continuous duty parts.  Metal parts may 
effectively be that way, but many low cost plastics, are not.  I suspect 
the clock has run on these dash parts, and to preserve them, you need to 
lower stress exposure.  I don't know if the liquid vehicle replacement 
spray-on products do anything below the surface, but some claim it.

  Random thoughts,
  John Wakefield