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Does PLA shrink if it gets wet?

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    Does PLA shrink if it gets wet?

    I have a printed chain which runs over two printed sprockets. It has, in the past, run successfully. I have just tried to re-install the chain on the sprockets and it is one link too short:
    Click image for larger version

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    The green-arrowed links are extras - they should not be there. Links 'A' and 'B' should connect directly; i.e. the white-arrowed axes should align.

    This chain and sprocket system operates underwater but it hasn't been in the tank for three months or more and the whole system (all printed in PLA) has dried. Nothing else has altered (as far as the chain drive is concerned).

    I have never heard of PLA shrinking because of that treatment - or any other treatment come to that. Has anyone heard of such a thing? Does anyone have a suggestion as to what might have cause this situation?

    Thanks

    #2
    Hi,

    Plastic can not shrink when getting wet. It absorbs a certain quantity but shrinking only happens when cooling down after having printed something.

    When it was alreadyd running then somthing else happened to your mechanics. IMHO

    Rainer

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      #3
      Firstly, PLA is not a plastic based on petrochemicals - it is made from cornstarch. As such it is probably about as reactive with water as early 19th century Ships Biscuit!
      Then, "Yes, you are right." PLA, like most things, should follow a linear process of expansion and contraction based on temperature. I would expect, however, that this expansion and contraction would affect both the chain and the frame of the machine equally thereby cancelling eachother out.
      Then again, nothing else had changed in the mechanics of the machine.

      To get the machine running again I printed sections of the machine columns 20mm shorter than the original and then shimmed up to chain tension. In operational state the total column length is 8mm shorter than the original. This means that the overall length of the 72-link chain has decreased by 16mm!

      My Mechanical Engineering mind is freaked out but, "The facts of the matter, Your Honour, are these!"

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        #4
        Hi,

        Thanks for the explanation what PLA is. My knowledge about this is not yet that far but if it melts is has to be some sort of plastic.

        Made some searches in Internet and did not find anything about PLA shrinking.

        Maybe a different material should ahve been used for that specific underwater use. ??

        What was the original total length of that chain ?

        Found in internet.

        PLA is an organic, biodegradable material also used in FDM printers. It is one of the most popular materials used in 3D printing because it is easy to print with and also non-toxic. PLA suffers from little shrinkage, hearing shrinkage rates of between 0.2%, up to 3% since it's a lower temperature thermoplastic.
        If we take the worst case scenario after printing a shrinkage of 16 mm would mean your chain is ~ 540mm long ? eg. ~ 21.25", but again if it worked under water and then suddenly anymore, it is a mistery for me.

        what about wetting the chain again and then mount it ?

        OK, remains a mistery

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          #5
          Chain is 32mm pitch so 72 links is 2304mm. Therefore shrinkage is ≈0.7%. Not much, eh! But annoying!

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