In the same case of water bottles, why are some bottles very flimsy, while others are very firm?
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It is hard to tell if you’re referring to different kinds of plastics or different kinds of bottles made of the *same* plastic.
Milk bottles and some (very few these days) water bottles are made from HDPE (high density polyethylene), which is a more flexible plastic than used in drink bottles made from PET (Polyethylene terephthalate).
PET is the most common material for plastic water bottles these days. As PET bottles go, the firmness depends on the wall thickness of the plastic as well as the rib structure. Bigger bottles generally need to be a bit thicker so they don’t collapse from holding them.
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