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Re: stopping dash cracks Posted by RS [Email] (#15) [Profile/Gallery] (more from RS) on Tue, 12 Jun 2001 22:53:17 In Reply to: Re: stopping dash cracks, SDBWORKS, Tue, 12 Jun 2001 19:42:28 Members do not see ads below this line. - Help Keep This Site Online - Signup |
I think that there are 2 things happening:
The plastic foam under the vinyl oxidizes and gets brittle under the temps we find here in the US. It also doesn't have much in the way of tensile strength, especially compared with the finyl overlay.
and
The black vinyl (if vinyl is what it is) shrinks under the same heat. The dash is held rigidly on both ends by being bolted to the body and by being fairly rigidly held in shape by the fascia. Because it is forced into shape by the fascia and by the body it develops stresses (sort of like heat-shrink plastic tubing - as you heat it, it shrinks. IF you put it around something unyielding and try to shrink it, it gets tight.)
The vinyl also loses elasticity by aging and losing plasticisers to oxidation and evaporation.
The tension in the vinyl is greatest around the speaker openings and in the coffee cup indent in the dash. (that's where a lot of the cracks start to propogate) When the tension in the vinyl from the shrinkage is greater than its strength at those high stress points, it yields and a crack forms.
If the dash were able to shrink without being held in place by the body or the fascia, it would probably wouldn't crack at all.
Since the tensile strength of the foam is low and it's bonded to the vinyl overlay, it yields along the crack in the vinyl overlay.
As an example, on my '88 900S, I noticed that, in the corners of the coffee cup indent, the vinyl overlay started to pull away from the foam because it shrank. Where it got to a point that it pulled away from the foam too far, it cracked.
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