@Alex: When it comes to environmental stress cracking, there is not necessarily any advantage to making the plastic thicker or thinner. The key variables are stress and chemical exposure. If by making the plastic thicker, you can reduce the stress below the threshold, then it might be a solution. But often the threshold stress is so low that this is impractical. And if the internal stresses in the material are high enough, it doesn't matter what you do with the external stress.
Here is a good introduction to residual stress in plastics. At some risk of oversimplification, thin-wall sections are more likely to have flow-induced residual stresses, while thick-wall sections are more likely to have thermal-induced residual stresses. But either way, molded-in stresses can be significant.
I never realized the commonplace chemicals like oil and grease were precursors to stress cracking in plastics. Good to know. Also didn't realize there's some built-in prestress. It seems that, in consumer systems, the plastic always ends up cracking at some point. Is that because thin(ner) plastics are always prone to cracking (and on the other side of the design equation, making them thick enough to be more crack resistant doesn't comport with weight and cost requirements. Or are the thicker plastics just as stress-crack prone?)
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